Research Note

Frozen to death? Detection of comet Hale-Bopp at 30.7 AU

Gy. M. Szabó^1,2,3, K. Sárneczky¹ and L. L. Kiss^1,4

¹ Konkoly Observatory, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: szgy@konkoly.hu
² Eötvös Fellow at the Astronomy Dept., University of Texas at Austin, 78712 TX, USA
³ Dept. of Exp. Physics & Astronomical Observatory, University of Szeged, 6720 Szeged, Hungary
⁴ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia

Received: 27 February 2011
Accepted: 21 April 2011

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 23ṃ3  ±  0ṃ2, which corresponds to an absolute brightness of R(1,1,0) = 8ṃ3. The profile of the coma was star-like at a seeing of 19, without any evidence of a coma or tail extending farther than 25 (=55 000 km in projection) and exceeding 26.5 mag/arcs² surface brightness. The measured total brightness corresponds to a relative total reflecting surface, a_RC, of 485 km², nine times less than three years before. The calculated a_RC 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.