Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low‐Mass White Dwarfs

Abstract

We present the first detailed study of the properties (temperatures,\ngravities, and masses) of the NGC 6791 white dwarf population. This unique\nstellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~\n0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that\nexhibits both a red giant clump and a much hotter extreme horizontal branch.\nFitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS\nspectra, suggests that most of these stars are undermassive, <M> = 0.43 +/-\n0.06 Msun, and therefore could not have formed from canonical stellar evolution\ninvolving the helium flash at the tip of the red giant branch. We show that at\nleast 40% of NGC 6791's evolved stars must have lost enough mass on the red\ngiant branch to avoid the flash, and therefore did not convert helium into\ncarbon-oxygen in their core. Such increased mass loss in the evolution of the\nprogenitors of these stars is consistent with the presence of the extreme\nhorizontal branch in the CMD. This unique stellar evolutionary channel also\nnaturally explains the recent finding of a very young age (2.4 Gyr) for NGC\n6791 from white dwarf cooling theory; helium core white dwarfs in this cluster\nwill cool ~3 times slower than carbon-oxygen core stars and therefore the\ncorrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well\nmeasured main-sequence turnoff age. These results provide direct empirical\nevidence that mass loss is much more efficient in high metallicity environments\nand therefore may be critical in interpreting the ultraviolet upturn in\nelliptical galaxies.\n