The Age and Progenitor Mass of Sirius B

The Sirius AB binary system has masses that are well determined from manydecades of astrometric measurements. Because of the well-measured radius andluminosity of Sirius A, we employed the TYCHO stellar evolution code todetermine the age of the Sirius A,B binary system accurately, at 225-250 Myr.Note that this fit requires the assumption of solar abundance, and the use ofthe new Asplund et al. primordial solar metallicity. No fit to Sirius A'sposition is possible using the old Grevesse & Sauval scale. Because the SiriusB white dwarf parameters have also been determined accurately from spaceobservations, the cooling age could be determined from recent calculations byFontaine et al. or Wood to be 124 +/- 10 Myr. The difference of the two agesyields the nuclear lifetime and mass of the original primary star, 5.056+0.374/-0.276 solar masses. This result yields in principle the most accuratedata point at relatively high masses for the initial-final mass relation.However, the analysis relies on the assumption that the primordial abundance ofthe Sirius stars was solar, based on membership in the Sirius supercluster. Arecent study suggests that its membership in the group is by no means certain.Comment: 12 pages, one figure (reduced resolution JPG posted), accepted for publication in the ApJ Letters. Replaced with revised version correcting a typo in the text (the derived age of Sirius A on page 5); conclusions unchange