Accurate V sin i measurements in M 67: The angular momentum evolution of 1.2 M⊙ stars

Abstract

By using FEROS spectrograph commissioning observations, we build a calibration of the FEROS cross-correlation function (CCF) to determine accurate projected rotational velocities V sin i for slow rotating F-K dwarf and giant stars. We apply this calibration to a sample of 28 main sequence, turnoff and giant stars belonging to the old open cluster M 67. We find that the stars behave in a very regular manner, depending on their position in the Color-Magnitude (C-M) diagram. Early main sequence G stars have a rotational velocity two times larger than the Sun, and they show a possible trend with (B-V) color, in that redder colors correspond to lower V sin i. The stars at the turnoff are the fastest rotators, with V sin i between 6.3 and 7.6 kms-1, while stars just above the turn-off are already significantly slower, with values between 4.6 and 4.9 kms-1. Along the Red Giant Branch (RGB), rotation decreases smoothly and for stars above (B-V) ≳ 1, only upper limits can be found, including for 4 clump stars. Analyzing the angular momentum history of 1.2 M⊙ stars with the help of theoretical evolutionary tracks, we see that these stars probably obey different angular momentum evolution laws on the main sequence and along the RGB: while on the main sequence some extra braking is required in addition to angular momentum conservation, along the RGB the data are well represented by the IΩ = C law. Finally, comparing the V sin i of the M 67 turnoff stars with their main sequence progenitors in the younger open clusters NGC 3680 and Hyades we find that the younger clusters show substantially higher rotation rates. This indicates that 1.2 M⊙ stars do experience main sequence braking. This could be relevant also for the interpretation of the nature of the "Lithium gap".