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

By using FEROS spectrograph commissioning observations, we build a calibrat ion of the FEROS crosscorrelation function (CCF) to determine accurate proj ected rotational velocities V sin i for slow rotating F-K dwarf and giant s tars. We apply this calibration to a sample of 28 main sequence, turnoff an d giant stars belonging to the old open cluster M 67. We find that the star s 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 vel ocity 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) greater than or simil ar to 1, only upper limits can be found, including for 4 clump stars. Analy zing the angular momentum history of 1.2 M-circle dot stars with the help o f theoretical evolutionary tracks, we see that these stars probably obey di fferent 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 represent ed by the I Omega = C law. Finally, comparing the V sin i of the M 67 turno ff stars with their main sequence progenitors in the younger open clusters NGC 3680 and Hyades we find that the younger clusters show substantially hi gher rotation rates. This indicates that 1.2 M-circle dot stars do experien ce main sequence braking. This could be relevant also for the interpretatio n of the nature of the "Lithium gap".