THE

No. 14 - September 1978

Large-Scale Internal Motions in NGC5128

An Ho. interferogram was obtained at the ESO 3.6 m telescope by Drs. J. Boulesteix and G. Courtes in April 1978, bymeans of the Marseille Fabry-Perot interferometer. See the article on page 2.

Interstellar Hydrogen Observed in the Arms and Disks of theNearest Southern GalaxiesJ. Boulesteix and G. Courtes

French astronomers have been studying thekinematics of interstellar hydrogen for more thana decade, mainly by means of Fabry-Perot inter­ferometers (cf. the article by M. Duval in Mes­senger No. 9). Recently, Drs. J. Boulesteix andG. Courtes trom Laboratoire d'AstronomieSpatiale in Marseille travelled to La Silla and ob­tained new and spectacular results about the hy­drogen in some nearby southern galaxies.

Our 3.6 m telescope observing programme on the kine­matics of the nearest galaxies was continued during arecent run of 3 nights (April 3-5, 1978).

The main purpose of these observations is to obtain thehighest possible space and spectral resolutions of thestructure of the ionized hydrogen in the spiral arms as weilas in the disko During the reductions, particular attention isgiven to the velocity gradients of the gas at the edges of themorphologically well-determined spiral structures. Similarwork has already been successfully done on M 33 (theTriangulum nebula) in the northern hemisphere in an at­tempt to verify the density-wave theory (see bibliographygiven by Courtes, invited review paper, lAU Grenoble,Symp. 1975 in Topics in Interstellar Matter, 209-242, ReidelPublishing Company, Dordrecht, Holland, Ed. Hugo vanWoerden).

• •

Figure 1: Three-minute exposure of NGC 2997 in the light of HCI.Note in particu/ar the centra/ hydrogen ring.

2

Some of the best southern candidates for this kind ofstudies are two Sc spiral galaxies, NGC 2997 andNGC 5236, with large apparent diameters.

The instrumentation was the Marseille focal reducer atthe Cassegrain focus of the 3.6 m telescope. This instru­ment gives a final focal ratio of 0.95 when used with aphotographic emulsion as detector and focal ratio 2 withan RCA image tube. The RCA image tube, in spite of a goodcooling, is definitely too noisy for long exposures (over75 min). Therefore, the faintest objects were observed withthe 0.95 focal ratio and the IIla-J photographic emulsion.

We obtained Fabry-Perot interferograms and f/0.95and f/3 direct photographs. In figure 1, we show a directphotograph of the spiral galaxy NGC 2997 in Ha; 3-minuteexposure through the RCA tube on a IIla-J plate. We noticethe interesting morphology of the gas around the nucleus.This complex structure was also pointed out by Sersic, butwe see here an almost complete, central ring of hydrogenwhich is weil defined thanks to the resolving power of the3.6 m telescope.

In another part of our programme, we obtained high-re­solution interferograms of the strong southern radiosource Cen A, also known as NGC 5128, thereby im provingupon the results of Carranza with the 1.5 m telescope at theCordoba Observatory. On the front page is shown the in­terferogram of the central part of this galaxy with verystrong internal motions. Note, for example, the shift of theHa interference rings by comparison with the Ha calibra­tion. The free spectral range between two successive ringscorresponds to approximately 300 km S-l in radial veloci­ty .

Figure 2: H 11 regions in the peculiar interacting ga/axies Arp 244(NGC 4038-39).

3.6 m Teleseope

1.52 m Speetrographie Teleseope

(Oetober 1, 1978-April 1, 1979)

Visiting Astronomers

Bouchet, F. and M. Spite, Bouchet.

Bouchet, Schöffel, Bouchet, Renson.

Renson, Mianes/LMC Group.

Bouchet, Wramdemark, Bastiaansen, Tinbergen,Loden.

Loden, Bastiaansen, Tinbergen, Bouchet.

Bouchet, Haug, Lagerkvist.

Bensammar, Vogt.

Vogt, Gieseking, Azzopardi.

Azzopardi, Gieseking.

Gieseking, Gahm/Andrews.

Gahm/Andrews, Gieseking.

Vogt.

Gahm.

Gahm, Gerbaldi.

Danziger/Brunt/Whelan, Walter.

Isserstedt.

Isserstedt, Pakull.

Pakull, Bastian/Mundt, Loden, Eist.

Eist.

Feb.1979:

March 1979:

Oct. 1978:

Nov. 1978:

Dec.1978:

Jan. 1979:

Tentative Meeting Schedule

Oct. 1978: Laustsen/Tammann, Danks/Alca(no, Bouchet,Bensammar, Nguyen/Wamsteker/Bouchet,Olander, Westerlund/Lundgren, Schoembs.

Nov. 1978: Schoembs, Vogt, Turon/Epchtein, Wamsteker,Knoechel, Bergvall/Ekman/Lauberts/Wester­lund, Schnur/Mattila.

Dec. 1978: Schnur/Mattila, Ardeberg/Lynga, Shaver/Danks/Wamsteker, van Woerden/Danks, F. and M.Querci, Mianes/LMC Group.

Jan. 1979: Mianes/LMC Group, Pakull, Wamsteker, Bouchet,Loden, Wlerick/Bouchet, Melnick.

Feb.1979: Melnick, Salinari/Tanzi/Tarenghi, Dachs,Moorwood, The, Adam.

March 1979: Adam, Mauder, Schultz/Costa, Wamsteker, Haug,Wlerick/Bouchet.

50 em ESO Photometrie Teleseope

1 m Photometrie Teleseope

The following dates and locations have been reserved formeetings of the ESO Council and Committees:

November 14/15 Finance Committee, Geneva

November 16 Committee of Council, Geneva

November 22/23/24 Observing ProgrammesCommittee, Paris

December 7/8 Council, Munich

40 em GPO Astrograph

Jan. 1979:

Feb.1979:

March 1979: Ritter/Schröder, Mauder, Bouchet, Ahlin, Haug,de Vries, Haug, Mauder, Stenholm, Wramdemark,Schnur.

61 em Boehum Teleseope

50 em Danish Teleseope

Oct. 1978:

Nov. 1978:

Dec.1978:

Jan. 1979:

Feb.1979:

March 1979:

Oct. 1978:

Nov. 1978:

Dec.1978:

Jan. 1979:

Feb.1979:Breysacher/Azzopardi, Bouchet, Nordström/Andersen, Ahlin, van Dessei, Bareau, F. and M.Spite, Foy.

Foy, Muratorio, Holweger, Bareau, Danks/Alcafno, Feitzinger, Feitzinger/Kühn/Reinhardt!Schmidt-Kaler, Borgman/Danks, Bergvall/Ekman/Lauberts/Westerl und.

Bergvall/Ekman/Lauberts/Westerl und, Monnet!Rosado, Wamsteker/Alcafno, Lohmann/Weigelt,Breysacher, F. and M. Querci, Schnur.

Schnur, Bastian/Mundt, Barbier/Swings,Bouchet, Renson, Baade, Swings, Hua/Doan,Gahm/Andrews.

Gahm/Andrews, Danks, Bouchet, Dachs, Ger­baldi, Bastiaansen, Ritter/Schröder.

The broadening of the Ha line is certainly less than 1 Aasa combined effect of the electron temperature and the in­ternal motions. This means that the gas behaves relativelyquietly in spite of the very strong large-scale motionsthat have been observed in the interstellar medium ofNGC 5128.

The peculiar, interacting galaxies Arp 244 (NGC4038-39) were also observed in order to detect H 11regions. Figure 2 shows that one of the components hasmany bright, condensed H 11 regions.

The excellent quality of the 3.6 m telescope permitted avery efficient use of the observing time, and the seeing andweather were also very good.

Nov. 1978:

Oct. 1978:

Dec. 1978:

Observing time has now been allocated for period 22 (October 1,1978 to April 1, 1979). The demand for telescope time was againmuch greater than the time actually available.

This abbreviated list gives the names of the visiting astronom­ers, by telescope and in chronological order. The complete list,with dates, equipment and programme titles, is available at re­quest from ESO/Munich.

Jan.1979:

Oct. 1978:

Nov. 1978:

West!S mith/Can non/Sch uster, Laustsen /Tam­mann, Lequeux/WestiSchuster/Laustsen,Feitzinger, Crane, van den Heuvel/Hammer­schlag/Henrichs, Crane, Materne, Tarenghi,Elvius.

Elvius, Lindblad, van den Heuvel/Hammer­schlag/Henrichs, Schoembs, Foy, Knoechel,Knoechel/Vogt, Borgman/Danks, Wamsteker,Schnur, Georgelin/Monnet.

Georgelin/Monnet, Ardeberg/Lynga, Hunger/Kudritzki, Weidemann, Lohmann/Weigelt,Alcafno, Houziaux/Nandy, Surdej/Swings, Vogt.

Vogt, Wlerick, Bouchet, Danks/Koornneef,Seitter/Duerbeck, Pakull, Lub, Melnick/Manfroid,Wlerick, Wamsteker.

Feb. 1979: Wamsteker, Breysacher, Wlerick/Bouchet, Adam,Ritter/Schröder, Veron.

March 1979: Veron, Danziger/Fosbury/Goss/Ekers/Wall,Courtin, Schultz/Kreysa, de Vries, Gyldenkerne,Schnur, Bergeron, Dennefeld, Kunth, Ulrich,Seggewiss.

Feb. 1979:

Dec.1978:

Jan. 1979:

3

Spectroscopic Observations of YY Orionis Stars at La SillaC. Bertout and B. Wolf

Trailblazing observations of stars in late stages oftheir initial formation were earried out somemonths ago by two astronomers from the Landes­sternwarte Heidelberg-Königstuhl, FRG, Drs.C. Bertout and B. Wolf. Speetroseopy with theESO 1.5 m teleseope on La Si/la during twelveeonseeutive nights revealed dramatie speetralehanges, and simultaneous photometrie observa­tions at the new observatory near San PedroMartir, Baja California, Mexieo, are now beingredueed.

What are the YY Orionis Stars?

Back in 1961, M. F. Wal ker noticed that the T Tau ri-I ike var­iable YY Orionis showed displaced absorption compo­nents located at the red edges of certain emission lines. Atthat time, ideas about the pre-main-sequence stellar evolu­tion were still mostly of a qualitative nature, and no particu­lar attention was paid to his discovery. In 1969, however,R. B. Larson published the first calculations of protostellarcollapse, which indicated that the protostar grew by accre­tion of matter from the surrounding interstellar cloud. Themodel predicted that the new-born star would be imbed­ded in a free-falling envelope for as long as a million years(i n the case of a 1 Me> star). It also pred icted that the starshould be optically visible during a major part of this time.

In 1972, Walker suggested that YY Orionis was indeedsuch a protostar at the end of its hydrodynamic evolution.He based this supposition on the redward displacement ofthe absorption components of the Balmer emission lines,which indicate that the absorbing material has a recessionvelocity of at least 300 km S-l with respect to the observer.This velocity is in good agreement with the end velocities ofthe infalling matter computed in theoretical collapse mod­els for a 1 Me> protostar. In the same paper, Walker re­ported the discovery of several other TTauri-like starsshowing line profiles similar to YY Orionis, and introducedthe term "YY Orionis stars".

Like most of the T Tauri stars, each of the YY Ori stars hasits own peculiarities, thus making it difficult to defineclear-cut class characteristics. However, the basic proper­ties of the YY Ori stars can be summarized as folIows. Theyare T Tauri variables, and most of them possess a strong ul­traviolet excess, defined by U-B $ O. Certain emissionlines, in particular the hydrogen Balmer lines, sometimesexhibit a displaced absorption component at the red edgeof the emission. Such line profiles are called YY Orionisprofiles, or inverse P-Cygni profiles. In their other proper­ties, YY Ori stars resemble T Tauri stars: their spectrum isoften "veiled" by continuous emission in the blue and vi­sual spectral ranges, so that it is often difficult to discerntheir photospheric late-type spectrum. Also, most YY Oristars exhibit strang IR excess. YY Ori stars, like other T Taustars, are aperiodic fast variables; large variations of theBalmer line profiles are recorded on time scales of hours todays, and variations of the continuum level and UV excessare indicated by photometric measurements. Examples ofobserved profile variations are given in the following sec­tions.

One can easily understand the fascination that thesestars exert on uso In the menagerie of young stellar objects,most of which show some evidence of mass loss ratherthan mass infall, and whose properties are stililargely un­explained, the YY Ori stars are the only link between theobserved pre-main-sequence evolution and today's pre­main-sequence evolutionary models. Detailed studies ofthese objects cannot only help us to gain insight into theearly phases of stellar evolution, but can also be used totest the validity of proposed theoretical models.

A Search for "Bright" YY Ori Stars

The YY Ori stars listed by Wal ker (1972) are rather faint, withmv between the 13th and 15th magnitude. Detailed spec­troscopic investigations of these stars are therefore verydifficult. In order to find additional and possibly brighterYY Ori stars, a spectroscopic survey is being carried out byseveral observers of the Heidelberg Observatory. Slit spec­trograms of young emission-I ine stars suspected to belongto the YY Ori class are being taken.

The brig htest YY Ori star so far, S Coronae Austral is, wasfound by Appenzellerduring his May 1976 observing run atthe ESO 1.5 m spectroscopic telescope. This star was

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Fig. 1: fntensity tracing ot the spectrum ot S CrA. The most prominent emission fines are identified. The inverse P Cygni ot the higherBafmer fines (trom Hy on) and the strong Fe 11 fines are cfearfy discernibfe.

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pers (Astron. Astrophys. 54,713, and 58, 163). In figure 1, weshow the intensity tracing of an image-tube spectrogram ofthis star. In spite of the rather moderate resolution (about3 A), the inverse P Cygni profiles of the Balmer lines (ex­cept Hß) and of the strongest Fe IIlines are readily recog­nized. Besides the hydrogen Balmer series and the Fe 11lines, prominent He I (n4026, 4471) and He I1 (1..4686)emission lines are present in the spectrum. The heliumlines are undisplaced and never exhibit absorption com-

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known as a TTauri-like variable, but apparently its promi­nent inverse P Cygni-line profiles had never been detectedin earlier spectroscopic observations. S CrA, which is as­sociated with the molecular cloud NGC 6729, has a meanvisual magnitude of 11 ':'5; and its range of variations is ~

mv ~ 0.8.The spectrum of S CrA is described in detail in two pa- Fig. 3: Same as tor tigure 2 tor CD -350 10525.

Fig.2: Intensity tracings ot the Hy profiles ot high-dispersion(20 A/mm) spectrograms ot the YY Ori star S CrA. The coudeplates 1 to 12 were taken during twelve consecutive nights (oneper night) between April 15, 1978 and April 27, 1978.

5

ponents. Their oeeurrenee is typieal of YY Ori stars andrequires exeitation and ionization temperatures of about30,000 to 50,0000K.

The seeond in brightness among the YY Ori stars re­ported so far is CD -35° 10525, with an average brightnessV = 11 m6. This star is assoeiated with the elongated inter­stellar dust eloud Barnard 228 in Lupus. The membership ofCD -35° 10525 to the YY Ori subelass of the T Tauri vari­ables was first established by Mundt and Wolf during anobserving run in July 1977 at La Silla. The speetrum of CD-35° 10525, deseribed in Astron. Astrophys. 63. 289, isagain eharaeterized by inverse P Cygni-line profiles, espe­eially of the Balmer lines. However, this star shows feweremission lines in the blue speetral range than S CrA. Forexample, Fe 11 does not show up in emission. Also in eon­trast to S CrA, CD -35° 10525 exhibits absorption featureseharaeteristie of an underlying late-type speetrum of spee­tral type around MO.

Coude Observationsand Balmer Une Profile Variations

The apparent brightness of the newly-diseoveredYY Orionis stars deseribed above allows for high disper­sion speetroseopie observations even with medium-sizedteleseopes. Highly-resolved speetrograms are very desir­able beeause they allow a detailed eomparison of the ob­served line profiles with profiles ealeulated using theore­tieal protostar models. To our knowledge, the first eoudeobservations in the blue speetral range of a YY Ori starwere earried out in August 1976 at La Silla, using the 1.5 mESO speetroseopie teleseope. These observations re­vealed the rather eomplex strueture of the S CrA Balmerline profiles with (besides the red-displaeed absorption at300 km s-') two emission peaks separated by a slightlyblue-shifted (about -30 km s-') eentral absorption. Aeomparison of these profiles with theoretiealline ealeula­tions allowed Wolf, Appenzeller and Bertout to present apossible eonfiguration of the outer layers of S CrA (Astron.Astrophys. 58, 163).

In order to study the full range of profile variations of theBalmer lines and to relate these variations to the eon­tinuum variability, one needs high dispersion speetros­eopy and simultaneous photometry. Sinee the broadphotometrie bands are eontam inated by the em issionlines, standard UBV photometry is not suitable to deter­mine the eontinuum level exaetly. What one needs here isnarrow-band photometry. We therefore applied for observ­ing time at the eoude ESO speetrograph at La Silla, and atthe 1.5 m photometrie teleseope of the Mexiean NationalObservatory loeated at San Pedro Martir, Baja California, ina joint programme with Dr. Luis Carraseo. The 13-eolourmedium narrow-band photometrie system developed byJohnson and available at San Pedro Martir is weil suited toour purposes.

Simultaneous Observations in Chile and in MexicoTwelve half nights were allotted to our projeet by ESO.Thanks to the flexibility of the Mexiean organization, Dr.Carraseo was able to obtain observing time during thesame period. The speetroseopie eoude observations ofS CrA and CD -35° 10525 were earried out by Wolf and thesimultaneous photometrie observations, extending from3300 A to 1 ~l, were made by Bertout and Carraseo, fromApril 15 to 27, 1978. Due to the eatastrophie floodingswhieh oeeurred in Baja California last winter, the SanPedro Martir Observatory had to be elosed from Februaryuntil our arrival. We are very mueh indebted to the

6

FIRST ANNOUNCEMENT OF AEUROPEAN WORKSHOP

ON

"Astronomical Uses of the SpaceTelescope"

The European Spaee Ageney (ESA) and the EuropeanSouthern Observatory (ESO) are jointly organizing aWorkshop on "Astronomieal Uses of the Spaee Tele­seope". It will be held in Geneva, on the premises ofCERN, on February 12-14, 1979. The purpose of theWorkshop is to give the European astronomieal eom­munity an oeeasion to diseuss in depth possible seien­tifie programmes in various astronomieal areas. A pre­liminary list of topies ineludes: Star Formation, GlobularClusters, Magellanie Clouds, External Galaxies, AetiveNuelei of Galaxies, Clusters of Galaxies, Cosmology. At­tention will also be given to the problem of ground­based observations required before the launeh in orderto optimize the use of the ST.

Seientists wishing to partieipate and possibly presenta short eontribution related to the use of the ST for per­forming speeifie programmes should write as soon aspossible to the following address:

Dr. M. TarenghiESO-CERN1211 Geneva 23Switzerland

The maximum number of partieipants will be about 120persons.

teehnieal team whose efforts kept the station in workingorder during our run.

Thanks to the exeellent weather and seeing eonditions atLa Silla, we eould take the best advantage of the allottedobserving time. We eould take one eamera I speetrogramof eaeh of our two programme stars eaeh night, i. e. we ob­tained a eomplete series of 24 speetrograms with a disper­sion of 20 A/mm. Due to the unusual elimatie eonditions,the weather in Baja California was not as good as in Chile.However, six and a half of our twelve nights were photo­metrie, so that we also obtained extensive data for ourprogramme stars.

A first glanee at the intensity traeings of the speetro­grams gained during this last observing run readily showsthe existenee of strong speetral variations. The eomplexstrueture of the Balmer line profiles and the profile varia­tions are illustrated by the intensity traeings of Hy shown infigures 2 and 3 for S CrA and CD -35° 10525. Espeeially thered-shifted emission eomponent shows dramatie ehangeseven within two eonseeutive nights. In faet, it ean be seenfrom these traeings that we never observed the same pro­file shape twiee. Unfortunately, the simultaneous photo­metrie observations are not yet eompl etely redueed, so thatwe eannot say mueh about possible eorrelations betweenprofile strengths and eontinuum level. However, it is al­ready apparent that speetaeular ehanges in the energy dis­tribution oeeur.

Now the most diffieult part of the work remains to bedone. That is, we must try to understand what physiealmeehanism might be responsible for these variations. Do­ing that will probably require even more luek than we had ineoordinating the simultaneous observations in Mexieo andChile without a telephone eonneetion!

The ESO Workshop on Infrared Astronomy

About 60 astronomers attended the workshop on IR as­tronomy jointly organized by the Stockholm Observatoryand ESO on the island of Utö on June 20-22, 1978. Thenumber of participants, significantly larger than originallyplanned, was in itself a sign of the increasing interestamong the European astronomers in the scientific and ob­servational aspects of ground-based IR astronomy.

The first day was devoted to the presentation of a numberof review talks on the contribution of infrared observationsto the understanding of various astrophysical problems inthe fields of extragalactic, galactic and solar-system as­tronomy.

The instrumental techniques were the topic of the sec­ond day, integrated by communications by European

groups active in the field and by reports on various projectsfor airborne and ground-based facilities for IR astronomy.

An extended discussion on which should be thepriorities for ESO in the development of infrared in­strumentation for the observatory at La Silla took place inthe morning of the last day.

Among the numerous suggestions and proposals, twoobtained wider support: the first was that of concentratingthe effort on intermediate-resolution spectroscopy aftercompletion of the presently developed IR spectrophoto­meters, and the second was to modify for optimal IR perfor­mances, in addition to the 3.6 m telescope, also a telescopeof smaller size to be used in the infrared for a prevailingfraction of ti me. Piero Salinari

The ESO Measuring Machines in Geneva

The ESO S-3000 plate-scanner and Grant machine are nowfully operational and are available for use by visiting as­tronomers for the reduction of direct plates and photo­graphic spectra.

The S-3000 microdensitometer can accommodate platesup to 14 x 14 inches in suitable plate-holders and muchlarger plates taped on top of the stage. The accuracy of thismachine is better than one micron for positional work andcomparable to that of a POS machine for density mea­surements up to 3.0 O. The data are stored on magnetictapes and can be processed with the ESO Interactive ImageProcessing Package (cf. Messenger No. 10, p. 16).

The ESO computer-controlled Grant machine can beused either in the conventional mode for manual mea­surements of radial velocities or as a one-dimensionalmicrodensitometer for scanning of photographic spectrawhich can be reduced with the Image Processing Packagementioned above.

PERSONNEL MOVEMENTS(A) Statt

ARRIVALS

Garehing

Ursula LIESE (German), shorthand-typist (telephone andtelex operations), 1.11.1978

GenevaPeter SCHABEL (Austrian), senior electronics engineer,1.9.1978Alan F. M. MOORWOOD (British), infrared astronomer,1.10.1978

La Silla

Patrick HALLEGU EN (French), electronics technician,1.8.1978Günter G. SCHUBA (German), electronics technician,1.11.1978

DEPARTURES

Geneva

Norbert RODGERS (British), administrative officer, 31.8.1978Susan KAY (British), secretary, 31.8.1978Peter SCHARNWEBER (German), electronics engineer,31.8.1978Dietmar PLATHNER (German), mechanical engi neer,31.8.1978

For further information and in order to apply for time onthese machines, please write to Jorge Melnick, ESO c/oCERN, CH-1211 Geneva 23, Switzerland, specifying thedates when you wish to use a given machine and enclosea short description of the proposed measurement pro­gramme.

ESO Astronomer Honoured!The following announcement can be read in Minor PlanetCircular No. 4358, which was published by the Center inCincinnati on April 30,1978:

New name of minor planet(2018) SCHUSTER = 1931 UC

Oiscovered 1931 Oct. 17 by K. Reinmuth at Heidelberg.Named in honour of Hans-Emil Schuster, astronomer at

the European Southern Observatory, who is active as anobserver and discoverer of minor planets and comets.

Bernard PILLET (French), laboratory assistant (photography),15.9.1978

La Silla

Christopher SMITH (Canadian), resident astronomer,15.9.1978

(8) Paid Associates - Fellows - Cooperants

ARRIVALS

Geneva (Scientific Group)

Peter A. SHAVER (Canadian), paid associate, 1.9.1978Danielle M. ALLOIN (French), fellow, September 1978Guillermo TENORIO-TAGLE (Mexican), fellow, 14.9.1978Hans R. OE RUITER (Outeh), fellow, 1.10.1978Eduardus J. ZUIDERWIJK (Outeh), fellow, 1.10.1978Per Olof L1NDBLAD (Swedish), paid associate, 1.11.1978

La Silla

Holger PEDERSEN (Danish), paid associate, 1.9.1978Christian PERRIER (French), coopenant, October 1978

DEPARTURES

Geneva (Scientific Group)

Jean MANFROID (Belgian), fellow, 30.9.1978Irena J. SEMENIUK (Polish), paid associate, 30.9.1978Marco SALVATI (Italian), paid associate, 7.8.1978Svend LAUSTSEN (Danish), scientific associate, 4.7.1978Franeo PACINI (Italian), paid associate, 31.10.1978

7

The X-ray Binary 4U 1700-37/HD 153919G. Hammerschlag-Hensberge and E. van den Heuvel

Figure 2: The radial-velocity deviations from the mean for same ofthe Balmer lines, plotted against X-ray phase.

measurements of radial veloeities diffieult. But thanks toour large number of exeellent plates, we were able to de­termine a rather aeeurate radial veloeity orbit. Figure 1shows the result of these radial veloeity measurements. Weput a lot of effort in eorreeting the measurements for differ­ent kinds of systematie errors. One of the most importanteorreetions is the following one: Oue to the steady outflowof the atmosphere of the Of star, lines whieh are formed atdifferent levels in the stellar atmosphere, show differentveloeities. Before oaleulating the mean veloeity of all ob­served lines on a plate, eorreetions for this systematie de­viation therefore had to be made. If the outflow of the at­mosphere is spherieally symmetrie, one would expeet thatthis radial veloeity deviation of a speetralline remains eon­stant through the binary eyele. But figure 2 shows that thisis not the ease! The radial veloeity deviation of Hy, for in­stanee, shows two peaks per orbital eyele, of whieh thelargest eoineides with the phase where we see the side ofthe star whieh is turned toward the X-ray eompanion. Atthis phase (0.5) the veloeity is more negative than average.

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New satellite data have provided ground-basedoptieal astronomers with a large amount of ex­tremely interesting work in eonneetion with X-raybinary stars. Speetroseopie and photometrie ob­servations are urgently needed to further unravelthe nature of these strange objeets. The presentreport by Drs. Godelieve Hammersehlag-Hens­berge and Edward P.J. van den Heuvel of the As­tronomieal Institute of the University of Amster­dam, the Netherlands, summarizes four years ofpainstaking observations of one of the brighteststars identified with an X-ray souree. Theirthorough study has already revealed some veryinteresting details about this binary system.

Sinee the diseovery of the first X-ray binaries by the Uhurusatellite, our group in Amsterdam, in eollaboration with theAstrophysieallnstitute of the Vrije Universiteit of Brussels,has put mueh effort in analysing the optieal behaviour ofthese systems. For ou r observations we made pri marily useof the ESO teleseopes and of the 92 em light eolleetor ofthe Leiden Southern Station in South Afriea.

In this note we will deseribe some reeent results forHO 153919, the optieal eomponent of the 3.41 day periodX-ray binary 4U 1700-37. HO 153919 is one of the brighteststars identified with an X-ray souree (only the supergiantbinary V 861 Seo, reeently identified with the X-ray soureeOAO 1653-40, is brighter). Its speetral type is 0 6.5f. The X­rays are eelipsed during 0.9 day of the 3.41 day orbitaleyele.

SpectroscopyBetween 1973 and 1977 we eolleeted 75 blue speetrogramsof this star with the eoude speetrograph of the 1.5 mESOteleseope. The speetra were taken by van den Heuvel, OeLoore (Brussels) and Hammersehlag-Hensberge. Thespeetral lines in this Of star are very broad, whieh makes

This is also elearly demonstrated in figure 3: the radial ve­loeity of Hy deviates strongly from the mean veloeity eurvenear binary phase 0.5 and beeomes more negative, whiehis indieative for a stronger outflow of material. This asym­metry in the stellar wind influenees the radial veloeities ofmost stellar lines and henee the derived masses of botheomponents.

-'50

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L...~_~~~~~_~~~_~ ~~----.J-'60

-0.2 0·0 0.2 0.4 0.6 O.B 1.0 1.2

XRAY PHASEFigure 3: Radial velocity variation of Hy, plotted against X-rayphase. The dashed curve drawn through the points representsthe best-fit orbital solution for the mean velocities of alliines to­gether.

-30 -3050

-'0 -'0-60

-50 -50-10

-60 -6060

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>- -90u -60 -600 .- -100-' '...JW -90 -90 0> -' -110w

-'00 -100-120

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-120 -120-\40

-130 -, JO-ISO

-l40 -140-0.2 0.0 0.2 O.• 0·6 0.6 1.0 1.2 -160

XRAY PHASEFigure 1: Variation of the average radial velocity for alliines ofHO 153919 with X-ray phase. Phase zero corresponds to mid X-rayeclipse time. Each point represents the average radial velocity forone ESO coude plate. The curve drawn through the points repre­sents the best-fit solution to the data.

8

Photometry: A Possible 97-Minute Periodicity

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The editors

About the "Messenger"We regret that this issue is somewhat delayed due to summerholidays. The next issue will appear as planned on Deeember 1,1978.

The Messenger is printed in approximately 2,200 eopies anddistributed to all major observatories in the world.lt is also sentto the lAU members in the ESO countries and to many otherfriends of astronomy, ineluding seienee journalists andamateurs. We shall always be happy to review the applieationfor a free eopy from others who are interested in ESO and inEuropean astronomy.

Most of the authors are professional astronomers who workin Europe and many of them deseribe their observations at theESO La Silla observatory. We try to bring the latests news andto inform the readers about what is going on in astronomy now.It is therefore unavoidable that some of the artieles have a "pre­liminary" look and that statements therein are often expressedwith some eaution. We eertainly do not attempt to eompetewith the professional journals.

Readers, who have suggestions or would like to proposeehanges and improvements over the present form are weleometo write to us-the address is on the last page.

whether this variability remains present over langer timeintervals than one month. Although the present data seemvery suggestive, we fee I that still more data are required todefinitely confirm the presence of this periodicity.

What is very mysterious aboutthis pulsar is why its pulseperiod is so lang. No theoretician has so far come up with asatisfactory answer to this question.

.9 .0 .1 .2 .3 .4 .5 .6 .7 .8 .9 .0 .1phase

Figure 5: The average light curve of a possible 97-minute variationof HO 153919 in the five channels of the Walraven system. The re­sults were obtained from observations between orbital phases0.44 and 0.59 when the X-ray source is in front of the Of star.

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0.2 0.0 0·2 0.4 0.6 0,6 1.0

At the same time we also studied this star photometrically.Tagether with Ed Zuiderwijk (Amsterdam), one of us(G.H.-H.) observed it in the uvby system with the Danish50 cm telescape at ESO in 1975. One year later, vanParadijs (Amsterdam) observed the star during one monthwith the Walraven 5-colour photometer in South Africa. Atthat time, we collected all existing photometry of this starand plotted it in the 3A1-day orbital period. The result isshown in figure 4. The double wave variation typical formassive X-ray binaries is clearly visible although there ismuch intrinsic scatter in the points.

In April 1978 Dr. T. Matilsky of Rutgers University and Dr.J. Jessen of the Massachusetts Institute of Technology re­ported the discovery of X-ray pulsations in 4U 1700-37 witha 97-minute periodicity from observations with the SAS-Csatellite. This is the langest reported period for any X-raypulsar. Most other X-ray pulsars have periods between0.7 sec. and 12 minutes. Although such lang periods mayalso be produced by rotating white dwarfs, there is a varietyof reasons why these pulsars-including 4U 1700-37-aremost Iikely to be neutron stars. The main reason is the X-rayspectrum: all the pulsating sources, including 4U 1700-37,appear to have very hard X-ray spectra, strongly suggest­ing that we are dealing with accreting neutron stars. Sameof the previously reported X-ray pulsars in binaries showedoptical pulses with the same period as the X-ray pulses. For4U 1700-37 our optical photometry comprised the largestavailable material, so that it was natural to use our data tosearch for possible optical variability. Dr. A. Kruszewskifrom Warsaw University Observatory searched for 97-minoptical pulsations in the yellow-filter photometry obtainedby van Paradijs and reported evidence for the presence ofthis 97-minute variability. According to him, the variabilityappears strongest ara und orbital phases 004 - 0.6 and itdisappears at X-ray eclipse time. We studied the variabilitynot only in the V channel but in all the five available spectralregions of the Walraven system. The 97-minute variabilityis probably present in all these channels and becomesstronger towards the ultraviolet as is shown in figure 5.Similar plots for other periods did not produce positive re­sults. More observations in the Walraven 5-colour system (byA. van Genderen) and in Hß-photometry (by H. Henrichs atESO) are being analysed at this moment to try to examine

XRRY PHRSE

Figure 4: V-light curve of HO 153919. The symbols denote obser­vations by the different observers, viz.:

Penny, Olowin, Penfold and Warren, 1973de Freitas Pacheco, Steiner and Quast, 1974

-t van Genderen, 1976van Paradijs, Hammerschlag-Hensberge and Zuiderwijk, 1978Hammerschlag-Hensberge and Zuiderwijk, 1976.

~ 6.56L

9

NEWS and NOTES

Computer Catalogue of ESO SchmidtPlatesVisiting astronomers to the ESO 1.52 m telescope are used to re­ceive, soon after their observing runs, a computer list of the platesthey obtained. The lists include all the plate data, from the expo­sure time and the celestial coordinates to the slit width and thequality of the plate.

No such service has until now been available for direct platestaken with the ESO telescopes. But during the past months, ESOprogrammer Klaus Teschner has implemented a very efficient sys­tem for storage and retrieval of Schmidt plate data. The systemworks so weil that it will soon be enlarged to include plates lromother telescopes as weil, in particular the GPO and the 3.6 m tele­scopes.

The programmes were written and tested with the ESO HP sys­tem on La Silla, but a regular exchange with ESO/Geneva will en­sure that all the information is available in both pi aces. At the mo­ment the more than 3,000 plates that were obtained with theSchmidt telescope are being put into the Catalogue, a task that willtake some time. Therealter, future plates will be entered as soon asthey have been taken.

The Teschner system consists of one input and two retrievalprogrammes. The input programme is only 01 interest to the nightassistants on La Silla who enter the new plates, but the two otherprogrammes will be extremely uselul lor the astronomers. Oneprogramme will search lor all plates with a certain characteristic orcombinations 01 these: e.g. which plates were taken in March 1976with lilter GG 385 in seeing better than 1:'5? The second pro­gramme will lind all the plates that show a certain position. Inthis way, anyone who is interested in a given celestial object willimmediately learn which plates are available at which epochs.

Further about ESO 113-IG45The readers 01 the Messenger may remember ESO 113-IG45, thequasar-galaxy that was described in issue No. 11 on page 24.Further observations were obtained 01 this remarkable object atother southern observatories at about the same time as the initialESO observations. It has also been investigated in some detailIrom La Silla.

Drs. A. C. Danks, W. Wamsteker, N. Vogt, P. Salinari and M.Tarenghi (ESO), together with Dr. H. W. Duerbeck Irom the HoherList Observatory near Bonn, FRG, observed 113-IG45 by means 01visible (UBV) and inlrared (JHKLM) photometry. They lind a stronginlrared excess, as olten seen in Seylert 1 galaxies. They alsonotice that the object is very variable; a decrease 01 more than onemagnitude in the L-band and somewhat less in the K-band wasseen between November 1977 and January 1978. This sets an up­per limit to the size 01 the inlrared emitting region and hel ps chos­ing between the possible models.

The ESO velocity (13,600 km/s) has been conlirmed by observ­ers with the Tololo and AAT 4 m telescopes and Dr. A. P. Fairall,who called attention to this object in August 1977, has recently ob­tained a spectrum 01 the elliptical companion galaxy to ESO 113-

New Publications from ESOPreprints: May-August 1978

23. J. LUB, J. VAN PARADIJS, J. W. PEL, P. R. WESSELlUS: UI­traviolet Photometry of the Cepheid B Doradus lrom theA.N.S. Satellite. May 1978. Submitted to: Astronomy and As­trophysics, Main Journal.

24. A. C. DANKS, L. HOUZIAUX: Spectroscopic Observations 0127 C Ma Irom 0.14 to 4.7 Microns. May 1978. Submitted to: As­tronomy and Astrophysics.

25. WILLEM WAMSTEKER: The Continuous Energy Distributionof Nova Cygni 1975. May 1978. Submitted to: Astronomy andAstrophysics.

26. G. CONTOPOULOS, L. GALGANI, A. GIORGILLI: On theNumber of Isolating Integrals in Hamiltonian Systems. June1978. Submitted to: Physical Review - A.

10

IG45 (see the Messenger photo). Although the spectrum showedfew lines, he was able to measure thevelocity and lound that it wasthe same as that 01 the main galaxy. They are therelore most likelyat the same distance and are physically connected.

Dr. Charles C. Wu 01 NASA obtained ultraviolet spectra 01ESO 113-IG45 with the International Ultraviolet Explorer (IUE)satellite in late June 1978. He is now reducing these observationstogether with Drs. A. Boggess and T. Gull and the preliminary re­sults are very interesting.

Objects of the ESO 113-IG45 class are lew-but they tell usmore about what is going on in the nuclei 01 Seylert galaxiesand in quasars. Observations over the widest possible spectralrange, Irom X-rays to radio, are continuing.

Spectra of the Brightest Starsin a Very Distant Globular ClusterAmong the vast number 01 new and exciting objects that havebeen lound on the ESO Quick Blue Survey plates, some have beenshown in the Messenger. One 01 these (No. 10, p. 13) has nowbeen observed in more detail with the ESO 3.6 m telescope.

The object, a globular cluster in the southern constellationEridanus, is rather lain!. The brightest stars have an apparentmagnitude lainter than 19, and the integrated magnitude 01 the en­tire cluster is about 15.8 in V, according to Dr. G. Wlerick, who ob­tained this measurement with the ESO 1 m telescope.

Spectra 01 the three brightest stars were obtained by ESO as­tronomer R. M. West with the Boiler and Chivens spectrograph inthe Cassegrain locus 01 the 3.6 m telescope. During aperiod 01very good seeing and thanks to the excellent perlormance 01 thetelesc0p,e and spectrograph, it was possible to obtain well-expos­ed 123 A/mm spectra (3900-7000 A) with a widening 010.1 mm.The spectral resolution is slightly lower (about 4 Ä) than wh at isneeded to perform MK-classilication, but enough to make a roughanalysis 01 the stars.

Together with Dr. R. A. Bartaya Irom the Abastumani Observa­tory, Dr. West linds that the spectral types 01 the three stars are inthe range G8-K2 111 and that they all are severely metal-delicient.This is a common leature of stars in the halo (Population 11).However, there are plenty 01 lines visible in the spectrum 01 thebrightest star, many 01 which are 01 metallic origin, and the starcannot be completely delicient in metals. This conlirms the con­clusion by Cowley, Hartwiek and Sargent (Ap. J. 220, 453) andCanterna and Schommer (Ap. J. 219, L 119) that there are metalseven in the most distant clusters and that so lar, there is no ob­servational indication 01 a possible Population 111 (no metals).

From the apparent magnitudes 01 the brightest stars in thepresent cluster and their absolute magnitudes (Irom the spec­tral types), it is possible to estimate that the distance is probablyweil over 100 kpc, thus making it one 01 the most distant knownin the neighbourhood 01 the Milky Way system. It may even bequestioned whether it really "belongs" to our galaxy, or wh etherit is an intergalactic "tramp". The only clue we have is the radialvelocity. From more than ten unblended spectral lines, a meanval ue 01 -42 ± 12 km/s is lound (hel iocentric, i.e. relative to theSun); the cluster is approaching. Adding the component of thesolar motion in the Galaxy, the measured velocity becomes evenmore negative. So who knows, may be this interesting cluster isreally one 01 "ours"?

27. I. R. KING: Astrometrie Accuracy with Large Rellectors. June1978. Colloqui um on Eu ropean Satell ite Astrometry, Padova,5-7 June 1978.

28. E. B. HOLMBERG, A. LAUBERTS, H.-E. SCHUSTER, R. M.WEST: The ESO/Uppsaia Survey 01 the ESO (B) Atlas 01 theSouthern Sky - VI. June 1978. Submitted to: Astronomy andAstrophysics.

29. G. CONTOPOULOS: The Dynamics 01 the Spiral Structure inGalaxies. August 1978. Presenled at lhe "Strämgren Sym­posium" in Copenhagen, May 1978.

30. D. KUNTH, W. L. W. SARGENT: Spectrophotometry 01 SixSeyfert Galaxies from lhe Zwicky Lists. August 1978. Sub­mitted to: Astronomy and Astrophysics.

Annual Report 1977.

Modern Techniques in Astronomical Photography. Proceedings01 ESO Colloquium. Editors R. M. West and J. L. Heudier, 304 p.

Be Stars Observed at La SillaL. Divan, D. Briot and J. Zorec

Be stars are hot stars with emission lines, butwhere are these lines tormed? Simultaneousspectroscopic observations at La Silla inJune 1978 by Drs. L. Divan, D. Briot and J. Zorectrom Institut d'Astrophysique (CNRS) in Paristhrow new light on the Be phenomenon.

Be stars have been known for a very long time and, as theiremission lines could not be formed in the photosphere,these objects were supposed to be surrounded by a thinhydrogen envelope extending to several stellar radii, theemission lines originating from the part of this atmo­sphere not projected on the central star. This picture ex­plained also the presence in some Be stars of the so-calledshell spectrum as due to the part of the extended atmo­sphere projected on the disk of the central star. Moreover,the rapid rotation of most Be stars could at first sightrender plausible the formation of this envelope.

But when the details are considered, so many difficultiesare encountered that even the reality of the extended at­mosphere in which we have believed during more thanforty years, now appears questionable. In fact, there is noreal correlation between the rotational velocity and the sizeof the envelope. Difficulties arise also because in the samestar, the emission lines and/or the shell spectrum can dis­appear in a few weeks or months and reappear again, moreor less rapidly; in the extended-atmosphere picture this isnot easy to explain.

On the other hand, the theoretical models for extendedatmospheres depend on so many parameters (opticallythin or not, dimensions, static or with any form of velocitygradient ...) that it is impossible to decide if they are or arenot in contradiction with the too few observed parameters.

We thought that this bad situation could be somewhatimproved if(1) more parameters were observed,(2) observations were made of the same stars at the same

time,and this is why simultaneous observations were done at LaSilla, by D. Briot with the 152 cm telescope and coudespectrograph, and by L. Divan and J. Zorec with the 50 cmESO telescope and Chalonge spectrograph.

Simultaneous Spectroscopic Observationson La Silla

The parameters observed by D. Briot are the equivalentwidth and profiles of Balmer lines (Ha included); the re­duction is made by classical methods but with improvedspectral types and effective temperatures thanks to (1'1, D)­classification obtained for the programme stars with theChalonge spectrograph.

The Chalonge spectrograph gives interesting and prac­tically never observed parameters in Be stars. The Balmerregion is particularly rich in information; in many casesBe stars have quite anormal Balmer Jump but sometimes,as Barbier and Chalonge announced long aga (Ap. J. 1939,90, p. 627) for ~ Tau, two Balmer Jumps occur atwavelengths differing by about 50 A. The first one issituated like in a normal star and the second one, at shorter

wavelengths near the theoretical Balmer limit, like insupergiant stars. Barbier and Chalonge concluded thathydrogen was present in the photosphere of the star and inan envelope at a very low pressure.

The Two Balmer Jumps

Following this idea we assume that the long-wavelengthBalmer Jump (D. on figure 1) is the Balmer Jump of thecentral star and that, together with the corresponding Al itgives the A1D spectral type (this had been done long ago, byD. Chalonge and L. Divan, Ann. d'Astroph. 15,1952,201, toclassify four stars with emission in the Balmer continuum:1 H Cam, 59 Cyg, u Cyg and 11 Cam) and the effectivetemperature (the A,D spectral type has been calibrated ineffective temperature). This assumption is justified by ob­servations made in stars with variable emission (see be­low).

The second Balmer Jump (d on figure 1) can be eitherin emission, like in a Ara (d < 0), or in absorption like in48 Lib (d > 0). In all cases the resultant Balmer Jump D isequal to (D* + d).

These stars are often variable, but the most interestingpoint is that the variations occur only in the short­wavelength Balmer Jump d due to the envelope which mayeven disappear entirely (1 H Cam, figure 2 or 59 Cyg, fig­ure 3) and reappear again. The long-wavelength BalmerJump is constant and then really defines the spectral type

Fig. 1: Microphotometer tracings ot aAra, (.\, Cru and 48 Lib.(.\, Cru is anormal B star showing a unique Balmer discontinuity,D. In aAra and 48 Lib two Balmer discontinuities can be seen, along-wavelength one, 0*, wh ich is due to the central star, and ashort-wavelength one, d, corresponding to the envelope.

dis in emission tor aAra (d < 0) and in absorption tor 48 Lib(d > 0). In the two cases 0 = D. + d.

11

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Fig. 2: Emission in the Ba/mer continuum which was c/ear/y visi­b/e on the 1948 spectrum has disappeared in 1965. Note also theHß line, absent in 1948 (filled in by emission) and normal in 1965.

(and luminosity class) of the central star as was supposedabove.

Fig. 3: Cf. figure 2.

Haute-Provence and Jungfraujoch Observatories with theChalonge spectrograph: there is a close relation betweenthe colour of thestar +envelope and the discontinuity d (0.being constant) both on the red side (eprb) and on the ul­traviolet side (epuv) of the Balmer Jump, showing that in this

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Spectral Variations

Naturally, variations in d are accompanied by variations ofthe emission in the Balmer lines (Hß in figures 2 and 3).What is less known (but was discovered by Barbier andChalonge for y Cas during its great burst in 1936-37) is thatvariations in d and Balmer emission lines are accompaniedby changes in the colour temperature of the star, on bothsides of the Balmer discontinuity. Figure 4 shows the re­sults obtained for HO 24534 (= X Per) by L. Divan at the

Fig.4: Spectrophotometric gradients eprb (spectra/ range6200-4000 .4) and epuv (3700-3150 .4) for HO 24534 (= X Per)as a function of 0 = O. + d.

The emission d in thel Ba/mer continuum of X Per is variable andwe can see here that the c%ur temperatures on both sides of theBa/mer Jump are strong/y corre/ated with d.

The normal c%ur of X Per (spectra/ type BO) is eporb = 0.73. /nFebruary 1977, the c%urof thestar is eprb = 1.36 and its spectrumshows practically no emission (d = 0, 0 = 0.); thus the c%urexcess qJrb-cporb = 0.63 is entire/y interstellar. /n all other cases thec%ur excess is the sum of two terms, the interstellar c%ur ex­cess and a c%ur excess due to the presence of the enve/ope. Ob­servations of this type permit a c/ear separation of these two~~~ ~

12

case the envelope cannot be considered as optically thin inthe continuum. Note that the extrapolation of this relationto d =0 (i n fact to the immediate vicinity of the pointFev 77for which the emissions have almost disappeared) givesthe interstellar colour excess of the star; this interstellarcolour excess is the same in all phases and can then besubtracted, to obtain the colour excess due to the envelopealone.

Future Observations

All these observations will be extended to many stars. Thefact that the discontinuity D. as it has been defined onfigure 1, is constant, justifies the picture of an unvariablecentral star. But the variable part of the spectrum is stilla mystery: extended atmosphere or perhaps a chromo-

sphere? A tentative representation of the Be phenomena bynon-thermal phenomena in the stellar atmosphere is inprogress (R. N. Thomas and J. Zorec) at the Institut d'As­trophysique in Paris and the present simultaneous obser­vations done by us at La Silla (high dispersion to measurethe emission in Balmer lines, low dispersion to measure allthe parameters of the continuum: D., A" d, <rrb, <ruv) willserve to test new models. Of particular interest are the cor­relations:(1) between d and emission in Balmer lines (Wa, Wß ... and

Balmer decrements),(2) between d and the colour temperatu res on both sides of

the Balmer Jump.We hope that the large set of new data obtained in June 78at La Silla together with theoretical efforts will throw somenew light on the Be phenomenon!

The Environments of Active Galactic NucleiR.A.E. Fosbury

It is becoming increasingly c1ear that strangethings happen in many galaxy nuclei and thattheirstudy is of extreme importance for our under­standing of the behaviour of matter. Observationsfrom satellites and with large telescopes haveyielded new and often unexpected results and thepossibility of the existence of black holes (or evenmore exotic objects) in the centres of active galax­ies is now taken seriously by most astronomers.Dr. Robert Fosbury, who joined the ESO ScientificGroup in Geneva in 1977, has been pursuing for along time the study of galaxy nuclei with some ofthe world's largest telescopes. He gives someexamples of recent work with the ESO 3.6 m tele­scope.

Small regions of intensely energetic activity are known toexist in the nuclei of many galaxies and there are a numberof arguments which, taken together, provide rather con­vincing evidence that asos are distant, luminous exam­pies of the same type of phenomenon. Understanding thenature of these energy sources is important because theextreme conditions encountered in them stretch our knowl­edge of basic physics. Also it would be possible to use theasos as cosmological probes with much more confidenceif there was a better knowledge of what they were andwhether their redshifts were entirely due to the universalexpansion.

Aside from the cosmological information implicit in thestudy of asos, by moving a little nearer home and lookingat activity in galactic nuclei, we are immediately presentedwith a number of observational and interpretative advan­tages. Not least of these is the chance to investigate the re­lationship between the nucleus and its galactic environ­ment. The interaction between the central energy source(10 15 cm from variability and other arguments) and its sur­roundings occurs on a very wide range of spatial scalesand, depending on the nature of these surroundings, pro­duces widely different manifestations of what are probablysimilar phenomena. This range of scales demands the ap­plication of a wide range of observational techniques. The

radio band does not in general contain a large fraction ofthe luminosity of active nuclei, radio continuum observa­tions do however provide a means of tracing events fromthe smallest to the largest angular scales. Optical and ul­traviolet spectrophotometry are powerful techniques forstudying the continuum radiation emitted from very closeto the energy source and also, from the emission lines, thestate of the ionized gas which is excited by the activity. Re­cently obtained X- and y- ray results are putting very severeconstraints on models of the energy source itself.

These are some interesting general correlations be­tween the outward appearance of the nuclear activity weobserve and the morphological type of the associatedgalaxy. For example, classical double radio sources seemonly to be found straddling elliptical galaxies, while class 1Seyfert nuclei (blue continuum, broad permitted emissionlines, narrow forbidden lines) exist predominantly in spiralgalaxies and are usually not strong radio sources. Whilecorrelations of this kind do not have the status of absoluterules, they do provide a starting point for a study of the linkbetween events in the nucleus and the evolution of thegalaxy as a whole.

As part of a larger programme studying the mor­phologies of radio galaxies in the Parkes Gatalogue usingthe SRG IIla-J sky survey, I obtained in April direct plates ofsome selected galaxies using the 3.6 m telescope at primefocus. Figure 1shows the galaxy identified with the invert­ed-spectrum radio source PKS 1934-63. The radio sourceitself is worthy of note since it was among the first discov­ered to show the characteristic low-frequency cut-off dueto synchrotron self-absorption (Bolton, Gardner andMackey, 1963, Nature, 199, 682). It is also extraordinarilypowerful as a compact source associated with a galaxyrather than a aso. Optical spectrophotometry has beenpublished by Penston and Fosbury (1978, M.N.RAS., 183,479), and figure 1 is a significant improvement over pre­viously published photographs. Rather than being a"double" galaxy it is possibly a giant elliptical girded bya dust belt reminiscent of our nearest radio galaxy NGG5128 (Gentaurus A). As in Gen A, the radio structure, whichis known to be a very close double from VLBI observa­tions, would be aligned perpendicular to the dust belt. It iseven possible that the dust is hiding a quasar-like nucleus,an idea which could be checked by infrared observations.

13

Fig. 1: From a 3.6 m prime foeus plate (60-min exposure on sen­sitized IIla-J with GG 385 filter) of the galaxy assoeiated with thepowerful eompaet radio souree PKS 1934-63. This may be a giantelliptieal erossed by a dust belt like Centaurus A.

Another topic receiving much attention in the current lit­erature is the 21-cm detection of the neutral hydrogen gasin same elliptical galaxies. From the limited sampie avail­able it has been noted that those ellipticals with detectableamounts of H I have same form of nuclear activity, evi­denced by optical emission lines and/or a compact nuclearradio source, and also occur in small groups with half adozen or so member galaxies. This has led to the nation

that the elliptical has not always contained the neutral gas(most ellipticals contain embarrassingly little) but that it iscurrently being accreted from gas lett over in the group andperhaps finding its way down to the nuclear regions topower the activity there.

The galaxy associated with the flat-spectrum radiosource PKS 1718-649 may have same relevance to thisproblem. It was originally classified as D-type (ellipticalwith extended halo) from the ESO (8) survey. The SRCIlla-J survey however showed that this halo was notsmooth but was in fact a pair of diffuse, very low surface­brightness spiral arms. (Fosbury, Mebold, Goss & vanWoerden, 1977, M.N,R,A.S" 179, 89) showed that thesystem contains a large mass (3 x 10 1°Mo ) of neutralhydrogen and that the nucleus has an emission-linespectrum reminiscent of the active ellipticals. The new3.6 m photograph (fig. 2) shows the spiral structure inmuch more detail (a) than seen previously and also (b) thetidal interaction between the main galaxy and the compan­ion spiral. Several fainter galaxies are visible in the viscinitywhich may be members of the same sm all group.

The Illa-J survey from the Schmidt telescape in Australiais giving us a much clearer view of galaxies than the pre­vious surveys. There is still, however, no substitute for theplates taken with a big reflector in a good site, and theseplates, coupled with the radio observations of H I, areproviding new insights into the galaxies around the nuclei.

I am happy to acknowledge thatthe prints reproduced infigure 2 were made by David Malin at the Anglo-AustralianObservatory.

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..Fig. 2: The flat speetrum radio souree PKS 1718-649 is in the nueleus of the big spiral galaxy with low surfaee-brightness arms. 80thprints are derived from the same 90-min exposure on sensitized IIla-J emulsion with the 3.6 m teleseope. (a) is made using an unsharpmask (Malin, 1977. AAS pholobuil. No. 16, 10) to filter out the low spatial frequeneies and show the fine detail from the densely exposedoriginal plate. (b) is a high-eontrast derivative (Malin, 1978, Modern Techniques in Aslronomical Pholography, Geneva. Ed. R.M. Westand J.L. Heudier) whieh shows the very faint tidal extension through the eompanion spiral.

14

SU Ursae Majoris-type Stars-The Most Interesting DwarfNovae

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for an accretion disc surrounding the white dwarf compo­nent of the system. This stream impacts onto the disc creat­ing a hot spot on its outer edge. Under advantageous orbit­al inclination conditions this hot spot manifests itself as ahump on the light curve seen during half the orbital per iodof the system. As a result of inhomogeneities in the impact­ing stream of matter the hot spo.t changes its luminosityand this exhibits itself as a rapid and irregular flickeringobserved in the light curves of dwarf novae. Observationsseem to indicate that the origin of outbursts is connectedeither with the outer layers of the white dwarf or with thecentral region of the disco

In the case of VW Hyi, normal eruptions occur in 80 percent of all observed cases. During the normal maxima thesystem increases its brightness by 3.9 magnitude on aver­age. The duration of normal maxima is about 4 days andthey repeat with a mean interval between them of 29 days.During the normal maxima no hump is observed. This is notsurprising, since one can easily imagine that during theoutburst the system brightens so much that the source ofradiation responsible for the periodically repeated hump isnegligible.

The supermaxima occur in VW Hyi with an average inter­val between them of 179 days. Their mean duration is17 days, i. e. they last about 4 times longer than the normal

-0.6

o5 0 6 0.7 JO Hel.

Observations of V 436 Gen in blue light during the superoutburstin May, 1978. A star of magnitude B= 12.4 was used as com­parison.

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I. Semeniuk

Among the large variety of cataclysmic variables there is asubgroup called SU Ursae Majoris-type stars. Althoughthis group of variables owes its name to a star in the north­ern sky, our contem po rary knowledge about these stars re­sults mainly from observations of southern members ofthis class. Similar to the U Geminorum-type stars-withwhich the reader is probably more familiar, as they at­tracted the attention of observers much earlier than theSU UMa-type stars-the SU UMa-type stars also showquasi-periodic recurrent rapid outbursts during which thestars' brightness increases by a few magnitudes during anight. The rapid rise to maximum is then followed by aslower decline to minimum light. But the SU UMa-type starsdiffer from the ordinary U Gem stars in demonstrating twoessentially different types of maxima. The firs~ type of max­ima, so-called normal maxima, occur most frequently andare characterized by rather steep rise and quick decline tominimum light. The second kind of maxima are so-calledflat maxima or supermaxima. They are on average 1 mag­nitude brighter than normal maxima and they last4-5 times Ionger than normal ones. Like the normal max­ima they also occur quasi regularly with the recurrenceperiods much longer than those between the normal max­ima.

The members of the Variable Stars Section of the RAS.of New Zealand, directed by F. Bateson, play an importantrole in observing SU UMa stars. Their extensive visual ob­servations make it possible to distinguish between thetypes of observed maxima, to classify a star of the SU UMatype, and to determine the mean recurrence period of out­bursts or superoutbursts. Sometimes they alert astronom­ers having better observational facilities at their disposalthat a superoutburst has begun and thus make it possibleto obtain precise photometry or spectroscopy duringthis interesting phase.

VWHyiOne of the brightest and the most investigated SU UMastars is VW Hyi. It was observed by N. Vogt and B. Warnerduring minimum and during outbursts and it is the best ex­ample to demonstrate the behaviour of the SU UMa stars.The light curve of the star during minimum light shows ahump of amplitude between 0.4-0.9 mag which repeatsperiodically with aperiod equal to 107 min (0907427). Thisis the orbital period of the binary system. As is generallyknown, all dwarf novae are binary systems of which onecomponent is a white dwarf and the second is a late-typestar filling its Roche lobe and losing mass through the in­ner Lagrangian point. This stream of matter is responsible

Dr. Irena Semeniuk is a senior lecturer at the As­tronomical Observatory of the Warsaw University,Poland, and joined the ESO Scientific Group inOctober 1977 with a one-year fellowship. She isinterested in extragalactic astronomy (distribu­tion of galaxies in clusters) as weil as in cataclys­mic variables. She recently observed at La Sillaand was very happy to catch one of these stars in a"superburst".

15

maxima and their mean amplitude is 4.8 magnitude, i. e.about 1 magnitude greater than that of the normal maxi ma.A nai've expectation would predict that no hump should beobserved during supermaxima as it is not observed du ringthe normal maxima. But in spite of this nai've expecta­tion-and it is the most curious thing-not only a hump buteven a superhump is observed during the supermaxima ofVW Hyi.lt is called the superhump because its amplitude inintensity un its is much greater than what is observed atminimum light. Moreover, what makes the phenomenonstill more curious is that the mean period of the super­hump of VW Hyi is 110 min (0907676), i. e. it is by about3 per cent larger than the orbital period observed atminimum light, and even more, the superhump period isdecreasing during the supermaxima. The same behaviourwas observed with WX Hyi, another southern star ofSU UMa type. In its case the mean period of superhump isalso greater by about 4 per cent than the orbital period ofthe system as obtained from humps at minimum light.

Interpretation?What is the real nature of such behaviour of SU UMa-typestars? Are the superhumps connected with appearance ofa "superspot" on the disc surrounding the white dwarfs? Isthe observed superhump period change related to theper iod change observed in Nova V 1500 Cyg? Is it indeedso that with all SU UMa stars no hump is observed duringthe normal maxima, but only during the supermaxima? Arethe physical processes responsible for the two types ofoutbursts completely different?

Observations on La SillaAll these questions motivated the author to place someSU UMa stars in her observational programme for La Silla.Most of these stars at minimum light are below thethreshold of visibility of the 60 cm Bochum telescopewhich was available. But every night the author started herobservations by checking whether or not any of the starshad exploded. The chance was rather small during the au­thor's short stay at La Silla. Thus she was very pleasantly

surprised when on the night of May 7, while making hernightly survey, she perceived a star in one of the previouslyempty fields. It was V 436 Cen which had just exploded andwas by then about 3 magnitudes brighter than the limitingmagnitude of the Bochum telescope. The author's excite­ment was so great that she could not believe that this wasthe correct star. Only after measuring colours of the starshe was sure there was no misidentification. The starshowed a great ultraviolet excess as is usually observedwith dwarf novae. The star was then monitored in blue lightalmost until the moment it was on the horizon.

It was not obvious after the observations of the first nightwhether or not the star was in a normal maximum or in oneof the rare supermaxima. According to the New Zealandobservers it was known that the normal maxima ofV 436 Cen last only about 2 days. On the following night thestar increased again in brightness and as it kept this highbrightness du ring the next two nig hts it became clear that itwas a supermaximum! In the course of the observations ahump which only started to develop on the first night in­creased its amplitude to the value of 0.3 magnitude so thatit was clearly seen from the counts displayed on themonitor screen.

Unfortunately cirrus clouds which often cover the skyabove La Silla at this time of the year interrupted these ex­citing observations. But when after an 8-day break, the au­thor again began the observations (due to the kindness ofN. Vogt and J. Breysacher who offered her their nights),the star was only about 1 magnitude fainter, and the humpwas still visible although its amplitude had decreased. Onthe following night the hump merged into a rapid flicker­ing. But the star was visible in the telescope even 16 daysafter the beginning of the outburst, giving strong evidencethat it was indeed a long-Iasting supermaximum. Thereader may see some of the observations of the star in thefigure. The observations are not yet fully reduced. Perhapstheir further careful analysis will make some contributionto the better understanding of the most interesting dwarfnovae.

two operation centres, one located at the Goddard SpaceFlight Center in Greenbelt, Maryland, USA, the other at theESA Tracking Station in Villafranca dei Castillo near Ma­drid, Spain.

The telescope field of view is seen by a television camerain the satellite and can be displayed on a TV screen at theground station to allow the observer to identify his target.The situation is like with anormal ground telescope: justimagine to observe with the ESO 3.6 m telescope, wherethe astronomer sits in the control room. With IUE the con­tro/ room for the European astronomers is at the Villa­franca station, the telescope is only a bit further than theother side of the window ... !

The telescope is a Ritchey Chretien of 45 cm aperture,with focal ratio f/15, an image qual ity of 1 arcsec and anacquisition field of 10 arcmin in diameter. At the focal planethere is an echelle spectrograph with two SEC vidiconcameras, one for the range 1150-2000 A, the other for therange 1800-3200 A. One can choose between a high-dis­persion mode (resolving power -10 4

) and a low-dispersionmode (resolution -6 A).

The scientific aims of the IUE mission can be sum­marized as folIows:

During the past months, astronomers have beenbusy at the IUE ground station near Madrid. Dr.Massimo Tarenghi was the first ESO astronomerto use this unique ultraviolet satellite to observeextragalactic objects. This is the first, brief reportabout his exciting observations, together with Dr.Perola from Milan. We expect to bring furthernews about the IUE in the next issue of the Mes­senger.

Observation of the M87 Jetwith the International Ultraviolet ExplorerM. Tarenghi, ESO, GenevaG.C. Pero/a, /stituto di Fisica, Mi/ano, /ta/y

The International Ultraviolet Explorer (IUE), a joint projectof NASA, the United Kingdom, and the European SpaceAgency, is the first satellite designed for use by the generalastronomical community which does not require a specialknowledge of space techniques on the part of the observer.IUE is a geosynchronous satellite equipped with a 45 cmCassegrain telescope for spectroscopic studies in thewavelength range 1000-3000 A. It is kept under control at

16

"blind" offset mode. We therefore moved the telescope offby 12" from the centre of the galaxy in the appropriate di­rection and then selected a bright star in the field of viewfor the automatic guide. After 6h30m of exposure time withthe short-wavelength camera we saw on the screen thespectrum of the brightest knot of the M87 jet. Unfortunate­Iy, some fairly wide sections of the spectrogram were dis­turbed by a comparatively strong microphonic noise pro­duced during the read-out of the camera. (This happensrather rarely.)

A preliminary version (there are still problems with thecalibration of the IUE camera) of the spectrum after thesubtraction of the background is presented in the figure. It

teils us two important things: the first is that the opticalcontinuum appears to extend into the far UV without chang­ing the slope of the power law. The second is that, despiteof the noise, there is one emission feature in the spectrumwhich looks undoubtedly real (we have carefully inspectedthe raw image of the spectrogram to make sure that it is nota fake). This feature sits at 1556 A, which corrected for theredshift of M87 corresponds to the line CIV "-1549, the mostprominent line after Lya found in this range of wavelengthsin high redshift asos and also in the spectra of 3C 273,NGC 4151 and NGC 1068 obtained with IUE. (Because ofthe large aperture of the slot, the strong Lya line at 1216 Aisdue to geocoronal light.)

This result is particularly important, because it im­med iately impl ies that the brightest knot at least cannot bemovi ng at a large speed (say greater than a few hundred kmsec-') relative to the galactic nucleus. This represents astrong constraint for dynamical models of the jet involvingejection of matter from the nucleus of the galaxy.

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- to obtain high-resolution spectra of stars of all spectraltypes,

- to study gas streams within binary star systems,- to observe at low resolution faint stars, galaxies and

quasars,- to obtain spectra of planets and comets,- to improve the knowledge of the physical conditions in

the interstellar matter by measuring its effect on the stel­lar spectra.

After the first few months of observations an exciting bodyof data has already accumulated on a large number of ob­jects, from planets to asos. At the end of September, a fullissue of Nature will be devoted to the data obtained in thevery first period of observations.

Here we would like to present the preliminary results ofan observation of the jet in M87, which represents a case ofthis instrument used at the limits of its technical pos­sibilities. The radio galaxy Virgo A (NGC 4486, M87) is abright eil iptical in the Virgo cl uster. It is known since 1918 tocontain a very peculiar feature near its centre, wh ich lookslike a jet emerging from the nucleus. The total magnitudeof this feature is mB = 15.8, but it consists of several brightspots, among which the brightest has mB = 16.77. The op­tical spectrum is a featureless continuum which follows apower law with a spectral index n = 1.7 (F(v)av-n

) and ishighly polarized.

The soviet astrophysicist L. Shklovski suggested in 1956that the optical radiation is produced via the synchrotronprocess by relativistic electrons and positrons in a magne­tic field. It was the first extragalactic source with spectraland polarization properties similar to those of the Crabnebula continuum. One of the main problems it poses ishow the electrons get continuously accelerated to catch upwith the synchrotron losses, which become more and moresevere as the emission frequency increases. The first aim ofour measurement with IUE was therefore to measure howthe continuum extends into the far UV.

On July 24,1978, we pointed the IUE telescope in the di­rection of M87, whose nucleus is bright enough to appearas a diffuse spot on the TV screen. The jet itself is too faintto be seen in the picture, so, in order to position the en­trance slot of the spectrograph on it, we made use of thepossibility offered by IUE to guide an observation in the

Optical Pulsations from 4U 1626-67Discovered with the ESO 3.6 m TelescopeS.A. lIavaisky, C. Match and C. Chevalier

A little over a year ago, Drs. Claude Chevalier andSergio 1I0vaisky reported the optical identificationof the X-ray source LMC X-4 (cf. Messenger No. 9,p. 4). Now, together with Dr. Christian Motch, alsofrom Observatoire de Meudon, France, they havesucceeded in measuring optical pulses in a 19m

star with the same 'period as the southern X-raysource 4U 1626-67, and therefore identical withthis source. To obtain a high time-resolution, O~8,

it was necessary to use the 3.6 m telescope. Con­trary to other X-ray sourees, no Doppler shift hasyet been detected in 4U 1626-67.

Much excitement has been generated in the astronomicalcommunity by the publication of more than 50 accuratepositions (± 20" to 30") of galactic X-ray sources obtainedwith the Rotation Modulation Collimator (RMC) experi­ment on the SAS-3 satellite. Even more numerous andaccurate X-ray positions are expected as a result of thesky survey being carried out at this moment by the giantHEAO-1 satellite. With this improved positional infor­mation, optical identifications can now be attempted witha high degree of confidence inside the small X-ray errorcircles.

Preliminary photometric detective work carried out byJeffrey McClintock and colleagues at Cerro Tololo last yearsingled out two sources for further study: the X-ray bursterMXB 1735-44 and the X-ray pulsar 4U 1626-67. In both er­ror circles faint blue stars with unusual colours (V = 17.5,B-V = +0.2, U-B = -0.8 and V = 18.6, B-V = +0.1,U-B = -1.2, respectively) were found. These two sug­gested optical counterparts have been scrutinized in detail

17

Fig. 1: Optical identification of 4U 1626-67. To the lett theSAS-3 RMC position with aSO" diameter 90 per cent confidencecircle. Pulsations were detected from star No. 4, which is also in­dicated in the right hand figure, reproduced from the ESO QBS At­las.

10 9 8 7 6.5

PERIOD (SECONDS)Fig. 2: A small part of the power spectrum of the 4U 1626-67 opti­cal counterpart. The strong peak corresponds to aperiod of7.6805 sec.

2.0o 0.5 1.0 1.5

PULSE PHASEFig. 3: The light-curve of 4U 1626-67. Note the relatively sharp riseand gradual decrease.

the light curve-the data folded module the bestperiod-in figure 3, where the pulse is repeated for clarity.Our observations were made in white light using an S-13,low-noise, uncooled photomultiplier. The time resolutionwas 0~9 with counts being actually accumulated only for0~8. A highly stable quartz-crystal oscillator integratedinto our microprocessor-controlled photometer gave usstrictly equi-spaced integrations-a must for Fourieranalysis. The high signal-to-noise ratio obtained for thisfaint object with our system (a net star-sky signal of1100 counts/sec with a sky count of 3800 cis using a10" aperture) has encouraged us to plan re-observing thestar next season with better ti me resol ution (0~1). We arealready re-programming the microprocessor and hope forimproved weather conditions!

The Nature of 4U 1626-67

With an amplitude of 4 per cent this source turns out to bethe one showing the largest optical modulation of the opti­cally identified X-ray pulsars in binary systems (Her X-1,Vela X-1, 3U 1700-37 and 3U 1223-62). However, and this iswhere the 4U 1626-67 system is really mysterious, there isno trace in the X-ray data of any Doppler effect. This effect,normally seen in other X-ray pulsars, is due to orbital mo­tion of the compact X-ray source around the centre of massof a binary system in which the optical primary is the moremassive component-and the one we see-and where or­bital periods range from 2 to 40 days. The optical pulsa­tions detected in these systems are thoug ht to be due to X­ray heating of the material close to the X-ray source, whereX-ray photons are absorbed and quickly re-emitted as op­tical photons.

The absence of a Doppler effect on the 4U 1626-67 sys­tem led theoreticians to conclude that the orbital periodwas most Iikely very short (P ~ 0.3 day) in order to have es­caped detection and that the companion optical star was alow-mass, low-Iuminosity object, actually of lower massthan the X-ray source! In such a highly compact binary sys­tem, it was argued, X-ray heating of the companion woulddominate and strong optical pulses might be detected.This prompted our observations, which indeed revealedstrong pulses, but not even half as strong as predicted. Ourdata show no Doppler effect within one hour. Perhaps fu­ture observations might reveal this effect and thereby de­termine the orbital period-or are we in for more sur­prises?

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this season: MXB 1735-44 from Cerro Tololo in June 1978and 4U 1626-67 from La Silla in May, and in both cases theoptical observations have successfu Ily shown that thesuggested stars are indeed the correct ones: precisely thesame time-signature known in X-rays has been discoveredin each star. For4U 1626-67 we found optical pulsations atexactly the X-ray period (7.68 seconds) and forMXB 1735-44 three sharp optical bursts identical in shapeand duration (0.1 sec rise and 10 sec decay) to the X-raybursts were observed by a joint Harvard-MIT group, onebeing exactly simultaneous with an X-ray burst observedwith SAS-3.

Optical ObservationsDuring our observing run at the 3.6 m in May 1978-ourfirst at this telescope-the weather was particularly badwith cirrus clouds and 100 km/h winds and it really lookedas if we would not observe much, if at all. But on the secondhalf of the night of 2/3 May we decided to attempt high­speed photometry of the 4U 1626-67 cand idate for about1 h-through thin cirrus and with some wind-using ourown special equipment brought from Meudon. Observingconditions on the Cassegrain cage were not ideal, due to aseverely inclined cage floor (more than 40° with respect tothe horizontal!), but we managed to locate the almostnineteenth-magnitude star and put it in the photometeraperture.

Fourier analysis of the data was done back home inMeudon a few weeks later, and how pleasant was our sur­prise when the computer print-out showed a huge spike inthe power spectrum at 7~6805, almost exactly the pre­dicted geocentric X-ray period at the date of the observa­tions! A portion of the spectrum is shown in figure 2 and

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18

A Search for High-Inclination Minor PlanetsL.D. Schmadel and J. Schubart

There is a renewed interest in the Solar Systemand its planets. Incredible photos are transmittedback trom distant space probes and on-site phys­ical investigations have become possible. An in­creasing number ot new planets are being disco v­ered with large, ground-based telescopes. How­ever, most ot the known objects lie in or near theplane ot the Earth's orbit, the Ecliptic. Little isknown about the objects that presumably existhigh above or tar below this plane. How many arethere and how did they get there? Drs. LutzSchmadel and Joachim Schubart trom As­tronomisches Recheninstitut in Heidelberg, Fed.Rep. ot Germany, describe a search programmewith the ESO Schmidt telescope that aims at thediscovery ot out-ot-the-Ecliptic minor planets.

Most searches for faint minor planets have been based onphotographic plates of fields close to the Ecliptic. How­ever, Schubart recently indicated the possible existence offaint, resonant asteroids of Hilda und Thule types at highinclination and predicted favourable conditions for dis­covery of such objects at rather high eclipticallatitudes (35to 60 degrees). We have therefore carried out two pilot sur­veys with the ESO 1 m Schmidt in order to discover highly­inclined asteroidal orbits. The ESO astronomers H.E.Schuster and R.M. West participated in this programme.

Earlier surveys that aimed at the detection of faint minorplanets clearly favoured objects with a small orbital incli­nation relative to the plane of the Ecliptic. This is particu­larly true for the now famous Palomar-Leiden Surveywhich was based on plates taken with the 48-inch PalomarSchmidt telescope in September-October 1960, since allthe observed fields were comparatively close to the Eclip­tic. Only a few attempts have so far been made in order todetect objects with high inclinations; e.g. the search bySchubart in 1960 with the 40 cm astrograph at the Son­neberg Observatory. By the way, the recently numberedminor planet (2000) HERSCHEL is a result of this work. It isof the so-called Phocaea-type (the elements of the pro­totype, (25) PHOCAEA, are a = 2.4 AU, e = 0.25 and i = 22°).

Orbital resonancesAsteroids with a resonance between their orbital periodsand that of Jupiter have been known for many decades.Typical cases are the Trojan asteroids (1:1 resonance, thatmeans that the respective periods are nearly equal) and theHilda group planets (3:2 resonance). However, othertheoretically possible resonances show no or only very fewexamples in nature. There are for instance only three num­bered objects corresponding to the 2:1 or Hecuba case.These three objects are characterized by rather high orbitalinclinations (20° to 35°). In this connection it is also in­teresting to note the recent ESO discovery of the Trojan­type planet 1976 UQ (see Messenger No. 8) which has a verylarge inclination, 39°.

Schubart has called attention to the possibility of the ex­istence of different ki nds of resonant motions in orbits withhigh inclinations. This refers to the Hilda (3:2) and Thule

(4:3) resonance types. Theoretical considerations show acertain analogy to the resonance between the majorplanets Neptune and Pluto. The orbital periods of theseplanets are approximately characterized by a 2:3 ratio, andthe orbit of Pluto is highly inclined with respect to that ofNeptune so that close encounters cannot occur. In thesame way the possible asteroids of Hilda and Thule typecan avoid e10se approach es to Jupiter, both because of theresonance and because of a high inclination. Especially,the parts of the orbits with the largest heliocentric dis­tances are always far above or below the orbital plane ofJupiter.

A Search with the ESO SchmidtMinor planet surveys require telescopes of considerableaperture and with sufficiently wide field. The PalomarSchmidt and the ESO Schmidt are very weil suited for thispurpose. In a cooperation between the AstronomischesRecheninstitut and ESO we have started a small-scale sur­vey for high-inclination asteroids. Two such searches tookplace during September/October and December 1977. Thefirst one was aimed at the discovery of Thule-type objects,the second one especially at Hilda's. No objects with thepredicted resonant periods were found in either of the in­vestigations. This is not too surprising in first attemptssince the objects are certainly very rare if they exist at all.Otherwise they would already have been detected bychance on plates taken for other purposes. Further at­tempts to discover such objects are sched uled for the nearfuture.

Even if the whole project should not lead to the discoveryof the expected types, it will make a valuable contributionto our knowledge about the statistics of highly-inclinedminor planets, as seen from the by-products of the firstsurvey. The most surprising event was the discovery ofComet Schuster 19770 (see Messenger No. 11). Besidesthis, Schuster found the fast-movi ng planet 1977 RC, a Pal­las-typ~ planet with a = 2.7 AU, e = 0.46, and i = 30°. Thisasteroid headed further south from the discovery field atlatitude -40° and reached -50° in October 1977. While itwas followed to this latitude, two further planets appearedon the plates. They are probably of the so-called Hungariagroup which is characterized by a small semi-major axisand a high inclination. Together with 1977 RC, we discov­ered six additional minor planets on the very first plates in­cluding another Hungaria asteroid and a new Phocaea­type object.

The December 1977 search comprised two fields cen­tered at ecl ipticallatitudes of _58° and -51 0. We discoveredno moving objects. Nevertheless, this result is statisticallyimportant since the observed field is larger than the field ofour first attempt. By-products at such very high latitudesmay sometimes occur, as was demonstrated by theabove-mentioned 1977 RC or, for instance, by ObjectLovas, which was discovered late in 1977 at a northern de­clination of about 80 degrees. This object was first believedto be a comet (1977 t) but was later found to be a minorplanet of Pallas type.

Our next survey is scheduled for September 1978 andwill be under way when this note appears in print. We shallagain look for Hilda's but at the more moderate latitude ofabout -42°. Hopefully, there will at least be some interest­ing Phocaea or Pallas objects this time.

19

LATEST NEWS

The "Hilda-Thule" minor planet programme that is presently (September 1978)being carried out in collaboration between Heidelberg and ESO has yielded'several new asteroids with peculiar motions (see page 19). Further news in thenext issue.

20

ALGUNOS RESUMENES

Fue honrado astr6nomode ESOEn la publicaci6n Minar Planet CircularNo. 4358, publicada por el Centro en Cin­cinnati el 30 de abril de 1978, se puedeleer el siguiente aviso:

Nuevo nombre para planeta menor

(2018) SCHUSTER = 1931 UCDescubierto el 17 de octubre de 1931por K. Reinmuth de Heidelberg..

Nombrado en honor de Hans-Emil Schus­ter, astr6nomo dei Observatorio EuropeoAustral, quien observa activamente y hadescubierto planetas menores y cometas.

Investigaci6n de PlanetasMenores de inclinaci6nelevadaLa mayoria de los nuevos planetas que sedescubren con los grandes telescopios seencuentran dentro 0 cerca dei plano de la6rbita terrestre, la ecliptica. En real idadsabemos poco sobre los objetos que pre­suntivamente existan muy por encima 0muy por debajo de este plano.

Por esta raz6n, los Drs. Lutz Schmadel yJoachim Schubart dei Astronomisches Re­cheninstitut en Heidelberg, Republica Fed­eral de Alemania, han recientemente hechouna investigaci6n de los asteroides debiles,resonantes dei tipo Hilda y Thule en inclina­ci6n elevada con el telescopio Schmidt de1 m de ESO en La Silla. Los astr6nomos deESO, Sres. Schuster y West, participaron eneste programa.

Por largo tiempo ya se conocfan as­teroides con una resonancia entre susperlodos orbitales y aquel de Jupiter. Uncaso t(pico son los asteroides dei tipo Trayacon una resonancia de 1:1 (10 que significaque los respectivos periodos son casiiguales).

Schubart ha lIamado la atenci6n sobre laposible existencia de diferentes tipas demovimientos resonantes en 6rbitas de in­clinaci6n elevada. Esto se refiere a los tiposde resonancia de Hilda (3:2) y Thule (4:3).

Se hicieron dos investigaciones, duranteseptiembre/octubre y en diciembre de1977. La primera fue dedicada al descu­brimiento de objetos dei tipo Thule y lasegunda especialmente a aquellos dei tipoHilda. En ninguna de las dos inves­tigacianes fueron encontrados objetos conlos periodos de resonancia anunciados.Esto no es sorprendente, ya que los ob­jetos, en caso de existir, san sumamenteescasos. Se han planeado mas ensayos enun futuro cercano.

Sin embargo, aunque el proyecto na lIeveal descubrimiento de los objetos es­perados, contribuira altamente a nuestrosconocimientos sobre las estadisticas deplanetas menores de inclinaci6n elevada.Como sub-producto de esta primera inves­tigaci6n fueron descubiertos el CometaSch uster 1977 0 Yseis nuevos planetas me­nores.

Fuentes de rayos X

Identificaciones 6pticas de fuentesde rayos X

Nuevos datos de satelites han propor­cionado una gran cantidad de trabajo al­tamente interesante en conexi6n con es­trellas de rayos X a astr6nomos 6pticos. Unbuen ejemplo es la investigaci6n lIevada acabo por el satelite SAS-3 que obtuvo masde 50 posiciones precisas de fuentes derayos X galacticas.

EI trabajo fotometrico preliminar efectua­do el ario pasado por Jeffrey McClintock ycolegas en el Cerro Tololo, seleccion6 dosfuentes para un futuro estudio. En amboscasos se encontraron estrellas azules de­biles con colores no usuales. Uno de estosejemplares 6pticos sugeridos, 4U 1626-67,fue investigada detenidamente por los Drs.Claude Chevalier, Sergio 1I0vaisky y Chris­ti an Motch dei Observatorio de Meudon,Francia, durante el mes de mayo dei ario encurso con el telescopio de 3,6 m de ESO. EIotro, MXB 1735-44, fue estudiado en elCerro Tololo. En ambos casos las obser­vaciones 6pticas mostraron que las estre­lias propuestas eran efectivamente correc­tas: Para 4U 1626-67 una pulsaci6n 6pticaexactamente en el perlodo de los rayos X(7,68 segundos) y para MXB 1735-44 se ob­servaron tres nltidas erupciones 6pticasidenticas en tamario y duraci6n a las erup­ciones de rayos X, una de estas sucediendoen exacta simultaneidad con una erupci6nde rayos X observada con el SAS-3.

La estrella doble de rayos X4U 1700-37/HD 153919

Otra estrella, HD 153919, el dupl icado 6p­tico de la estrella doble de rayosX 4U 1700-37 con un periodo de 3,41 dias,fue observada durante cuatro arios por losDrs. Hammerschlag-Hensberge, E.P.J. vanden Heuvel y sus colaboradores con lostelescopios de ESO. HD 153919 es una delas estrellas mas luminosas identificadascon una fuente de rayos X y el estudio deeste objeto ya ha revelado algunos detallesinteresantes con respecto a este sistema deestrellas dobles.