Distance and mass of pulsating stars from multicolour photometry and atmospheric models

For determining distance and mass of pulsating stars a new, purely photometric method is described in which radial velocity observations are not needed. From multicolour photometry the variation of angular diameter is determined in a conventional way by using the surface brightness of the theoretical atmospheric models ATLAS of Kurucz (1997). As a function of phase the following two parameters are introduced in the Navier-Stokes equation: gravity (of the appropriate static ATLAS model), and angular diameter. Distance and mass are derived from phases of standstill. Conclusions are drawn on the hydrodynamic behaviour of the atmosphere. The new method is compared with the Baade-Wesselink (BW) method. As an example the RR Lyrae variable SU Dra is given: using UBV RC photometry, a distance of 647± 16pc, and a mass of 0.66± .03M⊙ were found. In addition to the radius change (4.58 − 5.51)R⊙ radius undulations have been found with amplitude ≈ 0.2R⊙ and period P/5, which are synchronized with the main period P = 0. 66042. The present method has confirmed the distance value from the BW method; thus, in the problem of different distance scales from BW and other methods the short extragalactic distance scale has been bolstered by an independent argument. The mass is some 30 percent larger than the value from the equation of pulsation of van Albada & Baker (1971).