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

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 ≥one interstellar object every year. We advocate for future measurements of the production rates of H2O, CO2, and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. 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 CO2 relative to H2O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q(CO)/Q(H2O) < 0.2 and Q(CO)/Q(H2O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. 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 snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.

The Interstellar Interlopers

“…Adopting a nominal equilibrium sublimation rate of water ice at 2 4), and the bottom panel shows production rates of CN, C 2 , C 3 , NH 2 and HCN (Table 5). Adapted from Seligman et al (2022b). a Heliocentric distance at observation.…”

“…The top panel shows production rates of H 2 O, CO and OH (Table4), and the bottom panel shows production rates of CN, C 2 , C 3 , NH 2 and HCN (Table5). Adapted fromSeligman et al (2022b).…”

“…).The composition of the LPC C/2016 R2, 2I/Borisov, and typical carbon enriched and depleted solar system comets. This is a generalized version of an analogous figure inMcKay et al (2019), and adapted fromSeligman et al (2022b). The carbon depleted comet is representative of many of the solar system comets for which production rate measurements of CO 2 , CO and H 2 O exist (see Table1inSeligman et al (2022b)).…”

“…This is a generalized version of an analogous figure inMcKay et al (2019), and adapted fromSeligman et al (2022b). The carbon depleted comet is representative of many of the solar system comets for which production rate measurements of CO 2 , CO and H 2 O exist (see Table1inSeligman et al (2022b)). The carbon enriched comet is W3 Christensen(Ootsubo et al 2012).…”

Synthetic Population of Interstellar Objects in the Solar System

“…Measurements of production rates of CO 2 , CO and H 2 O of future interstellar comets can provide constraints on their primordial C/O ratios, which can be used as an indicator for their formation location within a protostellar disk. In this way, interstellar comets offer a key insight into mechanisms responsible for cometary ejection in exoplanetary systems (Seligman et al, 2022). ISOs could also be laboratories for examination of the interstellar environment which they passed through, the impact of this environment on their sizes, shapes and rotational states (Vavilov and Medvedev, 2019;Zhou, 2020;Phan et al, 2021), and galactic weathering processes (Jewitt et al, 2017;Fitzsimmons et al, 2018).…”