Close encounters of the stellar kind

TL;DR

This study uses advanced orbital integration and Monte Carlo simulations to identify stars that have closely approached or will approach the Sun, impacting the Oort cloud and potentially influencing comet influx into the inner solar system.

Contribution

It provides a comprehensive analysis of stellar encounters with the Sun using improved astrometric data and probabilistic methods, revealing new close encounter candidates and refining previous estimates.

Findings

42 stars within 2 pc of the Sun

Closest encounter with Hip 85605, 0.04-0.20 pc in 240-470 kyr

Potential perturbation by gamma Microscopii 3.8 Myr ago

Abstract

Stars which pass close to the Sun can perturb the Oort cloud, injecting comets into the inner solar system where they may collide with the Earth. Using van Leeuwen's re-reduction of the Hipparcos data complemented by the original Hipparcos and Tycho-2 catalogues, along with recent radial velocity surveys, I integrate the orbits of over 50 000 stars through the Galaxy to look for close encounters. The search uses a Monte Carlo simulation over the covariance of the data in order to properly characterize the uncertainties in the times, distances, and speeds of the encounters. I show that modelling stellar encounters by assuming instead a linear relative motion produces, for many encounters, inaccurate and biased results. I find 42, 14, and 4 stars which have encounter distances below 2, 1, and 0.5 pc respectively, although some of these stars have questionable data. Of the 14 stars coming within 1 pc, 5 were found by at least one of three previous studies (which found a total of 7 coming within 1 pc). The closest encounter appears to be Hip 85605, a K or M star, which has a 90% probability of coming between 0.04 and 0.20 pc between 240 and 470 kyr from now (90% Bayesian confidence interval). However, its astrometry may be incorrect, in which case the closest encounter found is the K7 dwarf GL 710, which has a 90% probability of coming within 0.10-0.44 pc in about 1.3 Myr. A larger perturbation may have been caused by gamma Microscopii, a G6 giant with a mass of about 2.5 Msol, which came within 0.35-1.34 pc (90% confidence interval) around 3.8 Myr ago.