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DQ White Dwarfs
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Title: Observations and Modelling of DQ White Dwarfs
Authors: Tommi Vornanen, Svetlana Berdyugina, Andrei Berdyugin

We present spectropolarimetric observations and modelling of 12 DQ white dwarfs. Modelling is based on the method presented in Berdyugina et al. (2005). We use the model to fit the C_2 absorption bands to get atmospheric parameters in different configurations, including stellar spots and stratified atmospheres, searching for the best possible fit. We still have problem to solve before we can give temperature estimates based on the Swan bands alone.

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SDSS J142625.71+575218.3
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SDSS J142625.71+575218

Position(2000): RA 14 26 25.71, Dec  +57 52 18.4

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Title: SDSS J142625.71+575218.3: The First Pulsating White Dwarf with a Large Detectable Magnetic Field
Authors: P. Dufour, G. Fontaine, J. Liebert, K. Williams, D. K. Lai

We report the discovery of a strong magnetic field in the unique pulsating carbon-atmosphere white dwarf SDSS J142625.71+575218.3. From spectra gathered at the MMT and Keck telescopes, we infer a surface field of B_s ~1.2 MG, based on obvious Zeeman components seen in several carbon lines. We also detect the presence of a Zeeman-splitted He I 4471 line, which is an indicator of the presence of a non-negligible amount of helium in the atmosphere of this Hot DQ star. This is important for understanding its pulsations, as nonadabatic theory reveals that some helium must be present in the envelope mixture for pulsation modes to be excited in the range of effective temperature where the target star is found. Out of nearly 200 pulsating white dwarfs known today, this is the first example of a star with a large detectable magnetic field. We suggest that SDSS J142625.71+575218.3 is the white dwarf equivalent of a roAp star.

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Pulsating White Dwarf Stars
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Title: Pulsating White Dwarf Stars and Precision Asteroseismology
Authors: D.E. Winget, S.O. Kepler

Galactic history is written in the white dwarf stars. Their surface properties hint at interiors composed of matter under extreme conditions. In the forty years since their discovery, pulsating white dwarf stars have moved from side-show curiosities to centre stage as important tools for unravelling the deep mysteries of the Universe. Innovative observational techniques and theoretical modelling tools have breathed life into precision asteroseismology. We are just learning to use this powerful tool, confronting theoretical models with observed frequencies and their time rate-of-change. With this tool, we calibrate white dwarf cosmochronology; we explore equations of state; we measure stellar masses, rotation rates, and nuclear reaction rates; we explore the physics of interior crystallisation; we study the structure of the progenitors of Type Ia supernovae, and we test models of dark matter. The white dwarf pulsations are at once the heartbeat of galactic history and a window into unexplored and exotic physics.

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Posts: 131433
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Pulsating carbon white dwarf
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University of Texas at Austin astronomers Michael H. Montgomery and Kurtis A. Williams, along with graduate student Steven DeGennaro, have predicted and confirmed the existence of a new type of variable star, with the help of the 2.1-meter Otto Struve Telescope at McDonald Observatory. The discovery is announced in today's issue of Astrophysical Journal Letters.
This research was funded by the National Science Foundation and the Delaware Asteroseismic Research Centre.
Called a "pulsating carbon white dwarf," this is the first new class of variable white dwarf star discovered in more than 25 years. Because the overwhelming majority of stars in the universe--including the sun--will end their lives as white dwarfs, studying the pulsations (i.e., variations in light output) of these newly discovered examples gives astronomers a window on an important end point in the lives of most stars.

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Posts: 131433
Date:
SDSS J142625.71+575218.3
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Title: SDSS J142625.71+575218.3, a Prototype for a New Class of Variable White Dwarf
Authors: M. H. Montgomery (1), Kurtis A. Williams (1 and 2), D. E. Winget (1), Patrick Dufour (3), Steven Degennaro (1), James Liebert (3) ((1) Univ. of Texas at Austin, (2) NSF Astronomy & Astrophysics Postdoctoral Fellow, (3) Univ. of Arizona)

We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. Based on our theoretical calculations, the star SDSS J142625.71+575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s (first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables, and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analogue of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.

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Posts: 131433
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DQ White Dwarfs
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Title: C_2 in Peculiar DQ White Dwarfs
Authors: Patrick B. Hall (1), Aaron J. Maxwell (1) ((1) York University, Toronto, Canada)

White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff<6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan bands. These "peculiar DQ bands" have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere. The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behaviour of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.

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Posts: 131433
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Carbon-dominated white dwarf stars
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Astronomers at the Steward Observatory in the University of Arizona have discovered white dwarf stars with pure carbon atmospheres. The stars were discovered among 10,000 new white dwarf stars found in the Sloan Digital Sky Survey. The survey, known as the SDSS, found about four times as many white dwarf stars previously known.
Claiming that this discovery could offer a unique view into the hearts of dying stars, astronomer Patrick Dufour said these stars possibly evolved in a sequence that were not known before.


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Title: Rare White dwarf stars with carbon atmospheres
Authors: P. Dufour, James Liebert, G. Fontaine, N. Behara

White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 msun and 8-10 msun, where msun 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. 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. These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follow the asymptotic giant branch.

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Posts: 131433
Date:
Hot DQ White Dwarf Stars
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Title: Hot DQ White Dwarf Stars: A New Challenge to Stellar Evolution
Authors: P. Dufour, J. Liebert, G. Fontaine, N. Behara

We report the discovery of a new class of hydrogen-deficient stars: white dwarfs with an atmosphere 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 (AGB) evolution, although these objects might be the cooler counter-part of the unique and extensively studied PG 1159 star H1504+65. These stars, together with H1504+65, might thus form a new evolutionary post-AGB sequence.

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