The Initial-final Mass Relation: First Constraints at the Low Mass End

The initial-final mass relation represents a mapping between the mass of a white dwarf remnant and the mass that the hydrogen burning main-sequence star that created it once had. The relation thus far has been constrained using a sample of ∼40 stars in young open clusters, ranging in initial mass from ∼2.75 – 7 M⊙, and shows a general trend that connects larger mass main-sequence stars with larger mass white dwarfs. In this paper, we present CFHT/CFH12K photometric and Keck/LRIS multiobject spectroscopic observations of a sample of 22 white dwarfs in two old open clusters, NGC 7789 (t = 1.4 Gyr) and NGC 6819 (t = 2.5 Gyr). At these ages, stars in these clusters with masses as low as 1.6 M⊙ have already evolved off the main sequence and formed white dwarfs. We measure masses for the highest signal-to-noise spectra by fitting the Balmer lines to atmosphere models and place the first constraints on the low mass end of the initial-final mass relation. Our results indicate that the observed trend at higher masses continues down to very low masses, with Minitial = 1.6 M⊙ main-sequence stars forming Mfinal = 0.54 M⊙ white dwarfs. When added to our new data from the very old cluster NGC 6791, the relation is extended down to Minitial = 1.16 M⊙ (corresponding to Mfinal = 0.51 M⊙). This extention of the relation represents a four fold increase in the total number of hydrogen burning stars for which the integrated mass loss can now be calculated, assuming a Salpeter initial mass function. For the first time, the new leverage at the low mass end is used to derive a purely empirical initial-final mass relation without the need for any indirectly motivated anchor points. The sample of white dwarfs in these clusters also shows three very interesting systems that we discuss further: a DB (helium atmosphere) white dwarf, a magnetic white dwarf, and a DAB (mixed hydrogen/helium atmosphere or a double degenerate DA+DB) white dwarf(s). Subject headings: open clusters and associations: individual (NGC 7789 and NGC 6819) stars: evolution techniques: photometric, spectroscopic white dwarfs