Turbulence Variations in Cepheids

From radial velocity measurements obtained with a cross-correlation technique, the variation of turbulence during the pulsation cycle is studied for a sample of 40 Cepheids. We will propose a new way to separate classical and s-Cepheids. More complete results will appear in a forthcoming paper (Bersier & Burki 1995)The radial velocities have been measured with the spectrometer CORAVEL (Baranne et al. 1979), whose cross-correlation function (CCF) is fitted with a Gaussian, giving the radial velocity Vr, the width σobs the depth H and the continuum, normalised to 1. The pulsation broadens the lines and thus also the CCF. In the Gaussian approximation one can writewhere σobs is the observed width, σinst is the instrumental width, σpuls is the additional width caused by the pulsational velocity field and σres contains all the other effects (turbulence, rotation, magnetic field, etc.). To be less affected by the noise in the data, a Fourier series has been fitted to each curve of σobs. With numerical simulations, one is able to synthesise the additional Doppler width due to pulsation, with a high accuracy. The instrumental width being well known for CORAVEL, the computation of σres is then straightforward. One then has a curve in phase for σres. From this curve, we determined the maximum residual broadening σmax (observed at or very close to minimum radius), and the width σo that the star would have if it did not pulsate. As shown by Bersier & Burki (1995), σo is slightly higher than the mean value of σres.