2010MNRAS.407.1657H

We have measured v sin i for a selected sample of dM1-type stars. We give 114 measurements of v sin i for 88 different stars, and six upper detection limits. These are the first measurements of v sin i for most of the stars studied here. This represents the largest sample of v sin i measurements for M dwarfs at a given spectral type. For these measurements, we used four different spectrographs: HARPS (ESO), SOPHIE (OHP), ÉLODIE (OHP) and UVES (ESO). Two of these spectrographs (HARPS and SOPHIE) are particularly stable in wavelength since they were designed for exoplanet searches.

We measured v sin i down to an accuracy of 0.3 km/s for the highest resolution spectrographs and a detection limit of about 1 km/s. We show that this unprecedented accuracy for M dwarfs in our data set is possible because all the targets have the same spectral type. This is an advantage and it facilitates the determination of the narrowest line profiles for v sin i ∼ 0. Although it is possible to derive the zero-point profiles using several spectral types at a time. These values were combined with other measurements taken from the literature. The total sample represents detected rotation for 100 stars (10 dM1e and 90 dM1 stars). We confirm our finding of Paper VII that the distribution of the projected rotation period is bimodal for dM1 stars with a much larger sample, i.e. there are two groups of stars: the fast rotators with P/sin i ∼ 4.5d and the slow rotators with P/sin i ∼ 14.4d. There is a gap between these two groups. We find that the distribution of stars as a function of P/sin i has two very abrupt cuts, below 10d and above 18d. There are very few stars observed out of this range 10-18d. We also observe that the distribution increases slightly from 18 to 10d.

We find that the M1 subdwarfs (very low metallicity dwarfs) rotate with an average period of P/sin i ∼ 7.2d, which is about twice faster as the main group of normal M1 dwarfs. We also find a correlation for P/sin i to decrease with stellar radius among dM1e stars. Such a trend is also observed in dM1 stars.

We also derive metallicity and radius for all our target stars using the same method as in Paper VII. We notably found that 11 of our target stars are subdwarfs with metallicities below -0.5dex.