C/2003 K4 (LINEAR)

On May 28, 2003 the LINEAR-Team reported an asteroidal object near the border of the constellations Cygnus/Equuleus, which turned out to be a comet during closer inspection. Comet C/2003 K4 (LINEAR) showed a 6" coma of magnitude 17.5 with a 3" central condensation. According to orbital calculations this comet is currently at a solar distance of 6 AU, but will come as near as 1.0 AU in autumn 2004. The follow-up observations forced the brightness to be upset by about 2 magnitudes, making this comet a rather bright object. However, earth and comet will be situated at opposite positions relative to the sun, which results in a proposed maximum apparent brightness of only 5^m. Furthermore this comet will be situated in the far southern sky at perihelion. This comet should become brighter than 10^m in April 2004. It should be visible from mid-Europe until beginning of September of this year (then magnitude 6.5) and again starting in March 2005 at magnitude 8 (IAUC 8139/45).

CCD-observations published during the summer months of 2003 indicated that this comet was about 0.5^m fainter than expected, with a coma of diameter 0.3'. According to CCD-observations by Jäger/Rhemann the comet stayed constant during August and September 2003 at 15-16^m, with a coma diameter of 10" and a tail length of 15".

2004/05: According to 62 observations by 10 members of the German Comet Section and 770 international reports the comet showed three different phases in the brightness evolution pre-perihelion, but a very smooth evolution post-perihelion. The comet increased its brightness very slowly until May 15. If this evolution would have continued until perihelion, the comet would not have gotten brighter than 7.8 mag. But then, between May 15 and June 25, a 40-day long period of extremely high activity followed, which could have made the comet up to 3 mag bright, would it have continued right to perihelion. Thereafter, the activity settled at average values, but as a result of the period of high activity the absolute brightness had increased by 2 mag. Due to the fact that the comet - while getting closer to the Sun - receded from Earth since July 7, the apparent brightness increased only slightly during August, expected to show a broad maximum of 5.5 mag during October/November. However when it reappeared it was of only magnitude 7.5 - nearly 2^m fainter than expected. During the following weeks the brightness decreased from its secondary maximum of 7.0 mag in mid-December 2004 to 10.5 mag in mid-March 2005. During fall 2005 it had faded further to 12.5 mag. The evolution can be described by the following formulae:

t < -140d: m = 6.7^m + 5 × log D + 5.7 × log r
-140d < t < -105d: m = -1.7^m + 5 × log D + 28.2 × log r
t > -105d: m = 4.1^m + 5 × log D + 8.4 × log r
t > 0d: m = 5.8^m + 5 × log D + 5.8 × log r

Thus the maximum brightness of magnitude 6.5 was reached at the end of August 2004.

Evolution of the heliocentric magnitude (pre-perihelion)

The apparent coma diameter remained around 0.7' for a long time, started to increase in May and reached a value of 2' at the end of May. During the first days of June the coma increased rapidly to 6' in mid-June, reaching a maximum of 18' in mid-July. Thereafter it decreased continuously to 12' in mid-August and to 6' at the beginning of September. After its reappearance the coma diameter measured 8' in mid-November, but had decreased to 4' in mid-March 2005. In fall 2005 it measured about 1'. The absolute coma diameter was 125.000 km until the beginning of May, increasing slowly to 200.000 km at the end of May. Since the early days of June it literally exploded and reached a maximum of 1.2 Mio. km in mid-July. Thereafter it decreased to 900.000 km in mid-August and 550.000 km at the end of August (with the latter value perhaps too small due to the fact that the observing circumstances became worse).During its second apparition the absolute coma diameter decreased only marginally from 500.000 km in mid-November 2004 to 450.000 km in mid-March 2005. It measured about 175.000 km in fall 2005. The coma itself was noticeably condensed with values of about DC 4-5 to DC 5. Visual tail observations have been reported since the beginning of May, increasing in number and reported tail lengths (then: 5') in June, reaching lengths of 0.3-0.4° (2.5 Mill. km) in mid-July. During the last weeks prior to perihelion the coma showed a pronounced central condensation incorporating a a star-like false nucleus. Post-perihelion the coma grew more and more diffuse: in mid-November the degree of condensation was estimated as DC 5, decreasing to DC 3-4 by mid-March 2005, where it still hovered in fall 2005.

Total Brightness and Coma diameter

Andreas Kammerer

FG observations