We present a comparative analysis of atmospheric parameters obtained with the so-called photometric and spectroscopic techniques. Photometric and spectroscopic data for 1360 DA white dwarfs from the Sloan Digital Sky Survey (SDSS) are used, as well a
s spectroscopic data from the Villanova White Dwarf Catalog. We first test the calibration of the ugriz photometric system by using model atmosphere fits to observed data. Our photometric analysis indicates that the ugriz photometry appears well calibrated when the SDSS to AB_95 zeropoint corrections are applied. The spectroscopic analysis of the same data set reveals that the so-called high-log g problem can be solved by applying published correction functions that take into account 3D hydrodynamical effects. However, a comparison between the SDSS and the White Dwarf Catalog spectra also suggests that the SDSS spectra still suffer from a small calibration problem. We then compare the atmospheric parameters obtained from both fitting techniques and show that the photometric temperatures are systematically lower than those obtained from spectroscopic data. This systematic offset may be linked to the hydrogen line profiles used in the model atmospheres. We finally present the results of an analysis aimed at measuring surface gravities using photometric data only.
We have paired the Second Data Release of the Large Area Survey of the UKIRT Infrared Deep Sky Survey with the Fifth Data Release of the Sloan Digital Sky Survey to identify ten cool white dwarf candidates, from their photometry and astrometry. Of th
ese ten, one was previously known to be a very cool white dwarf. We have obtained optical spectroscopy for seven of the candidates using the GMOS-N spectrograph on Gemini North, and have confirmed all seven as white dwarfs. Our photometry and astrometry indicates that the remaining two objects are also white dwarfs. Model analysis of the photometry and available spectroscopy shows that the seven confirmed new white dwarfs, and the two new likely white dwarfs, have effective temperatures in the range Teff = 5400-6600 K. Our analysis of the previously known white dwarf confirms that it is cool, with Teff = 3800 K. The cooling age for this dwarf is 8.7 Gyr, while that of the nine ~6000 K white dwarfs is 1.8-3.6 Gyr. We are unable to determine the masses of the white dwarfs from the existing data, and therefore we cannot constrain the total ages of the white dwarfs. The large cooling age for the coolest white dwarf in the sample, combined with its low estimated tangential velocity, suggests that it is an old member of the thin disk, or a member of the thick disk of the Galaxy, with an age 10-11 Gyr. The warmer white dwarfs appear to have velocities typical of the thick disk or even halo; these may be very old remnants of low-mass stars, or they may be relatively young thin disk objects with unusually high space motion.
We present medium resolution spectroscopy and multi-epoch VRI photometry for 21 new nearby (< 50 pc) white dwarf systems brighter than V ~ 17. Of the new systems, ten are DA (including a wide double degenerate system with two DA components), eight ar
e DC, two are DZ, and one is DB. In addition, we include multi-epoch VRI photometry for eleven known white dwarf systems that do not have trigonometric parallax determinations. Using model atmospheres relevant for various types of white dwarfs (depending on spectral signatures), we perform spectral energy distribution modeling by combining the optical photometry with the near-infrared JHK from the Two Micron All-Sky Survey to derive physical parameters (i.e., effective temperature and distance estimates). We find that twelve new and six known white dwarf systems are estimated to be within the NStars and Catalog of Nearby Stars horizons of 25 pc. Coupled with identical analyses of the 56 white dwarf systems presented in Paper XIX of this series, a total of 20 new white dwarf systems and 18 known white dwarf systems are estimated to be within 25 pc. These 38 systems of the 88 total studied represent a potential 34% increase in the 25 pc white dwarf population (currently known to consist of 110 systems with trigonometric parallaxes of varying qualities). We continue an ongoing effort via CTIOPI to measure trigonometric parallaxes for the systems estimated to be within 25 pc to confirm proximity and further fill the incompleteness gap in the local white dwarf population. Another 38 systems (both new and known) are estimated to be between 25 and 50 pc and are viable candidates for ground-based parallax efforts wishing to broaden the horizon of interest.
We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our G
alaxy, and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, <M> = 0.43 +/- 0.06 Msun, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791s evolved stars must have lost enough mass on the red giant branch to avoid the flash, and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.
