On Oct. 14, 2002 another comet was discovered by the LINEAR-team, positioned in the constellation Bootes. Comet C/2007 T7 (LINEAR) was an object of magnitude 17, showing a coma only slightly greater than the airy disks of nearby stars. According to the most recent elements it will pass perihelion in April 2004 at an solar distance of only 0.62 AU. Thereafter it will pass the earth at a distance of only 0.27 AU on May 20 (IAUC 8003). Assuming an average brightness evolution the comet could then reach 0m, displaying a coma of diameter 1° and a tail up to 40° long. However, observers in mid-Europe will not see much of this show, because the comet will be situated south of the sun (and at small elongations) during the most interesting weeks. It will leave dark skies at the beginning of March 2004 (expected magnitude 6), not to enter them again (morning apparition) until the end of October of this year (expected magnitude 11). Accepting a rather bright sky (sun depression 9°) it could be visisble during April near the morning horizon and during the first half of June near the evening horizon (expected magnitude 3 in both cases). However it will not be possible to view the tail, because its orientation will be nearly parallel to the horizon. If comet C/2007 T7 is a "new" one (n = 3), however, the maximum brightness would be only about 2.5m with a predicted coma diameter of 40' and a tail of 9° length.
The German Comet Section received 113 observations from 12 member observers. The analysis incorporates 760 international observations.
After a period of disappointingly slow brightness increase since the beginning of 2003 a significant change of the brightness parameters occurred around Oct. 25 at only magnitude 11.0m. During the following weeks the brightness increased exceptionally rapid, reaching 7.0m at the beginning of February 2004. If this evolution would have continued until perihelion or closest approach to Earth on May 19, the comet would have reached 0m at perihelion, thereby displaying a visual tail length of about 25°. In this scenario the comet would have become even more impressive at the time of minimum earth distance, when its brightness would have reached -1m and it would have appeared in the sky, according to my empiric formulae, as an 1.5° object with a 90° tail! Thus it would have become the 'Hyakutake' of the southern skies, albeit at smaller altitudes (appx. 30°).
Unfortunately the comet experienced a second break in its brightness evolution during the first days of February; thereafter the brightness evolved much slower until perihelion. This came as no big surprise, regarding the overall evolution. Fortunately the activity parameter did not decrease to the value during the first months of the apparition.
Thus the prediction had to be revised to the values of 1.5m, 0.9° und 15-20°, respectively. And there was even a certain possibility, that the maximum values would be only in the order of 2.0m, 0.8° and 10-15°, respectively.
In the end the comet remained even fainter and was even less prominent. Five different periods can be discerned in the brightness evolution: 3 pre- and 2 post-perihelion. At the transition between phase 4 and 5 an outburst of nearly one magnitude occurred!
Maximum brightness of magnitude 2.7 occurred around May 23. The overall brightness evolution can be described by the following formulae:
t < -180d: m=7.7m+5×log D + 3.4×log r
-180d < t < -82d: m=2.9m+5×log D + 13.1×log r
-82d < t <0d: m=4.9m+5×log D + 4.5×log r
0d < t <+26d: m=7.5m+5×log D + 15.7×log r
t >+26d: m=6.0m+5×log D + 8.2×log r
Evolution of the heliocentric magnitude pre-perihelion
Evolution of the heliocentric magnitude post-perihelion
Mimicking the rapid increase in brightness the coma increased from 1.2' at the end of October 2003 to 14' at the beginning of February 2004 (maximum value of the apparition). Thereafter it decreased significantly towards perihelion, increasing post-perihelion towards a secondary maximum of only 11' around May 20, due to the proximity to Earth. Until mid-July it had decreased to only 2'. After its reappearance near the end of 2004 the coma diameter was less than 1'. The absolute coma diameter reached its maximum of 1.2 Mio. km at the beginning of February, decreasing afterward until perihelion to 300.000 km. The further decrease to 125.000 km until perigee most probably was not real, but likely a result of the systematic problems in estimating comets at small perigee distances. After closest approach values of almost 400.000 km were observed around mid-June, which decreased to 200.000 km until mid-July. At the end of 2004 the coma diameter measured 125.000 km.
Total Brightness and Coma diameter
Until September 2003 the degree of condensation remained constant at DC 3-4. Thereafter - probably due to the period of high activity - it increased to DC 6-7 in November, decreasing to DC 4-5 in mid-January. During the days near perihelion the coma was highly condensed (DC 7-8), starting to decrease continuously shortly thereafter, reaching DC 5 around perigee and DC 2-3 in mid-July. The coma showed an interesting morphology during winter 2003/04 with a sometimes prominent false nucleus, surrounded by a compact coma which was of elliptical shape during fall. During January 2004 the false nucleus became less prominent and the coma less condensed. Observers at the end of 2004 estimated the degree of condensation as DC 3.
Right after the onset of the above-average activity the comet showed up a visual tail. Until Feb. 20, the tail length increased to almost 0.5° (appx. 5 million km). During perigee the visual tail length reached 8° (20 Mio. km). Thereafter the tail shrunk rapidly; last sightings were reported at the end of June (absolute length about 3 Mio. km).
As a general surprise a quite obvious anti-tail appeared around April 20, forming a 135° angle with the main tail, although Earth was located well above the comet's orbital plane at that time. According to a research by Z. Sekanina the anti-tail was formed by dust particles which were either ejected during a sudden increase in activity just before discovery or in the course of a short-lived dust outburst around conjunction with the Sun in mid-2003. Unless the - considerably less dense - material released since then is not distributed favourably in respect to comet-Earth geometry, there will be no observable anti-tail around June 26, when Earth will again cross the orbital plane of the comet (for the same reason no anti-tail was observed around Dec. 27, 2003). According to his calculations the dust strip showed the largest deviation of 135° from the main tail on Apr. 13 and May 21, with the anit-tail being most prominent. Between these dates and thereafter the orientation of the dust strip is turned towards the main tail direction (IAUC 8330).
Andreas Kammerer
