Hydrogen-model atmospheres for white dwarf stars

Rohrmann, R. D.

doi:10.1046/j.1365-8711.2001.04298.x

Cited by 32 publications

(40 citation statements)

References 79 publications

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“…Serenelli et al (2001) and Rohrmann et al (2002) calculated evolutionary tracks of the formation and cooling of helium white dwarfs. They also give colours based on model atmospheres of Rohrmann (2001). From these we infer a temperature of Fig.…”

Section: Temperature Gravity and Radial Velocitiesmentioning

confidence: 99%

Temperature and cooling age of the white dwarf companion of PSR J0218+4232

Bassa

Kerkwijk

Kulkarni

2003

A&A

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Abstract. We report on Keck optical BVRI images and spectroscopy of the companion of the binary millisecond pulsar PSR J0218+4232. A faint bluish (V = 24.2, B − V = 0.25) counterpart is observed at the pulsar location. Spectra of this counterpart reveal Balmer lines which confirm that the companion is a helium-core white dwarf. We find that the white dwarf has a temperature of T eff = 8060 ± 150 K. Unfortunately, the spectra are of insufficient quality to put a strong constraint on the surface gravity, although the best fit is for low log g and hence low mass (∼0.2 M ), as expected. We compare predicted white dwarf cooling ages with the characteristic age of the pulsar and find similar values for white dwarf masses of 0.19 to 0.3 M . These masses would imply a distance of 2.5 to 4 kpc to the system. The spectroscopic observations also enable us to estimate the mass ratio between the white dwarf and the pulsar. We find q = 7.5 ± 2.4, which is consistent with the current knowledge of white dwarf companions to millisecond pulsars.

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Section: Temperature Gravity and Radial Velocitiesmentioning

confidence: 99%

Temperature and cooling age of the white dwarf companion of PSR J0218+4232

Bassa

Kerkwijk

Kulkarni

2003

A&A

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“…Bergeron et al 1995a;Rohrmann 2001). It is the so-called ML2 (Tassoul et al 1990), with its own specific choice of a, b and c, different from the case of BV58.…”

Section: Introductionmentioning

confidence: 99%

Stellar models with the ML2 theory of convection

Salaris¹,

Cassisi²

2008

A&A

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Context. The mixing length theory (MLT) used to compute the temperature gradient in superadiabatic layers of stellar (interior and atmosphere) models contains in its standard form 4 free parameters. Three parameters are fixed a priori (and define what we denote as the MLT "flavour") whereas one (the so-called mixing length) is calibrated by reproducing observational constraints. The "classical" Böhm-Vitense flavour is used in all modern MLT-based stellar model computations and, despite its crude approximations, the resulting T eff scale appears -perhaps surprisingly -remarkably realistic, once the mixing length parameter is calibrated with a solar model. Aims. Model atmosphere computations employ parameter choices different from what is used in stellar interior modelling, raising the question of whether a single MLT flavour and mixing length value can be used to compute interiors and atmospheres of stars of all types. As a first step towards addressing this issue, we study whether the MLT flavour (the so-called ML2) and mixing length choice that have been proven adequate to model white dwarf atmospheres, are able to provide, when used in stellar models, results at least comparable to the use of the "classical" Böhm-Vitense flavour. Methods. We have computed solar models and evolutionary tracks for both low-and intermediate-mass Population I and II stars, adopting both solar calibrated Böhm-Vitense and ML2 flavours of the MLT in our stellar evolution code, and state-of-the-art input physics.Results. The two sets of models provide consistent results, with only minor differences. Both calibrations reproduce also the T eff of red giants in a sample of Galactic globular clusters. The ML2 solar model provides a mixing length about half the value of the local pressure scale height, thus alleviating -but not eliminating -one of the well known inconsistencies of the MLT employed in stellar models. This mixing length is also consistent with the value used in white dwarf model atmosphere computations.

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“…In our calculations, the outer boundary conditions were obtained using the pure-hydrogen LTE model atmospheres described at length in Rohrmann et al (2001Rohrmann et al ( , 2002Rohrmann et al ( , 2011. Specifically, we computed pressure, temperature, and outer mass fraction at a Rosseland mean optical depth τ Ross = 25.1189 (log τ Ross = 1.4) for 40 000 K ≤ T eff ≤ 2000 K and 6.5 ≤ log g ≤ 9.5.…”

Section: Numerical Toolsmentioning

confidence: 99%

“…The importance of using detailed boundary conditions was first addressed by Hansen (1998Hansen ( , 1999. Over the years, detailed model atmospheres have been developed that include a complete treatment of energy absorption processes such as collision-induced opacity (Bergeron et al 1991;Saumon & Jacobson 1999;Rohrmann 2001) and the Lyman α wing absorption (Kowalski & Saumon 2006;Rohrmann et al 2011). A&A 546, A119 (2012) Here we provide detailed boundary conditions that allow consistently computation of the evolution of cool white dwarfs with pure hydrogen atmospheres.…”

Section: Introductionmentioning

confidence: 99%

Outer boundary conditions for evolving cool white dwarfs

Rohrmann

Althaus

Garcı́a–Berro

et al. 2012

A&A

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Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the outer boundary conditions. Aims. We provide detailed outer boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen 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. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of outer boundary conditions provided by updated model atmospheres on the cooling times of 0.60 and 0.90 M white dwarf sequences. Results. Our results show that the use of detailed outer boundary conditions becomes relevant for effective temperatures lower than 5800 K for sequences with 0.60 M and 6100 K with 0.90 M . Detailed model atmospheres predict ages that are up to ≈10% shorter at log(L/L ) = −4 when compared with the ages derived using Eddington-like approximations at τ Ross = 2/3. We also analyze the effects of various assumptions and physical processes that are relevant in the calculation of outer boundary conditions. In particular, we find that the Lyα red wing absorption does not substantially affect the evolution of white dwarfs. Conclusions. White dwarf cooling timescales are sensitive to the surface boundary conditions for T eff < ∼ 6000 K. Interestingly enough, nongray effects have few 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 no noticeable effects on their cooling rates, except throughout the Rosseland mean opacity.

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Hydrogen-model atmospheres for white dwarf stars

Cited by 32 publications

References 79 publications

Temperature and cooling age of the white dwarf companion of PSR J0218+4232

Temperature and cooling age of the white dwarf companion of PSR J0218+4232

Stellar models with the ML2 theory of convection

Outer boundary conditions for evolving cool white dwarfs

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