2008MNRAS.390.1762D

I report on the implementation of the surface brightness (SB) technique aimed at deriving reliable Cepheid angular diameters at optical wavelengths and then Baade-Wesselink (BW) distances using spectroscopic linear radii. The technique is applied to calibrate the Galactic distance scale and the distance to the Large Magellanic Cloud (LMC) based on this Galactic calibration. The LMC distance benefits from the large number of Cepheids with high-precision photometry and reddening data available by the Optical Gravitational Lensing Experiment of Udalski et al. and from improvements and refinements, including (i) a least-squares (LS) fitting approach to the individual distances to recover the distance modulus unbiased by the period distribution of target Cepheids and (ii) a better understanding of the metallicity effects on the period-luminosity relations and SB scales. The resulting true distance modulus of the LMC is µ_O(LMC) = [18.559±0.003 (random)±0.026 (systematic)] mag.

The SB technique is also applied to confidently determine the Cepheid distance to the maser host galaxy NGC 4258. Cepheids in the inner region with near solar metallicity yield a true distance modulus of µ_O_(NGC4258) = [29.28±0.03 (random)±0.03 (systematic)] mag, in excellent agreement, at the 0.01 mag level, with the high-weight geometric water maser distance modulus. It is shown that the current data-reduction approach of averaging individual Cepheid distances can introduce significant systematic errors in the distance scale calibration. From a small sample of Hubble Space Telescope (HST) based galaxies, I derive an overall error of (12 ± 5) per cent, which would suggest a downward revision of the Freedman et al. Hubble constant to the value H₀= (63±7)km/s/Mpc, other things being equal.