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R. J. White, L. A. Hillenbrand (Caltech)
We present a high dispersion optical spectrum of St 34 and identify the system to be a spectroscopic binary with components of similar luminosity and temperature (both M3±0.5). Based on its systemic radial velocity, proper motion, strong accretion signatures, infrared excess and location on an H-R diagram, we conclude that St 34 is a classical T Tauri star and a member of the Taurus-Auriga T Association. Surprisingly, however, neither component of the system shows the Li I 6708 A absorption feature in its spectrum, 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. Comparison with pre-main sequence evolutionary models imply for each component a mass of 0.37 Msun and an isochronal age of 9 Myr, which is much younger than the predicted lithium depletion timescale of ~25 Myr. Although a distance closer than that of Taurus (90 pc versus 140 pc) or a hotter temperature scale could reconcile this discrepancy, similar discrepancies in other systems and the implication of an extremely long-lived accretion disk suggest a possible problem with evolutionary models. Regardless, with an age of at least ~10 Myr, St 34 is one of the oldest accreting T Tauri stars known. The implication is that there may be 10s of non-accreting Taurus members of early M spectral type that, even if identified and spectroscopically observed, have been excluded because they did not pass the lithium test for youth. If this proposed older population is confirmed, it would imply that there has been an extended period of star formation in Taurus. Finally, we speculate based on the high frequency of spectroscopic binaries among old accreting systems like St 34 that the presence of a sub-AU separation companion delays disk dissipation by tidally inhibiting, though not preventing, circumstellar accretion. Planets may therefore have a longer time to form in these circumbinary disks than in circumstellar disks around single stars.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.