Frozen to death? Detection of comet Hale-Bopp at 30.7 AU: (Research Note)

Abstract

Context.Comet Hale-Bopp (C/1995 O1) has been the most interesting comet ever encountered by modern astronomy, which continued to display significant activity at a solar distance of 25.7 AU. It is unclear when and how this activity will finally cease. Aims.We present new observations with the ESO 2.2 m telescope at La Silla to monitor the activity of Hale-Bopp at 30.7 AU solar distance. Methods.On 2010-12-04, 26 CCD images were taken with 180 s exposure times for photometry and morphology. Results. The comet was detected in R and had a total brightness of 23.m3 ± 0.m 2, which corresponds to an absolute brightness of R(1, 1, 0) = 8.m 3. The profile of the coma was star-like at a seeing of 1.′9, without any evidence of a coma or tail extending farther than 2.′5 (=55 000 km in projection) and exceeding 26.5 mag/arcs2 surface brightness. The measured total brightness corresponds to a relative total reflecting surface, aRC, of 485 km2, nine times less than three years before. The calculated aRC value would imply a nucleus with 60-65 km radius assuming 4% albedo. This size estimate contradicts significantly the previous results scattering around 35 km. Conclusions.We propose that the comet may still be in a low level of activity, despite the lack of a prominent coma. Alternatively, if the nucleus is already dormant, the albedo should be as high as 13%, assuming a radius of 35 km.With this observation, Hale-Bopp has become the most distant comet ever observed, far beyond the orbit of Neptune. © 2011 ESO.