Determining stellar macroturbulence using asteroseismic rotational velocities from Kepler

Abstract

The Rossiter-McLaughlin effect observed for transiting exoplanets often requires prior knowledge of the stellar projected equatorial rotational velocity. This is usually provided by measuring the broadening of spectral lines, however this method has uncertainties as lines are also broadened by velocity fields in the stellar photosphere known as macroturbulence. We have estimated accurate rotational velocity values from asteroseismic analyses of main sequence stars observed by Kepler. The rotational frequency splittings of the detected solar-like oscillations of these stars are determined largely by the near-surface rotation. These estimates have been used to infer the macroturbulence values for 28 Kepler stars. Out of this sample, 26 stars were used along with the Sun to obtain a new calibration between macroturbulence, effective temperature and surface gravity. The new calibration is valid for the temperature range 5200 to 6400 K and the gravity range 4.0 to 4.6 dex. A comparison is also provided with previous macroturbulence calibrations. As a result of this work, macroturbulence, and thus rotational velocity, can now be determined with confidence for stars that do not have asteroseismic data available. We present new spectroscopic rotational velocity values for the WASP planet host stars, using high resolution HARPS spectra.