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Session 18 - White Dwarfs.
Display session, Monday, January 15
North Banquet Hall, Convention Center
White dwarf stars are the evolutionary endpoint of some 99% of all stars formed, are a remarkably homogeneous class, and cool slowly enough that they still glow with an output of \sim10^-5 L_ødot at an age of \sim10 Gyr. Because the white dwarf stars are apparently a fundamentally homogeneous class with relatively well-understood cooling timescales, the luminosity function of white dwarfs provides a record of the age and star-formation history of the local Galactic disk. In this work, we present the results of a parametric study of the information contained in three recent determinations of the observed luminosity function of white dwarf stars. The evolutionary model sequences used are evolved to lower luminosities and should be more representative of the observed sample of stars than our previous work. Our recent preliminary determination of the luminosity function of white dwarfs in wide common proper motion binaries (Oswalt, Smith, amp; Wood 1995) suggests a minimum disk age of t_disk=11^+4_-2 Gyr using these same models. This estimate is consistent with globular cluster ages t_GC\approx15 Gyr, but is not determined well enough to select among the models for the early star-formation histories of spiral galaxies. Our results suggest a lower limit to the age of the Universe of t_U\approx12 Gyr which within the standard inflationary model (Ømega=1, \Lambda=0) implies H_0\ \hbox<\kern-1.7ex \lower0.85ex\hbox\sim 56 \rm\ km\ s^-1\ Mpc^-1. Recent determinations of the the Hubble constant have clustered near H_0\approx80\rm\ km\ s^-1\ Mpc^-1 with typical quoted uncertainties of 10 to 20 percent, which implies a Hubble expansion age of \sim8 Gyr. Our result, which is a lower limit, exceeds this by \sim2\sigma, and provides further evidence that the Universe may be characterized by a sub-critical density (Ømega<1) or non-zero cosmological constant.
The authors gratefully acknowledge support from NSF grants AST-9016284 (TDO) and AST-9217988 (MAW), NASA Graduate Student Research Fellowship NGT-51086 (JAS), NASA Astrophysics Theory Program grant NAG 5-3103 (MAW), and an Ernest F. Fullam Award from the Dudley Observatory (MAW amp; TDO).
