冻死了？--在30.7 AU探测到海尔-波普彗星
Frozen to death? -- Detection of comet Hale-Bopp at 30.7 AU
摘要：
海尔-波普彗星（C/1995 O 1）是现代天文学遇到的唯一最重要的彗星，在2007年底仍然在25.7 AU的太阳距离上显示出显着的活动。这一活动何时何地将最终停止是一个令人困惑的问题。在这里，我们提出了新的观测与ESO 2.2米望远镜在拉西拉检查活动的海尔-波普在30.7 AU的太阳距离。2010年12月4日，拍摄了26张CCD图像，曝光时间为180 s，用于光度测定和形态学检查。这颗彗星位于R轨道，总亮度为23.3+-0.2等，绝对亮度为R（1，1，0）=8.3。在视宁度为1.9”时，彗发的轮廓像恒星一样，没有任何证据表明彗发或尾延伸到2.5”（投影值= 55，000 km）以外，表面亮度超过26.5等/弧^[2]。测得的总亮度对应的相对总反射面a_RC为485 km^2，比3年前小了9倍。计算出的a_RC值意味着一个半径为60- 65公里的核（假设4%的α_C）。这一大小估计与以前的结果有很大的矛盾，分散在35公里左右。因此，我们认为，彗星可能仍然处于低水平的活动，尽管没有一个突出的昏迷。或者，如果核已经处于休眠状态，假设半径为35公里，那么辐射强度应该高达13%。有了这个观测，海尔-波普已经是有史以来观测到的最遥远的彗星，远远超出了海王星的轨道。
英文摘要：
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.
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