Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov

Abstract

The interstellar traveler, 2I/Borisov, is the first clearly active extrasolar comet, ever detected in our Solar system. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA), and multi-color optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact "pebbles" of radii exceeding ∼ 1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass loss rate of > ∼ 200 kg s −1 and a dust-to-gas ratio > ∼ 3. Our long term monitoring of 2I/Borisov with VLT indicates a steady dust mass loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide gas (CO) with ALMA and derived the gas production rate of Q(CO) = (3.3 ± 0.8) × 10 26 s −1 . We found that the CO/H 2 O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov's home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO.

The Volatile Carbon-to-oxygen Ratio as a Tracer for the Formation Locations of Interstellar Comets

“…This confirmed the existence of a galactic population of interstellar objects with spatial number densities of order n o ∼ 1-2 × 10 −1 au −3 (Trilling et al 2017;Laughlin & Batygin 2017;Jewitt et al 2017;Rafikov 2018b;Zwart et al 2018;Do et al 2018;. Object 2I exhibited a dusty coma (Jewitt & Luu 2019;Bolin et al 2020b;Fitzsimmons et al 2019;Ye et al 2020;McKay et al 2020;Guzik et al 2020;Hui et al 2020;Kim et al 2020;Cremonese et al 2020;Yang et al 2021) with typical cometary carbon-and nitrogen-bearing species detected (Opitom et al 2019;Kareta et al 2020;Lin et al 2020;Bannister et al 2020;Xing et al 2020;Aravind et al 2021). It was enriched in CO relative to H 2 O (Bodewits et al 2020;Cordiner et al 2020), indicating formation exterior to the CO snow line in its original protoplanetary disk (Price et al 2021).…”

On The Spin Dynamics of Elongated Minor Bodies with Applications to a Possible Solar System Analogue Composition for `Oumuamua

“…Moreover, Borisov's dust was consistent with grains mostly larger than 100 µm (Kim et al 2020). Yang et al (2021) presented VLT and ALMA observations that showed the presence of 1mm size pebbles in the coma. They measured a CO production rate of Q(CO) = 3.3 ± 0.8 × 10 26 s −1 and found that the CO/H 2 O mixing ratio changed drastically before and after perihelion, implying that the comet's home system experienced efficient radial mixing.…”

The Population of Interstellar Objects Detectable with the LSST and Accessible for In Situ Rendezvous with Various Mission Designs

“…In stark contrast to 'Oumuamua, the interstellar comet Borisov displayed physical characteristics that are broadly consistent with the census of cometary bodies in the solar system. It had dusty cometary activity (Fitzsimmons et al 2019;Jewitt & Luu 2019;Cremonese et al 2020;Guzik et al 2020;Hui et al 2020;Kim et al 2020;McKay et al 2020;Ye et al 2020;Bolin et al 2020b;Yang et al 2021), and early observations confirmed the presence of carbon-and nitrogenbased species in the outflow (Opitom et al 2019;Bannister et al 2020;Kareta et al 2020;Lin et al 2020;Xing et al 2020;Aravind et al 2021). It was particularly enriched in CO (Bodewits et al 2020;Cordiner et al 2020), which could be explained if it formed in the outer regions of a protoplanetary disk (Price et al 2021).…”