Rotational velocities of nearby young stars

Abstract

<i>Context. <i/>Stellar rotation is a crucial parameter driving stellar magnetism, activity and mixing of chemical elements. Measuring rotational velocities of young stars can give additional insight in the initial conditions of the star formation process. Furthermore, the evolution of stellar rotation is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are believed to be responsible for rotational deceleration during the accretion phase, and rotational spin-up during the contraction phase after decoupling from the disk for fast rotators arriving at the ZAMS. On the ZAMS, stars get rotationally braked by solar-type winds.<i>Aims. <i/>We investigate the projected rotational velocities <i>v<i/> <i>i<i/> of a sample of young stars with respect to the stellar mass and disk evolutionary state to search for possible indications of disk-braking mechanisms. Furthermore, we search for signs of rotational spin-up of stars that have already decoupled from their circumstellar disks.<i>Methods. <i/>We analyse the stellar spectra of 220 nearby (mostly <i><<i/>100 pc) young (2–600 Myr) stars for their <i>v<i/> <i>i<i/>, stellar age, H<i>α<i/> emission, and accretion rates. The stars have been observed with FEROS at the 2.2 m MPG/ESO telescope and HARPS at the 3.6 m telescope in La Silla, Chile. The spectra have been cross-correlated with appropriate theoretical templates. We build a new calibration to be able to derive <i>v<i/> <i>i<i/> values from the cross-correlated spectra. Stellar ages are estimated from the equivalent width at 6708 Å. The equivalent width and width at 10% height of the H<i>α<i/> emission are measured to identify accretors and used to estimate accretion rates . The is then analysed with respect to the evolutionary state of the circumstellar disks to search for indications of disk-braking mechanisms in accretors.<i>Results. <i/>We find that the broad <i>v<i/> <i>i<i/> distribution of our targets extends to rotation velocities of up to more than 100 km s<sup>-1<sup/> and peaks at a value of 7.8 <i>±<i/> 1.2 km s <sup>-1<sup/>, and that ~70% of our stars show <i>v<i/> <i>i<i/> <i><<i/> 30 km s<sup>-1<sup/>. Furthermore, we can find indications for disk-braking in accretors and rotational spin-up of stars which are decoupled from their disks. In addition, we show that a number of young stars are suitable for precise radial-velocity measurements for planet-search surveys.