We present the first detailed study of the properties ( temperatures , gravities , and masses ) of the NGC 6791 white dwarf population .
This unique stellar system is both one of the oldest ( 8 Gyr ) and most metal-rich ( [ Fe/H ] \sim + 0.4 ) open clusters in our Galaxy , and has a color-magnitude diagram ( CMD ) that exhibits both a red giant clump and a much hotter extreme horizontal branch .
Fitting the Balmer lines of the white dwarfs in the cluster , using Keck/LRIS spectra , suggests that most of these stars are undermassive , \langle M \rangle = 0.43 \pm 0.06 M _ { \odot } , and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch .
We show that at least 40 % of NGC 6791 ’ s evolved stars must have lost enough mass on the red giant branch to avoid the flash , and therefore did not convert helium into carbon-oxygen in their core .
Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD .
This unique stellar evolutionary channel also naturally explains the recent finding of a very young age ( 2.4 Gyr ) for NGC 6791 from white dwarf cooling theory ; helium core white dwarfs in this cluster will cool \sim 3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact \gtrsim 7 Gyr , consistent with the well measured main-sequence turnoff age .
These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies .