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RE: White Dwarfs
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Title: Constraining white-dwarf kicks in globular clusters : III. Cluster Heating
Authors: Jeremy S. Heyl (UBC)

Recent observations of white dwarfs in globular clusters 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 typically larger velocities than other stars of similar mass in the cluster. As these fast moving white dwarfs travel through the cluster they can impart their excess kinetic energy on the other stars in the cluster, providing a new heat source for globular clusters, especially during their youth.

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In the life of a star, the white-dwarf stage is kind of like a long, boring retirement.
After 10 billion years as a fiery sun -- followed by a brief middle age as a red giant -- the star collapses into nothing more than a small, dense core, slowly cooling off until the end of time.
Or so it was thought.
A couple of astronomers at the University of B.C. have discovered that, in the moment a star becomes a white dwarf, it somehow gets one last kick of real power -- enough to send it hurtling through space at about 15,000 kilometres an hour.
Harvey Richer, a professor at UBC, and PhD student Saul Davis made the discovery while analysing pictures of a globular star cluster taken by the Hubble space telescope. The two were trying to determine the age of the cluster, known as NGC 6397.
But what struck them when they looked at the image was that all of the newborn white dwarf stars, regardless of their size, were near the outer edges of the cluster. On the surface, this made no sense to the astronomers.
The gravitational field of a cluster slowly sorts stars by their mass, heavier stars near the centre and lighter ones farther out.


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Stars like the Sun may drift into space like ghosts when they die, Hubble Space Telescope observations reveal. But what propels them is still a mystery.
Relatively low-mass stars like the Sun do not explode as supernovae when they die. Instead, they bloat up into red giant stars before shedding their outer layers and becoming dense embers called white dwarfs.
But surprisingly few white dwarfs have been found in low-mass groupings of stars called open star clusters. Some scientists have suggested that the white dwarfs might be hiding in the glare of brighter companion stars, while others have argued that the dwarfs were kicked out of their clusters altogether.
Now, new observations of a globular star cluster a massive agglomeration of up to a million stars have reinforced the latter idea.
Saul Davis of the University of British Columbia in Vancouver, Canada, led a team that used Hubble to look at white dwarfs in a globular cluster in our galaxy called NGC 6397, which lies about 8500 light years from Earth.
Interactions between stars in a globular cluster cause the stars to separate by mass, with the heaviest stars sinking to the centre over time and the lightest stars moving to the periphery.

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NASA's Hubble Space Telescope is providing strong evidence that white dwarfs, the burned out relics of stars, are given a "kick" when they form.
The sharp vision of Hubble's Advanced Camera for Surveys uncovered the speedy white dwarfs in the ancient globular star cluster NGC 6397, a dense swarm of hundreds of thousands of stars.
Before the stars burned-out as white dwarfs, they were among the most massive stars in NGC 6397. Because massive stars are thought to gather at a globular cluster's core, astronomers assumed that most newly minted white dwarfs dwelled near the centre.
Hubble, however, discovered young white dwarfs residing at the edge of NGC 6397, which is about 11.5 billion years old.

"The distribution of young white dwarfs is the exact opposite of what we expected. Our idea is that as aging stars evolve into white dwarfs, they are given a kick of 7,000 to 11,000 miles an hour (3 to 5 kilometers a second), which rockets them to the outer reaches of the cluster" - astronomer Harvey Richer of the University of British Columbia in Vancouver.

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Title: Spitzer IRAC Observations of White Dwarfs. I. Warm Dust at Metal-Rich Degenerates
Authors: J. Farihi, B. Zuckerman, E. E. Becklin

This paper presents the results of a Spitzer IRAC 3-8 micron photometric search for warm dust orbiting 17 nearby, metal-rich white dwarfs, 15 of which apparently have hydrogen dominated atmospheres (type DAZ). G166-58, G29-38, and GD 362 manifest excess emission in their IRAC fluxes and the latter two are known to harbour dust grains warm enough to radiate detectable emission at near-infrared wavelengths as short as 2 micron. Their IRAC fluxes display differences compatible with a relatively larger amount of cooler dust at GD 362. G166-58 is presently unique in that it appears to exhibit excess flux only at wavelengths longer than about 5 micron. Evidence is presented that this mid-infrared emission is most likely associated with the white dwarf, indicating that G166-58 bears circumstellar dust no warmer than T~400 K. The remaining 14 targets reveal no reliable mid-infrared excess, indicating the majority of DAZ stars do not have warm debris disks sufficiently opaque to be detected by IRAC.

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Albus 1
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Title: CPD-20 1123 (Albus 1) is a bright He-B Subdwarf
Authors: Stephane Vennes, Adela Kawka, J. Allyn Smith

Based on photometric and astrometric data it has been proposed that Albus 1 (also known as CPD-20 1123) might be a hot white dwarf similar to G191-B2B or, alternatively, a hot subdwarf. We obtained a series of optical spectra showing that CPD-20 1123 is a bright He-B subdwarf. We analysed the HI Balmer and HeI line spectra and measured T_eff = 19800 ±400 K, log g = 4.55 ±0.10, and log N(He)/N(H) = 0.15 ±0.15. This peculiar object belongs to a family of evolved helium-rich stars that may be the products of double-degenerate mergers, or, alternatively, the products of post horizontal- or giant-branch evolution.

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GD 99
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Title: GD 99 - an unusual, rarely observed DAV white dwarf
Authors: Zs. Bognár (1), M. Paparó (1), B. Steininger (2), G. Virághalmy (1) ((1) Konkoly Observatory, Budapest, Hungary, (2) Institute of Astronomy, University of Vienna, Vienna, Austria)

New observations of GD 99 are analysed. The unusual pulsation behaviour, showing both long and short periods, has been confirmed. All the available periods show a grouping of short and long period modes with roughly regular spacing. If we interpret the groups separately, a binary nature can be a possible explanation as in the similar cases of WD 2350-0054 and G29-38.

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Albus 1
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Title: Albus 1: A very bright white dwarf candidate
Authors: Jose Antonio Caballero Enrique Solano

We have serendipitously discovered a previously-unknown, bright source (B_T = 11.75 ±0.07 mag) with a very blue V_T-K_s colour, to which we have named Albus 1. A photometric and astrometric study using Virtual Observatory tools has shown that it possesses an appreciable proper motion and magnitudes and colours very similar to those of the well known white dwarf G 191-B2B. We consider Albus 1 as a DA-type white dwarf located at about 40 pc. If confirmed its nature, Albus 1 would be the sixth brightest isolated white dwarf in the sky, which would make it an excellent spectrophotometric standard.

WhiteD_e1
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False-colour composite image, 5.6×5.6 arcmin² wide, centred on Albus 1. Blue is for BJ , green for R, and red is for IN (DSS1 and DSS2 photographic plates from ESO and MAMA). North is up, east is left.

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Hottest White Dwarfs
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Title: HST Spectroscopy of the Hottest White Dwarfs
Authors: Thomas Rauch, Klaus Werner (Institut fuer Astronomie und Astrophysik, Tuebingen, Germany)

Spectral analysis needs the observation of lines of successive ionisation stages in order to evaluate the ionisation equilibrium (of a particular species) which is a sensitive indicator for the effective temperature. Since stars with effective temperatures as high as 100000 K have their flux maximum in the extreme ultraviolet (EUV) wavelength range and due to the high degree of ionisation, most of the metal lines are found in the ultraviolet (UV) range. Thus, high-S/N and high-resolution UV spectra are a pre-requisite for a precise analysis. Consequently, we employed the Faint Object Spectrograph (FOS), the Goddard High Resolution Spectrograph (GHRS), and the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) in order to obtain suitable data. We present state-of-the-art analyses of the hottest (pre-) white dwarfs by means of NLTE model atmospheres which include the metal-line blanketing of all elements from hydrogen to nickel.

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Magnetic White Dwarfs
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Title: New Close Binary Systems from the SDSS-I (Data Release Five) and the Search for Magnetic White Dwarfs in Cataclysmic Variable Progenitor Systems
Authors: Nicole M. Silvestri, Mara P. Lemagie, Suzanne L. Hawley, Andrew A. West, Gary D. Schmidt, James Liebert, Paula Szkody, Lee Mannikko , Michael A. Wolfe, J. C. Barentine, Howard J. Brewington, Michael Harvanek , Jurik Krzesinski , Dan Long , Donald P. Schneider, Stephanie A. Snedden

We present the latest catalogue of more than 1200 spectroscopically-selected close binary systems observed with the Sloan Digital Sky Survey through Data Release Five. We use the catalogue to search for magnetic white dwarfs in cataclysmic variable progenitor systems. Given that approximately 25% of cataclysmic variables contain a magnetic white dwarf, and that our large sample of close binary systems should contain many progenitors of cataclysmic variables, it is quite surprising that we find only two potential magnetic white dwarfs in this sample. The candidate magnetic white dwarfs, if confirmed, would possess relatively low magnetic field strengths (B_WD < 10 MG) that are similar to those of intermediate-Polars but are much less than the average field strength of the current Polar population. Additional observations of these systems are required to definitively cast the white dwarfs as magnetic. Even if these two systems prove to be the first evidence of detached magnetic white dwarf + M dwarf binaries, there is still a large disparity between the properties of the presently known cataclysmic variable population and the presumed close binary progenitors.

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Dusty DAZ White Dwarfs
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Title: The New Class of Dusty DAZ White Dwarfs
Authors: Ted von Hippel, Marc J. Kuchner, Mukremin Kilic, Fergal Mullally, William T. Reach

 Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope and the IRAC imager has revealed an infrared excess associated with the white dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the fourth white dwarf observed to have circumstellar dust. All four are DAZ white dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal lines.
We discuss these four objects as a class, which we abbreviate "DAZd", where the "d" stands for "dust". Using an optically-thick, geometrically-thin disk model analogous to Saturn's rings, we find that the inner disk edges are at >~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model naturally explains the accretion rates and lifetimes of the detected WD disks and the accretion rates inferred from photospheric metal abundances.

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