探测到的最大奥尔特云天体：彗星C/2014 UN 271（Bernardinelli-Bernstein）
Size and albedo of the largest detected Oort-cloud object: comet C/2014 UN 271 (Bernardinelli-Bernstein)
摘要：
最近宣布的奥尔特云彗星C/2014 UN 271（Bernardinelli-Bernstein）至少在三个方面引人注目：（i）它是在距离太阳约29 au的地方被发现的（发现前图像高达约34 au）;（ii）它在近24 au时就显示出彗星活动;（iii）它的核等（Hr ~ 8.0）表明是一个非常大的物体。随着彗星在2031年朝着~11 au近日点前进，预计在未来几年内将探测到气体。我们使用扩展配置的ALMA（分辨率~0.064”）测量彗星的1287 μ m（233 GHz）连续通量。观测于2021年8月8日在距离太阳20.0 au的地方进行。选择高空间分辨率是为了过滤掉任何尘埃成分。我们探测到该物体在~10 sigma处的热发射，通量为0.128+/-0.012 mJy。根据观测限制和我们对尘埃贡献的理论估计，测量到的全部通量都可以归因于原子核。根据NEATM模型结合Hr星等，我们确定表面等效直径为137+/-17 km，红色几何膨胀率为5.3+/-1.2%。这证实了2014 UN 271是迄今为止发现的最大的奥尔特云天体（几乎是C/1995 O 1海尔-波普彗星的两倍），除了半人马座95 P/凯龙星，它显示出爆发般的活动，是太阳系中已知最大的彗星。另一方面，彗星的典型特征，增加了“普遍的”彗星核彗星的可信度。凭借其遥远的近日点和独特的大尺寸，2014 UN 271是遥远彗星的突出原型，其活动由高挥发物驱动。近日点后的热测量将允许研究可能的温度变化，例如与近日点前相比的表面增亮，如在海尔-波普观测到的那样。
英文摘要：
The recently announced Oort-cloud comet C/2014 UN271 (Bernardinelli-Bernstein) is remarkable in at least three respects: (i) it was discovered inbound as far as ~29 au from the Sun (with prediscovery images up to ~34 au); (ii) it showed cometary activity already at almost 24 au; and (iii) its nuclear magnitude (Hr ~ 8.0) indicates an exceptionally large object. Detection of gases is expected in the upcoming years as the comet is heading towards a ~11 au perihelion in 2031. We used ALMA in extended configuration (resolution ~0.064") to measure the 1287 um (233 GHz) continuum flux of the comet. Observations were performed on August 8, 2021 at a 20.0 au distance from the Sun. The high spatial resolution was chosen in order to filter out any dust contribution. We detected the thermal emission of the object at ~10 sigma, with a flux of 0.128+/-0.012 mJy. Based on observational constraints and our theoretical estimates of the dust contribution, the entirety of the measured flux can be attributed to the nucleus. From NEATM modelling combined with the Hr magnitude, we determine a surface-equivalent diameter of 137+/-17 km and a red geometric albedo of 5.3+/-1.2 %. This confirms that 2014 UN271 is by far the largest Oort-cloud object ever found (almost twice as large as comet C/1995 O1 Hale-Bopp), and except for the Centaur 95P/Chiron which shows outburst-like activity, the largest known comet in the Solar System. On the other hand, the object albedo is typical of comets, adding credence for a "universal" comet nucleus albedo. With its distant perihelion and uniquely large size, 2014 UN271 is the prominent archetype of distant comets, whose activity is driven by hypervolatiles. Post-perihelion thermal measurements will permit to study possible albedo changes, such as a surface brightening compared to pre-perihelion, as was observed for Hale-Bopp.
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