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Post Info TOPIC: White Dwarfs
Blobrana


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RE: White Dwarfs
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An international team of astronomers has made a major discovery about stars.
Scientists found that solar systems similar to our own could have existed elsewhere in the universe.
Team member Dr Jay Farihi, of the University of Leicester, presented the discovery at the European Week of Astronomy and Space Science conference.
The astronomers noticed that at least one in 100 white dwarf stars - the compact, hot remnants left behind when stars like our Sun reach the end of their lives - show evidence of orbiting asteroids and rocky planets.

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Blobrana


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Title: The Dynamical Effects of White Dwarf Birth Kicks in Globular Star Clusters
Authors: John M. Fregeau, Harvey B. Richer, Frederic A. Rasio, Jarrod R. Hurley

Recent observations of the white dwarf (WD) populations in globular clusters suggest that WDs receive a kick of a few km/s shortly before they are born. Using our Monte Carlo cluster evolution code, which includes accurate treatments of all relevant physical processes operating in globular clusters, we study the effects of the kicks on the cluster and on the WD population itself. We find that in clusters whose velocity dispersion is comparable to the kick speed, WD kicks are a significant energy source for the cluster, prolonging the initial cluster core contraction phase significantly so that at late times the cluster core to half-mass radius ratio is a factor of up to ~ 10 larger than in the no-kick case. WD kicks thus represent a possible resolution of the large discrepancy between observed and theoretically predicted values of this key structural parameter. Our modelling also reproduces the observed trend for younger WDs to be more extended in their radial distribution in the cluster than older WDs.

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Blobrana


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White dwarf masses
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Title: Intermediate polars in the Swift/BAT survey: Spectra and white dwarf masses
Authors: Jorg Brunschweiger, Jochen Greiner, Marco Ajello, Julian Osborne

White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. We investigate the X-ray spectral properties of a sample of Intermediate Polars detected above 15 keV to derive the masses of their accreting white dwarfs. We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars.

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Blobrana


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Posts: 131433
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White Dwarf Companions
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Title: The Age of White Dwarf Companions
Authors: S. Weston, R. Napiwotzki

We carried out a spectroscopic investigation of single lined white dwarfs (WDs) in double degenerate (DD) systems and discuss their binary evolution. Simulated spectra of the Halpha region are used to derive upper limits on the temperature of the invisible component and thus lower limits on the cooling age. This is done for a range of hypothetical secondary masses and a minimum cooling age deduced. Results are compared with the well known parameters of the visible primary, which allows us to determine a lower limit for the cooling age difference of both WDs. Most of the ten systems in our sample have a minimum age difference of not larger than 0.5Gyr and their small orbital separation is highly suggestive of at least one unstable mass transfer phase. However, a stable first mass transfer phase is feasible as the age difference is less then 1Gyr. The results imply that unstable mass transfer is the most likely final contact binary scenario to have occurred in DD systems but the first mass transfer phase is not constrained.

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Blobrana


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White-dwarfs
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Title: Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse
Authors: Jeremy S. Heyl, Matthew Penrice (UBC)

Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them larger energies than other stars in the cluster. As these energetic white dwarfs travel through the cluster they can impart their excess energy on the other stars in the cluster. A Monte-Carlo simulation of the white-dwarfs kicks combined with estimate of the phase-space diffusion of the white dwarfs reveals that as the white dwarfs equilibrate, they lose most of their energy in the central region of the cluster. They could possibly mimic the effect of binaries, puffing up the cluster and delaying core collapse.

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Blobrana


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Posts: 131433
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White dwarf stars
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Title: Axions and the cooling of white dwarf stars
Authors: J. Isern, E. Garcia-Berro, S. Torres, S. Catalan
(Version v2)

White dwarfs are the end-product of the lives of intermediate- and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m_a cos^2 \beta ~ 5 meV, where m_a is the mass of the axion and cos^2 \beta is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existing theoretical and observational uncertainties do not allow yet to confirm the existence of axions, but our results clearly show that if their mass is of the order of few meV, the white dwarf luminosity function is sensitive enough to detect their existence.

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Blobrana


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Posts: 131433
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White dwarfs
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The brightness of white dwarfs may point towards the existence of exotic dark matter particles.
Jordi Isern of the Institute of Space Sciences in Bellaterra, Spain, and colleagues modelled what would happen if white dwarfs - small, dense, dying stars - were emitting axions. These hypothetical particles are a candidate for dark matter, which makes up most of the universe.
The model showed that axion emission affected a white dwarf's brightness, and the distribution of stars of a given luminosity predicted by the model matched well with observations of 6000 white dwarfs by the Sloan Digital Sky Survey. This is the first indication that axions could exist, say the team.

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Blobrana


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Posts: 131433
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Title: Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalogue
Authors: A. Gianninas, P. Bergeron, M. T. Ruiz

For some years now, we have been gathering optical spectra of DA white dwarfs in an effort to study and define the empirical ZZ Ceti instability strip. However, we have recently expanded this survey to include all the DA white dwarfs in the McCook & Sion catalogue down to a limiting visual magnitude of V=17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs from this ongoing survey. We have several specific areas of interest most notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM binary systems. Furthermore, we present a comparison of the ensemble properties of our sample with those of other large surveys of DA white dwarfs, paying particular attention to the distribution of mass as a function of effective temperature.

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Blobrana


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Title: White Dwarfs in Globular Clusters
Authors: S. Moehler (ESO), G. Bono (INAF/Univ. Rome/ESO)

We review empirical and theoretical findings concerning white dwarfs in Galactic globular clusters. Since their detection is a critical issue we describe in detail the various efforts to find white dwarfs in globular clusters. We then outline the advantages of using cluster white dwarfs to investigate the formation and evolution of white dwarfs and concentrate on evolutionary channels that appear to be unique to globular clusters. We also discuss the usefulness of globular cluster white dwarfs to provide independent information on the distances and ages of globular clusters, information that is very important far beyond the immediate field of white dwarf research. Finally, we mention possible future avenues concerning globular cluster white dwarfs, like the study of strange quark matter or plasma neutrinos.

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Blobrana


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Title: Binary Star Origin of High Field Magnetic White Dwarfs
Authors: C. A. Tout, D. T. Wickramasinghe, J. Liebert, L. Ferrario, J. E. Pringle

White dwarfs with surface magnetic fields in excess of 1 MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. CVs emerge from common envelope evolution as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger the magnetic field. The originate from those common envelope systems that almost merge. We propose further that those common envelope systems that do merge are the progenitors of the single high field white dwarfs. Thus all highly magnetic white dwarfs, be they single stars or the components of MCVs, have a binary origin. This hypothesis also accounts for the relative dearth of single white dwarfs with fields of 10,000 - 1,000,000G. Such intermediate-field white dwarfs are found preferentially in cataclysmic variables. In addition the bias towards higher masses for highly magnetic white dwarfs is expected if a fraction of these form when two degenerate cores merge in a common envelope. Similar scenarios may account for very high field neutron stars.

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