Improved Calibration of Cosmic Distance Scale by Cepheid Pulsation Parallaxes

Abstract

I report on a modern empirical realization of the geometric Baade-Wesselink (BW) method aimed at providing accurate distances to Cepheid variable stars. The method is carefully calibrated using high-precision observational data now available from spectroscopic and interferometric techniques. The distance-dependent linear radius R of a Cepheid, derived from the period P according to the period-radius (PR) relation, is matched to the distance-independent angular diameter ? inferred from the surface brightness-color (SC) correlation using the infrared color (V - K) as brightness indicator. The resulting pulsation parallax is found to be largely insensitive to most of known biasing effects, notably the reddening and metallicity, because of the application of the magnitude-color combination (V, V - K) in deriving stellar angular sizes. The reliability of the actual BW realization is critically checked using 25 galactic Cepheids in clusters with available high-precision photometry, reddenings and distances by the zero-age main-sequence (ZAMS) fitting approach. I find that 20 of these stars show an average residual (BW - ZAMS) = -(0.01 ? 0.03) mag with a scatter ? = ?0.12 mag, whereas the remaining five stars deviate by up to 5 ?. The fairly good agreement with 20 primary calibrators gives high confidence on the reliability of the method which can provide now an independent route to the problem of cosmic distance scale calibration. According to the overall residual, the actual uncertainty on the absolute galactic distance scale is suggested to be ?0.04 mag, i.e., a factor of 2 smaller than that currently quoted for the Pleiades distance modulus which limits the ZAMS calibration. The BW approach is then applied to the extragalactic range by including the Magellanic Clouds Cepheids (LMC and SMC) with available high-precision photometry in V and K passbands. An accurate PR relation is recalibrated by a composite fit to galactic and extragalactic Cepheids. The relation shows a dispersion as low as ? = ?0.11 mag nearly close to the scatter affecting BW distances to calibrating galactic Cepheids. It is argued that this scatter is likely due to the asymptotic spread set by the finite width of the instability strip. The BW distances to the LMC and SMC are also determined to be ?0(LMC) = (18.58 ? 0.024) mag and ?0(SMC) = (19.00 ? 0.025) mag or d(LMC) = (52.0 ? 0.6) kpc and d(SMC) = (63.2 ? 0.7) kpc with uncertainties not including the contribution due to the absolute distance scale calibration. The Cepheid-based distance to the LMC shows close agreement with the geometric expansion parallax of the SN 1987A in LMC given by d(SN 1987A) = (51.1 ? 1.5) kpc (Panagia et al. 1996). The BW method is also calibrated by including the Johnson-Cousins color (V - I) relevant in the Hubble Space Telescope (HST) observations of Cepheids. By using sets of SMC, LMC, and galactic Cepheids with high-precision V, I, and K photometry, it is shown that the angular sizes predictable by the magnitude-color combination (V, V - I) can be affected by the metallicity. However, the induced effects on BW distances are found to be as small as 0.03 mag up to the metal content of the SMC. Furthermore, it is demonstrated that the current HST approach to the extragalactic distances using a linear combination of period-luminosity (PL) relations in V and I passbands yields the same observational relation as that of the BW realization in the magnitude-color combination (V, V - I). This achievement plays a major role in improving the extragalactic distance scale set by Cepheid data from HST observations. First, all distances spanning from the galactic to extragalactic range can be now sampled by the same BW relation strongly supported by the fundamental results of several observational techniques. Second, all distances will result to be affected by the metallicity as the BW data, i.e., as the SC correlation applied for inferring the Cepheid angular sizes. Third, all distances will suffer from random errors as the almost reddening-free BW data which cancel out the extinction on a star-by-star basis, notably the differential reddening internal to the parent galaxies. In order to show this relevant improvement, the BW distance to the Virgo galaxy M100 is determined to be ?0(M100) = (31.03 ? 0.06) mag or d(M100) = (16.1 ? 0.5) Mpc with a random error lowered by about a factor of 3 with respect to that derived according to the PL relations by the HST procedure.