Constraints on the Distance Moduli, Helium and Metal Abundances, and Ages of Globular Clusters From Their RR Lyrae and Non-Variable Horizontal-Branch Stars. III. M55 and NGC 6362

TL;DR

This study uses RR Lyrae and non-variable horizontal-branch stars in globular clusters M55 and NGC 6362 to precisely determine their distances, compositions, and ages, challenging some recent temperature scale assumptions.

Contribution

It provides new, highly accurate measurements of distances, metallicities, and ages for M55 and NGC 6362 using binary star data and stellar models, and discusses implications for stellar population analysis.

Findings

M55 has (m-M)_V = 13.95, [Fe/H] = -1.85, age 12.9 Gyr

NGC 6362 has (m-M)_V = 14.56, [Fe/H] = -0.90, age 12.4 Gyr

Binary star temperatures challenge low T_eff scale assumptions.

Abstract

M55 (NGC 6809) and NGC 6362 are among the few globular clusters for which masses and radii have been derived to high precision for member binary stars. They also contain RR Lyrae variables which, together with their non-variable horizontal-branch (HB) populations, provide tight constraints on the cluster reddenings and distance moduli through fits of stellar models to their pulsational and evolutionary properties. Reliable (m-M)_V estimates yield M_V and M_bol values of comparable accuracy for binary stars because the V-band bolometric corrections applicable to them have no more than a weak dependence on effective temperature (T_eff) and [Fe/H]. Chemical abundances derived from the binary mass-M_V relations are independent of determinations based on their spectra. The temperatures of the binaries, which are calculated directly from their luminosities and the measured radii, completely rule out the low T_eff scale that has been determined for metal-deficient stars in some recent spectroscopic and interferometric studies. If [alpha/Fe] = 0.4 and [O/Fe] = 0.5 +/- 0.1, we find that M55 has (m-M)_V = 13.95 +/- 0.05, [Fe/H] = -1.85 +/- 0.1, and an age of 12.9 +/- 0.8 Gyr, whereas NGC 6362 has (m-M)_V = 14.56 +/- 0.05, [Fe/H] = -0.90 +/- 0.1, and an age of 12.4 +/- 0.8 Gyr. The HB of NGC 6362 shows clear evidence for multiple stellar populations. Constraints from the RR Lyrae standard candle and from local subdwarfs (with Gaia DR2 parallaxes) are briefly discussed.