The Astrophysical Journal Letters, 896:L39 (9pp), 2020 June 20 https://doi.org/10.3847/2041-8213/ab99cb © 2020. The American Astronomical Society. All rights reserved. Outburst and Splitting of Interstellar Comet 2I/Borisov David Jewitt1,2, Yoonyoung Kim3 , Max Mutchler4 , Harold Weaver5 , Jessica Agarwal3,6 , and Man-To Hui7 1 Department of Earth, Planetary and Space Sciences, UCLA, Los Angeles, CA 90095-1567, USA; [email protected] 2 Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1547, USA 3 Max Planck Institute for Solar System Research, D-37077 Göttingen, Germany 4 Space Telescope Science Institute, Baltimore, MD 21218, USA 5 The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA 6 Technical University at Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany 7 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA Received 2020 May 20; revised 2020 June 1; accepted 2020 June 5; published 2020 June 19 Abstract We present Hubble Space Telescope observations of a photometric outburst and splitting event in interstellar comet 2I/Borisov. The outburst, first reported with the comet outbound at ∼2.8 au, was caused by the expulsion of solid particles having a combined cross section ∼100 km2 and a mass in 0.1 mm sized particles ∼2×107 kg. The latter corresponds to ∼10−4 of the mass of the nucleus, taken as a sphere of radius 500 m. A transient “double nucleus” was observed on UT 2020 March 30 (about 3 weeks after the outburst),havingacrosssection ∼0.6 km2 and corresponding dust mass ∼105 kg. The secondary was absent in images taken on and before March 28 and in images taken on and after April 3. The unexpectedly delayed appearance and rapid disappearance of the secondary are consistent with an origin through rotational bursting of one or more large (meter-sized) boulders under the action of outgassing torques, following their ejection from the main nucleus. Overall, our observations reveal that the outburst and splitting of the nucleus areminoreventsinvolvinga negligible fraction of the total mass: 2I/Borisov will survive its passage through the planetary region largely unscathed. Unified Astronomy Thesaurus concepts: Comets (280); Aperiodic comets (52); Comet surfaces (2161) 1. Introduction limit to the nucleus radius. Additional HST observations bracketing perihelion were used to more completely character- Comet 2I/Borisov (originally C/2019 Q4 and, hereafter, ize the comet, finding a mass-loss rate in dust ∼35 kg s−1,a simply “2I”) was discovered on UT 2019 August 30. The nucleus obliquity of ∼30°, and an isophotal asymmetry strongly hyperbolic orbit (eccentricity e=3.356) convincingly interpreted as thermally lagged emission (Kim et al. 2020). points to an interstellar origin (Higuchi & Kokubo 2019), The small nucleus renders 2I susceptible to torques induced by presumably beyond the snow line in the protoplanetary disk of sublimation mass loss (Jewitt & Luu 2019; Jewitt et al. 2020a). an unknown star. We do not know how long 2I has been adrift A small (∼0.7 mag) optical outburst was detected in the among the stars. Its discovery offers the opportunity to period UT 2020 March 4–9 and tentatively ascribed to a compare and contrast its properties with those of both solar nucleus-splitting event (Drahus et al. 2020). Here we report system comets and the first detected interstellar object, short-term changes in the near-nucleus region detected from 1I/’Oumuamua (“1I”). HST observations obtained before and after this event. Remarkably, 1I and 2I appear quite different. Whereas 1I appeared inert, 2I closely resembles a typical active solar system comet, with a prominent and persistent dust coma 2. Observations (Jewitt & Luu 2019; Guzik et al. 2020; Jewitt et al. 2020a; Kim et al. 2020; Ye et al. 2020) consisting of nonicy grains (Yang Our observations were obtained under the General Observer et al. 2020) and showing spectral lines indicative of ongoing programs GO 16009, 16041, and 16087. For each program, we sublimation (Fitzsimmons et al. 2019; Bodewits et al. 2020; used the WFC3 charge-coupled device (CCD) camera, which McKay et al. 2020; Xing et al. 2020). Both 1I (Micheli et al. has pixels 0 04 wide, giving a point-spread function (PSF) 2018) and 2I (Hui et al. 2020; Jewitt et al. 2020a) exhibit with a Nyquist-sampled resolution of about 0 08 (corresp- nongravitational accelerations, perhaps caused by recoil forces onding to 145 km at geocentric distance Δ=2.5 au). We read from anisotropic mass loss. Alternative explanations have been out only half of one of the two WFC3 CCDs, giving an advanced for the acceleration of 1I by radiation pressure (Bialy 80″×80″ field of view. The telescope was tracked at the & Loeb 2018; Flekkøy et al. 2019; Moro-Martín 2019). instantaneous nonsidereal rate of the comet (up to about 100″ Comet 2I passed perihelion on UT 2019 December 8 at hr−1) and also dithered to mitigate the effects of bad pixels. We distance q=2.007 au. Hubble Space Telescope (HST) obser- used the broadband F350LP filter in order to maximize the vations taken just before perihelion revealed a small nucleus throughput and hence the signal-to-noise ratios of the data. This fi (effective radius 0.2rn0.5 km) ejecting predominantly lter has peak transmission ∼28%, an effective wavelength submillimeter-size particles from a large fraction of its surface λe∼5846 Å, and an FWHM∼4758 Å. In each HST orbit, we (Jewitt et al. 2020a). In what follows, we adopt rn=0.5 km for obtained six images, each of 260 s duration. The geometric our calculations, while acknowledging that this is an upper parameters of the observations are summarized in Table 1. 1 The Astrophysical Journal Letters, 896:L39 (9pp), 2020 June 20 Jewitt et al. Table 1 Observing Geometry a b c d e f g h i UT Date and Time GO FLTR ΔT rH Δ α θe θ−V δ⊕ 2020 Jan 3 03:22–03:57 16009 F350LP 26 2.086 1.942 23.0 294.5 319,4 −9.0 2020 Jan 29 11:35–12:08 16041 F350LP 52 2.313 2.059 25.2 305.0 304.3 0.2 2020 Feb 24 02:42–03:19 16041 F350LP 78 2.638 2.269 21.6 321.9 2902 7.8 2020 Mar 23 10:17–10:54 16041 F350LP 105 3.072 2.565 17.6 349.9 285.6 12.8 2020 Mar 28 01:19–03:31 16040 Variousj 110 3.148 2.620 17.0 355.2 286.0 13.2 2020 Mar 30 06:00–06:37 16087 F350LP 112 3.184 2.646 16.7 357.8 286.3 13.3 2020 Apr 3 03:14–04:32 16044 F606W 116 3.250 2.695 16.2 2.4 286.8 13.6 2020 Apr 6 00:34–01:10 16088 F350LP 119 3.298 2.731 15.8 5.9 287.3 13.7 2020 Apr 13 16:53–17:34 16087 F350LP 126 3.310 2.740 15.7 6.7 287.4 13.7 2020 Apr 20 15:43–16:13 16087 F350LP 133 3.552 2.932 14.1 23.8 289.4 13.7 Notes. a HST General Observer program number. b Filter employed. c Number of days from perihelion (UT 2019 December 8). d Heliocentric distance, in au. e Geocentric distance, in au. f Phase angle, in degrees. g Position angle of the projected antisolar direction, in degrees. h Position angle of the projected negative heliocentric velocity vector, in degrees. i Angle of Earth above the orbital plane, in degrees. j F438W, F689M, F845M. 3. Results 3.1. Morphology A well-established feature of 2I is that the coma dust particles are large, with radii a0.1 mm (Jewitt & Luu 2019; Jewitt et al. 2020a; Kim et al. 2020), and also slowly moving, with a characteristic speed for 0.1 mm particles v∼9ms−1 (Kim et al. 2020). As a result, the large-scale morphology of the comet changes relatively little with time, and a single-epoch image (Figure 1) suffices to portray the characteristic rounded coma and stubby radiation pressure swept tail. The image was formed by shifting and combining six individual images taken over the ∼45 minute HST observing window. We used a clipped median combination (which rejects the highest pixel data number before computing the median of the remainder) in order to suppress cosmic rays and noise. Faint structures snaking in the background of the figure are the residuals left by imperfect removal of trailed field stars and galaxies. While Figure 1 closely represents the morphology of 2I in the first 6 months since its discovery, subtle changes related to the Figure 1. Comet 2I/Borisov on UT 2020 March 23, with 2″ and 104 km changing observing geometry and, to a lesser extent, to real scale bars. changes in the comet do exist (Kim et al. 2020). Here we focus attention on the near-nucleus morphology of others. Also in Figure 2, the image from April 3 (GO 16044, the comet, in particular on changes occurring within a few PI: Qicheng Zhang) employed both different filters and ×103 km of the nucleus. This central region is shown in polarizers, while that from UT April 6 (GO 16088, PI: Karen Figure 2 in the 2020 January (rH=2.1 au) to April Meech) used F350LP and an exposure sequence more similar (rH=3.6 au) period. In these images we suppressed low to ours. spatial frequency structures by subtracting a Gaussian-con- The most noteworthy feature of Figure 2 is the appearance of volved version (standard deviation σ=0 08) of the image a double or “split” structure on March 30, with a separation from itself.