On March 23, 2012 an apparently asteroidal object of magnitude 21 was discovered by the Mt.Lemmon project in the head of Leo. However, during follow-up observations its cometary nature was detected. Comet C/2012 F6 (Lemmon) showed a 10" coma of total magnitude 20.0, which was elongated towards p.a. 110° (CBET 3070 / MPEC 2012-G45). This comet will pass perihelion in spring 2013, presumably peaking at magnitude 9.5. However, at this time it will be situated deep in the southern sky. For mid-European observers it will not be brighter than 13 mag, before/after it will disappear/appear above the horizon, respectively.

The comet evolved surprisingly during the winter months 2012/13, based on the estimates of the observers in the Southern hemisphere. Between Nov. 20, 2012 and the first week of February 2013 the brightness increased from 12.0 mag to 5.5 mag. Based on this result I predicted a maximum brightness of 3.8 mag on Mar. 20, 2013. However, in those days the comet would be observable exclusively from the southern hemisphere, low above the horizon.

Actually, the comet reached its maximum brightness of 4.4 mag on Mar. 18, 2013 - 5 magnitudes brighter than expected originally! This positive trend continued post-perihelion, although in a more complex manner. Until May 8 the comet faded on average, followed by a two-weeks period of standstill, which was replaced by a faster than average fading, starting on May 23. Starting in the second week of May 2013 the comet could be observed from mid-latitude sites in the Northern hemisphere, when it was estimated at magnitude 7.0 mag. Based on 23 observations by 7 members of the German Comet Section and 380 international observations the brightness evolution may be described by the formulae:

post-perihelion:
t < +45d: m = 5.1 mag + 5×log D + 9.7×log r
+45d < t < +60d: m = 5.6 mag + 5×log D
t > +60d: m = 4.3 mag + 5×log D + 11.0×log r

Thus the comet exhibited a continuous and rather average evolution, leading to the assumption that the discovery brightness may have been underestimated (but not by 5 magnitudes!). Furthermore it is plausible, that the comet switched to a state of significantly greater activity sometimes between late summer and early fall 2012.

Total Brightness and Coma Diameter

Pre-perihelion, the apparent coma diameter steadily increased from short of 2' to short of 12' at the end of January 2013. It had decreased to 10' by mid-February, remaining constant at this value until the comet disappeared in the twilight. At its reappearance in mid-April the comet diameter was estimated at 8', decreasing to 6' until the beginning of June, 3.5' in early September and 1.0' at the start of November. In absolute dimensions the coma increased steadily from 225.000 km at the end of November 2012 to 550.000 km in mid-January 2013. Post-perihelion the coma measured constant 600.000 km until mid-June. Thereafter it shrunk slowly, measuring 375.000 km in early September and 160.000 km at the beginning of November. Until the beginning of 2013 the degree of condensation of the coma was constant at DC 4-5. The degree of condensation increased to DC 6-7 until the end of February, remaining constant at that value until the comet disappeared in the twilight. Post-perihelion the coma grew ever more diffuse until the end of July, when it was estimated at DC 3-4. Thereafter this value remained constant. First visual sightings of a tail were reported in mid-January. Until mid-March its length increased to 2.5° (10 mio. km). Post-perihelion maximum lengths of 0.7° were observed. The last visual tail sightings were reported at the end of August. Pre-perihelion the position angle changed from NW to SSE, post-perihelion from W to SE.

Andreas Kammerer

FGK observations