Frozen To Death? Detection Of Comet Hale-Bopp At 30.7 AU (Research Note)

Context. Comet Hale-Bopp 9C/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(m).3 +/- 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/arcs(2) surface brightness. The measured total brightness corresponds to a relative total reflecting surface, a(R)C, of 485 km(2), nine times less than three years before. The calculated a(R)C 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.