White dwarf atmosphere models with Ly-alpha opacity  in the analysis of the white dwarf cooling sequence of NGC 6397

Kowalski, Piotr M.

doi:10.1051/0004-6361:20077985

Cited by 14 publications

(25 citation statements)

References 20 publications

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“…A comparison in Fig. 10 of our new synthetic spectra (solid lines) with evaluations from Kowalski (2007) (symbols) for T eff = 3000 K and 5000 K shows an excellent agreement.…”

Section: Resultsmentioning

confidence: 53%

Lyman α wing absorption in cool white dwarf stars

Rohrmann¹,

Althaus²

2010

Monthly Notices of the Royal Astronomical Society

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Kowalski & Saumon identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Lyman α 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 α 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 in 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 α 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 Lyman α 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 broad‐band colour indices for model atmospheres with Teff < 5000 K. These results confirm those previously obtained by Kowalski & Saumon. 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.

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“…A comparison in Fig. 10 of our new synthetic spectra (solid lines) with evaluations from Kowalski (2007) (symbols) for T eff = 3000 K and 5000 K shows an excellent agreement.…”

Section: Resultsmentioning

confidence: 53%

Lyman α wing absorption in cool white dwarf stars

Rohrmann¹,

Althaus²

2010

Monthly Notices of the Royal Astronomical Society

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“…We first fit pure-hydrogen atmosphere models (Kowalski 2006b;Kowalski 2007;Kowalski & Saumon 2006) to the photometry of J1102+4113 (excluding the u band, whose uncertainties are too large to be useful), assuming a typical WD gravity of log g = 8 (e.g., Fontaine et al 2001). The resulting fit yields T eff = 3830 K. Synthetic photometry from this model is shown as the blue triangles in Figure 4.…”

Section: Comparison With Pure-hydrogen Modelsmentioning

confidence: 99%

A Nearby Old Halo White Dwarf Candidate From the Sloan Digital Sky Survey

Kowalski

Harris

Awal

et al. 2008

The Astronomical Journal

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We report the discovery of a nearby old halo white dwarf (WD) candidate from the Sloan Digital Sky Survey (SDSS). SDSS J110217.48+411315.4 has a proper motion of 1 .75 yr −1 and redder optical colors than all other known featureless (type DC) WDs. We present SDSS imaging and spectroscopy of this object, along with near-infrared photometry obtained at the United Kingdom Infrared Telescope (UKIRT). Fitting its photometry with up-to-date model atmospheres, we find that its overall spectral energy distribution is fit reasonably well with a pure-hydrogen composition and T eff ≈ 3800 K (assuming log g = 8). This temperature and gravity would place this WD at 35 pc from the Sun with a tangential velocity of 290 km sand space velocities consistent with halo membership; furthermore, its combined main-sequence and WD cooling age would be ≈11 Gyr. However, if this object is a massive WD, it could be a younger object with a thick disk origin. Whatever its origin, the optical colors of this object are redder than predicted by any current pure-hydrogen, pure-helium, or mixed hydrogen-helium atmospheric model, indicating that there remain problems in our understanding of the complicated physics of the dense atmospheres of cool WDs.

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“…Hansen et al (2004Hansen et al ( , 2007 and Bedin et al (2009) use these data to derive cooling ages of ≈12 Gyr for the two clusters. The coolest WDs in these clusters are about 650 ± 230 K cooler than the coolest WDs in the disk (Kowalski 2007). These studies demonstrate that the Galactic halo is older than the disk by 2 Gyr (Hansen et al 2002;Fontaine et al 2001;Kowalski 2007).…”

Section: Introductionmentioning

confidence: 99%

“…The coolest WDs in these clusters are about 650 ± 230 K cooler than the coolest WDs in the disk (Kowalski 2007). These studies demonstrate that the Galactic halo is older than the disk by 2 Gyr (Hansen et al 2002;Fontaine et al 2001;Kowalski 2007). Even though the WDs in globular clusters provide reliable age estimates, these clusters may not represent the full age range of the Galactic halo.…”

Section: Introductionmentioning

confidence: 99%

VISITORS FROM THE HALO: 11 Gyr OLD WHITE DWARFS IN THE SOLAR NEIGHBORHOOD

Kilic

Munn

Williams

et al. 2010

ApJ

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We report the discovery of three nearby old halo white dwarf (WD) candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 deg 2 proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3 m telescopes, and they display proper motions of 0. 4-0. 5 yr −1 . Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere WDs with T eff ≈ 3700-4100 K. For average mass WDs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km s −1 . Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and WD cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field WDs currently known. These three stars represent only a small fraction of the halo WD candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo WDs that can be used to constrain the age of the Galactic thick disk and halo.

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White dwarf atmosphere models with Ly-alpha opacity in the analysis of the white dwarf cooling sequence of NGC 6397

Cited by 14 publications

References 20 publications

Lyman α wing absorption in cool white dwarf stars

Lyman α wing absorption in cool white dwarf stars

A Nearby Old Halo White Dwarf Candidate From the Sloan Digital Sky Survey

VISITORS FROM THE HALO: 11 Gyr OLD WHITE DWARFS IN THE SOLAR NEIGHBORHOOD

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