Capture of exocomets and the erosion of the Oort cloud due to stellar encounters in the Galaxy

TL;DR

This study models how stellar encounters in the Galaxy can erode the Oort cloud and transfer exocomets into it, revealing that such captures are rare and that the OC has lost a significant portion of its original mass over time.

Contribution

It provides the first detailed simulations of exocomet capture and mass loss in the Oort cloud due to stellar flybys, considering various encounter parameters.

Findings

Exocomets are captured only during slow, close flybys, which are extremely rare.

The Oort cloud has lost 25-65% of its mass over the Sun's lifetime due to stellar encounters.

The fraction of exocomets in the OC is estimated to be between 10^{-5} and 10^{-4}.

Abstract

The Oort cloud (OC) probably formed more than 4$\,$Gyr ago and has been moving with the Sun in the Galaxy since, exposed to external influences, most prominently to the Galactic tide and passing field stars. Theories suggest that other stars might posses exocomets distributed similarly to our OC. We study the erosion of the OC and the possibility for capturing exocomets during the encounters with such field stars. We carry out simulations of flybys, where both stars are surrounded by a cloud of comets. We measure how many exocomets are transferred to the OC, how many OC's comets are lost, and how this depends on the other star's mass, velocity and impact parameter. Exocomets are transferred to the OC only during relatively slow ($\lesssim0.5\,$km$\,$s$^{-1}$) and close ($\lesssim10^5\,$AU) flybys and these are expected to be extremely rare. Assuming that all passing stars are surrounded by a cloud of exocomets, we derive that the fraction of exocomets in the OC has been about $10^{-5}$--$10^{-4}$. Finally we simulate the OC for the whole lifetime of the Sun, taking into account the encounters and the tidal effects. The OC has lost 25--65% of its mass, mainly due to stellar encounters, and at most 10% (and usually much less) of its mass can be captured. However, exocomets are often lost shortly after the encounter that delivers them, due to the Galactic tide and consecutive encounters.