Identifying Contaminated K-band Globular Cluster RR Lyrae Photometry

TL;DR

This paper investigates how unresolved stellar contamination affects K-band photometry of RR Lyrae stars in globular clusters, revealing systematic biases in distance and age estimates crucial for precision cosmology.

Contribution

It identifies the impact of unresolved close stars on K-band RR Lyrae photometry and suggests future high-resolution surveys to mitigate this issue.

Findings

Contamination causes underestimated cluster distances.

Clustercentric trends are not due to metallicity variations.

Empirical K-band period-magnitude relation shows negligible metallicity dependence.

Abstract

Acquiring near-infrared K-band (2.2 um) photometry for RR Lyrae variables in globular clusters and nearby galaxies is advantageous since the resulting distances are less impacted by reddening and metallicity. However, K-band photometry for RR Lyrae variables in M5, Reticulum, M92, omega Cen, and M15 display clustercentric trends. HST ACS data imply that multiple stars in close proximity to RR Lyrae variables located near the cluster core, where the stellar density increases markedly, are generally unresolved in ground-based images. RR Lyrae variables near the cluster cores appear to suffer from photometric contamination, thereby yielding underestimated cluster distances and biased ages. The impact is particularly pernicious since the contamination propagates a systematic uncertainty into the distance scale, and hinders the quest for precision cosmology. The clustercentric trends are probably unassociated with variations in chemical composition since an empirical K-band period-magnitude relation inferred from Araucaria/VLT data for RR Lyrae variables in the Sculptor dSph exhibits a negligible metallicity dependence: (0.059+-0.095)[Fe/H], a finding that supports prior observational results. A future multi-epoch high-resolution near-infrared survey, analogous to the optical HST ACS Galactic Globular Cluster Survey, may be employed to establish K-band photometry for the contaminating stars discussed here.