triton stellar occultation candidates: 2000...
TRANSCRIPT
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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
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-20˚
-10˚
-20˚
-10˚
-28˚
-26˚
-24˚
-22˚
-20˚
-18˚
-16˚
-14˚
-12˚
-10˚
-08˚
-06˚
δ δ
β β
α2α2
γ γ
ζ ζ
ε ε
θ θ
ω ω
ψ ψ
α1α1
ι ι
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
1818
77
3030
λ λ
3737
1919
88
1919
3535
3535
ξ2ξ2
44
4444
1717
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1717
4545
AGAG
AOAO
2727
33 ξ1ξ1
DXDX
6565
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3131
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.
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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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Tr2
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00M
ar11
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0.19
163
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2032
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1837
30.4
130
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Tr2
06..
..20
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0201
0.29
338
15.0
17.2
780
7420
3502
.565
[18
2650
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710
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....
2000
May
1910
560.
1535
47.
917
.29
112
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2035
27.8
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1825
46.7
629
.72
10b
Tr2
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..20
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n7
2019
0.81
178
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16.8
1213
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
7[
9620
3225
.803
[18
3703
.63
29.1
808
b,c,
09b
Tr2
11..
..20
00Ju
l18
2055
0.05
174
20.2
17.7
417
157
2030
51.9
94[
1842
29.9
529
.11
08a
Tr2
12..
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00Ju
l19
2235
0.45
163
19.9
15.5
2717
231
2030
46.0
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1842
46.5
929
.11
08a,
b,09
a,b
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0429
0.98
350
19.6
16.6
1017
7[
6320
3010
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4458
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a,b,
cT
r214
....
2000
Aug
1713
320.
0317
417
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.216
160
137
2027
37.9
30[
1854
13.8
429
.16
06a,
b,07
bT
r215
....
2000
Aug
1811
320.
9434
218
.517
.74
159
166
2027
33.3
89[
1854
30.2
529
.17
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16..
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00A
ug22
1159
0.50
355
20.3
17.3
515
515
620
2707
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[18
5614
.55
29.1
907
cT
r217
....
2000
Sep
1520
050.
8035
79.
314
.781
131
1020
2508
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[19
0319
.91
29.4
405
a,b,
06a,
bT
r218
....
2000
Oct
1212
100.
1114
81.
017
.422
104
102
2024
13.6
46[
1907
17.3
529
.85
04d
Tr2
19..
..20
00N
ov3
0635
0.86
359
11.7
16.6
1383
164
2024
37.8
95[
1905
58.5
230
.22
04d,
05a,
b,06
a,b,
cT
r220
....
2000
Nov
2520
320.
8916
625
.116
.114
60[
6720
2609
.560
[19
0101
.15
30.5
805
a,b,
06a,
bT
r221
....
2000
Dec
616
230.
0316
027
.716
.97
50[
1520
2715
.159
[18
5732
.53
30.7
406
c,07
cT
r222
....
2001
Mar
2605
530.
0234
217
.513
.514
857
3820
4220
.538
[18
0354
.44
30.6
313
a,b,
c,14
bT
r223
....
2001
Apr
102
200.
9834
914
.917
.27
6386
2042
52.4
66[
1801
55.4
630
.55
14b
Tr2
24..
..20
01A
pr4
2139
0.96
171
20.3
14.1
8567
153
2043
11.0
18[
1800
21.3
030
.48
13a,
b,c,
14b
Tr2
25..
..20
01A
pr12
1231
0.89
176
9.6
17.1
1074
[78
2043
43.3
80[
1758
40.2
430
.36
13b,
cT
r226
....
2001
Jun
1621
570.
9534
620
.117
.54
137
7620
4309
.808
[18
0119
.97
29.3
513
b,c,
14b
Tr2
27..
..20
01Ju
l14
1547
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
19.3
929
.09
11b
Tr2
29..
..20
01A
ug4
0143
0.31
172
23.9
16.8
817
5[
2920
3822
.363
[18
1945
.39
29.0
912
aT
r230
....
2001
Aug
2101
390.
8835
024
.617
.54
159
[45
2036
34.0
52[
1826
49.3
229
.16
11a
Tr2
31..
..20
01A
ug25
2148
0.29
162
25.1
12.9
186
154
720
3606
.539
[18
2835
.54
29.1
909
b,10
a,11
aT
r232
....
2001
Aug
2610
480.
9216
624
.810
.959
815
317
220
3602
.776
[18
2850
.10
29.1
909
b,10
a,b,
11a,
bT
r233
....
2001
Nov
2017
340.
1017
617
.417
.07
68[
1520
3435
.093
[18
3514
.85
30.4
609
aT
r234
....
2001
Nov
2022
360.
9817
716
.514
.665
68[
9120
3436
.136
[18
3513
.14
30.4
609
a,b,
10a,
bT
r235
....
2001
Dec
1509
570.
5317
023
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.54
4376
2037
02.5
34[
1826
47.8
430
.81
11a
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01D
ec20
2210
0.98
172
26.4
16.6
938
[11
320
3744
.025
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2414
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30.8
710
b,11
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r237
....
2001
Dec
2515
520.
8217
132
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33[
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3821
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111
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1504
0.46
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430
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15a,
16a
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1848
0.12
167
33.5
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8535
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420
4839
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4251
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30.9
016
a,b,
17a,
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r240
....
2002
Mar
2103
580.
6633
724
.918
.12
5074
2050
29.4
14[
1735
58.6
030
.72
17b
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41..
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02A
pr10
0110
0.19
170
19.3
17.9
369
9720
5219
.332
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2831
.30
30.4
318
bT
r242
....
2002
Apr
2103
210.
9716
215
.017
.36
8054
2052
58.8
12[
1725
54.6
930
.25
18b,
19a,
cT
r243
a..
.20
02M
ay4
1702
0.38
312.
417
.99
93[
165
2053
27.9
58[
1724
19.7
230
.02
19c
Tr2
43b
...
2002
May
2118
160.
9911
36.
817
.95
110
160
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2002
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2002
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2002
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...
2003
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1201
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...
2003
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1321
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...
2003
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1822
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29.1
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r259
...
2003
Aug
1007
100.
1034
419
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220
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29.0
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r260
...
2003
Aug
1706
340.
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167
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r261
...
2003
Aug
2014
410.
6017
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5502
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r262
...
2003
Aug
3008
270.
7134
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...
2003
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1820
170.
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0.47
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r266
...
2003
Nov
2903
480.
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30.4
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...
2004
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...
2004
Jun
2707
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...
2004
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...
2004
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29.0
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r275
...
2004
Aug
1900
280.
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...
2004
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704
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...
2004
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514
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r280
...
2004
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818
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...
2004
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...
2005
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3016
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...
2005
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1709
230.
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...
2005
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...
2005
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516
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2005
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...
2006
Mar
1802
380.
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...
2006
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2405
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...
2006
Jun
2919
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...
2006
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0243
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...
2006
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312
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...
2006
Aug
2116
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...
2006
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...
2007
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...
2007
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609
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...
2007
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2903
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...
2007
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405
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2007
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...
2007
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...
2008
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...
2008
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...
2008
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2313
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2008
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...
2009
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1823
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dT
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...
2009
Jul7
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...
2009
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2908
370.
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...
2009
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1420
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...
2009
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2022
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...
2009
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...
2009
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2009
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210
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bT
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...
2009
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311 312 313 314Right Ascension, degrees
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Residuals,arcsec
311 312 313 314Right Ascension, degrees
-1
-0.5
0
0.5
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DecResiduals,arcsec
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
Tr217 Tr218 Tr219 Tr220 Tr221 Tr222 Tr223 Tr224 Tr225 Tr226 Tr227 Tr228
Tr229 Tr230 Tr231 Tr232 Tr233 Tr234 Tr235 Tr236 Tr237 Tr238 Tr239 Tr240
Tr241 Tr242 Tr243aTr243b Tr244 Tr245 Tr246 Tr247 Tr248 Tr249 Tr250 Tr251
Tr252 Tr253 Tr254 Tr255 Tr256 Tr257 Tr258 Tr259 Tr260 Tr261 Tr262 Tr263
Tr264 Tr265 Tr266 Tr267 Tr268 Tr269 Tr270 Tr271 Tr272 Tr273 Tr274 Tr275
Tr276 Tr277 Tr278 Tr279 Tr280 Tr281 Tr282 Tr283 Tr284 Tr285 Tr286 Tr287
Tr288 Tr289 Tr290 Tr291 Tr292 Tr293 Tr294 Tr295 Tr296 Tr297 Tr298 Tr299
Tr300 Tr301 Tr302 Tr303 Tr304 Tr305 Tr306 Tr307 Tr308 Tr309 Tr310 Tr311
Tr312 Tr313 Tr314 Tr315 Tr316 Tr317 Tr318 Tr319 Tr320 Tr321 Tr322 Tr323
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
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0.5
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1
Minimum
Separation,
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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.
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