Mapping the differential reddening in globular clusters

TL;DR

This study creates detailed differential-reddening maps for 66 Galactic globular clusters using HST data, revealing spatially-variable extinction primarily caused by interstellar dust, and demonstrating that correction improves measurement precision without altering fundamental cluster parameters.

Contribution

It introduces a novel method for mapping internal differential reddening in globular clusters using stellar-density Hess diagrams, applicable to a large sample with varying coverage.

Findings

Detected spatially-variable extinction in all clusters studied.

Differential-reddening correction reduces scatter and improves measurement accuracy.

Main source of differential reddening is interstellar dust.

Abstract

We build differential-reddening maps for 66 Galactic globular clusters (GCs) with archival HST WFC/ACS F606W and F814W photometry. Because of the different GC sizes (characterised by the half-light radius $R_h$) and distances to the Sun, the WFC/ACS field of view ($200\arcsec\times200\arcsec$) coverage ($R_{obs}$) lies in the range $1\la R_{obs}/R_h\la15$ for about 85% of the sample, with about 10% covering only the inner ($R_{obs}\la R_h$) 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 $\approx7\arcsec\times7\arcsec$ to $\approx20\arcsec\times20\arcsec$. We detect spatially-variable extinction in the 66 globular clusters studied, with mean values ranging from $\mEBV\approx0.018$ (NGC\,6981) up to $\mEBV\approx0.16$ (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 \EBV, the distance to the Sun is conserved. Fits to the mean-ridge lines of the highly-extincted and photometrically scattered globular cluster Palomar\,2 show that 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 \mEBV\ with both the foreground reddening and the sampled cluster area indicates that the main source of differential reddening is interstellar.