The connection between mass loss on the red giant branch ( RGB ) and horizontal branch ( HB ) morphology in globular clusters ( GCs ) has long been acknowledged but the mechanisms governing mass loss remains poorly understood from a theoretical perspective .
The present study uses synthetic HB models to demonstrate for the first time that \alpha -enhancement and a simple relation between mass loss and metallicity can explain the entire range of HB morphology ( characterized by the HB type index ) observed in old , coeval GCs .
The mass loss-metallicity relation accounts naturally for the fact that the most metal poor GCs ( [ Fe/H ] < -2 ) have redder HBs than is typical of GCs with -2 < [ Fe/H ] < -1.5 without invoking younger ages .
These results may prove useful in studying the contribution of HB stars to integrated light via stellar population synthesis .