The first comet of 2005 was discovered by the LINEAR-team on Jan. 13 as a 14.5 mag object in the constellation Virgo. Comet C/2005 A1 (LINEAR) displayed a 15" coma and a significant 75" tail in p.a. 310°. It will pass perihelion in mid-April, then positioned in the far-southern sky, with a predicted brightness of 11 mag (IAUC 8463). Mid-European observers could follow it for only two weeks, before it dipped below the horizon.
Between Jan. 17 and Feb. 5 it brightened only slowly from magnitude 12.0 to 11.5. But on Feb. 14 D. Seargent found the comet to be of magnitude 9.5, implying an outburst. It reached maximum brightness of magnitude 8.0 at the end of March. Starting in July 2005 the comet could be observed again. Until the beginning of September it faded from 11.0 mag to 12.5 mag.
Analysing 9 observations of 3 members of the German Comet Section and 130 international observations it turns out, that the comet did not experience an outburst in February, but that it just brightened more rapidly than the average comet. In addition, the discovery magnitude turned out to be too faint, which added to the short-lived excitement. Actually, the brightness of the comet evolved according to the formulae below. These show that the activity of the comet decreased more slowly post-perihelion than it increased pre-perihelion. This fact most probably is the result of the splitting of the nucleus (see below), in the course of which fresh material was exposed. The brightness formulae are as follows:
pre-perihelion: m = 8.2m + 5×log D + 14.0×log r
post-perihelion: m = 7.9m + 5×log D + 8.7×log r
The apparent coma diameter increased from 2.5' in mid-January to almost 5.5' in mid-March, decreasing thereafter to 3.5' during the first days of April. Between July and September it decreased from 2.5' to 0.8'. The absolute coma diameter increased from 140.000 km at first to 200.000 km in mid-March and to 225.000 km at the beginning of April. Between July and October it decreased from 200.000 km to 100.000 km. Pre-perihelion the coma continuously condensed, with the degree of condensation increasing from DC 3 to DC 5 (mid-March). Post-perihelion it hovers around DC 3.
Total Brightness and Coma diameter
On images taken by S. Pastor and A. Reyes on June 25 a secondary nucleus was detected, which was about 0.7 mag fainter than the main component. Z. Sekanina calculated Apr. 23.4±0.8d as the date of the disintegration (IAUC 8562).Andreas Kammerer
