TRITON STELLAR OCCULTATION CANDIDATES: 2000...

THE ASTRONOMICAL JOURNAL, 119 :936È944, 2000 February2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

TRITON STELLAR OCCULTATION CANDIDATES: 2000È2009

S. W. MCDONALD AND J. L. ELLIOT1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 ;

mcdonald=mit.edu, jle=mit.eduReceived 1999 October 18 ; accepted 1999 November 10

ABSTRACTAs part of our ongoing program of predictions and observations of stellar occultations by solar system

bodies, we have completed a search for candidates for occultations by Triton over the decade 2000 to2009. Star positions near TritonÏs projected orbit as determined by the DE405 ephemeris and NEP016orbit model were measured on (unÐltered) CCD strip scans recorded with the 0.6 m telescope at theGeorge R. Wallace Astrophysical Observatory to a depth of 16th to 18th magnitude, depending on thequality of individual strip scans. Within of the predicted orbit of Triton during this period, 128 stars1A.0were found, including 12 stars brighter than 14th magnitude. Only appulses with geocentric minimumseparations of less than about will result in an occultation visible from Earth, but potential errors0A.37in the ephemeris and in the positions of our candidates preclude accurate prediction of actualoccultation events without further astrometry.Key words : astrometry È occultations È planets and satellites : individual (Triton)

1. INTRODUCTION

Stellar occultations have been used many times in thepast to study the rings and atmospheres of solar systembodies (Elliot, Dunham, & Olkin 1995 ; Elliot & Olkin1996). The atmosphere of Triton has proved to be particu-larly interesting since from recent stellar occultation datawe found that it exhibits an unexpectedly large ellipticity(Elliot et al. 1997) and an increasing pressure since theV oyager encounter (Elliot et al. 1998 ; Sicardy et al. 1998).The large ellipticity is likely due to winds circulating withnearly sonic velocities, while the increasing pressure iscaused by a slight warming of the nitrogen surface frost,since this frost is in vapor-pressure equilibrium with theprincipal atmospheric gas.

In order to facilitate continued probing of TritonÏs atmo-sphere with the high spatial resolution a†orded by stellaroccultations, we have extended our identiÐcations of poten-tial occultations by Triton (McDonald & Elliot 1992, 1995)into the Ðrst decade of the next century. This paper presents128 stars measured on CCD strip scans that lie within of1A.0the apparent path of Triton over the period of 2000 through2009. Occultations observed for stars from our previouscandidate lists (Table 1) show astrometric errors of a fewtenths of an arcsecond, and at TritonÏs distance from Earth,the angles subtended by TritonÏs atmospheric half-lightradius and by the radius of Earth are about and0A.07 0A.30,respectively. Hence, further astrometry of these candidateswill be necessary before we can predict which will beinvolved in an occultation visible from Earth, and an evenmore extensive astrometric e†ort will be required to predictthe path of TritonÏs shadow accurately enough to facilitateobservations.

2. OBSERVATIONS AND ANALYSIS

This section gives an overview of the analysis procedureswe used for this project, focusing on the changes introducedsince the last Triton candidate paper (McDonald & Elliot

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Also Department of Physics, Massachusetts Institute of Technology,

and Lowell Observatory.

1995). Some of these procedures have become moreautomated.

The data were recorded with the 61 cm telescope atMITÏs Wallace Astrophysical Observatory, located inWestford, Massachusetts. Our Portable CCD Camera(PCCD; Buie et al. 1993) has replaced the SNAPSHOTCCD camera (Dunham et al. 1985), which was used forprevious candidate searches. Images were taken in strip-scan mode on the PCCD with no Ðlter in order to maximizethe number of stars detected. The typical full width at half-maximum of the star images was about 2 pixels, at an imagescale of pixel~1. Each strip-scan Ðeld overlapped its2A.57neighbors by 50% to get redundant coverage of the entirestar Ðeld near TritonÏs orbit (see Fig. 1).

Each image was Ñattened in IRAF (Tody 1986) and pro-cessed by IRAFÏs DAOPHOT routines (Stetson 1987) toautomatically identify and measure the positions andinstrumental magnitudes of stars down to the limiting mag-nitude. Astrometric reduction of the strip scan is begun byvisually inspecting the frame and establishing three corre-spondences with stars in the US Naval ObservatoryÏs A2.0star catalog, which are then used for a preliminary astrom-etric solution. This solution is accurate enough so that allUSNO-A2.0 stars detected on the frame (several thousand)can be identiÐed, and a Ðnal astrometric solution is carriedout. We Ðnd that the rms residual of the USNO-A2.0 starpositions is about We have not found any signiÐcant0A.25.systematic errors in the USNO-A2.0 catalog, so registeringto a large number of stars should give us a good meanreference frame.

In previous astrometric analysis of CCD strip scans, wehave observed deviations between the observed coordinatesof stars on a single scan compared with either a goodcatalog of star positions or with a mean position taken fromnumerous observations of the same Ðeld (Dunham,McDonald, & Elliot 1991). These deviations are highly cor-related for neighboring stars. On a CCD strip scan, eachstar image is exposed for a time interval that depends on itsright ascension. Since the right ascension exposed within astrip scan is a linear function of time, a variation in rightascension and declination with respect to right ascensionimplies a time-dependent error in both coordinates. The

936

21h22h 20h20m20h40m21h21h20m21h40m22h

-20˚

-10˚

-20˚

-10˚

-28˚

-26˚

-24˚

-22˚

-20˚

-18˚

-16˚

-14˚

-12˚

-10˚

-08˚

-06˚

δ δ

β β

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γ γ

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ε ε

θ θ

ω ω

ψ ψ

α1α1

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ι ι

2424

ν ν

3636

ε ε

ξ ξ

µ µ

κ κ

ν ν

ρ ρ

η η

µ µ

4646

υ υ

4242

τ τ

φ φ

4141

π π

σ σ

2929

χ χ 3333

3030

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3737

1919

88

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2727

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CAPRICORNUS

AQUARIUS

Right Ascension and Declination [J2000]

TRITON STELLAR OCCULTATION CANDIDATES 937

TABLE 1

RESULTS OF PREVIOUS OCCULTATIONS

CLOSEST APPROACH CLOSEST APPROACH(candidate search) (observed)

OCCULTED OCCULTATION Min. Sep. P.A. of Triton Min. Sep. P.A. of Triton DIFFERENCESTAR DATE (arcsec) (deg) (arcsec) (deg) (arcsec) REFERENCES

Tr60 . . . . . . . 1993 Jul 10 0.02 360 0.03 180 [0.05 1, 2Tr148 . . . . . . 1995 Aug 14 0.42 348 0.28a 348 [0.14 2, 3Tr176 . . . . . . 1997 Jul 18 0.09 162 0.07 342 0.16 3, 4Tr180 . . . . . . 1997 Nov 4 0.06 158 0.21 338 0.27 3, 5

a Tr148 was discovered to be a double star ; the observed minimum separation is calculated for the center of light.REFERENCES.È(1) McDonald & Elliot 1992 ; (2) Olkin et al. 1997 ; (3) McDonald & Elliot 1995 ; (4) Elliot et al. 2000 ; (5) Elliot et al.

1998.

pattern of deviation varies with each frame but generallyexhibits irregular variability on timescales longer than 30 s.The best explanation we have for this e†ect is time-variableatmospheric distortion, which is consistent with both thelocal correlation and the timescale of the e†ect (each star isexposed for 90 s on our strip scans).

During most of our previous searches for occultationcandidates that used CCD strip scans, we did not have anadequate star catalog to remove the variable deviation, andwe could not take enough images to produce mean starpositions, so we were unable to correct the positions of ouroccultation candidates for the deviation. However, theUSNO-A2.0 catalog is dense enough for this purpose, so wewere able to correct for the deviation in this candidatesearch. After registering each image against the catalog and

converting the observed star positions into right ascensionand declination, we plotted the remaining residuals fromthe catalog versus right ascension using Mathematica 3.0(Wolfram 1996). We then Ðtted a Fourier series to theresiduals in right ascension, and independently to theresiduals in declination. The resulting functions were sub-tracted from the full set of observed star positions on theimage to produce what we believe is an improved positionfor each star (see Fig. 2). This is of course critically depen-dent on the lack of systematic errors in the USNO-A2.0catalog. If there are systematic errors in the catalog, thismethod will introduce them into our star positions.

The observed stellar magnitudes were calibrated in asimilar fashion. For most stars we have found that ourunÐltered CCD observations follow the R magnitude scale

FIG. 1.ÈLayout of strip-scan Ðelds. The odd-numbered Ðelds are depicted, along with a plot of TritonÏs orbit from 2000 through 2009. Even-numberedÐelds overlap the odd-numbered Ðelds by 50% on each side. Each Ðeld is about 15@ wide.

TABLE 2

STRIP SCANS USED IN SEARCH

MagnitudeStrip Date Limit Notes

04a . . . . . . 1996 Aug 15 16.5 104b . . . . . . 1996 Aug 6 16.704c . . . . . . 1996 Aug 23 16.7 1, 204d . . . . . . 1998 Aug 22 17.905a . . . . . . 1996 Aug 6 17.705b . . . . . . 1996 Aug 23 17.0 1, 206a . . . . . . 1996 Aug 6 17.0 306b . . . . . . 1996 Aug 23 16.806c . . . . . . 1998 Sep 18 18.107a . . . . . . 1996 Aug 6 17.0 207b . . . . . . 1996 Aug 22 17.107c . . . . . . 1998 Sep 18 17.908a . . . . . . 1996 Aug 7 17.108b . . . . . . 1996 Aug 23 17.1 208c . . . . . . 1998 Sep 18 16.4 109a . . . . . . 1996 Aug 7 16.509b . . . . . . 1996 Aug 23 17.110a . . . . . . 1996 Aug 7 17.110b . . . . . . 1996 Aug 23 17.111a . . . . . . 1996 Aug 7 17.011b . . . . . . 1996 Aug 23 17.312a . . . . . . 1996 Aug 7 17.012b . . . . . . 1996 Aug 23 17.113a . . . . . . 1996 Aug 8 16.513b . . . . . . 1996 Aug 22 17.213c . . . . . . 1996 Sep 4 17.014b . . . . . . 1996 Sep 4 17.2 315a . . . . . . 1996 Aug 8 17.215b . . . . . . 1996 Sep 4 17.2 216a . . . . . . 1996 Aug 9 17.3 116b . . . . . . 1996 Aug 21 17.2 117a . . . . . . 1996 Aug 9 17.817b . . . . . . 1996 Sep 4 17.518a . . . . . . 1996 Aug 9 17.918b . . . . . . 1996 Sep 4 17.6 218c . . . . . . 1998 Aug 22 17.819a . . . . . . 1996 Aug 9 17.619b . . . . . . 1996 Sep 4 17.4 319c . . . . . . 1998 Aug 22 17.620a . . . . . . 1996 Aug 7 17.020b . . . . . . 1996 Sep 4 17.2 320c . . . . . . 1998 Aug 22 17.621a . . . . . . 1996 Aug 8 16.721b . . . . . . 1996 Sep 20 17.821c . . . . . . 1998 Jul 21 15.522a . . . . . . 1996 Aug 8 16.922b . . . . . . 1996 Sep 20 17.722c . . . . . . 1998 Aug 3 17.623a . . . . . . 1996 Aug 8 17.023b . . . . . . 1996 Sep 20 17.623c . . . . . . 1998 Aug 3 18.324a . . . . . . 1996 Aug 8 16.624b . . . . . . 1996 Sep 20 17.624c . . . . . . 1998 Aug 3 18.225a . . . . . . 1996 Aug 9 17.325b . . . . . . 1996 Sep 20 18.125c . . . . . . 1998 Aug 3 17.9 326a . . . . . . 1996 Aug 9 17.326b . . . . . . 1996 Sep 20 17.526c . . . . . . 1998 Aug 3 17.9 327a . . . . . . 1996 Aug 9 16.6 227b . . . . . . 1996 Aug 21 17.427c . . . . . . 1998 Aug 3 18.128a . . . . . . 1996 Aug 12 16.4 128b . . . . . . 1996 Aug 18 16.9 1, 228c . . . . . . 1996 Aug 21 17.428d . . . . . . 1998 Aug 3 18.229b . . . . . . 1996 Aug 21 17.529c . . . . . . 1998 Aug 3 18.1 130a . . . . . . 1996 Aug 19 17.630b . . . . . . 1996 Aug 23 17.130c . . . . . . 1998 Aug 3 17.5 131a . . . . . . 1996 Aug 19 17.3

TABLE 2ÈContinued

MagnitudeStrip Date Limit Notes

31b . . . . . . 1996 Sep 20 17.531c . . . . . . 1998 Jul 21 17.932a . . . . . . 1996 Aug 19 18.232b . . . . . . 1996 Sep 20 17.832c . . . . . . 1998 Aug 2 18.433a . . . . . . 1996 Aug 19 18.033b . . . . . . 1996 Sep 21 17.1 133c . . . . . . 1998 Aug 2 17.734a . . . . . . 1996 Aug 19 18.134b . . . . . . 1996 Sep 21 17.234c . . . . . . 1998 Aug 2 18.235a . . . . . . 1996 Aug 19 17.435b . . . . . . 1996 Sep 21 17.635c . . . . . . 1998 Aug 2 17.936a . . . . . . 1996 Aug 19 18.136b . . . . . . 1996 Sep 21 17.436c . . . . . . 1998 Aug 2 17.937a . . . . . . 1996 Aug 19 17.837b . . . . . . 1996 Sep 21 17.337c . . . . . . 1998 Aug 2 18.038a . . . . . . 1996 Aug 19 17.338b . . . . . . 1996 Aug 21 17.938c . . . . . . 1998 Aug 2 17.639a . . . . . . 1996 Aug 19 18.139b . . . . . . 1996 Sep 21 18.139c . . . . . . 1998 Aug 2 17.740a . . . . . . 1996 Aug 19 18.140b . . . . . . 1996 Sep 21 18.340c . . . . . . 1998 Jul 25 17.841a . . . . . . 1996 Aug 19 18.141b . . . . . . 1996 Sep 21 18.041c . . . . . . 1998 Jul 21 18.142b . . . . . . 1996 Aug 21 17.0 242c . . . . . . 1998 Jul 25 17.5 343a . . . . . . 1996 Aug 20 17.9 143b . . . . . . 1996 Aug 23 16.943c . . . . . . 1996 Sep 21 18.243d . . . . . . 1998 Jul 25 17.8 444a . . . . . . 1996 Aug 20 17.9 1, 244b . . . . . . 1996 Sep 21 18.044c . . . . . . 1998 Jul 25 18.144d . . . . . . 1998 Sep 17 17.045a . . . . . . 1996 Aug 20 18.045b . . . . . . 1996 Sep 21 17.845c . . . . . . 1998 Jul 25 17.646a . . . . . . 1996 Aug 20 17.546b . . . . . . 1996 Sep 21 17.646c . . . . . . 1998 Jul 25 17.547a . . . . . . 1996 Aug 20 17.6 347b . . . . . . 1996 Sep 20 17.447c . . . . . . 1998 Jul 19 17.548a . . . . . . 1996 Aug 20 18.248b . . . . . . 1996 Sep 20 17.548c . . . . . . 1998 Jul 19 18.049a . . . . . . 1996 Aug 20 17.849b . . . . . . 1996 Sep 20 17.249c . . . . . . 1998 Jul 19 17.4 450b . . . . . . 1996 Sep 20 17.350c . . . . . . 1998 Jul 19 17.751a . . . . . . 1996 Aug 20 17.951b . . . . . . 1996 Sep 20 17.151c . . . . . . 1998 Jul 19 17.652a . . . . . . 1996 Aug 20 18.2 352b . . . . . . 1996 Sep 20 17.052c . . . . . . 1998 Jul 19 17.653a . . . . . . 1996 Aug 20 17.2 253b . . . . . . 1996 Sep 21 17.453c . . . . . . 1998 Jul 19 17.553d . . . . . . 1998 Sep 29 17.754d . . . . . . 1998 Jul 19 17.1 5

NOTES.È(1) High background ([15% ofsaturation) ; (2) part of image washed out ; (3) haze orclouds ; (4) focus problems ; (5) low altitude.

938

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S/N

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(deg

)(J

2000

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2000

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STR

IPS

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00M

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30.4

130

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780

7420

3502

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2000

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1910

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629

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178

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110

720

3435

.855

[18

2841

.05

29.4

410

a,b,

11a,

bT

r209

....

2000

Jun

1700

290.

2716

221

.317

.64

140

3520

3358

.645

[18

3121

.69

29.3

309

a,10

bT

r210

....

2000

Jul4

0803

0.30

340

25.4

17.3

515

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9620

3225

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3703

.63

29.1

808

b,c,

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0.05

174

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417

157

2030

51.9

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29.9

529

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160

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13.8

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Aug

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159

166

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515

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....

2000

Sep

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Oct

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2000

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6720

2609

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....

2000

Dec

616

230.

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027

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50[

1520

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....

2001

Mar

2605

530.

0234

217

.513

.514

857

3820

4220

.538

[18

0354

.44

30.6

313

a,b,

c,14

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....

2001

Apr

102

200.

9834

914

.917

.27

6386

2042

52.4

66[

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55.4

630

.55

14b

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24..

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pr4

2139

0.96

171

20.3

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153

2043

11.0

18[

1800

21.3

030

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13a,

b,c,

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25..

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0.89

176

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17.1

1074

[78

2043

43.3

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40.2

430

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13b,

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....

2001

Jun

1621

570.

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620

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137

7620

4309

.808

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.97

29.3

513

b,c,

14b

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27..

..20

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l14

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0.52

337

24.0

17.4

416

414

120

4038

.446

[18

1058

.19

29.1

212

bT

r228

....

2001

Jul25

0954

0.02

163

20.8

17.2

617

5[

142

2039

27.8

12[

1815

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929

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172

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16.8

817

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2920

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1945

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29.0

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....

2001

Aug

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390.

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159

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162

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186

154

720

3606

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b,10

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....

2001

Aug

2610

480.

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624

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815

317

220

3602

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2001

Nov

2017

340.

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617

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609

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2001

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360.

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68[

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3436

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3513

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609

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2001

Dec

1509

570.

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2001

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....

2002

Mar

2103

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....

2002

Apr

2103

210.

9716

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54.6

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a..

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2002

May

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110

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311 312 313 314Right Ascension, degrees

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942 MCDONALD AND ELLIOT

FIG. 2.ÈResiduals vs. right ascension, before correction. These twoplots show a typical example of the systematic deviation of the observedposition from the catalog position with respect to R.A. The pattern of thedeviation changes with each observation, but all show residuals in R.A.and decl. that independently vary systematically along the length of thestrip (i.e., along R.A. or time). Also plotted is a Fourier series Ðt to theresiduals that we use to correct our astrometry.

within a few tenths of a magnitude. Hence, we used the Rmagnitudes for the sample of USNO-A2.0 stars within eachstrip scan as standard stars to obtain approximate R magni-tudes for our candidates. Since each strip scan was 25minutes long, the seeing could change considerably fromthe beginning of the strip to the end. We corrected for time-variable seeing conditions over the course of each strip scanby Ðtting a Fourier series to the magnitudes as a function ofright ascension, analogous to our corrections for the time-dependent positional deviation.

The observed star positions were then compared with aTriton ephemeris to produce a list of candidates. We usedthe NEP016 ephemeris solution for Triton (Jacobson,Reidel, & Taylor 1991) combined with the DE405 ephem-eris for Neptune and Earth. The ephemerides were suppliedto us by JPLÏs Navigation Ancillary Information Facility(Acton 1990). All stars lying within of the ephemeris and1A.0more than 20¡ from the Sun at the time of the closestapproach of Triton were retained as occultation candidates.We examined each of these candidates on our images andon the Digitized Sky Survey and rejected several thatappeared to be nonstellar.

3. RESULTS AND DISCUSSION

Table 2 provides information about each of the strip-scanimages used in this occultation candidate search. Table 3

contains information about each occultation candidate wefound in our search. The format of the table is similar tothat used in McDonald & Elliot (1995). Each candidate hasbeen given an identiÐcation label beginning with ““ Tr ÏÏ andfollowed by a sequence number. The next columns show thedate and time of TritonÏs closest approach to the star andthe minimum separation and position angle of Triton rela-tive to the star (measured north through east) at that timefor a geocentric observer. The CCD magnitude of the star inthe next column is approximately equivalent to an R mag-nitude as described above. The velocity of the occultationshadow relative to the geocenter is in the next column.

An estimate for the expected signal-to-noise ratio (S/N)for the occultation event using the high-throughput modefor the High-Speed Occultation Photometer and Imager(HOPI ; Dunham, Elliot, & Taylor 1998) on the Strato-spheric Observatory for Infrared Astronomy (SOFIA;Becklin 1997) is given in the next column. For this calcu-lation, the signal is that expected from the unocculted starover a time interval corresponding to 20 km of shadowmotion, and the noise is the photon noise expected from thecombined light of Triton and the star. We have notremoved candidates with signal-to-noise values lower thanthe minimum of 20 that is necessary for atmospheric model-ing of the light curve, because higher values could beobtained if the occultation proves to be visible with a tele-scope larger than SOFIA. Also, the colors of these stars arenot known, and a very red or very blue star may providebetter signal-to-noise ratios if observed in the infrared orultraviolet, respectively.

The next column contains the solar elongation of Tritonat event time. Then comes the observed coordinates of thestar in J2000 equinox. The next column is the substar Earthlongitude of the star at the time of the event. The substarEarth latitude is the same as the starÏs declination. The lastcolumn contains a list of the CCD strip images on which thecandidate was measured.

Our previous occultation candidate search papers haveincluded Ðnder charts for the candidates. However, there isno longer a need to publish such Ðnder charts with thecurrent availability of the Digitized Sky Survey and othercharting resources. Figure 3 contains a view of Earth fromTriton at the time of closest approach to the star. The shad-owed region shows the area from which the Sun is morethan 12¡ below the horizon. This Ðgure is useful for seeingwhere on Earth the event might be visible, but we have notshown an actual path of the occultation shadow (which hasa width about a quarter of EarthÏs diameter).

For a geocentric observer, an appulse by Triton to a starcloser than about angle subtended by the sum of0A.37Èthethe radii of Earth and Triton at the mean distance ofTritonÈwould produce an occultation event observablefrom somewhere on Earth. To be sure of not missing anypotential occultations due to astrometric errors, ouroccultation candidate list includes events with minimumgeocentric separations up to The minimum separations1A.0.in Table 3 are only our best estimates, and they may bea†ected by errors in our observations, the USNO-A2.0catalog, and the ephemeris. In our earlier work we esti-mated the total e†ect of these errors would be a few tenthsof an arcsecond (McDonald & Elliot 1992), an estimate thathas been substantiated by several observed occultations(Table 1). Since the error is substantially larger than theangle subtended by the half-light radius in TritonÏs atmo-

Tr205 Tr206 Tr207 Tr208 Tr209 Tr210 Tr211 Tr212 Tr213 Tr214 Tr215 Tr216



Tr241 Tr242 Tr243aTr243b Tr244 Tr245 Tr246 Tr247 Tr248 Tr249 Tr250 Tr251







Tr324 Tr325 Tr326 Tr327 Tr328 Tr329 Tr330 Tr331 Tr332

FIG. 3.ÈVisibility zones for Triton appulse and occultation events. Each frame in this Ðgure shows Earth as seen from Triton at the time of closestapproach of Triton to the designated star. The shaded region of Earth indicates the areas where the Sun is more than 12¡ below the horizon.

0.5 1 1.5 2 2.5 3Log of Signal-to-Noise

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

Minimum

Separation,

arcsec

944 MCDONALD AND ELLIOT

FIG. 4.ÈSignal-to-noise ratio vs. geocentric minimum separation. Thehorizontal axis is the logarithm (base 10) of the signal-to-noise values listedin Table 3. Negative values of minimum separation indicate events withposition angles greater than 90¡ and less than 270¡, i.e., events that passsouth of the center of Earth. Two horizontal lines indicate plus and minus

If our astrometry had no errors, those events between the two lines0A.37.would be occultations visible from somewhere on Earth, while thoseoutside the lines would be appulses. However, uncertainties in the astrom-etry and ephemeris mean that the actual minimum separations will di†erfrom those marked on this Ðgure.

sphere, considerable astrometric reÐnement will be neces-sary to produce a prediction sufficiently accurate to observean occultation.

Taking the ratio of to the search path we used,0A.37 1A.0we would expect about 47 stars from our candidate list tobe occulted by Triton (visible from Earth). Of these events,several will likely be visible from existing large telescopefacilities. Occultations of the brightest stars can be observedwith portable instruments (see, e.g., Elliot et al. 2000).Figure 4 plots our estimated signal-to-noise ratios for eachevent (taken from Table 3) versus the nominal minimumseparation.

The most interesting events are those that would have thehighest signal-to-noise ratios and permit observation of the““ central Ñash ÏÏ (Elliot et al. 1977). The events with the bestpotential signal-to-noise ratio are, in decreasing order,

Tr316, Tr232, Tr282, Tr306, Tr231, Tr312, Tr266, Tr222,and Tr261. Of these, six have nominal minimum separa-tions less than Tr222, Tr312, Tr306, Tr261, Tr231, and0A.37 :Tr316, in order by increasing minimum separation. Tr316 isa particularly promising event, with a magnitude 12.9 starand a geocentric shadow velocity of only 0.8 km s~1. In thenext 2 years, Tr222 and Tr231, in 2001, provide the bestopportunities, unless a signiÐcant correction to the ephem-eris or to the position of Tr232 shifts the expected path of itsoccultation shadow onto Earth. One other star of note inthe candidate list is Tr243, which provides two appulses byTriton in 2002 separated by 17 days (Tr243a and Tr243b).Unfortunately, this star is magnitude 17.9 and neitherappulse is currently expected to produce an occultationvisible from Earth.

4. CONCLUSIONS

We have found 128 stars within of TritonÏs ephemeris1A.0over the years 2000 to 2009. Of these, 47 can be expected toproduce occultations visible from Earth. We emphasize thatthese stars should be considered only candidates foroccultations, and that further astrometric observations arenecessary to determine whether any speciÐc candidate islikely to be occulted and where it would be visible. We haveprovided views of Earth at the event times to aid in selectingthe most promising occultations for further study.

We thank Vincent Fish, Angie Hancock, Lisa Kwok,Lucy Lim, Mike Person, Lucy Crespo da Silva, VanessaThomas, and Rosa Villastrigo, for carrying out most of theobservations. Figure 1 was created by the STARCHARTprogram, written by Alan Paeth and Craig Counterman.The Triton ephemeris was calculated by Bob Jacobson atJPL and repackaged by the Navigation Ancillary Informa-tion Facility. The USNO-A2.0 catalog was produced at theUS Naval Observatory at Flagsta†. This work was sup-ported, in part, by grant NAG 5-3940 from NASAÏsPlanetary Astronomy Program and NSFÏs Research Expe-riences for Undergraduates (REU) program.

REFERENCESActon, C. H., Jr. 1990, in Second International Symposium on Space Infor-

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Becklin, E. E. 1997, in The Far Infrared and Submillimetre Universe, ed. A.Wilson (ESA SP-401) (Noordwijk : ESA), 201

Buie, M. W., et al. 1993, BAAS, 25, 1115Dunham, E. W., Baron, R. L., Elliot, J. L., Vallerga, J. V., Doty, J. P., &

Ricker, G. R. 1985, PASP, 97, 1196Dunham, E. W., Elliot, J. L., & Taylor, B. W. 1998, BAAS, 30, 1109Dunham, E. W., McDonald, S. W., & Elliot, J. L. 1991, AJ, 102, 1464Elliot, J. L., Dunham, E. W., & Olkin, C. B. 1995, in ASP Conf. Ser. 73,

Airborne Astronomy Symposium on the Galactic Ecosystem: From Gasto Stars to Dust, ed. M. R. Haas, J. A. Davidson, & E. F. Erickson (SanFrancisco : ASP), 285

Elliot, J. L., French, R. G., Dunham, E., Gierasch, P. J., Veverka, J.,Church, C., & Sagan, C. 1977, ApJ, 217, 661

Elliot, J. L., et al. 1998, Nature, 393, 765Elliot, J. L., & Olkin, C. B. 1996, Ann. Rev. Earth Planet. Sci., 24, 89Elliot, J. L., et al. 2000, in preparationElliot, J. L., Stansberry, J. A., Olkin, C. B., Agner, M. A., & Davies, M. E.

1997, Science, 278, 436Jacobson, R. A., Riedel, J. E., & Taylor, A. H. 1991, A&A, 247, 565McDonald, S. W., & Elliot, J. L. 1992, AJ, 104, 862ÈÈÈ. 1995, AJ, 109, 1352Olkin, C. B., et al. 1997, Icarus, 129, 178Sicardy, B., et al. 1998, BAAS, 30, 1107Stetson, P. B. 1987, PASP, 99, 191Tody, D. 1986, Proc. SPIE, 627, 733Wolfram, S. 1996, The Mathematica Book (3d ed. ; New York : Cambridge

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