EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT

EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT

A. Bemporad1, G. Poletto2, J. C. Raymond3 and S. Giordano4

1Astronomy and Space Science Dept. – University of Firenze – Firenze, Italy2INAF - Arcetri Astrophysical Observatory – Firenze, Italy

3Harvard - Smithsonian Center for Astrophysics, Cambridge, MA, USA4INAF – Torino Astrophysical Observatory, Torino, Italy

AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

“EUV observations of sungrazing comets with the SOHO/UVCS instrument”

Arcetri Astrophysical Observatory

Harvard-SmithsonianCFA

University of Florence

Torino Astrophysical Observatory



SUMMARY

• Introduction: state of art

• The origin of the observed cometary emission

• Determination of cometary parameters

• Results: sungrazers outgassing rate, nuclear size, fragmentation processes, bow shock

• Work in progress



INTRODUCTION: BEFORE & AFTER THE SOHO MISSION (1995)

• Before 1979 only about 12 sungrazers were discovered: 3 before the IX century, 5 in the IX century and 4 in the years 1945-1970. • In the decade 1979-1989 about 16 sungrazers were discovered: 6 by the SOLWIND instrument on the P78-1 satellite and 10 by the coronagraph aboard the Solar Maximum Mission.• Since 1996 the two SOHO/LASCO coronagraphs discovered more than 900 comets; over 700 belong to the Kreutz sungrazer group.



THE LASCO (Large Angle Spectroscopic Coronagraph) INSTRUMENT ON SOHO

LASCO/C2: from 2.0 to 6.0 Rsun

CCD 10242 - Pixel size: 11.4”

LASCO/C3: from 3.7 to 32 Rsun

CCD 10242 - Pixel size: 56.0”

C/1996 Y1 C/1996 Y1



THE UVCS (UltraViolet CoronagraphSpectrometer) INSTRUMENT ON SOHO

UVCS: from 1.4 to 10 Rsun - CCD 1024X360 Pixel size: 0.0925Å spectral, 7” spatial



UVCS SUNGRAZER OBSERVATIONS:STATE OF ART

YearYear Comets #Comets # Obs. DateObs. Date Comet NameComet Name Comet GroupComet Group PublishedPublished

1996 1 Dec 23 C/1996 Y1 SOHO-6 KreutzKreutz YesYes1997 1 May 1-2 C/1997 H2 SOHO-8 None No1998 1 Jun 1 C/1998 K10 SOHO-54 Kreutz No1999 2 May 20 C/1999 K1 SOHO-63 Kreutz No

Sep 17 C/1999 S1 SOHO-86 Kreutz No2000 4 Feb 10 C/2000 C6 SOHO-104 KreutzKreutz YesYes

Feb 29 C/2000 D1 SOHO-106 Kreutz NoSep 9-11 2P/ENCKE None YesOct 11 C/2000 T1 SOHO-204 Kreutz No

2001 1 Feb 6-7 C/2001 C2 SOHO-294 KreutzKreutz YesYes2002 2 May 14 C/2002 J8 SOHO-442 Kreutz No

Sep 18 C/2002 S2 SOHO-517 Kreutz No

2003 1 Jan 27-29 C/2002 X5 Kudo-Fujikawa None Yes

• UVCS observed only 13 comets: 10 of these belong to the Kreutz Group.• Until now, only 3 works have been published.

Sungrazing comets are important to understand physical evolution and disintegration of comets, but…



UVCS SUNGRAZER OBSERVATIONS:DISCUSSION PLAN

1. UVCS observations of sungrazers revealed a high cometary emission in the Hydrogen Lyman-Lyman-αα λ1216Å spectral line. This implies an enhanced number of H atomsH atoms.

2. Cometary H atoms may be created by different processes: from an analysis of the Lyα profile we can infer their origintheir origin.

3. From the observed Lyα emission we derive an estimate for the cometary outgassing rateoutgassing rate and the nucleus sizenucleus size.

4. From the variation with the heliocentric distance of the outgassing rate we discuss possible fragmentation processesfragmentation processes.



THE ORIGIN OF THE Lyα EMISSIONIN SUNGRAZING COMETS

In coronal conditions spectral lines emission is mainly due to:

• Collisional excitationCollisional excitation with thermal electrons• Radiative excitationRadiative excitation from the chromospheric radiation

In order to distinguish between these processes, we can look at the ratioLyLyβ/Lyαβ/Lyα between the H Lyman-β λ1025Å, also included in the UVCS spectra, and the Lyman-α spectral lines.

At typical coronal temperatures (~106 K): collisionalcollisional Lyβ/Lyα ~ 0.13-0.14Because of the small chromospheric ratio: radiative radiative Lyβ/Lyα ~ 0.001-0.002

In the UVCS observations of C/1996 Y1, C/2000 C6 and C/2001 C2 In the UVCS observations of C/1996 Y1, C/2000 C6 and C/2001 C2 the Lythe Lyβ line was absentβ line was absent

Cometary Lyα emission only Cometary Lyα emission only from Radiative excitationfrom Radiative excitation




H atoms responsible for this Lyα emission may arise from:

• PhotodissociationPhotodissociation of outgassed H2O molecules• Charge exchangeCharge exchange process between coronal p+ and H atoms from H2O

1. The velocity imparted to the H atoms from outgassing (Delsemme 1982) and photodissociation processes (Huebner 1992) is ~ 30-40 km/s, much smaller than typical sungrazer speed close to the Sun.

2. The momentum transfer in the charge exchange process between p+ and H atoms is very small (McClure 1966).

H atoms from HH atoms from H22O photodissociationO photodissociation move with the cometmove with the comet

Secondary H atoms Secondary H atoms have the coronal have the coronal plasma velocity distributionplasma velocity distribution




Hence Lyα emission from H atoms generated by H2O photodissociation:

• Is strongly reduced by the Swing effectSwing effect (dimmed by a factor 0.2-0.1 for vrad ~ 250-300 km/s, Kohl 1997)

• Would drop as soondrop as soon as the comet travels beyond the spectrograph slit,while UVCS observations revealed a persistent (30-40 minutes) Lyα emission.

• Would results in a narrow line narrow line profileprofile, while…

(from Uzzo et al. 2001)




(from Uzzo et al. 2001)

…while the FWHM of the cometary and coronal Lyα profiles are about equal.

H atoms responsible for the sungrazer emission form from the charge exhange between coronal p+ and H atoms from

H2O



ESTIMATE OF SUNGRAZERS OUTGASSINGRATE AND NUCLEUS SIZE

1. The comet leaves along its path a number of neutral H atoms Ncoma proportional to the outgassing rate Ndot and to the charge exchange ratecx (which depends on the proton flux and the local proton density np= ne ).

2. Then, after the comet transit, the total number of Lyα counts exponentially decaysexponentially decays with time as exp(-t/ion) , where ion is the ionization rate (mainly by collisions with electrons, ion ∝ ne

-1).

(from Bemporad et al. 2005)

From the observed (Lyα)counts vs. time curve we derive the cometary

outgassing rateoutgassing rate and the local plasma electron density ne



ESTIMATE OF SUNGRAZERS OUTGASSINGRATE AND NUCLEUS SIZE

1. The comet leaves along its path a number of neutral H atoms Ncoma proportional to the outgassing rate Ndot and to the charge exchange ratecx (which depends on the proton flux and the local proton density np= ne ).

2. Then, after the comet transit, the total number of Lyα counts exponentially decaysexponentially decays with time as exp(-t/ion) , where ion is the ionization rate (mainly by collisions with electrons, ion ∝ ne

-1).

From the observed (Lyα)counts vs. time curve we derive the cometary

outgassing rateoutgassing rate and the local plasma electron density ne

Assuming a spherical nucleusand isotropic outgassing we

estimate its equivalentequivalent radiusradius



UVCS OBSERVATION OF SUNGRAZERS: RESULTS

Comet nameComet name QQH2OH2O ( (kg/skg/s)) rr ( (mm)) hh ( (RRoo))

C/1996 Y1 20.0 3.4 6.80C/2000 C6 71.8 3.0 3.26

140 5.8 4.5634.6 3.4 5.7110.5 2.5 6.36

C/2001 C2 820a 20.3 3.6058.9a 7.8 4.9828.5b 5.4 4.98

UVCS observations revealed a “hidden masshidden mass” inconsistent with nominal models of coronagraphic observations and explained by slowly eroding subfragments (Sekanina 2003).

The observed sudden increases in the Lyα brightness could indicate the fragmentation fragmentation of the comet nucleus (Uzzo et al. 2001).

(LASCO running difference movies)

1. Occasional secondary brighteningssecondary brightenings in some sungrazer lightcurves have been explained by introducing one or more nearby companionsnearby companions traveling with the main nucleus (Sekanina 2003)

(from Sekanina 2003)



SUNGRAZING COMETS: FRAGMENTATION PROCESSES

Fragmentation processes play a key role in the origin and evolution ofsungrazing comets.

2. From a comparison between the orbital parameters of different comets, has been demonstrated (Sekanina 2002) that many sungrazers arriving in pairs or tripletspairs or triplets originated via fragmentation eventsfragmentation events from a single sungrazer far from the Sun.



SUNGRAZING COMETS: FRAGMENTATION PROCESSES

3. Recently an intriguing scenario has been published (Sekanina & Chodas 2004) involving splitting both close and far to the Sun. This could explain the generation, via runway frag-runway frag-mentationmentation, of the observed sungra-zers from a single progenitorprogenitor.

(from Sekanina & Chodas 2004)Close to the Sun, sungrazer fragmentation is more probable because of:

a) High tidal stressestidal stresses (∝r2/d3 ), in particular around the Roche limit (~ 3.2 Ro)

b) High thermal stressesthermal stresses, mainly on the nucleus surface

c) High sublimation fluxsublimation flux can create an averagesurface pressure overcoming the tensile strength



UVCS observations of C/2001 C2 at 4.98 Ro show the presence of 2 Ly2 Lyα α

tailstails. A first interpretation is that the comet is composed by two fragmentstwo fragments, but at least 2 more alternatives are possible:

Tail 1

Tail 2

1) 2 tails generated by a single object: 2 tails generated by a single object: however, at 5 Ro the nucleus is exposed to an extremely high solar flux (~ 2.5·109 ergs cm-2 s-1)

→ the whole surface should be active.

2) Dust particles ejected from a single object at 2 different times: Dust particles ejected from a single object at 2 different times: we cannot exclude this interpretation, even if in this scenario it’s not easy to explain why ~1h later the comet showsonly one tail at 3.60 Ro.

(from Bemporad et al. 2005)

SUNGRAZING COMETS: THE OBSERVEDFRAGMENTATION OF C/2001 C2



SUNGRAZING COMETS: THE OBSERVEDFRAGMENTATION OF C/2001 C2

We interpret the two observed Lyα tails as two fragments, unresolved by the LASCO images.

These fragments had an equivalent radius of 7.8 m and 5.4 m and were ~ 90200 km apart ( ~ 124”).

This interpretation seems to be confirmed by the C/2001 C2 lightcurve which shows a secondary secondary brightneningbrightnening below ~ 5.3 Ro.



SUNGRAZING COMETS:BOW SHOCK

(from http://www.astro.uwo.ca/~jlandstr/planets/)

Close to the Sun, magnetic field, solar wind velocity Vwind and the sungrazer velocity Vcomet are nearly radial.A shock should form with a shock velocity Vshock= Vwind + Vcomet .A fraction of the energy dissipated in the shock goes into plasma heatingplasma heating.

Sungrazers C/2000 C6 and C/2001 C2 were immersed in a slow windslow wind region; no significant plasma heating was observed from Lyα profiles.

Sungrazer C/1996 Y1 crossed a fast windfast wind region: its Lyα line profile was much broader (Tk ~ 9 ·106 K) than the background coronal profile. From the observed proton temperature, Raymond et al. 1998 inferred VVwindwind ~ 620 km/s ~ 620 km/s



C/2002 S2 SUNGRAZING COMET:WORK IN PROGRESS

LASCO/C2 & C3 movie

UVCS observations of sungrazer C/2002 S2 revealed the presence of 2 Ly2 Lyα tailsα tails at 6.84 Ro and possibly also at lower heights.

(S. Giordano, private comunication)

On September 18-19, 2002 UVCS observed the sun-grazer C/2002 S2 at 4 heliocentric distances.



C/2002 S2 SUNGRAZING COMET:OPEN QUESTIONS

The observed curves for the Lyα counts vs. time show a slow slow increaseincrease, difficult to explain with the actual model.

The puzzling Lyα Doppler shift image revealed the 2 observed tails to be red- and blue-red- and blue-shiftedshifted by ~ 100 km/s. These cannot be interpreted as the signature of 2 fragments.



Actually the formulation of a simple modelmodel to explain the observed Lyα features is in progress.

C/2002 S2 SUNGRAZING COMET:THE MODEL

vwind = 0 km/sne = 1.4 · 104 cm-3

Tk = 1.5 · 106 KNdot = 1 · 1028 s-1

vwind = 200 km/sne = 2.8 · 104 cm-3

Tk = 1.5 · 106 KNdot = 1 · 1028 s-1



CONCLUSIONS (1/2)

• Since 1995 LASCO discovered more than 700 sungrazers; however only 10 of these were observed also by UVCS and only 3 works are published.

• UVCS observations of sungrazers revealed a high cometary emission in the Hydrogen Lyman-Lyman-αα λ1216Å spectral line. H atoms responsible for this emission are those formed with a charge exchange processcharge exchange process between H atoms from H2O and coronal protons.

• From UVCS data it’s possible to give an estimate for the cometary outgassing rate and the equivalent radiusequivalent radius: the observed sungrazers have a radius of about 3 – 8 m below ~ 6 Rsun (“hidden mass”).



CONCLUSIONS (2/2)

• The observed increases in the outgassing rate indicate fragmentationfragmentation.Taking advantage of the UVCS higher spatial resolution (e.g. 42” for C/2001 C2), the presence of sub-fragmentssub-fragments, unresolved by LASCO coronagraphs, has been directly observed for the C/2001 C2 sungrazer.

• The formation of the cometary bow shockbow shock is strongly dependent on the coronal region encountered by the comet.

• The explanation for the puzzling behaviourpuzzling behaviour of the Lyman-α emission from the C/2002 S2 sungrazer is actually a “work in progress”: a good modelmodel of the cometary tail could help us in order to explain the observed features.

EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT

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