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

TL;DR

This study reports the detection of comet Hale-Bopp at 30.7 AU, showing it may still have low-level activity or a high-albedo dormant nucleus, making it the most distant comet observed to date.

Contribution

First detection of Hale-Bopp at 30.7 AU, providing new insights into its activity and size estimates at unprecedented solar distances.

Findings

Comet detected at 30.7 AU with no visible coma or tail.

Total brightness suggests a nucleus size of 60-65 km radius or high albedo.

Hale-Bopp is the most distant comet observed so far.

Abstract

Comet Hale--Bopp (C/1995 O1) has been the single most significant comet encountered by modern astronomy, still having displayed significant activity at 25.7 AU solar distance in late 2007. It is a puzzling question when and where this activity will finally cease. Here we present new observations with the ESO 2.2m telescope at La Silla to check the activity of Hale--Bopp at 30.7 AU solar distance. On 2010-12-04, 26 CCD images were taken with 180 s exposure times for photometry and morphology. The comet was detected in R and had a total brightness of 23.3+-0.2 mag, referring to an absolute brightness of R(1,1,0)=8.3. 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_RC, of 485 km^2, 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 is in significant contradiction with the previous results scattering around 35 km. Therefore we suggest that the comet may still be in a low-level 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 been the most distant comet ever observed, far beyond the orbit of Neptune.