The Coevality of Young Binary Systems

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, |Δlog τ|, have an overall dispersion σ_(|Δlog τ|) ~0.40 dex. Random pairs of Taurus members are coeval only to within σ_(|Δlog τ|) ~0.58 dex, indicating that Taurus binaries are indeed more coeval than the association as a whole. However, the distribution of |Δlog τ| suggests two populations, with ~2/3 of the sample appearing coeval to within the errors (σ_(|Δlog τ|) ~0.16 dex) and the other ~1/3 distributed in an extended tail reaching |Δlog τ| ~0.4-0.9 dex. To explain the finding of a multipeaked 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 follow-up 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.

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