Distance to M31 With the HST and Hipparcos Red Clump Stars

摘要

Following an approach by Paczy'nski & Stanek we compare red clump stars withparallaxes known to better than 10% in the Hipparcos catalog with the red clumpstars observed in three fields in M31 using the HST. There are $sim 600$ and$sim 6,300$ such stars in the two data sets, respectively. The local red clumpluminosity function is well represented by a Gaussian with the peak at$M_{I,m}=-0.23$, and the dispersion $sigma_{RC}pprox0.2 $mag. This allows asingle step determination of the distance modulus to M31 $mu_{0,M31} =24.471pm 0.035 pm 0.045 $mag (statistical plus systematic error) and thecorresponding distance $R_{M31}= 784pm 13pm 17 kpc$. The number of red clumpstars is large enough that the formal statistical error in the distance is only$lesssim 2$%. We also correct the treatment of the local interstellar extinction byPaczy'nski & Stanek and we obtain the Galactocentric distance modulus$mu_{0,GC}=14.57 pm0.04 pm 0.04 $mag (statistical plus systematic error),and the corresponding Galactocentric distance $R_0=8.2 pm0.15 pm 0.15 kpc$.