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RE: White Dwarfs
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Title: White dwarfs in the European Galactic Plane Surveys (EGAPS)
Authors: L. Morales-Rueda (1), P. J. Groot (1), R. Napiwotzki (2), J. Drew (3), the EGAPS collaboration ((1) Radboud University Nijmegen, (2) University of Hertfordshire, (3) Imperial College London)

The space density of white dwarfs is highly uncertain even nearby. This results from the fact that the known sample of white dwarfs is largely incomplete in part because most white dwarfs have been discovered as by-products in non-dedicated surveys. In order to obtain more accurate white dwarf space densities and scale heights we must build up a complete sample of white dwarfs. The European Galactic Plane Surveys (EGAPS) are the best database to search for white dwarfs as they will provide broad band (U, g', r', i') and narrow band (Halpha and HeI) measurements for one per cent of all the stars in the Galaxy. By looking at the Galactic Plane, where most stars are, we ensure that we are obtaining a complete sample. The space densities obtained from EGAPS can then be compared with those found in high latitude surveys such as the Sloan Digital Sky Survey (SDSS). The methods used to identify white dwarfs using the colours available in EGAPS are described and some preliminary results presented.

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White Dwarf Binaries
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Title: Population Boundaries for Galactic White Dwarf Binaries in LISA's Amplitude-Frequency Domain
Authors: Ravi Kumar Kopparapu, Joel E. Tohline

Detached, inspiralling and semi-detached, mass-transferring double white dwarf (DWD) binary systems are both expected to be important sources for the proposed space-based gravitational-wave detector, LISA. The mass-radius relationship of individual white dwarf stars in combination with the constraints imposed by Roche geometries permit us to identify population boundaries for DWD systems in LISA's ''absolute'' amplitude-frequency diagram. With five key population boundaries in place, we are able to identify four principal population sub-domains, including one sub-domain that identifies where progenitors of Type Ia supernovae will reside. Given one full year of uninterrupted operation, LISA should be able to measure the rate at which the gravitational-wave frequency f and, hence, the orbital period is changing in the highest frequency subpopulation of our Galaxy's DWD systems. We provide a formula by which the distance to each DWD system in this subpopulation can be determined; in addition, we show how the masses of the individual white dwarf stars in mass-transferring systems may be calculated.

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Posts: 131433
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RE: White Dwarf GD50
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An ultramassive white dwarf in Eridanus may have escaped from the Pleiades star cluster, say astronomers in England. If so, the lost Pleiad was once a bright blue star that outshone all the current cluster members. The discovery implies that the white dwarf evolved from a single star, contradicting theories that say ultramassive white dwarfs form only from the merger of two lesser white dwarfs.

whitedwarfGD50
Position(2000): RA: 03 48 50.2 Dec: -00 58 31
Size: 14'1 x 14'1

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V4334 Sgr
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Title: The Spitzer IRS view of V4334 Sgr (Sakurai's Object)
Authors: A. Evans (Keele University), V. H. Tyne, J. Th. van Loon, B. Smalley (Keele), T. R. Geballe (Gemini), R. D. Gehrz, C. E. Woodward (Minnesota), A. A. Zijlstra (Manchester), E. Polomski (Minnesota), M. T. Rushton (Keele), S. P. S. Eyres (Central Lancs), S. G. Starrfield (Arizona), J. Krautter (Heidelberg), R. M. Wagner (LBT)

We present an observation of the very late thermal pulse object V4334 Sgr (Sakurai's Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. The emission from 5-38 microns is dominated by the still-cooling dust shell. A number of features are seen in absorption against the dust shell, which we attribute to HCN and polyyne molecules. We use these features to determine the 12C/13C ratio for the absorbing gas to be ~ 3.2 (+3.2,-1.6}; this implies that, despite the H-content of the molecules, the hydrocarbon-bearing gas must have originated in material produced in the very late thermal pulse. We see no evidence of emission lines, despite the recently-reported optical and radio observations that suggest the effective temperature of the stellar remnant is rising.

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Posts: 131433
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White dwarf GD50
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Title: On the origin of the ultramassive white dwarf GD50
Authors: P. D. Dobbie (1), R. Napiwotzki (2), N. Lodieu (1), M. R. Burleigh (1), M. A. Barstow (1), R. F. Jameson (1) ((1) University of Leicester, UK, (2) University of Hertfordshire, UK)

We argue on the basis of astrometric and spectroscopic data that the ultramassive white dwarf GD50 is associated with the star formation event that created the Pleiades and is possibly a former member of this cluster. Its cooling age (~60Myrs) is consistent with it having evolved essentially as a single star from a progenitor with a mass M>6Msun so we find no need to invoke a white dwarf-white dwarf binary merger scenario to account for its existence. This result may represent the first direct observational evidence that single star evolution can produce white dwarfs with M>1.1Msun, as predicted by some stellar evolutionary theories. On the basis of its tangential velocity we also provisionally identify the ultramassive (M~1.2Msun) white dwarf PG0136+251 as being related to the Pleiades. These findings may help to alleviate the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers.

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Posts: 131433
Date:
GROJ16555-40
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White dwarfs and black holes – some of the most dense and exotic stars that exist – were the focus of new findings reported by UCL researchers at this year’s ‘New Results in X-Ray Astronomy’ conference held at the UCL Mullard Space Science Laboratory in July 2006.
Dr Gavin Ramsay, a galactic astronomy specialist based at UCL, presented surprising findings about ultra-compact binary stars. When two white dwarfs– stars that are similar in mass to our Sun but only roughly the size of the Earth – orbit each other, one sucks matter from its companion. His observations show that the energy produced during this process is emitted in ultraviolet wavelengths, not in X-rays as previously suspected.
Dr Ramsay also proved that the two most compact binary stars known to astronomers, which are powerful sources of gravitational radiation, are continuing to ‘spin up’; that is, their orbits of each other are getting steadily faster.
Dr Catherine Brocksopp of UCL’s extragalactic research team reported an ‘outburst’ of sudden brightenings from what is thought to be a binary star involving a black hole (also known as the X-ray transient GRO J16555-40). The brightenings are due to the dust and gas that become hot and viscous as they pass from the star to the black hole.
The star had been inactive for nearly a decade, but its reawakening in February 2005 was perfectly timed to be captured by the Swift observatory, launched into orbit in 2004. The observatory recorded the effects in gamma-ray, X-ray, ultraviolet and optical wavelengths simultaneously, providing an unprecedented amount of data on this phenomenon.

New Results in X-Ray Astronomy’ is an annual conference that provides a platform for young researchers in the astronomical community. This year, talks and poster presentations showcased the world-class research going on across the UK drawing on data gathered by the Chandra, XMM-Newton and Swift observatories. Over 50 X-ray astronomers from 11 universities took part, including two experts from the University of Cantabria in Spain.

UK-XRA 2006
UCL MSSL Astrophysics Group

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Posts: 131433
Date:
RE: White Dwarfs
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Title: Measuring M dwarf Winds with DAZ White Dwarfs
Authors: J.H. Debes

Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, show evidence for ongoing accretion of material onto their surfaces. Some DAZs are known to have unresolved M dwarf companions, which could account for the observed accretion through a stellar wind. I combine observed Ca abundances of the DAZs with information on the orbital separation of their M dwarf companions to infer the mass loss rate of the M dwarfs. I find that for three of the six known DAZs with M dwarf companions, a stellar wind can plausibly explain the observed accretion on the white dwarfs assuming Bondi-Hoyle accretion of solar abundance stellar winds on the order of 10^-14 -10^-16\solar masses year^-1. The rest of the sample have companions with orbits $\gtorder$ 1~AU, and require companion mass loss rates of >$ 10^-11\solar masses year^-1. I conclude that there must be an alternative explanation for accretion of material onto DAZs with widely separated companions. The inferred winds for two of the close binaries are orders of magnitude smaller than typically assumed for the angular momentum loss of red dwarf-white dwarf pairs due to magnetic braking from a stellar wind and may seriously affect predictions for the formation rate of CVs with low mass companions.

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Title: A Catalogue of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
Authors: Daniel J. Eisenstein, James Liebert, Hugh C. Harris, S.J. Kleinmann, Atsuko Nitta, Nicole Silvestri, Scott A. Anderson, J.C. Barentine, Howard J. Brewington, J. Brinkmann, Michael Harvanek, Jurek Krzesinski, Eric H. Neilsen Jr., Dan Long, Donald P. Schneider, Stephanie A. Snedden

Researchers present a catalogue of 9316 spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey Data Release 4. They have selected the stars through photometric cuts and spectroscopic modelling, backed up by a set of visual inspections. Roughly 6000 of the stars are new discoveries, roughly doubling the number of spectroscopically confirmed white dwarfs.
They analyse the stars by performing temperature and surface gravity fits to grids of pure hydrogen and helium atmospheres. Among the rare outliers are a set of presumed helium-core DA white dwarfs with estimated masses below 0.3 solar masses, including two candidates that may be the lowest masses yet found. The researchers also present a list of 928 hot subdwarfs.

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