The Monitor project: Rotation of low-mass stars in the open cluster M34

Abstract

We report on the results of a V and i-band time-series photometric survey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), achieving better than 1% precision per data point for 13 <~ i <~ 17. Candidate cluster members were selected from a V vs V-I colour-magnitude diagram over 14 < V < 24 (0.12 <~ M/Msun <~ 1.0), finding 714 candidates, of which we expect ~ 400 to be real cluster members (taking into account contamination from the field). The mass function was computed, and found to be consistent with a log-normal distribution in dN/dlogM. Searching for periodic variable objects in the candidate members gave 105 detections over the mass range 0.25 < M/Msun < 1.0. The distribution of rotation periods for 0.4 < M/Msun < 1.0 was found to peak at ~ 7 days, with a tail of fast rotators down to periods of ~ 0.8 days. For 0.25 < M/Msun < 0.4 we found a peak at short periods, with a lack of slow rotators (eg. P >~ 5 days, consistent with the work of other authors (eg. Scholz & Eisloffel 2004) at very low masses. Our results are interpreted in the context of previous work, finding that we reproduce the same general features in the rotational period distributions. A number of rapid rotators were found with velocities ~ a factor of two lower than in the Pleiades, consistent with models of angular momentum evolution assuming solid body rotation without needing to invoke core-envelope decoupling.