Occultations by (357) Ursula - updated 2003 November 11

375 Ursula occultation observed in Japan - updated 2003 Nov. 11

The occultation of 9.6-mag. SAO 189284 by the large C-class asteroid 375 Ursula was timed from 15 stations in Japan on October 19. An occultation by Ursula observed in 1982 in the western USA showed that the asteroid was a little more than 200 km in diameter, much larger than expected (even current tables incorrectly give the diameter as 106 km). The outline obtained for this month's occultation is almost 200 km across; it can be seen at the Japanese occultation Web site here with just the outline shown here. The plot prepared by Rohith Adavikolanu and I from the observations sent to us by Tsutomu Haymizu is here with the observations (Occult .obs ASCII file) here.

The 1982 November 15th occultation by Ursula - new 2003 Oct. 29

A circular outline with diameter 216 km was fitted to 6 observations of the 1982 November 15th occultation as published by Millis, R. L.; Wasserman, L. H.; Bowell, E.; Franz, O. G.; Klemola, A.; and Dunham, D. W. in their 1984 paper, "The diameter of 375 Ursula from its occultation of AG +39 303" in the Astronomical Journal, Vol. 89, pages 592-596. This paper can be found here. A modern analysis with Occult reproducing those results is shown here with the Occult vers. 3.0 OBS input file (an ASCII plain text file) here.

The 1999 September 21st occultation by Ursula - new 2003 Oct. 29

The 1999 occultation was observed from three stations, but unfortunately two of them were almost at the same distance from the central line. The observations were visual and poorly timed because the star was unexpectedly a close double star; the observers were confused by the resulting intermediate level changes during the step events, which seemed gradual to them (at least the primary star's contacts appeared gradual either due to the stellar diameter or Fresnel diffraction). I have fitted the observations by McCormick and Sanford with a circular outline for Ursula shown here. The observations by Buchheim are shown in the plot (station 2, and 7 for the secondary star) but are given zero weight because they are in poor agreement with Sanford's chords (station 1, and 6 for the secondary star). The derived radius is 146 km, but the real error in that is much greater than the formal 8 km error; I suspect that the real radius is a few tens of km greater. For the double star, the plot heading incorrectly gives the separation in "; the units are really mas (milliseconds of arc). It gives the separation as 57 mas +/-2 mas and PA 71 deg. +/-3 deg. Again, the formal errors must be too small, with the real errors probably of the order of 25 mas in separation and 20 deg. in PA. The fact that Buchheim failed to see the reappearance brightening indicates to me that the secondary star was probably about 0.6 mag. fainter than the primary, causing just enough of a brightening so that Sanford did notice it. With the large real errors of these results, the diameter of Ursula derived from these observations should not be used in conjunction with the better results from 1982 and 2003 to constrain the shape of Ursula. The observations do clearly show that the star, TYC 2916 02502, is a close double, but the separation and position angle are poorly determined. The Occult vers. 3.0 OBS input file is here.