1999ApJ...518..262S

We present new VI photometry for the halo globular cluster M30 (NGC 7099=C2137-174) and compute luminosity functions (LFs) in both bands for samples of about 15,000 hydrogen-burning stars from near the tip of the red giant branch (RGB) to over 4 mag below the main-sequence (MS) turnoff (TO). We confirm previously observed features of the LF that are at odds with canonical theoretical predictions: an excess of stars on subgiant branch (SGB) approximately 0.4 mag above the turnoff and an excess number of RGB stars relative to MS stars. Based on subdwarfs with Hipparcos-measured parallaxes, we compute apparent distance moduli of (m-M)_V=14.87±0.12 and 14.65±0.12 for reddenings of E(V-I)=0.06 and 0.02, respectively. The implied luminosity for the horizontal branch (HB) at these distances is M^HB_V=0.11 and 0.37 mag. The two helium indicators we have been able to measure (R and Δ) both indicate that M30's helium content is high relative to those in other clusters of similar metallicity. M30 has a larger value for the parameter ΔV^HB_TO than any of the other similarly metal-poor clusters for which this quantity can be reliably measured. This suggests that M30 has either a larger age or higher helium content than all of the other metal-poor clusters examined. The color-difference method for measuring relative ages indicates that M30 is coeval with the metal-poor clusters M68 and M92.