Astronomy Letters, Vol. 30, No. 9, 2004, pp. 630633. Translated from Pisma v Astronomicheski Zhurnal, Vol. 30, No. 9, 2004, pp. 692696.Original Russian Text Copyright c 2004 by Denissenko.
been obtained so far. The angular sizes of even the 200300 mas of the occulted star. In combination
largest objects are about 0.04 0.01 (Quaoar),which is at the resolution limit of the Hubble SpaceTelescope. Most of the size estimates are based onindirect methods that depend strongly on assump-tions. The albedos of TNOs are commonly assumedto be 0.040.08, but their values clearly vary amongdierent objects.
At present, the only direct method of measuringthe sizes of single TNOs is to observe the occultationsof stars by them. Over 500 occultations of stars byMain Belt asteroids have been observed so far, withve or more chords having been measured for 17%of them. For some objects, the sizes and shapes weredetermined with an equivalent angular resolution of0.002 (Dunham et al. 2002). The predictions ofclose conjunctions with stars from 1999 until 2005have been published for Centaurs (5145), Pholus, and(10 199) Chariklo (Stone et al. 2000), but no positive
with the very slow angular motion of TNOs (0.11 mas s1), this makes the rate of stellar occultationsby them about three orders of magnitude lower thanthat for Main Belt asteroids and a factor of about100 lower than that for Jovian Trojans, i.e., inverselyproportional to a2.
Estimates show that any given TNO will occult astar brighter than 15m, on average, once every fouror ve years. We might expect one occultation in10 years by 20 largest TNOs for 105 HIPPARCOSstars brighter than 1011mand only one event in acentury for104 stars brighter than 6.5m.
THE ACCURACY OF STAR CATALOGS
An error in the coordinates of the occulted starcan increase the uncertainty in both the time andposition of the event. At a distance of 40 AU, 1 mascorresponds to 30 km. A 0.25 error in the star co-Occultations of Stars Brighter ThanObjects in
D. V. DenSpace Research Institute, Russian Academy of Scienc
AbstractWe computed the occultations of starsobjects (TNOs) for the next ten years. In our searchwith the coordinates of 2 838 666 stars taken fromet al. 2003) with 16 356 096 stars between 12m. 00 athe occultations of stars by the seventeen largest nand four known binary Kuiper Belt objects. We sele30, including the extremely rare occultation of a 6mon October 4, 2007. Observations of these events bthey can provide unique information about the sizesdramatically. c 2004 MAIK Nauka/InterperiodicKey words: trans-Neptunian objects, Kuiper Belt, s
Over 800 trans-Neptunian objects (TNOs) havebeen discovered since 1992, 67 of which are num-bered and 7 have proper names as of February 2004.No reliable measurements of their diameters haveresults have been obtained as yet.
1063-7737/04/3009-0630$26.00 c15m by the Largest Trans-Neptunian20042014
es, Profsoyuznaya ul. 84/32, Moscow, 117997 Russiarch 2, 2004
brighter than 15m by the largest trans-Neptunian, we used the following catalogs: Hipparcos; Tycho2UCAC2 (Herald 2003); and UCAC2 (Zachariasnd 14m. 99 north of 45 declination. We predictedumbered TNOs, the recently discovered 2004 DW,cted 64 events at solar elongations of no less than. 5 star by the double asteroid (66 652) 1999 RZ253y all available means are extremely important, sinceof TNOs and improve our knowledge of their orbitsa.
OCCULTATIONS BY TNOs
The distances of TNOs from the Earth are typi-cally 3050 AU, corresponding to a horizontal equa-torial parallax of 0.20.3. This implies that an oc-cultation will occur somewhere on the Earth onlyif the geocentric path of the object will pass withinordinates will shift the occultation path by 1 Earthradius. Since the orbital positions of TNOs are knownwith 0.25 accuracy, an error of 0.05 in the star
2004 MAIK Nauka/Interperiodica
OCCULTATIONS OF STARS 631coordinates is acceptable. Before 2003, the coordi-nates with an accuracy of 1 mas were known for100 000 stars in the HIPPARCOS catalogue. TheTycho2 catalogue, which includes about two millionstars that are mostly brighter than 12m, is essentiallya by-product of the same HIPPARCOS project. Thesystematic and statistical errors in the Tycho2 coor-dinates are typically within the range 0.10.2, butreach 0.51.0 in some elds of the sky. Thus, Ty-cho2 is unsatisfactory for predicting the occultationsby Kuiper Belt objects.
The situation in astrometry changed radicallyin 2003 with the release of the second USNOCCD Astrograph Catalogue UCAC2 (Zacharias etal. 2003) with coordinates and proper motions for48 million stars between 8m and 16m. The positionalerrors in UCAC2 are 1520 and 4050 mas for 12mand 15m stars. This catalogue does not cover areasnorth of +45 . . .+52 declination, but this is nota problem, since none of the objects included in thecurrent search will go beyond 45 declinations overthe next 10 years.
PREDICTIONS FOR 20042014
The occultations of stars from January 27, 2004,until December 31, 2014, were computed for the fol-lowing objects:
(a) The recently discovered largest known TNO2004 DW with an absolute magnitude of H0 = 2.22.4 (Marsden 2004);
(b) 17 numbered objects withH0 < 5.52.6 (50 000) Quaoar3.2 (28 978) Ixion3.3 (55 565) 2002 AW1973.3 (55 636) 2002 TX3003.6 (55 637) 2002 UX253.7 (20 000) Varuna4.2 (42 301) 2001 UR1634.5 (19 308) 1996 TO664.7 (26 375) 1999 DE94.7 (38 628) Huya4.8 (24 835) 1995 SM554.9 (19 521) Chaos4.9 (47 171) 1999 TC36 (binary)5.2 (26 181) 1996 GQ215.3 (55 638) 2002 VE955.4 (15 874) 1996 TL665.4 (48 639) 1995 TL8(c) 3 numbered binary TNOs5.8 (26 308) 1998 SM1655.9 (66 652) 1999 RZ2536.6 (58 534) 1997 CQ29ASTRONOMY LETTERS Vol. 30 No. 9 2004(Trujillo and Brown 2002; Brown and Trujillo2002; Noll and Stephens 2002; Noll and Stephens2003).
The diameters of single bodies were estimatedfrom their absolute magnitudes H0 using the for-mula logD[km] = 3.520.2H0 , which corresponds tothe assumed albedo of 0.16 and is a conservativeestimate. For (50 000) Quaoar, this formula yields1000 km, in good agreement with the lower limit fromthe Hubble direct imaging (1260 300 km). If thealbedo is 0.04, the actual TNO size will be exactlytwice the value listed in the table. For binary asteroidswith components of the same size and albedo, thediameter of each body will be 0.71 D.
The orbits of the selected TNOs from astorb.datas of January 27, 2004 (Bowell 2004) were integrateduntil the end of 2014 by taking into account theperturbations from major planets. Geocentric con-junctions within 0.5 with HIP and UCAC2 starswere found. For 2004, the occultations by 36 largestunnumbered objects with H0 < 5.5 were also com-puted, and nine events were found. The mean value ofthe current ephemeris uncertainty (CEU) was 0.26and 1.0 for the numbered and unnumbered objects,respectively.
The results of our search are presented in the table.The table gives the time intervals (rounded o by5 min) when an occultation can occur on the nightside of the Earth with 0.25 uncertainty. Due to thedierent velocity of TNOs relative to the Earth (from25 km s1 at opposition to 5 km s1 or less nearthe stationary point), these intervals can vary from 10min to 1.5 h for the occultation by Varuna in 2012.The extra 0.25 were added for double asteroids, sincethe orbital distances in these pairs are also estimatedto be 60008000 km.
The exceptional occultation of the 6m. 5 K0 starHIP 111 398 = HD 213 780 = SAO 146 161 = ZC3313 in Aquarius by the binary object (66 652) 1999RZ253 in 2007 should be particularly noted. It isextremely important to observe this TNO in 20042007 with large telescopes to rene the relativepositions of the components at the occultation epoch.An international campaign involving observers inAustralia, New Zealand, China, Japan, eastern Rus-sia and Hawaii October 4, 2007 should be organizedto measure the sizes and shape of this binary system.
Finding charts and preliminary path plots for theoccultations listed here are available at http://hea.iki.rssi.ru/denis/TNOocc.html or may be requested byE-mail at email@example.com.