258 research outputs found
SDSS J142625.71+575218.3: A Prototype for A New Class of Variable White Dwarf
We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71 + 575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s ( first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.NSF AST-0507639, AST-0602288, AST-0607480, AST-0307321Astronom
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Limits On Planets Around Pulsating White Dwarf Stars
We present limits on planetary companions to pulsating white dwarf stars. A subset of these stars exhibit extreme stability in the period and phase of some of their pulsation modes; a planet can be detected around such a star by searching for periodic variations in the arrival time of these pulsations. We present limits on companions greater than a few Jupiter masses around a sample of 15 white dwarf stars as part of an ongoing survey. One star shows a variation in arrival time consistent with a 2M(J) planet in a 4.5 yr orbit. We discuss other possible explanations for the observed signal and conclude that a planet is the most plausible explanation based on the data available.NASA Origins NAG5-13094Astronom
Graduate Sessions 1: Sylvia Lavin
Sylvia Lavin is Professor of Architecture at UCLA and writes widely on contemporary architecture and theory. She recently completed a year as a Getty Scholar where she was working on her next book, The Flash in the Pan and Other Forms of Architectural Contemporaneity. She is co-editor of Crib Sheets (Monacelli Press, 2005) and the author of Form Follows Libido: Architecture and Richard Neutra in a Psychoanalytic Culture (MIT Press, 2005)
White dwarf luminosity and mass functions from sloan digital sky survey spectra
We present the first phase in our ongoing work to use Sloan Digital Sky Survey (SDSS) data to create separate white dwarf (WD) luminosity functions (LFs) for two or more different mass ranges. In this paper, we determine the completeness of the SDSS spectroscopic WD 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 eff≲ 12,000 K 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 WDs with Teff< 12,000 K. Finally, we present a WD LF with nearly an order of magnitude (3358) more spectroscopically confirmed WDs than any previous work.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
We explore the physics of crystallization in the deep interiors of white
dwarf stars using the color-magnitude diagram and luminosity function
constructed from proper motion cleaned Hubble Space Telescope photometry of the
globular cluster NGC 6397. We demonstrate that the data are consistent with the
theory of crystallization of the ions in the interior of white dwarf stars and
provide the first empirical evidence that the phase transition is first order:
latent heat is released in the process of crystallization as predicted by van
Horn (1968). We outline how this data can be used to observationally constrain
the value of Gamma = E_{Coulomb}/E_{thermal} near the onset of crystallization,
the central carbon/oxygen abundance, and the importance of phase separation.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letter
Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology
We present some of the results of a survey aimed at exploring the
asteroseismological potential of the newly-discovered carbon-atmosphere white
dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere
white dwarfs may drive low-order gravity modes. We demonstrate that our
theoretical results are consistent with the recent exciting discovery of
luminosity variations in SDSS J1426+5752 and some null results obtained by a
team of scientists at McDonald Observatory. We also present follow-up
photometric observations carried out by ourselves at the Mount Bigelow 1.6-m
telescope using the new Mont4K camera. The results of follow-up spectroscopic
observations at the MMT are also briefly reported, including the surprising
discovery that SDSS J1426+5752 is not only a pulsating star but that it is also
a magnetic white dwarf with a surface field near 1.2 MG. The discovery of
-mode pulsations in SDSS J1426+5752 is quite significant in itself as it
opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ
Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
New Techniques to Determine Ages of Open Clusters Using White Dwarfs
Currently there are two main techniques for independently determining the
ages of stellar populations: main sequence evolution theory (via cluster
isochrones) and white dwarf cooling theory. Open clusters provide the ideal
environment for the calibration of these two clocks. Because current techniques
to derive cluster ages from white dwarfs are observationally challenging, we
discuss the feasibility of determining white dwarf ages from the brighter white
dwarfs alone. This would eliminate the requirement of observing the coolest
(i.e., faintest) white dwarfs. We discuss our method for testing this new idea,
as well as the required photometric precision and prior constraints on
metallicity, distance, and reddening. We employ a new Bayesian statistical
technique to obtain and interpret results.Comment: 15 pages, 5 figures. accepted to Astrophysical Journa
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DA Stars
We measure apparent velocities (v_app) of the Halpha and Hbeta Balmer line
cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical
spectra taken for the ESO SN Ia Progenitor surveY (SPY; Napiwotzki et al.
2001). Assuming these WDs are nearby and co-moving, 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 (2009). 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 12000 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.Comment: Accepted for publication in ApJ, 14 pages, 12 figure
White dwarf luminosity and mass functions from sloan digital sky survey spectra
We present the first phase in our ongoing work to use Sloan Digital Sky Survey (SDSS) data to create separate white dwarf (WD) luminosity functions (LFs) for two or more different mass ranges. In this paper, we determine the completeness of the SDSS spectroscopic WD 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 eff≲ 12,000 K 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 WDs with Teff< 12,000 K. Finally, we present a WD LF with nearly an order of magnitude (3358) more spectroscopically confirmed WDs than any previous work.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Customer Specific Transaction Risk Management in eCommerce
Increasing potential for turnover in e-commerce is inextricably linked with an increase in risk. Online retailers (e-tailers), aiming for a company-wide value orientation should manage this risk. However, current approaches to risk management either use average retail prices elevated by an overall risk premium or restrict the payment methods offered to customers. Thus, they neglect customer-specific value and risk attributes and leave turnover potentials unconsidered. To close this gap, an innovative valuation model is proposed in this contribution that integrates customer-specific risk and potential turnover. The approach presented evaluates different payment methods using their risk-turnover characteristic, provides a risk-adjusted decision basis for selecting payment methods and allows e-tailers to derive automated risk management decisions per customer and transaction without reducing turnover potential
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