The Volatile Carbon to Oxygen Ratio as a Tracer for the Formation Locations of Interstellar Comets

TL;DR

This paper proposes using the carbon to oxygen ratio in interstellar comets as a tracer for their formation locations within protoplanetary disks, emphasizing the importance of future measurements to understand comet origins and ejection mechanisms.

Contribution

It introduces a method to infer formation locations of interstellar comets based on their C/O ratios, accounting for processing effects in the interstellar medium and Solar System.

Findings

High C/O ratio in 2I/Borisov suggests formation outside the CO snowline.

Production rate ratios can indicate formation interior or exterior to the CO snowline.

Measurement facilities are identified for future compositional analysis.

Abstract

Based on the occurrence rates implied by the discoveries of 1I/`Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect $\ge1$ interstellar objects every year (Hoover et al. 2021). We advocate for future measurements of the production rates of H$_2$O, CO$_2$ and CO in these objects to estimate their carbon to oxygen ratios, which traces formation locations within their original protoplanetary disks. We review similar measurements for Solar System comets, which indicate formation interior to the CO snowline. By quantifying the relative processing in the interstellar medium and Solar System, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO$_2$ relative to H$_2$O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of ${\rm Q}({\rm CO})/{\rm Q}({\rm H_2O})<.2$ and ${\rm Q}({\rm CO})/{\rm Q}({\rm H_2O})>1$ likely indicate formation interior and exterior to the CO snowline, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snowline. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snowline. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.