526 research outputs found
Preliminary Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology
For many years, astronomers have promised that the study of pulsating white
dwarfs would ultimately lead to useful information about the physics of matter
under extreme conditions of temperature and pressure. In this paper we finally
make good on that promise. Using observational data from the Whole Earth
Telescope and a new analysis method employing a genetic algorithm, we
empirically determine that the central oxygen abundance in the
helium-atmosphere variable white dwarf GD 358 is 84+/-3 percent. We use this
value to place preliminary constraints on the 12C(alpha,gamma)16O nuclear
reaction cross-section. More precise constraints will be possible with
additional detailed simulations. We also show that the pulsation modes of our
best-fit model probe down to the inner few percent of the stellar mass. We
demonstrate the feasibility of reconstructing the internal chemical profiles of
white dwarfs from asteroseismological data, and find an oxygen profile for GD
358 that is qualitatively similar to recent theoretical calculations.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 6
figures, 2 tables, uses emulateapj5.st
Spitzer White Dwarf Planet Limits
We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of for 8 stars assuming
ages of , and for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
Dwarf Worksho
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Fine Grid Asteroseismology Of G117-B15A And R548
We now have a good measurement of the cooling rate of G117-B15A. In the near future, we will have equally well determined cooling rates for other pulsating white dwarfs, including R548. The ability to measure their cooling rates offers us a unique way to study weakly interacting particles that would contribute to their cooling. Working toward that goal, we perform a careful asteroseismological analysis of G117-B15A and R548. We study them side by side because they have similar observed properties. We carry out a systematic, fine grid search for best-fit models to the observed period spectra of those stars. We freely vary four parameters: the effective temperature, the stellar mass, the helium layer mass, and the hydrogen layer mass. We identify and quantify a number of uncertainties associated with our models. Based on the results of that analysis and fits to the periods observed in R548 and G117-B15A, we clearly define the regions of the four-dimensional parameter space occupied by the best-fit models.NSF AST 05-07639Astronom
Driving in ZZ Ceti stars - Problem solved?
There is a fairly tight correlation between the pulsation periods and
effective temperatures of ZZ Ceti stars (cooler stars have longer periods).
This seems to fit the theoretical picture, where driving occurs in the partial
ionization zone, which lies deeper and deeper within the star as it cools. It
is reasonable to assume that the pulsation periods should be related to the
thermal timescale in the region where driving occurs. As that region sinks
further down below the surface, that thermal timescale increases. Assuming this
connection, the pulsation periods could provide an additional way to determine
effective temperatures, independent of spectroscopy. We explore this idea and
find that in practice, things are not so simple.Comment: 4 pages, 3 figure
White Dwarfs In Ngc6397 And M4: Constraints On The Physics Of Crystallization
We explore the physics of crystallization in the dense Coulomb plasma of the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from Hubble Space Telescope photometry of the globular cluster M 4 and compare it with our results for proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with a binary mixture of carbon and oxygen crystallizing at a value of Gamma higher than the theoretical value for a One Component Plasma (OCP). We show that this result is in line with the latest Molecular Dynamics simulations for binary mixtures of C/O. We discuss implications for future work.Astronom
The search for ZZ Ceti stars in the original Kepler mission
We report the discovery of 42 white dwarfs in the original Kepler mission
field, including nine new confirmed pulsating hydrogen-atmosphere white dwarfs
(ZZ Ceti stars). Guided by the Kepler-INT Survey (KIS), we selected white dwarf
candidates on the basis of their U-g, g-r, and r-H_alpha photometric colours.
We followed up these candidates with high-signal-to-noise optical spectroscopy
from the 4.2-m William Herschel Telescope. Using ground-based, time-series
photometry, we put our sample of new spectroscopically characterized white
dwarfs in the context of the empirical ZZ Ceti instability strip. Prior to our
search, only two pulsating white dwarfs had been observed by Kepler.
Ultimately, four of our new ZZ Cetis were observed from space. These rich
datasets are helping initiate a rapid advancement in the asteroseismic
investigation of pulsating white dwarfs, which continues with the extended
Kepler mission, K2.Comment: 9 pages, 6 figures, accepted for publication in MNRA
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