Mapping the differential reddening in globular clusters

Abstract

We build differential-reddening maps for 66 Galactic globular clusters (GCs) with archival Hubble Space Telescope WFC/ACS F606W and F814W photometry. Because of the different GC sizes (characterized by the half-light radius Rh) and distances to the Sun, the WFC/ACS field of view (200 arcsec × 200 arcsec) coverage (Robs) lies in the range 1 < ~ Robs/Rh < ~ 15 for about 85 per cent of the sample, with about 10 per cent covering only the inner (Robs < ~ Rh) parts.We divide the WFC/ACS field of view across each cluster in a regular cell grid and extract the stellar-density Hess diagram from each cell, shifting it in colour and magnitude along the reddening vector until matching the mean diagram. Thus, the maps correspond to the internal dispersion of the reddening around the mean. Depending on the number of available stars (i.e. probable members with adequate photometric errors), the angular resolution of the maps range from ≈7 arcsec × 7 arcsec to ≈20 arcsec × 20 arcsec. We detect spatially variable extinction in the 66 GCs studied, with mean values ranging from ‹ δE(B − V )› ≈ 0.018 (NGC 6981) up to ‹δE(B − V )› ≈ 0.16 (Palomar 2). Differential-reddening correction decreases the observed foreground reddening and the apparent distance modulus but, since they are related to the same value of E(B − V), the distance to the Sun is conserved. Fits to the mean-ridge lines of the highly extincted and photometrically scatteredGCPalomar 2 showthat age and metallicity also remain unchanged after the differential-reddening correction, but measurement uncertainties decrease because of the reduced scatter. The lack of systematic variations of ‹δE(B − V )› with both the foreground reddening and the sampled cluster area indicates that the main source of differential reddening is interstellar.