Vol. 6 No. 1 January 1, 2010

Journal of Double Star Observations

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New Double Stars from Asteroidal Occultations,1971 - 2008
Dave Herald, Canberra, Australia
International Occultation Timing Association (IOTA)
Robert Boyle, Carlisle, Pennsylvania, USA
Dickinson College
David Dunham, Greenbelt, Maryland, USA;
Toshio Hirose, Tokyo, Japan;
Paul Maley, Houston, Texas, USA;
Bradley Timerson, Newark, New York, USA
International Occultation Timing Association (IOTA)Tim Farris, Gallatin, Tennessee, USA
Volunteer State Community College
Eric Frappa and Jean Lecacheux, Paris, France
Observatoire de Paris
Tsutomu Hayamizu, Kagoshima, Japan
Sendai Space Hall
Marek Kozubal, Brookline, Massachusetts, USA
Clay Center
Richard Nolthenius, Aptos, California, USA
Cabrillo College and IOTA
Lewis C. Roberts, Jr., Pasadena, California, USA
California Institute of Technology/Jet Propulsion Laboratory
David Tholen, Honolulu, Hawaii, USA
University of Hawaii
E-mail: DRHerald@bigpond.net.au

Abstract: Observations of occultations by asteroids and planetary moons can detectdouble stars with separations in the range of about 0.3” to 0.001”. This paper lists all doublestars detected in asteroidal occultations up to the end of 2008. It also provides a generalexplanation of the observational method and analysis. The incidence of double stars with aseparation in the range 0.001” to 0.1” with a magnitude difference less than 2 is estimatedto be about 1%.

Vol. 6 No. 1 January 1, 2010

Journal of Double Star Observations

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New Double Stars from Asteroidal Occultations, 1971 - 2008

Introduction

tions. More detail about the method of analysis is set
out in the Appendix.

Asteroids and planetary moons will naturally occult many stars as they move through the sky. The big- Results – 1971 to 2008
Double stars have been detected in occultations bygest challenge in observing such occultations is to accurately predict the time of the occultation event and the asteroids and planetary satellites since 1971. While
location of the shadow path as it crosses the Earth. some of these detections have been published in the
Since the Hipparcos mission, and the availability of literature, almost none are listed in the Interferometriccatalogues like Tycho2 and UCAC2, predictions of oc- Catalogue or the Washington Double Star catalogue.
cultations by asteroids and planetary moons with un- Table 1 presents the measures of all double stars obcertainties of 100km in the path location have become served or discovered in these occultations up to the end
routine. For the last few years almost 200 occultation of 2008 – whether or not they have been reported elseevents per year have been observed around the world. where. They include the bright stars γ Gem (HIP
In the early days of asteroidal occultations, most 31681), λ Vir (HIP 69974), β2 Sco (HIP 78821), 1 Vul
observations were made visually. For the last several (HIP 78821) and 16 Psc (HIP 116495). References are
years the great majority of observations are made us- given for those events known to have been published ining video cameras with a time signal obtained from the the literature.
GPS satellite system inserted into the video stream.
At the time these observations were made, little
Compared to visual observing, video provides enhanced attention was given to determining the star magnitime resolution, and a record that can be measured to tudes. Component magnitudes determined from photodeduce accurate event times. Asteroid profiles can be electric recordings were published for the three events
measured at the 1-km scale using small telescopes and listed below observed before 1983. Most of the otherinexpensive video detectors, as has been illustrated by occultations listed were video recorded, but until reTimerson [1].
cently there was no easy way to quantify the magniIn a typical occultation the asteroid is not visible. tudes of the component stars from video recordings.
The occultation is recorded as the sudden disappear- The authors plan to digitize the older video recordings
ance of the star, followed some seconds (or tens of sec- and publish a list of component magnitudes for most ofonds) later with a sudden reappearance of the star. For the listed events in a future publication.
a small number of events, the occultation may involve:
There are three types of entries in Table 1:
A partial drop in light, followed by a complete drop
Single-line entry. The double star solution forin light. The order of the light change may be either
these stars is unique and well determined.
the same, or reversed, when the star reappears; orSingle-line entry with a 3-point range (TYC
There is simply a partial drop in light.
5614-00026-1 and HIP 38465). For these stars
These are characteristics of the star being a double
the data is insufficient to provide a unique sostar – with the changes of light occurring as the twolution, but are constrained by the geometry ofstars are sequentially hidden by the asteroid. Whenthe event. The values given are the extreme
this occurs, a double star is typically ‘discovered’. Ifvalues in PA (where the separation is also
there are several observers at spaced locations, it isgreatest) - and the intermediate value of PA
possible to combine the observations in a way that alwhere the separation is least. If the asteroid
lows the PA and separation to be solved. The typicalprofile is significantly elliptical, the range inmeasurement accuracy obtained is a milli-arcsec in
PA can be significantly greater than 180 deseparation, and a few degrees in PA.
grees. The values are given in order of increasThe practical minimum separation detectable bying PA.
this method is about 1 milli-arcsec – a limit arising
Double-line entry (TYC 1808-00641-1 and HIPfrom the time resolution of the video cameras being
69974). For these stars there are two wellused, and Fresnel diffraction. The maximum separadetermined solutions. The observations aretion detectable is limited by the apparent diameter of
insufficient to resolve the ambiguity, or associthe asteroid involved. If the diameter is less than theate a formal uncertainty with the solution.
separation, an occultation of only one star might occur– with there being no data to measure the separation
On the night following the occultation of TYC 1879
from the second star, but these cases usually involve(Continued on page 91)
doubles already known from visual or other observa-

Vol. 6 No. 1 January 1, 2010

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Journal of Double Star Observations
New Double Stars from Asteroidal Occultations, 1971 - 2008

Table 1: Double stars discovered or measured in asteroidal occultations: 1971 - 2008

Star number,
J2000 RA/Dec
WDS/4thInterfer. ID
TYC 0622-00932-1
01 46 51.55 +08 16 08.3
TYC 1808-00641-1
03 53 33.26 +27 18 06.1
TYC 1879-02151-1
06 31 17.55 +22 47 50.9
COU 581TYC 1908-00844-1
07 14 41.22 +29 52 38.7
TYC 1947-00293-1
08 25 01.65 +28 33 55.3
TYC 2411-01645-1
05 31 33.75 +35 01 45.8
TYC 2916-02502-1
05 52 44.43 +40 10 45.0
TYC 5614-00026-1
15 54 10.20 -09 44 49.8
TYC 6154-00401-1
14 41 45.96 -16 36 09.2
HIP 26902
05 42 41.48 +36 08 59.4
HIP 30570
06 25 32.94 +23 19 37.8
MCA 26
HIP 31681
06 37 42.70 +16 23 57.3
BUP 90A
HIP 31694
06 37 51.49 +06 40 59.7
HD 47239
HIP 32525
06 47 13.67 +16 55 34.9
HIP 34452
07 08 30.95 +21 59 09.4
OCC 337;
HIP 36189
07 27 09.12 +11 57 18.0
HD 58686

Position Angle(PA)(degrees)

Angular Separation (masec)

248.7 ± 2.5

18.8 ± 1.1

2005 Nov 11
2005.865

116 Sirona-

278.1 or
272.1

5.1 or
25.1

2008 Jan 14
2008.034

1258 Sicilia-

240.0 ± 0.5

261.9 ± 1.5

2006 Sep 19
2006.715

144 Vibilia-

302.9 ± 0.1

170.4 ± 0.3

2006 Nov 29
2006.909

578 Happelia

-

105.6

106.3

2006 Dec 18
2006.961

87 Sylvia

2248.7 ± 0.5

6.3 ± 0.1

1996 Nov 25
1996.899

93 Minerva-

70.4 ± 3.9

57.5 ± 2.3

1999 Sep 21
1999.719

375 Ursula-

165 - 270 - 36

41.0 - 7.0 16.0

2002 May 10
2002.353

638 Moira-81.9 ± 1.4

5.4 ± 0.2

2003 Apr 21
2003.301

210 Isabella-

30.3 ±19.7

1.1 ± 0.3

2005 Dec
2005.915

1

99 Dike

3344.9 ± 1.3

36.5 ± 3.7

2001 Sep
2001.682

7

9 Metis4

126.3 ± 4.7

63.4 ± 1.0

1991 Jan 13
1991.032

381 Myrrha

526.1 ± 1.4

45.9 ± 2.1

1978 Dec 11
1978.942

18 Melpomene

670.9 ± 1.9

9.7 ± 0.3

2007 Mar 28
2007.235

72 Feronia

-

89.0

39.0

1974 Aug 29
1974.657

Saturn 5(Rhea)

7

230.7 ± 2.5

12.8 ± 0.5

2003 Mar 23
2003.221

704 Interamnia

-

Date

Asteroid

Publ
Ref

Table continued on next page.

Vol. 6 No. 1 January 1, 2010

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Journal of Double Star Observations
New Double Stars from Asteroidal Occultations, 1971 - 2008

Table 1 (continued): Double stars discovered or measured in asteroidal occultations: 1971 - 2008
Star number,
J2000 RA/Dec
WDS/4thInterfer. ID
HIP 38465
07 52 45.59 +18 49 37.4
HD 64072
HIP 66446
13 37 15.76 +04 25 03.4
HD 118510
HIP 69974
14 19 06.59 -13 22 15.9
HD 125337
HIP 76293
15 35 06.23 -25 06 20.2
HD 138790
HIP 78821
16 05 26.57 -19 48 06.9
MCA 42 CE
HIP 94703
19 16 13.04 +21 23 25.5
HJ 2862 AB
HIP116495
23 36 23.29 +02 06 08.0
THP 1

Position Angle(PA)(degrees)

Angular Separation (masec)

342 - 103 - 226

43 - 8 - 52

2002 Dec 24
2002.977

334 Chicago-

309.3 ± 5.4

19.7 ± 1.5

2001 Mar 15
2001.201423 Diotima

8

13.0 or
210.0

8.0 or
18.1

2006 Apr 12
2006.277

305 Gordonia-

81.0 ± 3.3

39.5 ± 2.0

2007 May 18
2007.374

1177 Gonnessia-

311.6 ± 2.3

98.6 ± 3.0

1971 May 14
1971.363

Jupiter I(Io)

9272.0 ±13.6

3.0 ± 0.6

1983 May 29
1983.405

2 Pallas10

117.6 ± 5.1

9.8 ± 0.8

2006 May
2006.340

7 Iris11

Date

Asteroid

5

Publ
Ref

(Continued from page 89)
technique from about 2002. For the years 2003 to-02151-1 Jean Lecacheux measured the pair from Pic- 2008, there have been a total of 1009 observed occultadu-Midi observatory, using the 1.05-m reflector at 43- tions. Over that period 11 double stars were detected
meters focal length and fast CCD imaging. The posi- in the occultation. From this it may be inferred thattion angle and separation were measured as 241 the incidence of double stars with a separation in the
deg and 260 masec, which can be compared with the range 0.001” to 0.1” with a magnitude difference lessoccultation values of 240.0 ± 0.5 deg and 261.9 ± than 2 is about 1%.
1.5masec.

Observers

References

1. Timerson et al, 2009, “A Trio of well-observed AsterThe determination of the double star details fromoid occultations in 2008”, Minor Planet Bulletin
an asteroidal occultation is usually the result of comvol. 36, #3, pp. 98-100
bining the results from all observers who successfully
observed the event. The names of the observers for 2. Chi-Long Lin et al. “A Close Binary Star Resolved
from Occultation by 87 Sylvia”, Publications of theeach double star determination are given in Table 2.
Astronomical Society of the Pacific, vol. 121, pp.
A Statistical Observation
359–364, 2009 April
Asteroidal occultations will expose a double star if 3. Dunham, D. W. 2006, “Occultations and the Brightthe separation is in the range of 0.001” to 0.l”. There isStar Catalog”, in The Hoffleit Centennial: A Yearalso a magnitude limitation arising from the brightof Celebration, ed. A. G. Davis Philip, William vanness of the stars being observed and the equipment
Altena, and Rebecca A. Koopmann (Schenectady,being used to record the event; generally, if the magniL. Davis Press), pp. 159-166.
tude difference is less than 2 magnitudes any component would be expected to be identified in the video
recording.
Video observations became the dominant recording(Continued on page 93)

Vol. 6 No. 1 January 1, 2010

Journal of Double Star Observations

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New Double Stars from Asteroidal Occultations, 1971 - 2008
Table 2: Observers for double star determinations.
Star number
0622-00932-1
1808-00641-1
1879-02151-1
1908-00844-1
1947-00293-1
2411-01645-1
2916-02502-1
5614-00026-1
6154-00401-1
HIP 26902
HIP 30570
HIP 31681

HIP 31694
HIP 32525
HIP 34452

HIP 36189

HIP 38465

HIP 66446
HIP 69974
HIP 76293
HIP 78821

HIP 94703

HIP116495

Observers
T. Farris, M. Good, J. Goss, R. James, D. Snyder, B. Stevens
R. Venable
L. Blommers, I. Bryukhanov, O. Canales, J. Caquel, R. Casas, E. Frappa, R. Goncalves,
A. Klotz, C. Labordena, M. Lavayssiere, J. Lecacheux, C. Perello, J. Ripero, L. Smelcer, S. Sposetti, G. Vaudescal, I. Vinyaminov.
M. Castets, F. Colas, O. Dechambre, A. Eberle, O. Farago, J. Lecacheux, M. MeunierKing, C. Lin, Lin, Zhang
H. Akazawa, N. Ohkura
F. Anet, R. Buchheim, J. McCormick, B. Owen, J. Sanford
R. Baldrige, D. Dunham, R. Nolthenius, R. Venable
A. Gilmore, D. Herald, R. Price, P. Purcell
D. Dunham, G. Fishkorn, J. Sedlak, W. Warren, J. Wetmore
D. Dunham, R. Innes, W. Morgan, R. Nolthenius, K. Okasaki, S. Preston
Y. Ban, S. Fukushima, A. Hagiwara, T. Hasegawa, A. Hashimoto, Y. Hirose, T. Homma, J.
Horiuchi, W. Kakei, S. Kaneko, H. Karasaki, T. Kiyokawa, H. Koyama, Y. Moriya, H. Oya,I. Sato, M. Sato, H. Shibuya, K. Shima, M. Soma, Y. Suenaga, M. Sugai, A. Suzuki, K.
Tanaka, S. Waku, M. Yamada
M. A'Hearn, R. Bolster, D. Dunham, J. Dunham, F. Espenak, T. Van Flandern, Schmidt, D.
SkillmanK. Coughlin, R. Fleishman
M. De Coca, V. De Gaia, A. Quintanilla
A. Nakanishi, A. Tsuchikawa, A. Yaeza, A. Miyamoto, J. Bedient, B. Brevoort, S. Bus,
C. Sakaki, E. Cleintuar, D. Dunham, O. El, W. El, V. Fukunaga, W. Fukunaga, H. Nihei,
H. Hamanowa, H. Nagai, H. Sugai, H. Sato, H. Watanabe, I. Ootsuki, K. Kitazaki, K.
Usuki, M. Sato, P. Maley, M. Momose, M. Koishikawa, M. Ishida, M. Okamoto, M. Fujita,
M. Ida, N. Kita, S. O'Meara, L. Roberts, R. Ikeshita, S. Uehara, S. Kaneko, S. Suzuki,
R. Savalle, S. Yoneyama, J. Swatek, R. Sydney, H. Takashima, D. Tholen, Ts. Sato, A.
Watanabe, Y. Tonomura, Y. Hirose, Y. Itou, Y. Sugiyama, M. Kouda, H. Yoshida, M. Yokokawa, E. Konno, K. Sasaki, To. Sato, T. Oribe, H. Sugawara, H. Tomioka, A. Kuboniwa
C. Bader, G. Billings, M. Damashek, R. D'Egidio, D. Dunham, R. Emmons, J. Holtz, A.
Lowe, C. Roelle
G. Akrivas, M. Appakutty, P.M. Berge, M. Bonnet, J. Caquel, F. Colas, P. Gilabert, K.
Jayakumar, H. Kulkarni, J. Lecacheux, C. Leyrat, M. Maillard, P. Martinez, A. Paranjpye, J. Piraux, R. Poncy, M. Prabhunne, O. Ruau, M. Senegas, K. Shah, B. Tregon, R.
Vasundhara, C. Velu.
R. Venable
L. Barak, S. Gebhard, T. Janik, F. Lomoz, J. Manek, Z. Moravec, M. Parl, V. Priban, H.
Raab, L. Smid
Bartholdi, Fallon, Devinney, Oliver, Owen, Smith
J. Abdias, M. Adams, M. A'Hearn, S. Austin, D. Baker, J. Barton, Bartz, R. Binz, B.
Birdsong, B. Blagg, Brady, J. Bragg, J. Breazeale, M. Brewster, D. Brunett, J. Camp,
T. Campbell, J. Cannizzo, J. Chauvin, D. Clark, J. Craft, Crawford, J. Culberson, B.
Cuthbertson, S. Dale, B. Darnell, C. Davis, L. Dedear, L. Delaney, J. Dellinger, R.
Dietz, R. Diiulio, J. Doryk, W. Douglass, D. Dunham, J. Dunham, G. Ellis, S. Elsner,
J. Erickson, G. Felos, D. Foster, T. Fox, T. Freeman, M. Frueh, D. Garland, D. Garnet,
G. Gonzales, C. Graham, J. Graves, D. Green, Greer, J. Hagan, R. Harper, F. Harvey, N.
Henderson, D. Hensch, C. Herold, W. Hoffler, Hok, A. Hradesky, Hubbard, B. Hudgens, S.
Ireland, B. Jansen, A. Jon, A. Jones, K. Jurgens, C. Kennedy, R. Kennedy, T. Kenyon,
G. Kiser, E. Kolarich, D. Kornbluh, M. Lawson, D. Leblanc, J. Lucke, P. Maley, T.
Malone, E. Manual, M. McCants, R. McCormack, C. McDougal, J. McGaha, J. Meadows, G.
Metcalf, B. Mitchell, M. Mooney, Moreno, S. Morgan, J. Mote, W. Nissen, R. Nolthenius,
D. Oliver, T. Oswalt, G. Page, R. Peters, J. Petersen, H. Povenmire, P. Powell, B.
Prislovsky, J. Ratley, L. Reed, A. Reeves, B. Riefer, G. Roark, J. Robicheaux, S.
Roby, P. Romig, J. Rose, Rouw, P. Roy, S. Sawyer, D. Schinkevic, G. Schneider, R.
Schnurr, J. Schroeder, T. Schult, M. Seslar, W. Settle, C. Sexton, J. Shinner, D. Siner, J. Smith, B. Snow, J. Stafford, H. Stanley, D. Strum, D. Sventek, P. Sventek,
Tafliger, R. Taibi, R. Teed, M. Treadway, R. Weber, A. Whipple, T. Williams, R. Wood,
L. Woods, Wren, J. Young, G. Zentz, J. Zitwer
R. Boyle, L. Garrett, J. Holtz, M. Kozubal, F. Melillo, B. Thompson

Vol. 6 No. 1 January 1, 2010

Journal of Double Star Observations

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New Double Stars from Asteroidal Occultations, 1971 - 2008
the Earth by the star. That shadow will cross the(Continued from page 91)
Earth as a result of the asteroid’s motion. Typically the
4. Dunham, D. W., “Planetary Occultations for 2002”,
Sky and Telescope, vol. 103, no. 3, March 2002, pp. shadow will cross the Earth in about 20 minutes, andthe path of the shadow will have a width equal to the
92 – 97.
diameter of the asteroid. An observer within the path
5. Sato, I, Soma, M and Hirose, T. “The occultation of
can record the shadow passing over them by recording
gamma Geminorum by the asteroid 381 Myrrha”,
the time at which the star light disappears and reapAstronomical Journal, vol. 105, no. 4, April 1993,
pears. Place several observers across the shadow path,
pp. 1553-1561.
and the occultation times recorded by the observers
effectively map out the size and shape of the shadow –6. Dunham, D. W., 1979, “Duplicity of both (18) Melpomene and SAO 114159 discovered during occul- and hence of the asteroid. This has been the main motation”, Occultation Newsletter, vol. 2, pp. 12 – 16. tivation for observing asteroidal occultations.
The greatest difficulty with observing asteroidal
7. Taylor, G. E.; Walker, E. N.; Quintanilla, A. R.;
occultations is accurately predicting the location of the
López de Coca, P., “Results of the occultation of the shadow path. For a well-known main belt asteroid, thestar SAO 79100 by RHEA on 1974 August 29”, As- uncertainty in the path location is rarely less thantronomy and Astrophysics, vol. 50, no. 1, July 1976, 50km, and is often well over 100km. For a transpp. 121-123.
Neptunian asteroid, the uncertainty is typically thou8. Vasundhara, R., “Observations of occultation of HIP sands of km or larger!
For a small number of events, the occulted star is66446 by (423) Diotima on 2001 March 15 fromobserved
to be a double star. This is typically detected
India - Detection of a companion to the star”, Bull.
asatwo-stepchange in light level as the occultation
Astr. Soc. India, vol. 29, 2001, pp. 577-584.
starts or ends. For a double star to be detected and9. Taylor, G.E. et al, “Occultation of Beta Scorpii C by measured, the separation cannot be much larger than
Io on May 14, 1971”, Nature vol. 234, pp. 405 - 406 the apparent diameter of the asteroid. In practice, this17 December 1971.
means that most double stars discovered in an asteroidal occultation have a separation less than 0.1”, and10. Dunham, D. W., et al, “The size and shape of (2)Pallas from the 1983 occultation of 1 Vulpeculae”, none have been recorded with a separation greater
Astronomical Journal, vol. 99, May 1990, pp. 1636- than 0.3”. The minimum separation is limited by Fresnel diffraction, and the time resolution of the video
1662.
cameras used. So far no double star with a separation
11. B. Thompson and T. Yeelin, “Duplicity in 16 Pisof less than 1 masec has been identified.
cium Confirmed from Its Occultation by 7 Iris on
The majority of observations are presently made2006 May 5”, Publications of the Astronomical So- using low-light video cameras, running at either 30 fps
ciety of the Pacific (PASP), 118, 1648-1655, Decem- (NTSC) or 25fps (PAL). The vertical synchronisationber 2006
pulse for each video frame is used to trigger the reading of a GPS-controlled clock, with a correspondingtime-stamp being video-inserted near the bottom ofthat frame. The video recording is analysed using atool such as Limovie (http://www005.upp.so-net.ne.jp/
Appendix:
k_miyash/occ02/limovie_en.html) to extract a timed
light curve.
Deriving Double Star Measurements
The process for reducing an asteroidal occultationfrom an Asteroidal Occultation
involves the following:
An asteroidal occultation results in the sudden ex• The process uses the "Fundamental plane".
tinction of light from a star as an asteroid passes inThis is a plane through the centre of the Earthfront of it. The typical duration of an occultation is athat is normal to the apparent direction of the
few seconds. A ‘good’ occultation will last for over 10
star;seconds; on rare occasions an occultation will last for• A moving reference frame is defined on thatover a minute. You can visualise an asteroidal occultaplane – with its motion corresponding to thetion by thinking of the shadow of the asteroid cast on
ephemeris motion of the asteroid’s shadow on

Vol. 6 No. 1 January 1, 2010

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New Double Stars from Asteroidal Occultations, 1971 - 2008
the plane. A nominal zero position is arbitrar- teramnia on 2003 March 23 – an occultation involving
a double star. In this plot, only the events for the priily defined using one of the observations.
• For each observed event by each observer, the mary star are included. (The points in a line at the top
observer’s location on the surface of the oblate and bottom of the plot represent the time of closest approach for observers who recorded no occultation. The
Earth is projected onto the moving reference
direction of motion of the asteroid is normal to theseframe.
The projection of the observers’ locations results in lines.)
Normal processing of the occultation observations
a series of data points in the moving reference framethat represents the outline of the asteroid. Figure 1 is entails a least-squares fitting of an ellipse to the datasuch a plot from an occultation of HIP 36189 by In- points. The least-squares solution involves 5 unknowns– the X and Y coordinates of the centre of the ellipse,
the major and minor axes of the ellipse, and the orientation of the ellipse.
When a double star is involved, the effect is to create a second set of data points that is displaced fromthe first set. Importantly, the shadow of the asteroid
cast by the companion star will have the same size,
shape and orientation as the shadow cast by the primary star – it will merely be displaced from the main
shadow by an amount dependent upon the separation,and in a direction dependent upon the PA of the companion. As a result, the double star solution merely
requires solving for two additional unknowns - theseparation and position angle that will bring the two
shadow profiles into mutual alignment.
In Figures 2 - 4, Figure 2 shows the data points
corresponding to the secondary star. Figure 3 has thedata points of both the primary and secondary stars
plotted together, with no offset. Figure 4 has the data
points offset so that the two profiles are in full alignment. The offset is indicated by the double star representation in the centre of the plot, which is drawn at
the scale of the asteroid. In this case, the apparent diFigure 1: Projection of observers' locations from occultation of ameter of Interamnia was 0.177”, and the measured
HIP 36189.
separation of the stars was 12.8masec. The duration of
the occultation was up to 71 seconds, with the time

Figure 2: Data points corresponding
to the secondary star.

Figure 3: Data points for both stars
with no offset.

Figure 4: Data points for both stars
with offset so that profiles arealigned.

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New Double Stars from Asteroidal Occultations, 1971 - 2008
served chord of the secondary matches the profile of
the asteroid – leading to two possible solutions forseparation and PA, as indicated by the double star representation in each diagram. In this case both solutions have a similar PA. However in general the two
PAs can differ by up to 180 deg. Note that one of thesolutions can be excluded if it is clear that stations
that ‘should have’ recorded the second star occulted
under that solution definitely did not see that star occulted.
When there is a large magnitude difference beFigure 5: Illustration of possible amibiguity if only one obtween the stars, it may be that the companion star isserver records double star events.
detected at only one of the disappearance and reappearance events. If there are several spaced observers
difference between the two components being up to 9 who record the companion star, a reliable unique soluseconds.
tion can still be obtained. However if only one observer
Where the occultation is well observed with several (or two closely spaced observers) record the companion,
chords, and the double star is recorded at both the dis- a unique solution is not possible. Rather there will be aappearance and reappearance events, the solution for range of possible solutions extending over a range ofthe separation and position angle can have very high position angles of about 180 degrees. The following diaprecision.
grams (Figure 6) are for the occultation of HIP 38465.
If only one of several observers records double star The first diagram shows the outline of the asteroid as
events, the solution may have an ambiguity. In such determined by nine observers. One observer detected acircumstances the observed chord might fit on both step event on the reappearance (in the diagram, thesides of the asteroid – leading to two solutions for the star is moving from right to left). The location of theseparation and position angle. This is illustrated by star path is constrained by the diameter of the asterthe following two diagrams (Figure 5), which relate to oid. The 2nd diagram shows the solution for the seconthe occultation of TYC 1808-00641-1. Two observers dary star assuming its chord is at its maximum posi(actually for this event, one observer running two tion north. The fourth diagram is for the chord being at
widely-separated stations, with video techniques mak- the maximum position south, and the third diagram is
ing it possible to run separate stations remotely) re- the solution for minimum separation of the compocorded the occultation of the main star – with chords nents.
being near the extremities of the asteroid. One of those
The situation differs from a single lunar occultastations (the left-most in the diagram) also recorded an tion with a vector separation in three ways:
occultation by a secondary star. As shown in the dia1. For a lunar observation the range of possible PAgrams, there are two possible locations where the ob- values is 180°. For an asteroidal observation the possi-

Figure 6: Possible amibiguities in separation and position angle if only one observer sees the secondary as a step event in the light
curve. See text.

Vol. 6 No. 1 January 1, 2010

Journal of Double Star Observations

Page 96

New Double Stars from Asteroidal Occultations, 1971 - 2008
ble range of PA is usually greater than 180°. (In this ship is more complex, and depends upon the shape ofcase, it is 244°)
the asteroid.
2. In a lunar observation, no limit on the separaThese are illustrated in the following plot of thetion can be derived from a single observation. In an separation against position angle for this star, together
asteroidal occultation the diameter of the asteroid im- with a plot of the separation curve for an equivalentposes an upper limit on the separation. In this case it lunar occultation, Figure 7. As can be seen, the lunar
is 0.052” – assuming the solution of the fourth dia- solution is symmetric, and is asymptotic at 90° fromgram.
the central location (of 105°). In contrast the asteroid
3. In a lunar occultation the separation is related solution is asymmetric, has no asymptotes, but has
to the difference between the PA of the star compo- limiting values of PA with corresponding maximum
nents and of the occultation vector by a simple secant values of separation.
relationship. For an asteroidal occultation the relation-

Figure 7: Separation versus position angle for a double star for both lunar and asteroidal occultations.