Program: GN-2014B-Q-11

Extended horizontal branch (EHB) stars are common but poorly understood stars observed in the field (subdwarf B/O, sdB/sdO), in globular clusters, and in the metal-rich open cluster NGC 6791. These stars are very hot with extremely thin hydrogen-rich envelopes, which requires that they have undergone significant mass loss beyond that explained by models. A common theory is that their envelopes have been stripped by binary companions. While a majority of the field sdB stars are found to be short-period binaries, spectroscopically observed EHB stars in globular cluster are predominantly single stars. Does this suggest that their mechanisms of formation are distinct in these differing environments? We propose to test this by spectroscopically targeting the EHB stars in the open cluster NGC 6791. The nature of the EHB population in this cluster provides two potentially exciting results. If we find that the EHB stars are binaries, it suggests that in the significantly more dense environment of globulars their binarity has been disrupted. Otherwise, if we find they are singly evolved, this suggests that either the multiple populations observed in both environments or the very high metallicity, if not both, plays an important role. Multiple epochs of brief spectroscopic observations of these EHB stars in NGC 6791 will verify their cluster membership, detect if they are short period binaries, determine their v sin i, and be used to fit models for determining their stellar parameters.