Lyman-alpha wing absorption in cool white dwarf stars

Kowalski & Saumon (2006) identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by the collisions between atomic and molecular hydrogen and successfully explained entire spectra of many cool DA-type white dwarfs. Owing to the important astrophysical implications of this issue, we present here an independent assessment of the process. For this purpose, we compute free-free quasi-molecular absorption in Lyman-alpha due to collisions with H and H2 within the one-perturber, quasi-static approximation. Line cross-sections are obtained using theoretical molecular potentials to describe the interaction between the radiating atom and the perturber. The variation of the electric-dipole transition moment with the interparticle distance is also considered. Six and two allowed electric dipole transitions due to H-H and H-H2 collisions, respectively, are taken into account. The new theoretical Lyman-alpha line profiles are then incorporated in our stellar atmosphere program for the computation of synthetic spectra and colours of DA-type white dwarfs. Illustrative model atmospheres and spectral energy distributions are computed, which show that Ly-alpha broadening by atoms and molecules has a significant effect on the white dwarf atmosphere models. The inclusion of this collision-induced opacity significantly reddens spectral energy distributions and affects the broadband colour indices for model atmospheres with Teff<5000 K. These results confirm those previously obtained by Kowalski & Saumon (2006). Our study points out the need for reliable evaluations of H3 potential energy surfaces covering a large region of nuclear configurations, in order to obtain a better description of H-H2 collisions and a more accurate evaluation of their influence on the spectrum of cool white dwarfs.Comment: 11 pages, 12 figures, 1 table, to be published in MNRA

The ages and colours of cool helium-core white dwarf stars

The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-gray model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 solar masses and follow their evolution from the end of mass loss episodes during their pre-white dwarf evolution down to very low surface luminosities. We find that when the effective temperature decreases below 4000K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour-colour and colour-magnitude diagrams and we find that helium-core white dwarfs with masses ranging from approx. 0.18 to 0.3 solar masses can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M_V approx. 16.5. In view of these results, many low-mass helium white dwarfs could have had time enough to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.Comment: 11 pages, 9 figures. Accepted for publication in MNRA

Revisiting the luminosity function of single halo white dwarfs

White dwarfs are the fossils left by the evolution of low-and intermediate-mass stars, and have very long evolutionary timescales. This allows us to use them to explore the properties of old populations, like the Galactic halo. We present a population synthesis study of the luminosity function of halo white dwarfs, aimed at investigating which information can be derived from the currently available observed data. We employ an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences for metal poor progenitors as well as an accurate modeling of the observational biases. We find that because the observed sample of halo white dwarfs is restricted to the brightest stars only the hot branch of the white dwarf luminosity function can be used for such purposes, and that its shape function is almost insensitive to the most relevant inputs, like the adopted cooling sequences, the initial mass function, the density profile of the stellar spheroid, or the adopted fraction of unresolved binaries. Moreover, since the cut-off of the observed luminosity has not been yet determined only lower limits to the age of the halo population can be placed. We conclude that the current observed sample of the halo white dwarf population is still too small to obtain definite conclusions about the properties of the stellar halo, and the recently computed white dwarf cooling sequences which incorporate residual hydrogen burning should be assessed using metal-poor globular clusters.Comment: 9 pages, 9 figures, accepted for publication in A&

Outer boundary conditions for evolving cool white dwarfs

White dwarf evolution is essentially a gravothermal cooling process, which,for cool white dwarfs, sensitively depends on the treatment of the outer boundary conditions. We provide detailed outer boundary conditions appropriate for computing the evolution of cool white dwarfs employing detailed non-gray model atmospheres for pure H composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are computed taken into account non-ideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman alpha quasi-molecular opacity. Our results show that the use of detailed outer boundary conditions becomes relevant for effective temperatures lower than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively. Detailed model atmospheres predict ages that are up to approx 10% shorter at log L/L_sun=-4 when compared with the ages derived using Eddington-like approximations at tau_Ross=2/3. We also analyze the effects of various assumptions and physical processes of relevance in the calculation of outer boundary conditions. In particular, we find that the Ly_alpha red wing absorption does not affect substantially the evolution of white dwarfs. White dwarf cooling timescales are sensitive to the surface boundary conditions for T_eff < 6000K. Interestingly enough, non-gray effects have little consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in their cooling rates, except throughout the Rosseland mean opacity.Comment: 6 pages, 9 figures, to be published in Astronomy and Astrophysic

DQ white-dwarf stars with low C abundance: Possible progenitors

The present paper focuses on the evolution of hydrogen-deficient white dwarfs with the aim of exploring the consequences of different initial envelope structures on the carbon abundances expected in helium-rich, carbon-contaminated DQ white dwarfs. In particular, the evolutionary link between the DQs with low detected carbon abundances and the PG1159, extreme horizontal branch, and helium-rich R Coronae Borealis (RCrB) stars is explored. We present full evolutionary calculations that take a self-consistent treatment of element diffusion into account as well as expectations for the outer layer chemical stratification of progenitor stars upon entering the white dwarf regime. We find that PG1159 stars cannot be related to any DQ white dwarfs with low C abundances. Instead, we suggest that the latter could constitute the progeny of the giant, helium-rich RCrB stars.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and Astrophysic

Gravitational settling of 22Ne and white dwarf evolution

We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies, which adopted a simplified treatment of the effects of 22Ne sedimentation, this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior the delay introduced by the ensuing chemical differentation is minor for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd about 1.0 Msun, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of about 2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize as well on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.Comment: To apper in The Astrophysical Journa

Prácticas de manejo e higiénico-sanitarias en las explotaciones lecheras y recuento de células somáticas

This study was done to establish relationships between management and sanitary practices on ewe dairy farms and the quality of milk produced. For this purpose, a survey was carried out on 118 farms and a total of 121,117 animals in the Castilla-La Mancha region (Southeast Spain) in which the veterinarians of the Association of Sanitary Defence participated. Flocks varied considerably in size, ranging from 120-7,200 sheep, with an average milking period lasting 3-5 months and milk yields of between 50 and 150 litres per lactation by mechanical systems on 75.0% of farms. Sanitary practices during milking are still not commonplace; 53.3% performed post milking teat disinfection, 37.0% did the California mastitis test, 31.1% had a dry therapy programme and only 18.6% of farms kept records of animals with clinical mastitis. Nonetheless, the results revealed that 42.7% of the farms surveyed showed good quality hygienic milk (bulk milk somatic cell count, BMSCC&lt;600 &times; 103 cells mL-1). On most farms, the rate of animals with clinical mastitis was less than 5.0%, while that of subclinical mastitis cases was less than 5.0% on 25.6% of farms. Certain factors like mechanical milking, post milking teat disinfection, recording animals with clinical mastitis and controlling the frequency of animals with subclinical mastitis did not exceed 5.0%. Those risk factors for increased BMSCC levels were significant. Therefore, farmers are recommended to bear these factors in mind, and to instil the importance of carrying out these practices when implementing a good dairy farm practices system.El trabajo se llev&oacute; a cabo para conocer las condiciones higi&eacute;nico-sanitarias de las explotaciones de ovino manchego, y su posible relaci&oacute;n con la sanidad de la ubre y la calidad de la leche obtenida. Para ello se elabor&oacute; una encuesta que se dirigi&oacute; a los veterinarios responsables de Agrupaciones de Defensa Sanitaria de Castilla-La Mancha. Se analizaron las encuestas de 118 explotaciones, que incluyen un total de 121.117 animales de orde&ntilde;o. Los resultados obtenidos mostraron que, a pesar de la enorme variabilidad en el tama&ntilde;o de las explotaciones (desde 120 hasta 7.200 ovejas), la mayor&iacute;a de ellas poseen sala de orde&ntilde;o (75%), siendo la duraci&oacute;n media del orde&ntilde;o entre 3-5 meses y la producci&oacute;n de leche por oveja y lactaci&oacute;n entre 50-150 L. Las pr&aacute;cticas sanitarias durante el orde&ntilde;o no est&aacute;n muy extendidas, ya que el 53,3% realiza sellado postorde&ntilde;o, el 37,0% lleva a cabo el test de California, el 31,1% sigue la terapia de secado y el 18,6% de las explotaciones registra a los animales con mastitis. Del total de explotaciones encuestadas, 42,7% producen leche con un recuento de c&eacute;lulas som&aacute;ticas &lt; 600&nbsp;&times; 103 c&eacute;lulas mL&ndash;1. La mayor&iacute;a de ellas posee valores por&nbsp; debajo del 5% de animales con mastitis cl&iacute;nicas, y entre las subcl&iacute;nicas, m&aacute;s de la cuarta parte de las granjas (25,6%) tienen una incidencia &lt; 5%. Pr&aacute;cticas como el orde&ntilde;o mec&aacute;nico, el sellado postorde&ntilde;o, el registro de animales con mastitis cl&iacute;nicas y controlar que la frecuencia de animales con mastitis subcl&iacute;nica no supere el 5% fueron significativos factores de riesgo del incremento del recuento de c&eacute;lulas som&aacute;ticas en tanque. En conclusi&oacute;n, cuando se pretenda implantar un programa de buenas pr&aacute;cticas ganaderas en un grupo de explotaciones de ovino de leche, es recomendable incorporarlas para mejorar el recuento celular y por lo tanto la calidad higi&eacute;nico-sanitaria de la leche producida

Hierarchic Superposition Revisited

Many applications of automated deduction require reasoning in first-order logic modulo background theories, in particular some form of integer arithmetic. A major unsolved research challenge is to design theorem provers that are "reasonably complete" even in the presence of free function symbols ranging into a background theory sort. The hierarchic superposition calculus of Bachmair, Ganzinger, and Waldmann already supports such symbols, but, as we demonstrate, not optimally. This paper aims to rectify the situation by introducing a novel form of clause abstraction, a core component in the hierarchic superposition calculus for transforming clauses into a form needed for internal operation. We argue for the benefits of the resulting calculus and provide two new completeness results: one for the fragment where all background-sorted terms are ground and another one for a special case of linear (integer or rational) arithmetic as a background theory