A portrait throughout perihelion of the NH$_2$-rich interstellar comet 2I/Borisov

TL;DR

This study provides a comprehensive optical analysis of interstellar comet 2I/Borisov throughout its perihelion, revealing its compositional and morphological characteristics, including a predominantly smooth dust emission and NH$_2$-rich composition.

Contribution

First detailed perihelion observation campaign of 2I/Borisov, revealing its smooth coma morphology and NH$_2$-rich, carbon-depleted composition compared to Solar System comets.

Findings

Dust emission was smooth with no seasonal effects.

Gas production rates declined then increased around outburst.

2I/Borisov is NH$_2$-rich and carbon-depleted.

Abstract

The interstellar comet 2I/Borisov is the first interstellar object where compositional characterisation was possible throughout its entire perihelion passage. We report all 16 epochs of a comprehensive optical observation campaign with ESO VLT's integral field spectrograph MUSE, spanning 126 days from 2019 November 14 to 2020 March 19. The spatial dust emission of 2I/Borisov was predominantly smooth, with no seasonal effect. A jet-like feature was consistently visible. The gas production morphology of its coma was also smooth and similar for C$_2$, NH$_2$, and CN: symmetric around the photocentre. The production rates of these species gently declined into and beyond perihelion, until 2I's outburst and splitting event in early 2020 March. C$_2$, NH$_2$, and CN production rates all increased, with NH$_2$ being the most significant; the dust emission also slightly reddened. 2I/Borisov is a carbon-depleted, relatively NH$_2$-rich comet when compared to those comets yet measured in the Solar System.