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 ∼1mm, 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 > ∼200kgs 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)×1026 s−1. We found that the CO/H2O 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. Planetary systems are born out of interstellar clouds of gas and dust grains, where dust plays an important role in radiative cooling of collapsing clouds as well as serving as seeds for condensation and accretion of the building blocks of planetary bodies. Dust particles are rich in information on their formation and evolution history, such as the transport and collisional processes in protoplanetary disks. However, it is nearly impossible to perform in depth investigations on the dust properties around other stars due to the large distances and faintness of these systems. Interstellar objects (ISOs) are planetesimals, the building blocks of planets, kicked out of their native planetary systems1. Some of these interstellar wanderers eventually pass through our Solar system, providing us rare opportunities to characterize exo-planetesimals in unprecedented detail. The first ISO, 1I/‘Oumuamua, was discovered in 2017 and exhibited a point-like appearance with no sign of cometary activity2, 3. In contrast, the second ISO, 2I/ Borisov, which was discovered in August 2019, unambiguously exhibited a coma and tail upon discovery4. The detection of typical cometary emissions such as CN (ref.5) and C2 (ref.6) makes it the first obviously active extrasolar comet ever detected in our Solar system. In addition, this comet is rich in supervolatile CO7, 8, regardless of its small nucleus9, 10, which indicates that the nucleus of 2I/Borisov is likely to be pristine. Solar system comets consist of ices and dust, where the major ice species are H2O, CO, and CO2 (ref.11) and the dust generally consists of silicates, oxides, and sulfides, as well as high-molecular weight refractory organics and amorphous carbon materials12, 13. When a comet enters the inner solar system, a coma of dust and gas and/or tails of dust and plasma begin to develop around the nucleus due to sublimation of surface ices in the heat of the Sun and the embedded dust particles are dragged out by the expanding gas14. In recent years, in-situ observations of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by the ESA Rosetta spacecraft greatly enhanced our understanding of comets. The nucleus of 67P is thought to be a primordial rubble pile15 and its dust particles have an irregular, fluffy structure13, with sizes varying widely from ∼1 μm to nearly 1 m (ref.16, 17). Rosetta measurements suggest that comets in our Solar system formed in a wide region beyond proto-Neptune and were scattered by giant planet migration to their present reservoirs14. We obtained high angular resolution continuum observations of 2I/Borisov with ALMA in Bands 3 and 4 (2019 Dec. 03) and Bands 6 and 7 (2019 Dec. 02), respectively, which is less than a week before the comet reached perihelion at rh = 2.007au on 2019 December 8. The ALMA dust continuum images of 2I/Borisov from 850 μm to 2.84mm are shown in Fig. 1. We detected the dust continuum emission