An Upper Bound to the Space Density of Interstellar Comets

TL;DR

This paper uses white dwarf observations to place an upper limit on the density of interstellar comets, suggesting they are less common than some models predict, based on hydrogen accretion evidence.

Contribution

It provides a novel method to constrain interstellar comet density using hydrogen measurements in white dwarf atmospheres.

Findings

Interstellar comets contain less than 1% of interstellar oxygen.

Most stars likely produce fewer comets than optimistic models suggest.

White dwarf hydrogen levels imply low interstellar comet impact rates.

Abstract

Two well-studied white dwarfs with helium-dominated atmospheres (DBs) each possess less hydrogen than carried by a single average-mass comet. Plausibly, the wind rates from these stars are low enough that most accreted hydrogen remains with the star. If so, and presuming their nominal effective temperatures, then these DBs have been minimally impacted by interstellar comets during their 50 Myr cooling age; interstellar iceballs with radii between 10 m and 2 km contain less than 1% of all interstellar oxygen. This analysis suggests that most stars do not produce comets at the rate predicted by "optimistic" scenarios for the formation of the Oort cloud.