On Aug. 27, 2004 the well-known comet hunter Donald Machholz discovered his 10th comet, again visually, with his 15 cm (6 inch) reflector. Comet C/2004 Q2 (Machholz) stood as a 11.0 mag object in the constellation of Eridanus, displaying a moderately condensed coma of 2' apparent diameter. CCD-images taken the same day show a 6' long tail pointing towards p.a. 245° as well as a 14.5 mag false nucleus (IAUC 8394/95). Two pre-discovery photos taken by the Australian amateur Vello Tabur showed the comet at magnitude 12.5-13.0 on May 27. In addition, on a photo taken by Jäger/Rhemann on May 15 in Namibia it is noticeable as a 13 mag object in the proximity of the tail of comet C/2002 T7 LINEAR - but only a bit more diffuse than nearby stars. Thus this comet did not experience an outburst shortly before discovery, and thus could have been discovered much earlier (Comet Mailing List, email). The comet will remain within the mentioned sky region during fall 2004, thus remaining at low altitudes for mid-European observers. In mid-December it will start to move rapidly northward, predicted to reach 4-5 mag at the beginning of January 2005, mainly due to the proximity to earth (0.35 A.U.). The comet should fade only slowly thereafter, with the comet moving through the constellations Perseus, Cassiopeia and Camelopardalis.
Based on the evolution the comet showed until the start of November 2004 I predicted a maximum brightness of 3.5 mag at the beginning of January 2005. Due to the then close proximity to Earth the coma should swallow to about 35' with the comet displaying a visual tail length of about 4-5°. (which, however, should show a rather low surface brightness due to the relatively large solar distance of the comet at that time).
Comet Machholz fulfilled the expectations. Not only was it visible by the unaided eye for several weeks (even though it could not be described as being prominent), it also showed a gas tail as well as a dust tail at right angles during January. For the following analysis 362 observations by 13 members of the German Comet Section and 1630 international observations (until start of Sep., 2005) were used.
The brightness evolution is remarkably smooth, without short-term variations or sudden changes in the brightness parameters. Only the activity parameter n has become a bit smaller after perihelion - not unusual for a comet. The brightness evolution can be described by the following formulae:
which results in a maximum brightness of 3.7m around Jan. 8 and a brightness of 4.0m or brighter for four weeks!
Total Brightness and Coma diameter
At the beginning of the apparition the apparent coma diameter was in the order of 3', increasing to 8' at the start of November 2004. Thereafter it increased at a faster rate due to the decreasing distance to Earth, reaching 16' at the beginning of December. The apparent coma diameter peaked around Jan. 10, 2005, reaching 32', being larger than 25' over a period of four weeks as well. Thereafter it began to decrease rapidly, reaching 18' in mid-February and 14' at the beginning of March. During the following six weeks it hovered around this value, only to start shrinking again in mid-April: reaching 6' at the start of June and 1.5' at the beginning of Sep. 2005. The absolute coma diameter measured 250.000 km at the beginning of the apparition, increasing continuously until mid-December to 500.000 km, where it paused until the beginning of March 2005. Thereafter it started to swallow again, reaching 725.000 km at the start of April, where it paused until the end of May. Since then it shrinks, reaching 300.000 km at the beginning of Sep. 2005.
The morphology of the coma was described by most observers as follows: a bright inner coma was well discerned within an outer, fainter coma. In the centre of the coma a central condensation could be seen with binoculars, but it was not prominent. With a telescope a prominent star-like false nucleus became visible. Scrutinizing the innermost coma with instruments larger than 25 cm (10") in aperture near-nucleus structures showed vaguely. Their contrast compared with the bright coma background was very low so that high magnifications had to be used. Reports state a brighter sector on the sun-facing side of the nucleus, one fountain and up to three jets. For the latter detailed analysis of CCD images taken in the R-Band in January 2005 showed the following lengths/position angles: Jan. 2.66 UT: 1: 150"/291°, 2: 150"/252°, 3: 60"/216°, Jan. 15.63 UT: 1: 150"/282°, 2: 150"/239°, 3: 30"/211° (IAUC 8480/84).
The coma itself showed only a moderate degree of condensation at first (DC 4), but condensed continuously during the next weeks and reached DC 6 at the change of the year. This value remained constant for four weeks. Between mid-February and the end of March it decreased to DC 3-4, where it hovered until the end of June, only to decrease further to DC 2-3 by the beginning of September 2005.
Comet Machholz displayed two tails over a period of several weeks. The more diffuse and broader one was the dust tail, which was observed between the first days of October 2004 and the end of March 2005. In addition, between mid-November 2004 and the end of March 2005 the thin, and at times rather long, ion tail was observed visually. Both, especially the ion tail, showed brightness variations from one night to the other. At one night the ion tail, at another the dust tail was more prominent. Although the surface brightnesses of both tails were low, they could be seen well in binoculars under dark skies. The ion tail reached its maximum length of 4-5° around Jan. 10, at the same time as the dust tail, which reached 2.0-2.3° at maximum. Photographically several disconnection events could be recorded within the ion tail. The maximum absolute length of the ion tail measured about 7-8 Mio. km, the dust tail reached 3.0-3.5 Mio. km.
The evolution of the position angle of both tails was interesting. As can be seen in the diagram both pointed in similar directions (North-West and West, respectively) when they were first recognized. During the next weeks the ion tail turned slowly towards easterly directions while the dust tail started to rapidly turn southward at the beginning of December 2004, later pointing to South-East. Thus both formed a right angle during the first days of December, even pointing in opposite directions for some days around the turn of the year. During the climax of the apparition both tails again formed nearly a right angle for several weeks. In February the difference in both position angles began to decrease at first, only to increase again short to 90° at the time when both tails disappeared for the visual observer.
Evolution of the position angle of the ion and dust tail
Andreas Kammerer
