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M55 (NGC 6809) and NGC 6362 are among the few globular clusters for whichmasses and radii have been derived to high precision for member binary stars.They also contain RR Lyrae variables which, together with their non-variablehorizontal-branch (HB) populations, provide tight constraints on the clusterreddenings and distance moduli through fits of stellar models to theirpulsational and evolutionary properties. Reliable (m-M)_V estimates yield M_Vand M_bol values of comparable accuracy for binary stars because the V-bandbolometric corrections applicable to them have no more than a weak dependenceon effective temperature (T_eff) and [Fe/H]. Chemical abundances derived fromthe binary mass-M_V relations are independent of determinations based on theirspectra. The temperatures of the binaries, which are calculated directly fromtheir luminosities and the measured radii, completely rule out the low T_effscale that has been determined for metal-deficient stars in some recentspectroscopic 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 6362shows clear evidence for multiple stellar populations. Constraints from the RRLyrae standard candle and from local subdwarfs (with Gaia DR2 parallaxes) arebriefly discussed.

astrophysical journal/the astrophysical journal

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