Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

Dufour, Patrick; Bergeron, P.; Fontaine, G.

doi:10.1086/430373

Cited by 142 publications

(228 citation statements)

References 32 publications

(86 reference statements)

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“…This model predicts a maximum contamination of carbon at an effective temperature of ∼10,000 K (corresponding approximately to the temperature at which the surface convection zone is maximal) before gradually decreasing with lower temperature, in agreement with atmospheric analysis determinations 9 . We note that although some of these objects show a very high level of carbon pollution, helium always remains the dominant constituent of the atmosphere (the highest carbon abundances by number (N) found are around logN(C)/N(He) ≈ −3).…”

supporting

confidence: 84%

White dwarf stars with carbon atmospheres

Dufour

Liebert

Fontaine

et al. 2007

Nature

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148

136

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White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 M ⊙ and 8-10 M ⊙ , where M ⊙ is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ∼80 per cent of known white dwarfs, by an additional hydrogen layer 1−3 . All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follow the asymptotic giant branch.Traces of carbon are typically observed as either neutral carbon lines or molecular C 2 Swan bands (defining the DQ spectral type) in cool helium-rich white dwarfs with effective temperatures (T eff ) below ∼ 13,000 K. The presence of carbon in the atmospheres of these objects has been explained successfully by a model in which carbon is dredged-up from the underlying carbon/oxygen core by the deep helium convection zone 8 . This model predicts a maximum contamination of carbon at an effective temperature of ∼10,000 K (corresponding approximately to the temperature at which the surface convection zone is maximal) before gradually decreasing with lower temperature, in agreement with atmospheric analysis determinations 9 . We note that although some of these objects show a very high level of

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supporting

confidence: 84%

White dwarf stars with carbon atmospheres

Dufour

Liebert

Fontaine

et al. 2007

Nature

Self Cite

148

136

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“…Plotting the photospheric densities of a DQ star cooling sequence in Fig. 2, which is based on the carbon abundances derived by Dufour et al (2005), we see that the atmospheres of DQ stars reach fluid-like densities and therefore high pressures at the temperatures of the observed DQ → DQp transition. This suggests the pressure effects as a good candidate to be responsible for the spectral distortions observed in DQp stars.…”

Section: Introductionmentioning

confidence: 90%

The origin of peculiar molecular bands in cool DQ white dwarfs

Kowalski¹

2010

A&A

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Aims. The DQ white dwarfs are stars whose atmosphere is enriched with carbon, which for cool stars (T eff < 8000 K) is indicated by the Swan bands of C 2 in the optical part of their spectra. With decreasing effective temperature these molecular bands undergo a significant blueshift (∼100-300 Å). The origin of this phenomenon has been disputed over the last two decades and has remained unknown. We attempt to address this problem by investigating the impact of dense helium on the spectroscopic properties of molecular carbon, the electronic Swan band transition energy T e and the vibrational frequency ω e , under the physical conditions encountered inside helium-rich, fluid-like atmospheres of cool DQ white dwarfs. Methods. In our investigation we use a density functional theory based quantum mechanical approach. Results. The electronic transition energy T e increases monotonically with the helium density (ΔT e (eV) ∼ 1.6 ρ (g/cm 3 )). This causes the Swan absorption to occur at shorter wavelengths compared with unperturbed C 2 . On the other hand the pressure-induced increase in the vibrational frequency is insufficient to account for the observed Swan bands shifts. Our findings are in line with the shape of the distorted molecular bands observed in DQp stars, but the predicted photospheric density required to reproduce these spectral features is one order of magnitude lower than the one predicted by the current models. This indicates pollution by hydrogen or reflects incomplete knowledge of the properties of fluid-like atmospheres of these stars. Conclusions. Our work shows that at the physical conditions encountered in the fluid-like atmospheres of cool DQ white dwarfs the strong interactions between C 2 and helium atoms cause an increase in T e , which should produce a blueward shift of the Swan bands. This is consistent with the observations and indicates that the observed Swan-like molecular bands are most likely the pressure-shifted bands of C 2 .

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“…Lacking information on its circumstellar environment, Desharnais et al (2008) had to invoke convective dredge-up combined with a highly unusual chemical profile of the white dwarf core to account for the extreme O/C > 1000 found in GD 61. However, all white dwarfs suspected of harboring dredged-up core material exhibit atmospheric C and effective temperatures near or below 12,000 K (Dufour et al 2005), including the recently detected class of O-rich white dwarfs (Gänsicke et al 2010). At 17,300 K, the depth of the convective envelope in GD 61 is only 2.5% of the maximum depth attained in He atmosphere white dwarfs (Koester 2009), which occurs between 12,000 and 10,000 K (Pelletier et al 1986).…”

Section: Infalling Debris From a Destroyed Minor Planetmentioning

confidence: 96%

Possible Signs of Water and Differentiation in a Rocky Exoplanetary Body

Farihi

Brinkworth

Gänsicke

et al. 2011

ApJ

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Spitzer observations reveal the presence of warm debris from a tidally destroyed rocky and possibly icy planetary body orbiting the white dwarf GD 61. Ultraviolet and optical spectroscopy of the metal-contaminated stellar photosphere reveal traces of hydrogen, oxygen, magnesium, silicon, iron, and calcium. The nominal ratios of these elements indicate an excess of oxygen relative to that expected from rock-forming metal oxides, and thus it is possible that water was accreted together with the terrestrial-like debris. Iron is found to be deficient relative to magnesium and silicon, suggesting the material may have originated as the outer layers of a differentiated parent body, as is widely accepted for the Moon.

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Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

Cited by 142 publications

References 32 publications

White dwarf stars with carbon atmospheres

White dwarf stars with carbon atmospheres

The origin of peculiar molecular bands in cool DQ white dwarfs

Possible Signs of Water and Differentiation in a Rocky Exoplanetary Body

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