Seminaria

"Geometrical distances of pulsating stars. Naughty projection factor"

Piotr Wielgórski (CAMK PAN, Warsaw)

Radially pulsating stars, like Cepheids (Classical, Type II) and RR Lyrae are widely used as precise distance indicators thanks to their period-luminosity relations which serve as a standard candle. However, there is another, semi-geometrical method which can be used to derive distances of such stars, known as the Baade-Wesselink or parallax-of-pulsation technique. In this method, change of the angular diameter of the star, which is calculated from photometric or interferometric measurements, is compared to the physical radius displacement obtained from integrating the pulsational velocity curve (velocity of the atmosphere of the star during pulsations). Pulsational velocity can be obtained from radial velocity measured from doppler shift of absorption lines in spectra once the so-called projection factor (p-factor) is known. The value of this parameter is primarily the result of the geometrical projection of the pulsational velocity of different parts of the stellar disk onto the line of sight, but factors like limb darkening, velocity gradient in the atmosphere etc. modify the value of p-factor. Precision of the Baade-Wesselink method is currently limited to ~5-10% due to poor knowledge of this parameter. During my talk I will present the efforts of the Araucaria group in calibrating the projection factor.