Direct Calibration of the Cepheid Period-Luminosity relation (Based on data from the ESA HIPPARCOS astrometry satellite)

After the first release of HIPPARCOS data, Feast & Catchpole gave a new value to 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 bears a crucial importance, and the discrepancy they show leads to the conclusion that the choice of the best weighting system can be provided through a Monte-Carlo simulation. On the basis of such a simulation it is shown that: - A cut in \pi or in \sigma_{\pi} / \pi introduces a strong bias. - The zero-point is more stable when only the brightest Cepheids are used. - The Feast & Catchpole weighting gives the best zero-point and the lowest dispersion. After correction, the adopted visual Period-Luminosity relation is: <M_V> = -2.77 log P - 1.44 \pm 0.05. Moreover, we extend this study to the photometric I-band (Cousins) and obtain: <M_I> = -3.05 log P - 1.81 \pm 0.09.