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Unusually High CO Abundance of the First Active Interstellar Comet

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Unusually High CO Abundance of the First Active Interstellar Comet Unusually High CO Abundance of the First Active Interstellar Comet Cordiner, M. A.1;2, Milam, S. N.1, Biver, N.3, Bockelee-Morvan,´ D.3, Roth, N. X.1;4, Bergin, E. A.5, Jehin, E.6, Remijan, A. J.7, Charnley, S. B.1, Mumma, M. J.1, Boissier, J.8, Crovisier, J.3, Paganini, L.9, Kuan, Y.-J.10;11, Lis, D. C.12 1Solar System Exploration Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA. 2Department of Physics, Catholic University of America, Washington, DC 20064, USA. 3LESIA, Observatoire de Paris, Universite´ PSL, CNRS, Sorbonne Universite,´ Universite´ de Paris, 5 place Jules Janssen, 92195 Meudon, France. 4Universities Space Research Association, Columbia, MD 21046, USA. 5Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109, USA. 6STAR Institute, Universite´ de Liege,` Allee´ du 6 Aout, 19C, 4000 Liege,` Belgium. 7National Radio Astronomy Observatory, Charlottesville, VA 22903, USA. 8IRAM, 300 Rue de la Piscine, 38406 Saint Martin d’Heres, France. 9NASA Headquarters, Washington, DC, United States of America. 10National Taiwan Normal University, Taipei 116, Taiwan, ROC. 11Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106, Taiwan, ROC. 12Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. Accepted for publication in Nature Astronomy, March 2020 Comets spend most of their lives at large dis- outer regions of a distant protoplanetary accretion tances from any star, during which time their in- disk, as part of a population of small, icy bodies terior compositions remain relatively unaltered. analogous to our Solar System’s own proto-Kuiper Cometary observations can therefore provide di- Belt. rect insight into the chemistry that occurred dur- During the last few decades, remote and in situ mea- ing their birth at the time of planet formation.1 surements of volatiles and dust in comets belonging To-date, there have been no confirmed observa- to our own Solar System have revealed a wealth of tions of parent volatiles (gases released directly information on the chemical inventory and physical from the nucleus) of a comet from any plan- processes that occurred during the formation of our etary system other than our own. Here we planets, 4.5 billion years ago1. The chemical condi- present high-resolution, interferometric observa- tions prevalent during planet formation around other tions of 2I/Borisov, the first confirmed interstel- stars have become accessible for some nearby systems, lar comet, obtained using the Atacama Large Mil- thanks to advances in astronomical remote sensing limeter/submillimeter Array (ALMA) on 15th-16th methods2–4, but this work is hindered by the extreme December 2019. Our observations reveal emis- difficulty of observing ice and gas in the dense, opaque sion from hydrogen cyanide (HCN), and carbon mid-planes of the disks where extrasolar planets form. monoxide (CO), coincident with the expected posi- Measurements of cometary compositions provide a tion of 2I/Borisov’s nucleus, with production rates unique method for probing the composition of the disk Q(HCN) = (7:0 ± 1:1) × 1023 s−1 and Q(CO) = mid-plane, thus providing crucial input for theories (4:4 ± 0:7) × 1026 s−1. While the HCN abundance concerning protoplanetary disk chemical evolution5, 6, relative to water (0.06–0.16%) appears similar to and helping to improve our understanding of the in- that of typical, previously observed comets in our gredients available for forming planetary bodies and Solar System, the abundance of CO (35–105%) is their atmospheres7, 8, 11. Observations of the interstel- among the highest observed in any comet within lar comet 2I/Borisov provide unique new insights into 2 au of the Sun. This shows that 2I/Borisov must the chemistry that occurred during planet formation have formed in a relatively CO-rich environment in another stellar system. — probably beyond the CO ice-line in the very cold, Comet 2I/Borisov (initially designated C/2019 Q4), Unusually High CO Abundance of the First Active Interstellar Comet was detected on 2019 August 30 by amateur as- 2I/Borisov HCN J=4-3 2019/12/15+16 tronomer G. Borisov, and later shown to be on a hyper- 6 bolic orbit (with eccentricity of 3.36), consistent with 6000 an interstellar origin in the direction of Cassiopeia12. Although the apparition of interstellar objects (ISOs) 4000 5 ) was predicted for decades9, 2I/Borisov is only the sec- 1 m 2000 a ond confirmed ISO. The first — 1I/’Oumuamua — was 4 e b discovered in October 2017 when it was already leaving 1 s 0 m the Solar System, making detailed studies difficult. As- 3 k y J tronomical observations were able to partially constrain Distance (km) m 2000 ( 13 x ’Oumuamua’s shape and surface colors , and possible u 2 l F outgassing activity was implied by non-gravitational 4000 acceleration10, but a lack of gas spectroscopic detec- 1 tions made a cometary classification for 1I/’Oumuamua 6000 uncertain. 0 The presence of a gas and dust coma surrounding 6000 4000 2000 0 2000 4000 6000 2I/Borisov, on the other hand, has been confirmed Distance (km) by numerous observations, including the detection of 2I/Borisov CO J=3-2 2019/12/15+16 cyanide (CN) and hydroxyl (OH) emission in the near- 14–16 8 UV and optical . The observations reported to-date 6000 have revealed a CN/H2O ratio and optical dust colours 7 consistent with typical Solar System comets, while the 4000 ) 17, 18 1 C2 abundance may be somewhat depleted . The OH 6 m 2000 a radical is believed to be a product of water photodis- e b 5 sociation in the coma, but the origin of 2I/Borisov’s 1 s 0 CN is less certain, as cometary CN can be produced m 4 k y from the photodissociation of HCN and other nitriles, J Distance (km) m 2000 ( 19 3 and from the degradation of organic-rich dust grains . x u l In contrast, observations of rotational emission in the F 4000 2 microwave and sub-mm part of the spectrum20 can Trail reveal parent volatiles — sublimating gases that have 6000 1 been stored (as ices) inside 2I/Borisov’s nucleus, for Sun 0 the duration of its interstellar journey — and therefore 6000 4000 2000 0 2000 4000 6000 provide a more direct probe of its chemical composi- Distance (km) tion. We used the Atacama Large Millime- Figure 1: Spectrally integrated flux maps of HCN (J = 4 − 3) ter/submillimeter Array (ALMA) to obtain spectra and CO (J = 3−2) emission from interstellar comet 2I/Borisov. and images of 2I/Borisov in the frequency range Positive flux contours are shown with dashed lines, and nega- 342.9-355.6 GHz (0.84-0.87 mm), during its passage tive contours are dotted, with a contour spacing equal to 2.5 through the inner Solar System. The comet reached times the RMS noise. FWHM of the spatial resolution element (1:000 × 0:700 ≈ 1500 × 1000 km) is indicated lower-left. Sky- perihelion on 2019 December 8th, and our obser- projected solar and orbital trail vectors are shown lower-right. vations were carried out over two sessions on 2019 Coordinate axes are aligned with the equatorial system, with December 15th and 16th, at a heliocentric distance the origin at the HCN peak. rH = 2:01 au and a geocentric distance ∆ = 1:96 au. Further details of the observations and data analysis procedures are presented in the Methods section. Molecular production rates (Q) were obtained by The targeted emission lines of HCN (J = 4 − 3) and performing nonlinear least-squares fits to the extracted CO (J = 3 − 2) were both detected at the center of spectra, shown in Figure 2, using a three-dimensional the ALMA field of view (see Figure 1). Spectrally in- radiative transfer model (see Methods). This resulted tegrated fluxes are given in Table 1, and correspond in Q(CO) = (4:4 ± 0:7) × 1026 s−1 and Q(HCN) = to a statistical significance of 5:2σ for HCN and 5:9σ (7:0 ± 1:1) × 1023 s−1, assuming CO and HCN were for CO. Our HCN and CO detections are robust, but well mixed, with a constant kinetic temperature of the detailed coma morphology in Figure 1 is largely Tkin = 50 K. For CO, the spectral line profile (with hidden by noise; HCN shows a central peak, slightly ex- FWHM = 1:1±0:1 km s−1) is consistent with an isotrop- tended in the north-south direction, while an extended ically expanding coma, with best-fitting outflow veloc- −1 flux component is detectable for CO, at distances up ity vout = 0:47±0:04 km s . HCN exhibits a narrower to 4000 km from the nucleus. Our observations also spectral line shape (with FWHM = 0:57±0:09 km s−1), targeted CS and CH3OH, but these molecules were not that could arise as a result of localised HCN outgassing detected. in the form of a jet-like stream primarily confined to Page 2 of 9 Unusually High CO Abundance of the First Active Interstellar Comet 15 HCN 4 3 observation CO 3 2 autocorrelation spectrum Isotropic Model Isotropic Model (v =0.45 km/s) 12 out Jet Model 300 Hemispheric Model (vout(1)=0.49; vout(2)=0.26 km/s) 9 ) 1 ) m 6 1 a 200 e b m a y 3 J e b m ( y J x 0 u m 100 l ( F 3 x u l F 6 0 9 12 CO 3 2 observation Isotropic Model 100 9 8 6 4 2 0 2 4 6 8 Cometocentric Velocity (km/s) ) 1 6 m a Figure 3: ALMA autocorrelation (total power) spectrum of e 3 b y 2I/Borisov’s CO, averaged over all 42 antennas (observed UT J m ( 0 2019-12-15).
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