Browsing by Subject "White dwarfs"
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Item Do metal-polluted stars of the ZZ ceti instability strip have a distinct asteroseismic signature?(2015-08) Jumper, Kevin Arthur; Winget, Donald Earl, 1955-; Montgomery, Michael Houston; Sneden, ChrisCooling DA stars that pass through the ZZ instability strip, a region between temperatures of approximately 12,600 K to 11,100 K, tend to experience the driving of g-mode pulsations near their surface layers. These pulsations cause variations in the luminosities of such stars, leading them to be known as DAVs. A fraction of DAVs also have photospheres contaminated by metals, usually thought to be from the tidally disrupted remnants of planetary systems. The high resolution spectroscopy needed to make definite identifications of these metal lines is relatively demanding, whereas it is simple to obtain photometric data on the pulsation periods of DAV stars. Therefore, if known metal-polluted DAVs (DAZVs) have systematic differences in their photometric data compared to that of DAVs that lack such pollution, photometry could provide an easy way to determine which stars are likely to contain metals in their photospheres in the future. However, we find that the known DAZV population is not large enough to permit its behavior to be distinguished from that of the normal DAV population at the present time, though extremely low-mass white dwarfs may help expand the populations and improve the quality of our fits.Item A gravitational redshift determination of the mean mass of white dwarfs. DA stars.(2010-08) Falcon, Ross Edward; Winget, Donald Earl, 1955-; Montgomery, Michael H.We measure apparent velocities (v_app) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, : we find = = 32.57+/-1.17 km/s. Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647+0.013-0.014 Msun. We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic Teff both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing Teff. Our results are consistent with no significant change in mean mass: ^hot = 0.640+/-0.014 Msun and ^cool = 0.686+0.035-0.039 Msun.Item White dwarfs and the ages of stellar populations(2009-12) De Gennaro, Steven Andrew; Winget, Donald Earl, 1955-; von Hippel, Ted; Robinson, Edward L.; Bromm, Volker; van Dyk, DavidOur group has developed a Bayesian modeling technique to determine the ages of stellar populations (in particular, open and globular clusters) using white dwarf (WD) cooling physics. As the theory of WD cooling is both simpler than, and essentially independent of, main sequence evolutionary theory, white dwarfs provide an independent measure of the ages of Galactic populations. We have developed a Bayesian technique that objectively incorporates our prior knowledge of stellar evolution, star cluster properties, and data quality estimates to derive posterior probability distributions for a cluster's age, metallicity, distance, and line-of-sight absorption, as well as the individual stellar parameters of mass, mass ratio (for unresolved binaries) and cluster membership probability. The key advantage of our Bayesian method is that we can calculate probability distributions for cluster and stellar parameters with reference only to known, quantifiable, objective, and repeatable quantities. In doing so, we also have more sensitivity to subtle changes in cluster isochrones than traditional ``chi-by-eye'' cluster fitting methods. As a critical test of our Bayesian modeling technique, we apply it to Hyades UBV photometry, with membership priors based on proper motions and radial velocities, where available. We use secular parallaxes derived from Hipparcos proper motions via the moving cluster method to put all members of the Hyades at a common distance. Under the assumption of a particular set of WD cooling and atmosphere models, we estimate the age of the Hyades based on cooling white dwarfs to be 610 +- 110 Myr, consistent with the best prior analysis of the cluster main-sequence turn-off age (Perryman, et al. 1998). Since the faintest white dwarfs have most likely evaporated from the Hyades, prior work provided only a lower limit to the cluster's white dwarf age. Our result demonstrates the power of the bright white dwarf technique for deriving ages (Jeffery, et al. 2007) and further demonstrates complete age consistency between white dwarf cooling and main-sequence turn-off ages for seven out of seven clusters analyzed to date, ranging from 150 Myr to 4 Gyr. We then turn our attention to the white dwarf luminosity function. We use Sloan Digital Sky Survey (SDSS) data to create a white dwarf luminosity function with nearly an order of magnitude (3,358) more spectroscopically confirmed white dwarfs than any previous work. We determine the completeness of the SDSS spectroscopic white dwarf sample by comparing a proper-motion selected sample of WDs from SDSS imaging data with a large catalog of spectroscopically determined WDs. We derive a selection probability as a function of a single color (g-i) and apparent magnitude (g) that covers the range -1.0 < g-i < 0.2 and 15 < g < 19.5. We address the observed upturn in log g for white dwarfs with Teff < ~12,000K and offer arguments that the problem is limited to the line profiles and is not present in the continuum. We offer an empirical method of removing the upturn, recovering a reasonable mass function for white dwarfs with Teff < 12,000K. Finally, we outline several other current and future applications of our method and our code to determine not only ages of Galactic stellar populations, but helium abundances of clusters, ages of individual field WDs, and the initial (main sequence) to final (WD) mass relation.