Possible fragmentation of interstellar comet 2I/Borisov
ATel #13613; Bryce T. Bolin (Caltech/IPAC), Dennis Bodewits (Auburn), Carey M. Lisse (JHU-APL), Yanga R. Fernandez (UCF), George Helou (IPAC), and S. B. Cenko (NASA/GSFC)
on 3 Apr 2020; 02:22 UT
Credential Certification: S. Bradley Cenko (brad.cenko@nasa.gov)
Referred to by ATel #: 13618
Images obtained by HST/WFC3 GO programs 16041 (PI Jewitt; ATEL #13611) and 16040 (PI Bolin) on 2020 March 23 and 28 indicate the presence of a small fragment separated by ~0.3 arcsec in the anti-solar direction, west of north, from the coma's optocenter in addition to the bimodal appearance described by Jewitt et al. ATEL #13611. The distance between the optocenter and the fragment changes by 1-2 pixels (0.0395 arcsec/pixel) between the 2020 March 23 and 2020 March 28 observation dates. This implies a lower limit on the relative velocity between the nucleus and the fragment due to projection effects of at least ~0.5 m/s, consistent with the escape velocity from a sub km-scale radius body. The angular separation and motion detected in the 2020 March 23 and 28 data are consistent with a release date in 2020 March 7 around the time of the comet's ~0.7 mag outburst observed by ground-based facilities (Drahus et al. ATel #13549., Jehin et al. 2020, CBET 4729). The fragment is not seen in the most recent HST/WFC3 data taken on 2020 February 24 (GO 16041, PI Jewitt) which were acquired before the comet's outburst. The fragment has an estimated upper size limit of <100 m, assuming a geometric albedo of 0.04, and is confirmed in HST/WFC3 2020 March 30 data taken by HST GO 16087, PI Jewitt. In addition, the post-outburst HST/WFC3 images show the existence of two new jets with position angles of 240 and 310 degrees not observed in previous HST/WFC3 images (Bolin 2020, arxiv:1912.07386).
HST/WFC3 Mosaic of 2I/Borisov from 2020 February 24 to March 30
