Improved Baade-Wesselink surface-brightness relations

Recent, and older accurate, data on (limb-darkened) angular diameters is compiled for 221 stars, as well as BVRIJK[12][25] magnitudes for those objects, when available. Nine stars (all M-giants or supergiants) showing excess in the [12-25] colour are excluded in the analysis as this may indicate the present of dust influencing the optical and near-infrared colours as well. Based on this large sample, Baade-Wesselink surface-brightness (SB) relations are presented for dwarfs, giants, supergiants and dwarfs in the optical and near-infrared. M-giants are found to follow different SB-relations from non-M giants, in particular in V-(V-R). The preferred relation for non-M giants are compared to earlier relation by Fouque & Gieren (1997, based on 10 stars) and Nordgren et al. (2002, based on 57 stars). Increasing the sample size does not lead to a lower rms value. It is shown that the residuals do not correlate with metallicity at a significant level. The finally adopted observed angular diameters are compared to those predicted by Cohen et al. (1999) for 45 stars in common, and there is reasonable overall, to good agreement when \theta <6 mas. Finally, I comment on the common practice in the literature to average, and then fix, the zero point of the V-(V-K), V-(V-R) and K-(J-K) relations, and then re-derive the slopes. Such a common zero point at zero colour is not expected from model atmospheres for the (V-R) colour and depends on gravity. Relations derived in this way may be biased.Comment: accepted for publication in the MNRA

Theoretical Limb Darkening for Classical Cepheids: II. Corrections for the Geometric Baade-Wesselink Method

The geometric Baade-Wesselink method is one of the most promising techniques for obtaining a better calibration of the Cepheid period-luminosity relation by means of interferometric measurements of accurate diameters. In this paper we present new wavelength- and phase-dependent limb darkening corrections based on our time-dependent hydrodynamic models of the classical Cepheid zeta Gem. We show that a model simulation of a Cepheid atmosphere, taking into account the hydrodynamic effects associated with the pulsation, shows strong departures from the limb darkening otherwise predicted by a static model. For most of its pulsational cycle the hydrodynamic model predicts a larger limb darkening then the equivalent static model. The hydrodynamics affects the limb darkening mainly at UV and optical wavelengths. Most of these effects evolve slowly as the star pulsates, but there are phases, associated with shocks propagating into the photosphere, in which significant changes in the limb darkening take place on time-scales of the order of less than a day. We assess the implication of our model LD corrections fitting the geometric Baade-Wesselink distance of zeta Gem for the available near-IR PTI data. We discuss the effects of our model limb darkening on the best fit result, and analyze the requirements needed to test the time-dependence of the limb darkening with future interferometric measurements.Comment: 22 pages, 5 figures, to be published on the Astrophysical Journal, June 1 2003 issu