PULSATIONAL PARALLAXES AND CALIBRATION OF THE COSMIC DISTANCE SCALE BY CEPHEID VARIABLE-STARS

The recent empirical calibration of surface brightness colour (Fv, V - K)-relation for supergiants has been applied to Cepheid variable stars. Taking advantage of the V, K photometry suitable for inferring precise stellar angular sizes, an accurate period-radius (PR)-relation has been calibrated through Cepheids in the Large Magellanic Cloud (LMC) and Baade-Wesselink radii (BW) of galactic Cepheids. In the range of periods 0.9 < log P < 2.0 the relation is found to be: log[R] = 1.139+0.716 log P with uncertainties of +/-0.010 in the zero-point and slope. It enables the Cepheid radius to be estimated from observable period with accuracy of about 4% in good agreement with theoretical results to within this error. The relation is then applied for determining true distance moduli to any Cepheid with available P, V, K data. The major achievement is a reddening-free formulation of the cosmic distance scale calibration. The approach is also expected to be largely insensitive to abundance effects as well as to stellar gravities. The derived true distance modulus to LMC is 18.68+/-0.07. By using the improved knowledge of the infrared interstellar absorption achievable from the (Fv, V - K)-calibration the distance scale has also been calibrated by the classical period-luminosity (PL)-relation. The PL distance to LMC is found to be 18.69+/-0.08 in fairly good agreement with the previous one.