THE CHEMICAL EVOLUTION OF COOL WHITE-DWARFS AND THE AGE OF THE LOCAL GALACTIC DISK

Photometric and spectroscopic observations of 110 cool white dwarf sta rs are presented and analyzed with state-of-the-art model atmosphere c alculations appropriate for cool white dwarfs with pure hydrogen and p ure helium, as well as mixed H/He compositions. High signal-to-noise s pectroscopy reveals the presence of H alpha in 20 white dwarfs previou sly classified as DC stars, four of which are magnetic. Cool white dwa rfs are shown to form a narrow sequence in both color-color and color- magnitude diagrams, with little evidence for separation between hydrog en- and helium-rich compositions in these diagrams. The observed energ y distributions are obtained from a combination of both optical BVRI a nd infrared JHK photometric data and used to derive both the effective temperature and the atmospheric composition of each star. Stellar mas ses are also obtained for 60 white dwarfs with known trigonometric par allaxes. Some discrepancies between the observed energy distributions and those predicted by the model atmospheres are described. In particu lar, evidence for the presence of a UV opacity source in the coolest h ydrogen-rich white dwarfs is interpreted in terms of a pseudo continuu m opacity originating from the Lyman edge. The simultaneous analysis o f the photometric and spectroscopic observations provides a detailed p icture of the chemical composition of cool white dwarfs. Most cool whi te dwarfs have energy distributions that are well reproduced by either pure hydrogen or pure helium models, with little evidence for objects with mixed atmospheric compositions. We identify a peculiar class of non-DA star with T-eff > 6000 K whose energy distributions are well re produced by pure hydrogen models but whose spectra do not show Ha. Our results reveal an inhomogeneous temperature distribution of hydrogen- and helium-rich white dwarfs, and in particular the presence of a non -DA gap in the range 5000 less than or similar to T-eff less than or s imilar to 6000 K. The chemical evolution of cool white dwarfs is discu ssed at length with respect to our findings. We show that no known phy sical mechanisms (e.g., convective mixing, convective dredge-up, accre tion from the interstellar medium) can account for the temperature dis tribution of hydrogen- and helium-rich white dwarfs. Possible new mech anisms that could explain our results are presented. We propose a mech anism by which hydrogen is accreted onto the surface of helium-rich wh ite dwarfs while remaining spectroscopically invisible. Observational evidence that supports our hypothesis is discussed. Lower limits for t he age of the local Galactic disk are obtained by determining the age of the oldest white dwarfs in our sample. Ages in the range 6.5-10 Gyr are derived from evolutionary models with various core compositions a nd helium envelope masses.