Astronomy Letters, 2004, vol. 30.…

                                     Astronomy Letters, 2004, vol. 30. Translated from Pis'ma v Astronomicheskij Zhurnal.


                                     Occultations of HIP and UCAC2 stars downto 15m by large TNO in 2004-2014

                                                                        © 2004 D.V. Denissenko
                                                             Space Research Institute (IKI), Moscow, Russia
arXiv:astro-ph/0403002 28 Feb 2004




                                                                          Received 27 February 2004

                                         Occultations of stars brighter than 15m by largest TNOs are predicted. Search was
                                         performed using the following catalogues: Hipparcos; Tycho2 with coordinates of
                                         2838666 stars taken from UCAC2 (Herald, 2003); UCAC2 (Zacharias et al., 2003) with
                                         16356096 stars between 12.00 and 14.99 mag to the north from -45 declination.
                                         Predictions were made for 17 largest numbered transneptunian asteroids, recently
                                         discovered 2004 DW and 4 known binary Kuiper Belt objects. 64 events occuring at
                                         solar elongation of 30° and more are selected, including exceptionally rare occultation
                                         of 6.5m star by double (66652) 1999 RZ253 on 2007 October 4th. Observations of these
                                         events by all available means are extremely important since they can provide unique
                                         information about the size of TNOs and improve their orbits dramatically.


                                                  INTRODUCTION                                        OCCULTATIONS BY TNO

                                     Over 800 Transneptunian Objects (TNO) are            TNOs are typically at 30-50 AU distances
                                     discovered since 1992 with 67 of them being          from the Earth corresponding to 0.2"-0.3"
                                     numbered and 7 having proper names as of             parallax. This means that occultation will
                                     February 2004. No reliable measurements              only happen somewhere on the Earth if the
                                     of their sizes have been obtained so far.            geocentric path of the object passes within
                                     Angular sizes even of the largest objects are        200-300 mas from the star. Combined with
                                     about 0.04"±0.01" (Quaoar) which is at the           a very slow angular motion of TNO (0.1-1
                                     limit of Hubble Space Telescope resolution.          mas per second of time) it makes rate of
                                     Most size estimates are based on the indirect        stellar occultations by them about three
                                     methods strongly dependant on assumptions.           orders of magnitude smaller than by main
                                     Albedoes 0.04-0.08 are generally supposed            belt asteroids and about 100 times less than
                                     for TNOs, but they are definitely varying            that of Jovian Trojans, approximately
                                     among different objects.                             inversely proportional to (a-1)2.

                                     The only direct method to measure sizes of           Estimates show that any given TNO will
                                     single TNOs at present is observing stellar          occult a star brighter than 15m on average
                                     occultations by them. Over 500 occultations          approximately every 4 or 5 years. For ~105
                                     of stars by Main Belt asteroids have been            HIPPARCOS stars brighter than 10-11m we
                                     observed so far, with 17% of them having 5           should be expecting one occultation in 10
                                     or more chords measured. For some objects            years by 20 largest transneptunian objects,
                                     size and shape were determined with an               and only one event per century for ~104
                                     equivalent angular resolution of 0.002".             stars brighter than 6.5m.
                                      DENISSENKO D.V.


   STAR CATALOGUES ACCURACY                          3.3 (55565) 2002 AW197
                                                     3.3 (55636) 2002 TX300
Error in coordinates of occulted star will           3.6 (55637) 2002 UX25
significantly increase the uncertainty of both       3.7 (20000) Varuna
event time and position. At 40 AU distance,          4.2 (42301) 2001 UR163
1 milliarcsecond is corresponding to 30 km.          4.5 (19308) 1996 TO66
0.25" error in star coordinates will shift the       4.7 (26375) 1999 DE9
occultation path by ~1 Earth radius. Since           4.7 (38628) Huya
the positions of Transneptunian objects are          4.8 (24835) 1995 SM55
known to 0.25" uncertainty, 0.05" accuracy           4.9 (19521) Chaos
of the star coordinates will be acceptable.          4.9 (47171) 1999 TC36 (binary)
Before 2003, coordinates were known to               5.2 (26181) 1996 GQ21
milliarcecond accuracy for ~100000 stars in          5.3 (55638) 2002 VE95
HIPPARCOS catalogue. Tycho2 catalogue                5.4 (15874) 1996 TL66
including ~2 Mln stars usually brighter than         5.4 (48639) 1995 TL8
12m is essentially a by-product of the same
HIPPARCOS project. It has systematic and             c) 3 numbered binary TNO
statistical errors typically within 0.1"-0.2"
but reaching 0.5"-1.0" in some fields. Thus          5.8 (26308) 1998 SM165
Tycho2 is not satisfactory for predicting            5.9 (66652) 1999 RZ253
occultations by TNO.                                 6.6 (58534) 1997 CQ29

Situation in astrometry was revolutionized           Single body diameters were calculated from
in 2003 with the release of 2nd USNO CCD             H0 by the formula log D[km]=3.52-0.2*H0.
Astrograph Catalogue UCAC2 (Zacharias et             This corresponds to assumed albedo of 0.16
al.) with coordinates and proper motions for         and is a conservative estimate. For (50000)
48 Mln stars between 8m and 16m. Position            Quaoar this formula gives 1000 km which is
errors of 12m stars in UCAC2 are 15-20 mas           in a good agreement with the lower limit
and 40-50 mas for 15m. This catalogue does           from Hubble direct imaging (1260±300 km).
not cover areas to the north from +45..+52           If albedo is 0.04, actual size of TNO will be
declination, but that is not a problem since         exactly twice the value listed in table. For
no objects involved in current search will           binary asteroids with two bodies of the same
reach ±45 declination during next 10 years.          size and albedo diameter of each component
                                                     will be 0.71*D.
      PREDICTIONS FOR 2004-2014
                                                     Orbits of selected TNO from 2004 Jan. 27th
Occultations from 2004 Jan 27 to 2014 Dec            version of astorb.dat (Bowell, 2004) were
31 were computed for the following objects:          integrated through the end of 2014 and all
                                                     geocentric conjunctions within 0.5" to HIP
a) Recently discovered largest known TNO             and UCAC2 stars were found. For the year
2004 DW with absolute mag H0=2.2-2.4;                2004 occultations by 36 largest unnumbered
                                                     objects with H0