Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements

A photometric and spectroscopic analysis of 152 cool white dwarf stars ispresented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 newmagnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported,as well as 19 known or suspected double degenerates. The photometric energydistributions, the Halpha line profiles, and the trigonometric parallaxmeasurements are combined and compared to model atmosphere calculations todetermine the effective temperature and the radius of each object, and also toconstrain the atmospheric composition. New evolutionary sequences with C/Ocores with thin and thick hydrogen layers are used to derive masses and ages.We confirm the existence of a range in Teff between 5000 and 6000K where almostall white dwarfs have H-rich atmospheres. There is little evidence for mixedH/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfswhich possibly have mixed H/He/C atmospheres. The DQ sequence terminates near6500K, below which they are believed to turn into C2H stars. True DC starsslightly above this temperature are found to exhibit H-like energydistributions despite the lack of Halpha absorption. Attempts to interpret thechemical evolution show the problem to be complex. Convective mixing isnecessary to account for the non-DA to DA ratio as a function of temperature.The presence of helium in cool DA stars, the existence of the non-DA gap, andthe peculiar DC stars are also explained in terms of convective mixing,although our understanding of how this mechanism works needs to be revised. Theoldest object in our sample is about 7.9 Gyr or 9.7 Gyr old depending onwhether thin or thick hydrogen layer models are used. The mean mass of oursample is 0.65 +/- 0.20 Msun.Comment: Accepted by ApJ Suppl (~April 2001); 79 pages incl. 25 figure