Photometric and spectroscopic analysis of cool white dwarfs with trigonometric parallax measurements

A detailed photometric and spectroscopic analysis of cool (T-eff less than or similar to 12,000 K) white dwarf stars is presented. The sample has been drawn from the Yale Parallax Catalog and from a proper motion survey in th e southern hemisphere. Optical BV RI and infrared JHK photometry, as well a s spectroscopy at H alpha, have been secured for a sample of 152 white dwar fs. The discovery of seven new DA white dwarfs, two new DQ white dwarfs, on e new magnetic white dwarf, and three weak magnetic white dwarf candidates, is reported. Our sample also identifies 19 known or suspected double degen erates. The photometric energy distributions, the Ha line profiles, and the trigonometric parallax measurements are combined and compared against the predictions of model atmosphere calculations to determine the effective tem perature and the radius of each object in the sample and also to constrain the atmospheric composition. New evolutionary sequences with carbon/oxygen cores with thin and thick hydrogen layers are used to derive stellar masses and ages. The results are used to improve our understanding of the chemica l evolution of cool white dwarfs. We confirm the existence of a range in ef fective temperature between similar to 5000 and 6000 K where almost all whi te dwarfs have hydrogen-rich atmospheres. Our sample shows little evidence for mixed H/He white dwarfs, with the exception of two helium-rich DA stars , and four (possibly five) C2H white dwarfs which have been interpreted as having mixed H/He/C atmospheres. The observed sequence of DQ stars is found to terminate abruptly near 6500 K, below which they are believed to turn i nto C2H stars. True DC stars slightly above this temperature are found to e xhibit hydrogen-like energy distributions despite the lack of Ha absorption features. The mean mass of our complete sample is 0.65 M. with a dispersio n of sigma similar to 0.20 M.. Attempts to interpret the chemical evolution of cool white dwarfs show the problem to be complex. Convective mixing is called upon to account for the increase of the non-DA to DA ratio below 12, 000 K, as well as the reappearance of helium-rich stars below similar to 50 00 K. The possible presence of helium in cool DA stars, the existence of th e non-DA gap, and the nature of the peculiar DC stars are also explained in terms of convective mixing, although our understanding of how this mechani sm works needs to be revised in order to account for these observations. Gi ven this chemical evolution uncertainty, it is not clear whether thick or t hin hydrogen layer models should be used to determine cooling ages. The old est object in our sample is similar to7.9 Gyr or similar to9.7 Gyr old depe nding on whether thin or thick hydrogen layer models are used, respectively .