Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations

Creators: Bolin, Bryce T.; Lisse, Carey M.; Kasliwal, Mansi M.; Quimby, Robert M.; Tan, Hanjie; Copperwheat, Chris M.; Lin, Zhong-Yi; Morbidelli, Alessandro; Abe, Lyu; Bendjoya, Philippe; Burdge, Kevin B.; Coughlin, Michael; Fremling, Christoffer; Itoh, Ryosuke; Koss, Michael; Masci, Frank J.; Maeno, Syota; Mamajek, Eric E.; Marocco, Federico; Murata, Katsuhiro; Rivet, Jean-Pierre; Sitko, Michael L.; Stern, Daniel; Vernet, David; Walters, Richard; Yan, Lin; Andreoni, Igor; Bhalerao, Varun; Bodewits, Dennis; De, Kishalay; Deshmukh, Kunal P.; Bellm, Eric C.; Blagorodnova, Nadejda; Buzasi, Derek; Cenko, S. Bradley; Chang, Chan-Kao; Chojnowski, Drew; Dekany, Richard; Duev, Dmitry A.; Graham, Matthew; Jurić, Mario; Kulkarni, Shrinivas R.; Kupfer, Thomas; Mahabal, Ashish; Neill, James D.; Ngeow, Chow-Choong; Penprase, Bryan; Riddle, Reed; Rodriguez, Hector; Smith, Roger M.; Rosnet, Philippe; Sollerman, Jesper; Soumagnac, Maayane T.

Abstract

We present visible and near-infrared (NIR) photometric and spectroscopic observations of interstellar object (ISO) 2I/Borisov taken from 2019 September 10 to 2019 December 20 using the GROWTH, the Apache Point Observatory Astrophysical Research Consortium 3.5 m, and the NASA Infrared Telescope Facility 3.0 m combined with pre- and postdiscovery observations of 2I obtained by the Zwicky Transient Facility from 2019 March 17 to 2019 May 5. Comparison with imaging of distant solar system comets shows an object very similar to mildly active solar system comets with an outgassing rate of ∼10²⁷ mol s⁻¹. The photometry, taken in filters spanning the visible and NIR range, shows a gradual brightening trend of ~0.03 mag day⁻¹ since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The light curve from recent and prediscovery data reveals a brightness trend suggesting the recent onset of significant H₂O sublimation with the comet being active with super volatiles such as CO at heliocentric distances >6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 4, we estimate a diameter for 2I's nucleus of ≾ 1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size distribution, resulting in a slope of ~3.4 ± 1.2, similar to solar system comets and bodies produced from collisional equilibrium.

Additional Information

© 2020. The American Astronomical Society. Received 2019 November 5; revised 2020 May 7; accepted 2020 May 12; published 2020 June 17. We would like to thank the anonymous reviewers for helpful comments in substantially improving the manuscript. This work was supported by the GROWTH project, funded by the National Science Foundation under PIRE grant No. 1545949. Our work includes observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium. We thank the Director (Nancy Chanover) and Deputy Director (Ben Williams) of the Astrophysical Research Consortium (ARC) 3.5 m telescope at Apache Point Observatory for their enthusiastic and timely support of our Director's Discretionary Time (DDT) proposals. We also thank Russet McMillan, Ted Rudyk, Candace Gray, Jack Dembicky, and the rest of the APO technical staff for their assistance in performing the observations on the same day our DDT proposals were submitted. Based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. The ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank Jim Lyke and Carlos Alvarez for guiding the planning and executing the AO observation with the OSIRIS instrument on Keck I. C.-C.N. is thankful for the funding from MoST grant 104-2923-M-008-004-MY5. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. This publication has made use of data collected at Lulin Observatory, partly supported by MoST grant 108-2112-M-008-001. The results presented in this paper are based in part on observations collected with the Liverpool Telescope, which is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. F.M. is supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by the Universities Space Research Association under contract with NASA. The authors would like to thank E. Turner, B. Draine, S. Tremaine, and M. Mac-Low for many useful discussions concerning the nature and provenance of 1I and 2I. In addition, the authors would like to thank R. Jedicke and G. Helou for helpful discussion on the size distribution of comets. We would also like to thank J. Bauer and Y. Fernandez for fruitful discussion on comet nuclei. The authors would like to thank the astrophysics masters students of the Université Côte d'Azur who recorded the data taken by the C2PU telescope at Observatoire de la Côte d'Azur, Calern observing site. Facilities: Astrophysical Research Consortium 3.5 m, C2PU Omicron telescope, Bisei Observatory 101 cm, Lulin Optical Telescope, Keck:I (LRIS, OSIRIS), Liverpool Telescope, Mount Laguna Observatory 40 inch, NASA/Infrared Telescope Facility, Zwicky Transient Facility, SED Machine.

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