Multiple populations on the horizontal branch

A number of globular clusters have gaps in the distribution of stars on the horizontal branch. These gaps are not predicted by current theory. The author shows, through reconstructed mass distributions, that the distribution of stars is clearly bimodal. In other words, there are multiple populations of stars on the HB. She tests several theories for their origin by placing the HB stars in the log T/sub eff/-log g plane. A number of the possible origins for the gap predict that the stars on the blue side of the gap should not lie along the extrapolation of the red sequence of stars. She also uses the log T/sub eff/-log g diagram to look for confirmation of the Y-Z anticorrelation hypothesized by Sandage. This was originally indicated from the observed period shift at constant temperature as a function of metallicity in RR Lyrae variables. The author observed the gap clusters M15, NGC 288 and M92. In addition, she used M3, M5, M15, and M92 to look for the Sandage shift. Between the two clusters she felt are best used to look for the Sandage shift, M5 and M92, she found that the difference in surface gravity at a given temperature is consistent with a value of zero at about the 1 σ level. It is consistent with the value predicted by the Sandage shift (0.09) at the 1.5σ level