Direct calibration of the Cepheid period-luminosity relation

Abstract

After the first release of Hipparcos data, Feast & Catchpole gave a new value for the zero-point of the visual Cepheid period-luminosity relation, based on trigonometric parallaxes. Because of the large uncertainties on these parallaxes, the way in which individual measurements are weighted is of crucial importance. We therefore conclude that the choice of the best weighting system can be aided by a Monte Carlo simulation. On the basis of such a simulation, it is shown that (i) a cut-off in π or in σ_ππ introduces a strong bias; (ii) the zero-point is more stable when only the brightest Cepheids are used; and (iii) the Feast & Catchpole weighting gives the best zero-point and the lowest dispersion. After correction, the adopted visual period-luminosity relation is <M_V>=-2.77logP-1.44+/-0.05. Moreover, we extend this study to the photometric I band (Cousins) and obtain <M_I>=-3.05logP-1.81+/-0.09.