TL;DR
This study measures the size and albedo of the largest known Oort-cloud object, comet C/2014 UN271, confirming its exceptional size and typical cometary albedo, and highlighting its significance as a distant, active comet driven by hypervolatiles.
Contribution
First direct measurement of the size and albedo of the largest Oort-cloud object, using high-resolution ALMA observations and thermal modeling.
Findings
Diameter of 137±17 km confirms it as the largest Oort-cloud object.
Albedo of 5.3±1.2% is typical for comets.
Object's size exceeds that of Hale-Bopp, making it the largest known comet.
Abstract
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…
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