Rotational velocities of nearby young stars

Abstract

Context. 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. Aims. We investigate the projected rotational velocities v sin 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 spinup of stars that have already decoupled from their circumstellar disks. Methods. We analyse the stellar spectra of 220 nearby (mostly <100 pc) young (2-600 Myr) stars for their v sin i, stellar age, Hα 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 v sin i values from the cross-correlated spectra. Stellar ages are estimated from the Li i equivalent width at 6708 Å.The equivalent width and width at 10% height of the Hα emission are measured to identify accretors and used to estimate accretion ratesṀ acc . The v sin i is then analysed with respect to the evolutionary state of the circumstellar disks to search for indications of disk-braking mechanisms in accretors. Results. We find that the broad v sin i distribution of our targets extends to rotation velocities of up to more than 100 km s −1 and peaks at a value of 7.8 ± 1.2 km s −1 , and that ∼70% of our stars show v sin i < 30 km s −1 . 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.

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs in wide orbits

“…We measured an RV difference of about 18 km s −1 between the components with a line depth for the secondary that is about 30% that of the primary at 6450 Å. Our projected rotational velocity and Li EW are significantly different from those reported in the literature by Wichmann et al (2003) and Weise et al (2010). The first discrepancy might be due to a different RV separation between the components at the time of observation.…”

“…Such a value is not consistent with v sin i and the stellar radius. TYC 7743-1091-1 = HD 99409: (ASAS 112558−4015.8;1SWASP+J112558.28-401549.9) This is a late type star with a moderately strong lithium line, as measured by Wichmann et al (2003); Weise et al (2010). Luminosity class II-III is quoted in the Michigan Spectral Survey Catalog (Houk 1982).…”

HD 135344B: a young star has reached its rotational limit

“…In contrast, for the majority of TTSs only moderate v sin i values are observed, which indicates that a disk-braking mechanism operates during the accretion phase (e.g. Weise et al 2010).…”

“…Typical v sin i values for HAeBe stars are in the range from 60 to 225 km s −1 (Davis et al 1983;Finkenzeller 1985;Boehm & Catala 1995), whereas most TTSs have v sin i values of about 10 km s −1 (e.g. Weise et al 2010). This result indicates that mechanisms of angular momentum dispersal are much less efficient in HAeBe stars than in TTSs (Boehm & Catala 1995).…”

Young stars inϵChamaleontis and their disks: disk evolution in sparse associations

“…TWA-30A may harbor a disk because its optical and infrared spectra show emission lines due to accretion activity (Looper et al 2010a). However, there is no infrared data to suggest whether TWA-30A shows excess emission at wavelengths Gauvin & Strom (1992); (8) Hughes et al (1994); (9) Weise et al (2010). shorter than 8 μm. Thus, we do not count this source when calculating the disk fraction in TW Hya.…”