A Long-lived Accretion Disk around a Lithium-depleted Binary T Tauri Star

Abstract

We present a high-dispersion optical spectrum of St 34 and identify the system as a spectroscopic binary with components of similar luminosity and temperature (both M3 ± 0.5). Based on kinematics, signatures of accretion, and location on an H-R diagram, we conclude that St 34 is a classical T Tauri star belonging to the Taurus-Auriga T association. Surprisingly, however, neither component of the binary shows Li I λ6708 absorption, the most universally accepted criterion for establishing stellar youth. In this uniquely known instance, the accretion disk appears to have survived longer than the lithium-depletion timescale. We speculate that the long-lived accretion disk is a consequence of the sub-AU separation companion tidally inhibiting, although not preventing, circumstellar accretion. Comparisons with pre-main-sequence evolutionary models imply, for each component of St 34, a mass of 0.37 ± 0.08 M_☉ and an isochronal age of 8 ± 3 Myr, which is much younger than the predicted lithium-depletion timescale of ~25 Myr. Although a distance 38% closer than that of Taurus-Auriga or a hotter temperature scale could reconcile this discrepancy at 21-25 Myr, similar discrepancies in other systems and the implications of an extremely old accreting Taurus-Auriga member suggest instead a possible problem with evolutionary models. Regardless, the older age implied by St 34's depleted lithium is the first compelling evidence of a substantial age spread in this region. Additionally, since St 34's coeval comembers with early M spectral types would likewise fail the lithium test for youth, current membership lists may be incomplete.