Analysis of past comet apparitions

C/2000 WM1 (LINEAR)


An 18.0m faint object discovered already on Nov. 16, 2000, by LINEAR in the constellation of Cassiopeia was reported again on Dec. 16 by the same team. On a picture taken on Mt. Hopkins with the 1.2m telescope on Dec. 20, T. Spahr noticed a 10" small coma and a broad, faint 10-20" long tail pointing NE. Comet C/2000 WM1 (LINEAR) reached a minimum distance to the Sun of 0.55 AU in January 2002, passing Earth on Dec. 3, 2001 at 0.32 AU (IAUC 7546). Supposing an average brightness evolution it was expected to get as bright as 4mwith a predicted apparent coma diameter of 25' and a tail length of 6°. Unfortunately it would vanish from mid-European skies in December, so that Europeans could only expect a 3° long tail.

Since August 2001 this comet was under better coverage by the observers. However, the estimates published during the first weeks showed much scatter. Taking together all brightness estimates it seemed at that time as if the comet was about 0.5m fainter than expected. Thus the brightness would be 5.0m in mid-December and 4.5m in mid-January 2002. During the second half of September the number of observations increased significantly and since mid-October dramatically. After its trip to the southern sky it can be followed again by northern latitude observers since beginning of March 2002.

Until mid-September 2002 87 observations by 14 members of the German comet section were received. In addition 650 estimates by international observers were used for the analysis. Until Nov. 10, 2001, (t = -73d) the comet showed an average evolution, although with an absolute brightness about 1m fainter than previously expected. Thereafter a period of increased activity occured that lasted until the last days of that month (t = -55d). This increased activity seems to have exhausted the gas reservoirs, because during the following weeks until about perihelion the comet experienced a period of subdued activity.

Evolution of the heliocentric magnitude (pre-perihelion)

From mid-December 2001 to the beginning of March 2002 the comet could only be observed from the southern hemisphere - having in store a big surpise. On Jan. 27 the brightness raised sharply from 6.2m to a maximum of 2.9m on Jan. 31 (IAUC 7809/14)! For the second time in 12 months the observers in the southern hemisphere were able to see a comet with the unaided eye - although rather close to the horizon! The brightness evolution prior to perihelion can be described best with two formulae:

T < -55d: m = 7.2m + 5×log D + 10×log r

T > -55d: m = 7.5m + 5×log D + 6×log r

indicating a maximum brightness of 5.6m (around Dec. 7/8) prior to the outburst. After the outburst (t=+4d to t=+8d) the evolution can be described with the formula

T < +65d: m = 6.4m + 5×log D + 15×log r

T > +65d: m = 6.5m + 5×log D + 12.5×log r

When this comet was again visible from mid-northern latitudes during the very first days of March it showed a brightness of magnitude 7.0.

The apparent coma diameter increased slowly from 0.7' in August 2001 to 2' at the beginning and 3' at the end of October. During November, however, it swallowed rapidly, reaching a maximum of 20' at the end of that month (it passed Earth on Dec. 2). Thereafter the coma diameter decreased comparably fast, measuring 12' around mid-December and only 5' around perihelion. In mid-February 2002 the coma diameter reached a secondary maximum, due to the outburst, of 6-7', declining thereafter to less than 3' at the start of April and to 1.0' in mid-July. The absolute coma diameterwas about 125.000 km in August, only increasing to 150.000 km until the start of November. Thus the increase of the apparent coma diameter was strictly the result of the dinimishing distance to Earth during this period. Thereafter, however, the coma literally exploded, reaching a first maximum diameter of 300.000 km around Nov. 25. Until perihelion the absolute diameter decreased slowly towards 200.000 km. The outburst then caused the diameter to swallow again, to 280.000 km at the start of February and to 340.000 km around Feb. 20. At the beginning of April it had decreased to 180.000 km, and in mid-July to 110.000 km.The coma was only moderately condensed (DC 2-3) at first, becoming more concentrated (DC 5) until mid-December. In the course of the outburst it peaked at DC 8, getting more diffuse during the following days, reaching DC 3-4 at the beginning of April and DC 2-3 in mid-July.

Visual tail observations were reported from mid-October 2001 onwards. A maximum of 1° (1.3 Mill. km) was reached around Dec. 10. Due to the outburst the length increased considerably and reached 2.5° (10 Mill. km) around Feb. 10, 2002. Thereafter it shortened rapidly, but was still visible at a length of few arcminutes at the beginning of April. Since the end of April no additional visual sighting was reported; however on photographs taken at the end of May the 12m comet still spotted a 0.5° long tail. The tail pointed westward at first, rotating rapidly through south to ENE around Nov. 20 (Earth-crossing of comet's orbital plane), returning slowly to southwestward directions since then.

Total Brightness and Coma diameter

Andreas Kammerer

FG observations


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