Water production rates and activity of interstellar comet 2I/Borisov

TL;DR

This study monitored interstellar comet 2I/Borisov's water production and activity using UV/Optical observations, revealing rapid post-perihelion decline, high activity fraction, and compositional characteristics distinct from typical comets.

Contribution

First detailed water production rate measurements of 2I/Borisov across perihelion, providing insights into its activity, nucleus size, and composition, and comparing it to known comet behaviors.

Findings

Water production increased before perihelion, then decreased rapidly after.

Nucleus radius constrained to at least 0.37 km.

Comet is carbon-chain depleted and NH₂ enriched.

Abstract

We observed the interstellar comet 2I/Borisov using the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope. We obtained images of the OH gas and dust surrounding the nucleus at six epochs spaced before and after perihelion (-2.56 AU to 2.54 AU). Water production rates increased steadily before perihelion from $(7.0\pm1.5)\times10^{26}$ molecules s$^{-1}$ on Nov. 1, 2019 to $(10.7\pm1.2)\times10^{26}$ molecules s$^{-1}$ on Dec. 1. This rate of increase in water production rate is quicker than that of most dynamically new comets and at the slower end of the wide range of Jupiter-family comets. After perihelion, the water production rate decreased to $(4.9\pm0.9)\times10^{26}$ molecules s$^{-1}$ on Dec. 21, which is much more rapidly than that of all previously observed comets. Our sublimation model constrains the minimum radius of the nucleus to 0.37 km, and indicates an active fraction of at least 55% of the surface. $A(0)f\rho$ calculations show a variation between 57.5 and 105.6 cm with a slight trend peaking before the perihelion, lower than previous and concurrent published values. The observations confirm that 2I/Borisov is carbon-chain depleted and enriched in NH$_2$ relative to water.