On the Absolute Calibration of the Cepheid Distance Scale Using [ITAL]Hipparcos[/ITAL] Parallaxes

Abstract

The fundamental Hipparcos parallaxes (HIPP) of 219 Cepheids are used for the absolute calibration of the Galactic distance scale sampled by a modern Baade-Wesselink (BW) distance indicator, which reliably accounts for pulsation and thermal properties of Cepheid variable stars. Notably we map thermal properties into the Johnson-Cousins color (V-I). The BW realization is found to be much less affected than previously adopted optical luminosity laws by intrinsic scatter and systematic errors in representing individual Cepheid distances and thus is best suited for a calibration of the galactic distance scale using the fundamental Hipparcos parallaxes (HIPP). Comparisons between the actual Hipparcos calibration and three independent ground-based calibrations of the same BW distance scale show very close agreement at the 0.04 mag level, i.e., at the 1 σ level of the absolute accuracy claimed for BW realizations, although the Hipparcos calibration is affected by an uncertainty of ±0.10 mag due to propagation of parallax errors alone. Comparisons include the zero-age main-sequence calibration by Cepheids in clusters (Pleiades distance modulus at 5.57 mag), the calibration by pulsation parallaxes of Cepheids, and the calibration by updated model calculations of synthetic stellar spectra of Cepheids. Notably, the resulting galactic distance scale is found to be ∼0.1 mag shorter than the value obtained in the original calibration of Feast & Catchpole. The implications of the actual calibration on the Cepheid-based distance to the Large Magellanic Cloud (LMC) and the extragalactic distance scale are briefly discussed. A true distance modulus of 18.59 ± 0.04 mag is presently achieved. Evidence from Hipparcos-based calibrations by different methods strongly supports the actual upward revision of 0.09 ± 0.04 mag for the LMC distance of 18.50 mag adopted in the Hubble Space Telescope Key Project program, corresponding to no more than a 5% decrease in the value of the Hubble constant.