The Mass and Age of Very Low Mass Members of the Open Cluster Alpha Persei

Abstract

We present spectroscopic optical and photometric infrared observations of 12 faint candidate members of the young open cluster Alpha Persei found by Prosser (1994). Keck HIRES echelle spectra provide radial and rotational velocity measurements for five objects, two of which are clearly nonmembers based on the radial velocities. These kinematic nonmembers also do not fit well in the V-I vs I-J cluster sequence. One additional faint object is likely a nonmember based on a low-resolution spectrum. Using HIRES, we have searched for the Li I resonance line. Combining the absence/presence of lithium and photometry of the faint Alpha Persei targets with confirmed membership constrains their ages and masses. The lack of lithium in AP J0323+4853 implies its age is greater than about 65 Myr, which is older than the cluster classical upper main-sequence turnoff age of 50 Myr. A similar age discrepancy is found in the Pleiades. We detect lithium in the faintest of our program stars, AP270, which implies a mass for it just at the substellar mass limit, given our adopted age and its luminosity. The membership of AP281 is in question because of its high radial velocity compared with the cluster mean. On the other hand AP281 lies on the photometric cluster sequence, and has a very high rotation velocity and H-alpha emission, indicating youth. If a member, its lack of lithium would push the minimum age of the cluster to 75 Myr, in agreement with a very recent upper main-sequence determination. In that case, AP270 would not be a brown dwarf.

Accepted by ApJ. The first lithium detection of a VLM star in Alpha Per

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