Observation and modelling of main-sequence star chromospheres - XVI. Rotation of dK5 stars

Using two different spectrographs, High Accuracy Radial velocity Planet Search (HARPS) (European Southern Observatory) and SOPHIE (Observatoire de Haute Provence), we have measured v sin i for a sample of dK5 stars. These are the first measurements of v sin i for most of the stars studied here. We measured v sin i to an accuracy of 0.3 km s−1 and a detection limit of about 0.5-1 km s−1. All our targets have similar (R − I)c colour. This is an advantage and facilitates the determination of the narrowest line profiles for v sin i ∼ 0. In our total sample, we detected rotation for 22 stars (three dK5e and 19 dK5 stars), and we did not detect rotation in a further 22 stars. This result shows that there are many dK5 slow rotators, and many more than for dM1 stars. We also report on a newly discovered dK5e star, McC 522, which is also the fastest rotator in our sample. We determine radii and effective temperatures for all our target stars. The effective temperatures were derived using the (R − I)c colour and empirical far-red colour-effective temperature correlations, and the radii were derived from the standard formulae relating M bol, bolometric correction and T eff. We find that the distribution of P/sin i (the projected rotation period) is rather homogeneous, i.e. the distribution of the 22 detected stars as a function of P/sin i is approximately flat and does not show any maximum, unlike for dM1 stars, a close spectral type. We find that the distribution in v sin i is bimodal, as in dM1 stars, with fast dK5e rotators and slower dK5 rotators.