The Coevality of Young Binary Systems

TL;DR

This study investigates the coevality of young binary star systems in Taurus-Auriga, finding most are coeval within errors, but a significant tail suggests some systems may have complex formation histories or observational effects.

Contribution

It provides a detailed analysis of the age distribution of binary systems in Taurus-Auriga, revealing a mostly coeval population with a multi-peaked age distribution indicating possible hierarchical multiples or observational biases.

Findings

Most binary systems are coeval within 0.16 dex.

Binary systems wider than 0.7 pc show the full age spread.

Approximately 2/3 of the sample are coeval within errors.

Abstract

Multiple star systems are commonly assumed to form coevally; they thus provide the anchor for most calibrations of stellar evolutionary models. In this paper we study the binary population of the Taurus-Auriga association, using the component positions in an HR diagram in order to quantify the frequency and degree of coevality in young binary systems. After identifying and rejecting the systems that are known to be affected by systematic errors (due to further multiplicity or obscuration by circumstellar material), we find that the relative binary ages, |Delta log(tau)|, have an overall dispersion of sigma~0.40 dex. Random pairs of Taurus members are coeval only to within sigma~0.58 dex, indicating that Taurus binaries are indeed more coeval than the association as a whole. However, the distribution of |Delta log(tau)| suggests two populations, with ~2/3 of the sample appearing coeval to within the errors (sigma~0.16 dex) and the other ~1/3 distributed in an extended tail reaching |Delta log(tau)|~0.4-0.9 dex. To explain the finding of a multi-peaked distribution, we suggest that the tail of the differential age distribution includes unrecognized hierarchical multiples, stars seen in scattered light, or stars with disk contamination; additional followup is required to rule out or correct for these explanations. The relative coevality of binary systems does not depend significantly on the system mass, mass ratio, or separation. Indeed, any pair of Taurus members wider than ~10' (~0.7 pc) shows the full age spread of the association.