Abstract
Context. 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 already showed cometary activity 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 heads toward a perihelion of ∼11 au in 2031. Aims. The goal is to determine the object’s diameter and albedo from thermal measurements. Methods. We used ALMA in extended configuration (resolution ∼0.064″) to measure the 1287 μm (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 also used a recently published Afρ value to estimate the dust production rate and the expected dust thermal signal for various assumptions on particle size distribution. Results. We detected the thermal emission of the object at ∼10σ, 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 modeling 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 C/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 C/2014 UN271 albedo is typical of comets, adding credence to a “universal” comet nucleus albedo. Conclusions. With its distant perihelion and uniquely large size, C/2014 UN271 (Bernardinelli-Bernstein) is the prominent archetype of distant comets whose activity is driven by hypervolatiles. Monitoring of dust and gas emission as the comet approaches and passes perihelion will permit its activity time pattern to be studied and compared to the distant (outbound) activity of Hale-Bopp. Post-perihelion thermal measurements will permit the study of possible albedo changes, such as a surface brightening compared to pre-perihelion, as was observed for Hale-Bopp.
Highlights
Comet C/2014 UN271 (Bernardinelli-Bernstein) was discovered as part of the search for outer Solar System objects with the Dark Energy Survey (DES) performed over the period 2013–2019 (Bernardinelli et al 2021a). 2014 UN271 was observed in 42 DES survey images on 25 nights from October 2014 to November 2018, with a heliocentric distance rh ∼ 29 au (Bernardinelli et al 2021b)
We report here on such measurements based on the detection of thermal emission with the Atacama Large Millimeter Array (ALMA) and the application of the radiometric technique
We find D = 137±17 km 3 and pR = 5.3±1.2 %, where error bars are dominated by model errors – and principally by the uncertainty on ǫr
Summary
Comet C/2014 UN271 (Bernardinelli-Bernstein) (hereafter 2014 UN271 for brevity) was discovered as part of the search for outer Solar System objects with the Dark Energy Survey (DES) performed over the period 2013–2019 (Bernardinelli et al 2021a). 2014 UN271 was observed in 42 DES survey images on 25 nights from October 2014 to November 2018, with a heliocentric distance rh ∼ 29 au (Bernardinelli et al 2021b). Comet C/1995 O1 Hale-Bopp was observed to be active and outbound 11 years after perihelion at 25.7 au from the Sun (Szabó et al 2008) and may be even have been at 30.7 au (Szabó et al 2011) Activity in these objects may occur even farther away: modeling of the rh dependence of the dust production rate in C/2017 K2 indicates activity was already present at 35 au, presumably driven by the sublimation of CO or other supervolatile ices (Jewitt et al 2021). Based on Hale-Bopp experience (Biver et al 2002; Rauer et al 2003), detection of CO, CN, and other species in 2014 UN271 may be expected as it heads toward and beyond its ∼ 11 au perihelion in the upcoming years, and a determination of its diameter and albedo is needed to complete its physical characterization. We report here on such measurements based on the detection of thermal emission with the Atacama Large Millimeter Array (ALMA) and the application of the radiometric technique
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