1,721,073 research outputs found
White dwarf luminosity and mass functions from sloan digital sky survey spectra
Full text linkWe 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 < g − i < 0.2 and 15 <g< 19.5. We address the observed upturn in log g for WDs with Teff 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.Fil: DeGennaro, Steven. University of Texas at Austin; Estados UnidosFil: von Hippel, Ted. University of Texas at Austin; Estados UnidosFil: Winget, D. E.. University of Texas at Austin; Estados UnidosFil: Kepler, S. O.. Universidade Federal do Rio Grande do Sul; BrasilFil: Nitta, Atsuko. Gemini Observatory; Estados UnidosFil: Koester, Detlev. Universität Kiel; AlemaniaFil: Althaus, Leandro Gabriel. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentin
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Fine Grid Asteroseismology Of G117-B15A And R548
Full text linkWe 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
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Limits Of Perturbative Nonlinear Light Curve Analyses: The Case Of G117-B15A
Full text linkThis poster examines the nonlinear combination frequencies found in the DAV G117-B15A. Using multiple epochs of observations we show that the ratio of the amplitude of the combination frequencies to those of the parent frequencies is not constant in time but rather varies on a time scale of years. This variation is difficult to explain in any model in which the modeled nonlinearities are second order, such as that of Brassard et al. [2], which accounts for these nonlinearities via the conversion from temperature variations to flux variations. In contrast, due to its extreme sensitivity, the convective light curve model can accomodate these variations with only modest variations in parameters such as T(eff).Astronom
Dark Stars: Improved Models And First Pulsation Results
Full text linkWe use the stellar evolution code MESA to study dark stars (DSs). DSs, which are powered by dark matter (DM) self-annihilation rather than by nuclear fusion, may be the first stars to form in the universe. We compute stellar models for accreting DSs with masses up to 10(6) M-circle dot. The heating due to DM annihilation is self-consistently included, assuming extended adiabatic contraction of DM within the minihalos in which DSs form. We find remarkably good overall agreement with previous models, which assumed polytropic interiors. There are some differences in the details, with positive implications for observability. We found that, in the mass range of 10(4)-10(5) M-circle dot, our DSs are hotter by a factor of 1.5 than those in Freese et al., are smaller in radius by a factor of 0.6, denser by a factor of three to four, and more luminous by a factor of two. Our models also confirm previous results, according to which supermassive DSs are very well approximated by (n = 3)-polytropes. We also perform a first study of DS pulsations. Our DS models have pulsation modes with timescales ranging from less than a day to more than two years in their rest frames, at z similar to 15, depending on DM particle mass and overtone number. Such pulsations may someday be used to identify bright, cool objects uniquely as DSs; if properly calibrated, they might, in principle, also supply novel standard candles for cosmological studies.Department of EnergyDOE-FG02-95ER40899Michigan Center for Theoretical Physics at the University of Michigan, Ann ArborNational Science Foundation AST-0909107, AST-1312983, PHYS-1066293NASA NNX12AC96GAstronom
The ZZ Ceti Red Edge
Full text linkWith a time-series CCD photometric survey, we have demonstrated clearly that the observed red edge for the ZZ Ceti stars instability strip at 11 000 K is real, with the pulsation amplitude decreasing at least by a factor of 50. Previous surveys for variability among hydrogen atmosphere white dwarfs around 11 000 K have been carried out using time-series photoelectric photometry, not differential photometry, insensitive for small amplitude periodicities of 15 min and longer. In our survey we constantly monitor the sky brightness as well as one or more comparison stars through the same color filter, reducing the adverse effects of differential extinction and sky fluctuations, obtaining true differential photometry.Chandra award GO3-14012NASA NAS8-03060Austrian Science Fund (FWF)Astronom
A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs: DBA And DB Stars
Full text linkWe measure apparent velocities (nu(app)) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres-16 dbAs and 20 dbs-using optical spectra taken for the European Southern Observatory SN Ia progenitor survey. We find a difference of 6.9 +/- 6.9 kms(-1) in the average apparent velocity of the H alpha lines versus that of the He I 5876 angstrom lines for our dbAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. to use the apparent velocity of the He I 5876 angstrom line and conduct the first gravitational redshift investigation of a group of WDs without visible hydrogen lines. We use biweight estimators to find an average apparent velocity, <nu(app)>(BI), (and hence average gravitational redshift, <nu(g)>(BI)) for our WDs; from that we derive an average mass, < M >(BI). For the dbAs, we find <nu(app)>(BI) = 40.8 +/- 4.7 kms(-1) and derive < M >(BI) = 0.71(-0.05)(+0.04) M-circle dot. Though different from <nu(app)> of DAs (32.57 km s(-1)) at the 91% confidence level and suggestive of a larger dbA mean mass than that for normal DAs derived using the same method (0.647(-0.014)(+0.013) M-circle dot; Falcon et al.), we do not claim this as a stringent detection. Rather, we emphasize that the difference between <nu(app)>(BI) of the dbAs and <nu(app)> of normal DAs is no larger than 9.2 kms(-1), at the 95% confidence level; this corresponds to roughly 0.10 M-circle dot. For the dbs, we find <nu(He)(app)>(BI) = 42.9 +/- 8.49 km s(-1) after applying the blueshift correction and determine < M >(BI) = 0.74(-0.09)(+0.08) M-circle dot. The difference between <nu(He)(app)>(BI) of the dbs and <nu(app)> of DAs is <= 11.5 kms(-1) (similar to 0.12 M-circle dot), at the 95% confidence level. The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by Voss et al. Given the small sample sizes, it is possible that systematic uncertainties are skewing our results due to the potential of kinematic substructures that may not average out. We estimate this to be unlikely, but a larger sample size is necessary to rule out these systematics.National Science Foundation AST-0909107, AST-0602288Norman Hackerman Advanced Research Program 003658-255-2007, 003658-0252-2009Institute for High Energy Density ScienceUniversity of Texas SystemSandia National LaboratoriesNational Physical Science ConsortiumDelaware Asteroseismic Research CenterNASA NNX11AG82G, HST-GO-11141McDonald Observator
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Limits On Planets Around Pulsating White Dwarf Stars
Full text linkWe 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
A New Timescale For Period Change In The Pulsating DA White Dwarf WD 0111+0018
Full text linkWe report the most rapid rate of period change measured to date for a pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s mode of WD0111+0018. The observed period change, faster than 10(-12) s s(-1), exceeds by more than two orders of magnitude the expected rate from cooling alone for this class of slow and simply evolving pulsating WDs. This result indicates the presence of an additional timescale for period evolution in these pulsating objects. We also measure the rates of period change of nonlinear combination frequencies and show that they share the evolutionary characteristics of their parent modes, confirming that these combination frequencies are not independent modes but rather artifacts of some nonlinear distortion in the outer layers of the star.Norman Hackerman Advanced Research Program 003658-0255-2007, 003658-0252-2009NASA Origins Program NAG5-13094National Science Foundation AST-0909107McDonald Observator
Two New Tidally Distorted White Dwarfs
Full text linkWe identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, <= 0.2 M-circle dot)WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 M-circle dot, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59% +/- 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074R(circle dot). For J2119, the radius exceeds 0.10R(circle dot). These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23% +/- 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.NSF AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009McDonald Observator
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A Gravitational Redshift Determination Of The Mean Mass Of DBA White Dwarfs
Full text linkWe measure apparent velocities (v(app)) of the H alpha and H beta Balmer line cores for 16 helium-dominated white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Following the gravitational redshift method employed by Falcon et al. [1], we find a mean apparent velocity of (v(app)) = 39.58 +/- 4.41 km s(-1) and use it to derive a mean mass of < M > = 0.701(-0.046)(+0.042) M(circle dot). Though the sample is small, the mean mass appears to be larger than the mean mass of DAs derived using the same method [0.647(-0.014)(+0.013) M(circle dot), 1]Astronom
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