FLAMINGOS-2 infrared photometry of 2I/Borisov

TL;DR

This study uses infrared photometry from FLAMINGOS-2 on Gemini South to estimate the size of interstellar comet 2I/Borisov's nucleus, finding it to be approximately 1.5 km, which improves upon previous estimates.

Contribution

First infrared-based size estimate of 2I/Borisov's nucleus, providing a more precise measurement than optical observations affected by coma.

Findings

Nucleus size estimated at 1.5 km

Infrared observations reduce coma effects

Results are more stringent than previous estimates

Abstract

2I/Borisov is the second interstellar object (ISO) after 'Oumuamua (Meech et al. 2017), but differs from 'Oumuamua drastically with its extensive cometary activity. A key ingredient to understand the nature of this comet is its size. However, due to its cometary activity and extended coma in the optical, only rough estimates and upper limits can be made for 2I/Borisov, ranging in a wide spread from 0.7 to 3.8 km (Guzik et al. 2019; Fitzsimmons et al. 2019; Jewitt, & Luu 2019; Bolin et al. 2019). It has been shown that observations at longer wavelengths (i.e. infrared) are less susceptible to the effects of coma, and can provide a better estimate of the size of the comet nucleus (see, e.g., Fern\'andez et al. 2013; Bauer et al. 2017). Here we present an estimate of the nucleus of 2I/Borisov from infrared observations by FLAMINGOS-2 on-board the Gemini South telescope (under Fast Turnaround program GS-2019B-FT-207), and infer a comet nucleus size of 1.5 km, comparable to but more stringent than the estimate from Keck AO imaging by Bolin et al. (2019).