Comet C/2001 A2 (LINEAR) stood as a 17m faint object in the constellation Cancer on discovery date Jan. 15, 2001, showing a 0.3' coma, which displayed a 19m faint false nucleus. The comet will get close to the Sun and pass Earth on July 1 at only 0.24 AU (IAUC 7564), brighter but was not expected to get brighter than 10m since it is a very small body.
However, the evolution turned out to be completely different. Since the beginning of March the brightness increased considerably and on March 29/30 an outburst occured, which was analyzed in detail by Mark Kidger (The Astronomer, No. 444): According to this analysis the comet was an object of magnitude 15, only slightly diffuse, until March 22. Then, on March 23, José Navarro already estimated it at 11.5m, with a 2.5' large coma. On March 28 the coma had further grown and eventually on April 1 - at a brightness of 7.5m - it was possible to detect a faint, thin gas tail extending over several million km photographically. Until March 30 CCD-observations revealed a false nucleus of 15m. After that date its brightness increased, reaching about 12m. Spectral observations on April 7/8 revealed strong gas emission lines, indicating that this comet is a gas-abundant object - not really surprising for such an intrinsically faint object experiencing such a great increase in its activity level.
Around mid-April this comet disappeared in the evening twilight for observers in the northern hemisphere, but could be followed well from the southern hemisphere. Contrary to the expectations the brightness surge was not short-lived. The comet rather showed an ongoing high level of activity, and eventually could be seen with the unaided eye, displaying a medium bright, several degrees long tail in binoculars.
Observations with the 1.54m-Catalina reflector of C/2001 A2 taken on Apr. 30.12 UT showed a double nucleus. The two components were nearly equal in brightness and 3.5" apart and aligned precisely on an east-west line. Both components are highly condensed. Observations (with the same telescope) on Apr. 24.14 showed only a single nucleus. (IAUC 7616). CCD-observations by J. Broughton (Australia) on May 9.3 UT with a 25cm-SC showed the brighter (western) component at m2 = 14.7m, with the secondary, 0.3m fainter, perhaps 5" distant in p.a. 100°, although some elongation of the secondary could make the separation as much as 7". On May 11.3 the western component had brightened to m2 = 13.2m, and the eastern component had virtually disappeared. On May 12.4 the western component had faded to m2 about 14.0m, while the secondary was then perhaps 2m fainter, 8.5" away in p.a. 100°. Using the astrometry data till the start of May (with the western primary labeled as B and the eastern secondary labeled as A), Z. Sekanina obtained a preliminary separation time of Mar. 17 +/- 12 (IAUC 7625).
On May 14/15 comet C/2001 A2 was observed using the 8.2-m Very Large Telescopes Melipal and Yepun. Two components were seen, the eastern, tailward one (component A) about 1m fainter than component B at a separation of 12".6 in p.a. 105°. Both components had individual comae elongated approximately in the antisolar direction. Component B showed sunward-extended isophotes in the very inner part of the coma. On May 16/17 the distance between the components had increased to 14".6 (in the same position angle). In addition, the sunward fragment appeared to have split into two components (B/C) with a separation of 1" in p.a. 135-315°; these components were of about the same brightness and surrounded by a joint coma. V-band images revealed very extended isophotes perpendicular to the separation direction of this new pair. This could indicate the presence of a large amount of gas in the coma in addition to the dust. J. Broughton reports further CCD astrometry on May 14.4 and 16.3 UT, noting that on the latter occasion component A was at least two magnitudes fainter than and was separated from component B by about 14" in p.a. 102° (IAUC 7627).
Based on the astrometric offsets of component A from B reported between Apr. 30 and May 18 Z. Sekanina calculated that the splitting occurred on Mar. 29.9 +/- 1.6 UT (thus coinciding with the major outburst). Predicted separations and position angles: May 21: 19"/110°, 26: 28"/115°, 31: 39"/121°, June 5: 54"/128°, 10: 74"/136°, 15: 97"/145°, 20: 122"/156°. Assuming that the separation of component C from B coincided with the outburst of May 11, the preliminary predicted ephemeris is: May 21: 3"/138°, 26: 8"/142°, 31: 16"/148°, June 5: 29"/154°. The Australian amateur D. A. J. Seargent told that on May 14.4 UT there was a narrow ion tail, intense for about 1°, traced for approximately 4° in p.a. 133°; on May 15.4 a diffuse "cloud" was visible, detached from the tail, at about 1.5° from the coma (IAUC 7630).
Observations on June 16-21 indicated that the comet continues to fragment. The images obtained at ESO in the thermal infrared with the 3.6-m telescope and in the optical region with the 3.5-m New Technology Telescope showed faint companions drifting away from the principal nucleus (B) in an approximately antisolar direction. Analysis by Z. Sekanina, Jet Propulsion Laboratory, shows that all the observations can be satisfied by three fragments, D, E, and F. The analysis implies that fragment D separated from B on June 3.5 +/- 1.8 with an initial velocity of 1.0 +/- 0.1 m/s (approximately normal to the orbit plane); fragment E on June 9.5 +/- 0.7 with a velocity of 0.3 +/- 0.1 m/s; and fragment F on June 11.3 +/- 0.5 with a velocity of 0.8 +/- 0.2 m/s. These breakup events apparently triggered another major outburst, reported by visual observers to have peaked on June 12 (IAUC 7656).
D. Schleicher and R. Greer obtained the following averaged gas production rates (molecules/s), based on a set of narrowband photometry on June 27 and 28: OH = 4.8×1028, CN = 1.2×1026, H2O: 6.5×1028, dust: Af(rho) = 288 (IAUC 7653).
Observations at Lowell Observatory on June 29-30 showed that the comet displayed CN jet(s) symmetrical about p.a. 250 deg. Three successive arcs separated by approximately 12.000 km were observed on each side; outward motion of the arcs was detected. These arcs were not observed in the dust continuum (IAUC 7666).
Radio observations during July 1.5-2.4 UT showed that the H2O coma is elongated along the comet-sun line. The line width indicates an H2O outflow velocity of 0.7 km/s. The estimated water-production rate for July 1.7-2.0 was 3.8×1028 molecules/s (IAUC 7706). Monitoring of the inner coma by the Spanish Comet Observers Team revealed two small photometric events that are possibly related to the ejection of small fragments from the nucleus. The lightcurve indicates events of amplitude 0.4 and 0.2 mag initiating at approximately July 25.0 and 30.0 respectively. For comparison, the major event around July 12 had an amplitude of 1.5 mag (10" aperture). This indicates that, if due to fragmentation, these later events were probably caused by the separation of small, short-lived splinters that may not have been directly observable. In the ultrviolet, this comet was detected on July 12 (IAUC 7681).
During the last days of June the comet became visible again from Europe. In those days it was a rather unspectacular object near the horizon, despite its brightness of 4.5m. Due to its rapidly increasing altitude it became more prominent during the days following, although without a visually noticeable tail. Unfortunately around July 4 the Moon started to disturb observations, what meant that the best period of its apparition for Europe began not prior to mid-July. At that time it was already one magnitude fainter, but now a mostly broad tail was visually observable. The most fascinating fact of comet C/2001 A2 (LINEAR) was the daily changing appearance: one night unspectacular with no tail it could be a much more prominent object for binoculars during the next night, with the tail easily visible. In addition the shape of the coma varied from round to elliptical. Two different regions within the coma could be discerned: a diffuse outer part and a well condensed central inner part. In the center of the inner coma a small knot of material could be seen, its prominence and morphology showing daily changes too (condensed versus diffuse). Within this knot of material a faint, false nucleus could be seen most of the time, being always star-like up to magnifications of 500x. Until August 15 the brightness decreased continuously, although with some variations. Thereafter the comet experienced a minor drop in brightness (about 0.5m) from which it has not recovered yet.
The following analysis is based on 177 group member observations and 980 international ones! The brightness of the comet was clearly variable with short outbursts of up to 1.0m. These can be seen in the accompanying diagram, which shows the moving weighted 3-days mean value for each day, including the standard deviation.
Brightness variations (moving, weighted 3-days-means)
Besides the great outburst of March 30 the following minor outbursts, at about 30 days-intervals, can be seen: May 11: dm=0.5m, June 11: dm=1.0m and July 7: dm=1.0m. In addition the diagram shows a slow increase, compared to the average evolution, between April 17 and 25 by 0.6m.
The average brightness evolution - after the outburst at the end of March - can be described surprisingly well by one standard formula before and after perihelion:
pre-perihelion: m = 7.5m + 5×log D + 13.7×log r
post-perihelion: m = 7.1m + 5×log D + 10.5×log r
This results in a maximum brightness of 4.0m around June 22. However, the comet was in outburst during this time so that the true maximum brightness was 3.4m around June 12. Thus comet C/2001 A2 (LINEAR) was the second comet easily visible with the naked eye after Hale-Bopp, beside C/1998 J1 (SOHO), alas again only for observers in the southern hemisphere.
Total Brightness and Coma diameter
At the end of March the coma showed an apparent diameter of 5', increasing to 11' until mid-May, thereafter decreasing slightly towards perihelion. This temporary decrease, however, is probably a result of the unfavorable observation conditions at that time. After perihelion the apparent coma diameter increased significantly, reaching a maximum of 25' in the first week of July. Thereafter it decreased to 12' at the end of July, to 3.5' at the end of August, and 1.5' in mid-October. The absolute coma diameter increased from 200.000 km at the end of March to 425.000 km around Apr. 20 - probably as a result of the outburst. It is unclear to which degree the decrease to 250.000 km around the time of perihelion can be attributed to the unfavourable observing conditions. It would be equally comprehensible that the coma really shrunk after the large outburst. However, throughout June and July the absolute coma diameter increased in reality, reaching 370.000 km around July 20. Since then it shrinks slowly, showing a diameter of 270.000 km at in mid-August and of 130.000 km in mid-September, where it hovers since. At the end of March the coma was only moderately condensed (DC 3), becoming more condensed during the following weeks. Between mid-May and mid-June the degree of condensation reached its maximum at DC 6-7. Since then it continuously became more diffuse, reaching DC 1-2 at the end of August. Visual tail observations were reported between mid-April and mid-July, with a maximum of 7° at mid-June and maximum lengths of 1.5° during the European visibility period. The resulting absolute lengths are 6 Mill. km and 2 Mill. km respectively.
Andreas Kammerer
