THE COEVALITY OF YOUNG BINARY SYSTEMS

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, vertical bar Delta log tau vertical bar, have an overall dispersion sigma(vertical bar Delta log tau vertical bar) similar to 0.40 dex. Random pairs of Taurus members are coeval only to within sigma(vertical bar Delta log tau vertical bar) similar to 0.58 dex, indicating that Taurus binaries are indeed more coeval than the association as a whole. However, the distribution of vertical bar Delta log tau vertical bar suggests two populations, with similar to 2/ 3 of the sample appearing coeval to within the errors (sigma(vertical bar Delta log tau vertical bar) similar to 0.16 dex) and the other similar to 1/3 distributed in an extended tail reaching vertical bar Delta log tau vertical bar similar to 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 similar to 10 (similar to 0.7 pc) shows the full age spread of the association.