We present an investigation of the horizontal branch (HB) of M15, using theoretical sequences calculated on the assumption of various values of the envelope helium and oxygen abundances. We discuss the effect of CNO-enhanced compositions on the structure and morphology of the horizontal branch and find that, at least for low metallicities, nucleosynthesis effects dominate those due to opacity. In addition, we argue that the ratio [O/H] is a better indicator of the expected stellar distribution than [Fe/H]. Our detailed matches of our theoretical sequences to the cluster observations indicate that the high envelope helium abundances (YHB ≈ 0.29) required by recent work on RR Lyrae pulsation are incompatible with the observed morphology. We find that there is a clear preference for values of 0.21 ≲ Y ≲ 0.25, independent of the value of [O/Fe]. Tighter constraints are possible, in principle but not in practice, due to uncertainties in the observations and in the available synthetic temperature-bolometric-correction relations. The oxygen-enhanced zero-age HB models are found to have a period-color relationship which is almost identical to that of their scaled-solar counterparts, but they reduce significantly the predicted double-mode variable masses. Importantly, we find that for lower estimates of the reddening to M15-E(B-V) ≈ 0.09 mag - there is no discrepancy between the predicted and observed periods for the RR Lyrae variables, after evolution is taken into account. However, the period shift between M3 and M15 can be explained by canonical models only if the helium abundance in both clusters is low (7HB ∼ 0.21), and the bulk of the RR Lyrae star population in M15 is at late stages of evolution.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science