* Astronomy

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RE: White Dwarfs

Blobrana · L

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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GD 99

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Albus 1

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Hottest White Dwarfs

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Magnetic White Dwarfs

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Dusty DAZ White Dwarfs