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

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 M I,m =-0.23 and the dispersion σ RC ≈ 0.2 mag. This allows a single-step determination of the distance modulus to M31, μ 0, M31 =24.471 ± 0.035 ± 0.045 mag (statistical plus systematic error), and the corresponding distance, R M31 =784 ± 13 ± 17 kpc. The number of red clump stars is large enough that the formal statistical error in the distance is only ≲2%. We also correct the treatment of the local interstellar extinction by Paczyński & Stanek, and we obtain the Galactocentric distance modulus, μ 0, GC =14.57 ± 0.04 ± 0.04 mag (statistical plus systematic error), and the corresponding Galactocentric distance, R 0 =8.2 ± 0.15 ± 0.15 kpc.