Toward a Better Understanding of the Distance Scale from RR Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723

Abstract

We present BV photometry for 54 variables in the metal-rich inner halo globular cluster NGC 6723. With the discovery of new RR Lyrae variables (RRLs), we obtain langP _abrang = 0.541 ± 0.066 and langP _crang = 0.292 ± 0.030 day, n(c)/n(ab+c) = 0.167, and langV(RR)rang_int = 15.459 ± 0.055. We carry out the Fourier decomposition analysis and obtain [Fe/H]_ZW = -1.23 ± 0.11 and E(B - V) = 0.063 ± 0.015 for NGC 6723. By calibrating the zero point from the recent absolute trigonometric parallax measurements for RR Lyr, we derive the revised M_V (RR)-[Fe/H] relation, providing M_V (RR) = 0.52 at [Fe/H] = -1.50 and (m - M)₀ = 18.54 for the Large Magellanic Cloud, in excellent agreement with others. We obtain (m - M)₀ = 14.65 ± 0.05, equivalent to a distance from the Sun of 8.47 ± 0.17 kpc, for NGC 6723 from various distance measurement methods using RRLs. We find that RRLs in NGC 6723 do not have magnitude dependency on the radial distance because there is not a severe degree of apparent crowdedness. Finally, we show that a relation exists between the degree of photometric contamination and the apparent crowdedness of the central region of globular cluster systems, |\bar{\mu } - \mu _r| \propto log \rho _c d^2. The use of this relation can play a significant role in mitigating the discrepancy in establishing a cosmic distance scale using RRLs in resolved stellar populations in near-field cosmology.

Partially based on observations made with telescopes operated by the SMARTS consortium.