Distance to M31 with the [ITAL]Hubble Space Telescope[/ITAL] and [ITAL]Hipparcos[/ITAL] Red Clump Stars

Abstract

Following the approach by Paczyński & Stanek, we compare the red clump stars with parallaxes known to better than 10% in the Hipparcos catalog with the red clump stars observed in three fields in M31 using the Hubble Space Telescope. There are ~600 and ~6300 such stars in the two data sets, respectively. The local red clump luminosity function is well represented by a Gaussian with the peak at MI,m=-0.23 and the dispersion σRC~0.2 mag. This allows a single-step determination of the distance modulus to M31, μ0,M31=24.471+/-0.035+/-0.045 mag (statistical plus systematic error), and the corresponding distance, RM31=784+/-13+/-17 kpc. The number of red clump stars is large enough that the formal statistical error in the distance is only