Oort Cloud Formation and Evolution in Star Clusters

TL;DR

This study uses N-body simulations to investigate how the timing of maximum Oort cloud mass relative to star cluster escape influences comet retention, revealing a logarithmic relationship between cluster residence time and comet stripping.

Contribution

It introduces a detailed simulation framework analyzing the impact of cluster escape time and maximum cloud mass timing on Oort cloud survival, highlighting a new logarithmic relationship.

Findings

Longer cluster residence leads to increased comet stripping.

Retention rates vary with the timing of maximum cloud mass.

A logarithmic model describes the fraction of comets stripped.

Abstract

It is unknown if an Oort cloud reaches its maximum mass within its star's birth cluster or millions of years later. Complicating the Oort cloud evolution process is the fact that comets can be stripped from orbit due to perturbations from passing stars. We explore how a star's cluster escape time (t$_{ \rm esc}$) and the time its Oort cloud reaches maximum mass (t$_{ \rm max}$) affect the Oort cloud's ability to survive via $N$-body simulations. In a 14 M$_\odot$/pc$^3$ cluster, we identify 50 stars of 1 M$_\odot$ with a range of t$_{ \rm esc}$ to host Oort clouds, each with 1000 comets at t$_{ \rm max}$. For each host, we consider Oort clouds that reach maximum mass 0, 50, and 250 Myr after the cluster's formation. Each Oort cloud's evolution is simulated in the cluster from t$_{ \rm max}$ to t$_{ \rm esc}$. Only a fraction of comets tend to remain in orbit, with this amount depending on t$_{ \rm max}$ and t$_{ \rm esc}$. We observe that 12%, 22%, and 32% of Oort clouds with a t$_{ \rm max}$ of 0, 50 and 250 Myr retain >50% of their comets at t$_{ \rm esc}$, respectively. We find that the fraction of comets stripped has the relationship, $\rm f=m\log_{10}(\frac{t_{ \rm esc}-t_{ \rm max}}{Myr})$ where m = 0.32$\pm$0.04, indicating that the longer the Oort cloud remains in the cluster, the more comets are stripped, with this fraction increasing logarithmically at approximately the same rate for each t$_{ \rm max}$.