Pure-hydrogen 3D model atmospheres of cool white dwarfs

A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs is presented for a surface gravity of log g = 8 and effective temperatures from 6000 to 13,000 K. We show that convective properties, such as flow velocities, characteristic granulation size and intensity contrast of the granulation patterns, change significantly over this range. We demonstrate that these 3D simulations are not sensitive to numerical parameters unlike the 1D structures that considerably depend on the mixing-length parameters. We conclude that 3D spectra can be used directly in the spectroscopic analyses of DA white dwarfs. We confirm the result of an earlier preliminary study that 3D model spectra provide a much better characterization of the mass distribution of white dwarfs and that shortcomings of the 1D mixing-length theory are responsible for the spurious high-log g determinations of cool white dwarfs. In particular, the 1D theory is unable to account for the cooling effect of the convective overshoot in the upper atmospheres.Comment: 14 pages, 17 figures, accepted for publication in Astronomy and Astrophysic

Spectroscopic analysis of DA white dwarfs with 3D model atmospheres

We present the first grid of mean three-dimensional (3D) spectra for pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use CO5BOLD radiation-hydrodynamics 3D simulations instead of the mixing-length theory for the treatment of convection. The simulations cover the effective temperature range of 6000 < Teff (K) < 15,000 and the surface gravity range of 7 < log g < 9 where the large majority of DAs with a convective atmosphere are located. We rely on horizontally averaged 3D structures (over constant Rosseland optical depth) to compute spectra. It is demonstrated that our spectra can be smoothly connected to their 1D counterparts at higher and lower Teff where the 3D effects are small. Analytical functions are provided in order to convert spectroscopically determined 1D effective temperatures and surface gravities to 3D atmospheric parameters. We apply our improved models to well studied spectroscopic data sets from the Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm that the so-called high-log g problem is not present when employing spectra and that the issue was caused by inaccuracies in the 1D mixing-length approach. The white dwarfs with a radiative and a convective atmosphere have derived mean masses that are the same within ~0.01 Msun, in much better agreement with our understanding of stellar evolution. Furthermore, the 3D atmospheric parameters are in better agreement with independent Teff and log g values from photometric and parallax measurements.Comment: 15 pages, 18 figures, 10 pages online appendix, accepted for publication in Astronomy and Astrophysic

Spin susceptibility of interacting electrons in one dimension: Luttinger liquid and lattice effects

The temperature-dependent uniform magnetic susceptibility of interacting electrons in one dimension is calculated using several methods. At low temperature, the renormalization group reaveals that the Luttinger liquid spin susceptibility $\chi (T)$ approaches zero temperature with an infinite slope in striking contrast with the Fermi liquid result and with the behavior of the compressibility in the absence of umklapp scattering. This effect comes from the leading marginally irrelevant operator, in analogy with the Heisenberg spin 1/2 antiferromagnetic chain. Comparisons with Monte Carlo simulations at higher temperature reveal that non-logarithmic terms are important in that regime. These contributions are evaluated from an effective interaction that includes the same set of diagrams as those that give the leading logarithmic terms in the renormalization group approach. Comments on the third law of thermodynamics as well as reasons for the failure of approaches that work in higher dimensions are given.Comment: 21 pages, latex including 5 eps figure

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows the comparison of convective processes in significantly different environments. We show that the relative intensity contrast is correlated with both the Mach and Peclet numbers in the photosphere. The horizontal size of granules varies between 3 and 10 times the local pressure scale height, with a tight correlation between the factor and the Mach number of the flow. Given that convective giants, dwarfs, and white dwarfs cover the same range of Mach and Peclet numbers, we conclude that photospheric convection operates in a very similar way in those objects.Comment: 16 pages, 17 figures, 37 pages online appendix, accepted for publication in Astronomy and Astrophysic

Spectroscopic and photometric studies of white dwarfs in the Hyades

The Hyades cluster is known to harbour ten so-called classical white dwarf members. Numerous studies through the years have predicted that more than twice this amount of degenerate stars should be associated with the cluster. Using the PPMXL catalog of proper motions and positions, a recent study proposed 17 new white dwarf candidates. We review the membership of these candidates by using published spectroscopic and photometric observations, as well as by simulating the contamination from field white dwarfs. In addition to the ten classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades origin and three more objects that have an uncertain membership status due to their unknown or imprecise atmospheric parameters. Among those, two to three are expected as field stars contamination. Accurate radial velocity measurements will confirm or reject the candidates. One consequence is that the longstanding problem that no white dwarf older than ~340 Myr appears to be associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and Astrophysics journa

A Methodological Approach to Selective Cutbacks

At times - when budget must be severely reduced on short notice and when sensible decisions must arise from complex situations - planners have to work out simple but reliable measures of resource allocation and reduction. This paper recognizes the need for applied research in this area and proposes a budget reduction formula that has the advantage of relating financial planning to program evaluation, faculty resources, and activity levels of departmental units. The tool developed is flexible, as it can be manipulated without endangering its basic assumptions to describe best each unit's intrinsic characteristics. Furthermore, its redistributive effects can be readily measured and made to respond to institutional priorities.Lorsque les budgets doivent être réduits rapidement de façon substantielle et que des décisions acceptables doivent être prises à partir de situations compliquées, les planificateurs et gestionnaires ont besoin de s'appuyer sur des approches simples et fiables d'allocation et de réduction de ressources. Cet article reconnaît la nécessité de la recherche appliquée dans ce domaine et propose une formule de réduction budgétaire qui a l'avantage d'établir la relation entre la planification financière et l'évaluation des programmes, les ressources professorales et les activités d'enseignement des unités académiques. La flexibilité de l'instrument permet de respecter les caractéristiques disciplinaires de chaque unité. De plus, \les résultats de l'application de la méthodologie donne la possibilité d'évaluer rapide-ment les effets redistributifs des réductions budgétaires et d'y porter un jugement à la lumière des priorités institutionnelles

3D Model Atmospheres for Extremely Low-Mass White Dwarfs

We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg = 5-6.5 (cgs units) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., logg = 5-9). For low-mass WDs, the correction in temperature is relatively small (a few per cent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.Comment: 11 pages, 8 figures, accepted for publication in the Astrophysical Journa