Observation and modelling of main‐sequence stellar chromospheres – VII. Rotation and metallicity of dM1 stars ★

We have measured v sin  i and metallicity from high‐resolution spectroscopic observations of a selected sample of dM1‐type stars. To measure v sin  i , we first selected three template stars known for their slow rotation or their very low activity levels and then cross‐correlated their spectra with those of our target stars. The excess broadening of the cross‐correlation peaks gives v sin  i . For metallicity, we compiled all available measurements from the literature and correlated them with the stellar radius. Provided the parallax is known, this new method allows us to derive metallicities for all our target stars. We measured v sin  i to an accuracy of 2 km s −1 . These values were combined with other measurements taken from the literature. We have detected rotation in seven dM1e stars and 11 dM1 stars and upper limits for 20 other dM1 stars. Our results show that the distribution of the rotation period may be bimodal for dM1 stars, i.e. there are two groups of stars: the fast rotators with P rot ∼ 6 d and the slow rotators with P rot ∼ 24 d. There is a gap between these two groups. We obtained a correlation between metallicity and stellar radius which allows us to derive metallicities for all stars in our sample and more generally for all dM1 stars with [M/H] in the range −1.5 to 0.5 dex, with a reasonable accuracy. We compare this correlation to models and find a significant disagreement in radii. However, the observed shape of the correlation is globally reproduced by the models. We derive the metallicity for 87 M1 dwarfs and subdwarfs.