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Session 62 - Comet Hyakutake Encounter.
Display session, Wednesday, June 12
Formerly continuum observations of comets typically resulted in an intensity at a single frequency at the predicted position of the comet. The size of the cometary emission, the separation of the contribution of nucleus and halo, and the estimate of the particle size in the halo could not be derived this way. In addition the comet intensity varied occasionally strongly on time scales of days. Therefore we organized a "network" of four telescopes, covering frequencies from 23 GHz to 860 GHz with 9 different RX systems and spatial resolutions ranging from a synthesized beam size of about 1 arcsec to a HPBW of about 27 arcsec. The observations of the comet were made simultaneously. At the Heinrich-Hertz-Telescope and at the IRAM 30m-telescope the comet was monitored from March 19 into early April, mostly at 250 GHz. Measurements and data reduction are presently under way.
The preliminary results are:
The integrated intensity at closest approach was about 260 mJy at 250 GHz; this is about 10 % of the emission of P/Halley, scaled to the same distance. (2) The diameter of the nucleus is significantly below 5 km, deduced from the upper limits observed with the VLA and the PdBI. (3) The gaussian half power width of the radio halo is about 1500 km, probably without significant changes of the size during the observing period. (4) The bulk of the particles in the halo has a size of at least millimeter, with some fraction of submillimeter particles. (5) There is possibly some structure in the cometary halo; confirmation requires a more involved data analysis.
Even though the radio measurements did not show rapid time variations, the concept of simultaneous observations seems to be successful and is advisable for future comet observations.
Program listing for Wednesday