Occultation of star GSC 1898-00944 by asteroid 1107 Lictoria observed photoelectrically

A good weather allowed an observation of rare event: occultation of star GSC 1898-00944 by asteroid 1107 Lictoria using the observational facilities of the Astronomical Institute of the Slovak Academy of Sciences in Tatranska Lomnica, Slovakia. The possibility of this observation was pointed out by PhD student J. Koza who noticed it in the orbit refinement (see E.A.O.N homepage ). According to the refinment, the observatory was at the border of the path of totality. The shadow should have passed from southwest to northeast. The maximum duration of the occultation corresponding to the estimated diameter of the asteroid (81 km) was 13.3 seconds. Although the star is quite faint (V = 11.5mag), it was still within the reach of the photoelectric photometer installed in the focus of 0.6m Cassegrain telescope located at Stara Lesna. The photometer, after an innovation by Dr. R. Komžík , enables a fast photometry with the integration time 0.1 or 0.01 second. With respect to the relatively low brightness of the star, we decided to use 0.1 second integration. Since a stellar occultation is a rare phenomenon, the common observation program of the telescope, just performed by Dr. R. Gális and Dr. L. Hric , was interrupted and observation of the occultation was preferred in spite of several negative visual observations performed by Dr. Gális in the previous decade.

The observation started at 19:02 UT. The observation conditions were practically ideal (seeing 2-3"). No brightness decrease was observed till 19:06 UT. Then, the star suddenly disappeared and reappeared, despite the prediction of 13.3 second, after about 16 seconds. We continued the fast photometry for about 2 minutes to reveal a possible second dimming, but no such an event occurred.

No more than after about a half an hour, the observation was reduced: ingress and egress lasted less than 0.2 seconds, the entire duration of the occultation was 16.3 seconds. Later Dr. Neslusan integrated the orbit of 1107 Lictoria and found the geocentric velocity, its component perpendicular to the line of sight equal to 6.09 km/s. It implied the occultation cross-section of asteroid 99.3 km. So, the diameter of the asteroid has to be larger and albedo lower than those previously determined.

Theodor Pribulla
Last modification March 18, 2002