2006年4月3日のNewScientistの記事によれば、(1) いくつかの白色矮星からの過剰な赤外線の存在から、それらの白色矮星のまわりにダストが存在し、(2) そのスペクトルから、いくつかの白色矮星に重い元素が存在することがわかっていた。
[Maggie McKee: "Rocky planets may circle many white dwarfs" (2006/04/03) on NewScientist]

astronomers discovered heavy elements such as magnesium in the spectra of some white dwarf stars. White dwarfs are the burned-out embers of stars up to eight times more massive than our Sun.

The heavy elements, or metals, were surprising because white dwarfs contain about as much mass as the Sun squeezed into bodies the size of the Earth, giving them surface gravities 10,000 times stronger than the Sun's. That should cause heavy elements to sink towards their centres - and out of sight.

Some astronomers suggested metals could appear in the spectra when the white dwarfs passed through interstellar gas clouds and swept up heavy elements. But independent teams led by Eric Becklin at the University of California in Los Angeles and Ted von Hippel of the University of Texas in Austin, both in the US, suggested another explanation in 2005.



天文学者たちの中には、スペクトルに現れた金属は、白色矮星が星間ガス雲を通過したときに、ひきずってきた重い元素ではないかと示唆した。しかし、UCLAのEric Becklin率いる研究チームと、University of Texas in AustinのTed von Hippel率いる研究チームは独立に、別の説明を2005年に示唆した。

Debris discs

They found that two metal-bearing white dwarfs showed an excess of infrared radiation. They attributed this to the glow of dust created by asteroids or comets that were gravitationally ripped apart when they wandered too close to the dead star. Dust and debris from the break-up would then fall into the white dwarf, depositing heavy elements there.

Now, both von Hippel and Becklin's teams have found a total of five white dwarfs with such debris discs. All five also contain heavy elements, or metals, suggesting they are surrounded by asteroids and probably rocky planets.

About 10% of all white dwarfs show signs of metals - and thus probable planetary systems, but von Hippel estimates the true fraction is 50% or more because the spectral effect is short-lived and therefore difficult to detect. Coupled with the fact that 98% of all stars become white dwarfs when they run out of nuclear fuel, he says that suggests "the fraction of stars that create rocky planets is high".




[Rachel Courtland: "Alien asteroid dust hints at Earth-like planets" (2009/01/06) on NewScientist]

Michael Jura of the University of California, Los Angeles, and colleagues measured the infrared light from these stars using NASA's Spitzer Space Telescope.

The team found the dust contains a glassy silicate material similar to olivine, which is common on Earth and has also been seen on the Moon and Mars.

The dust also seems to have no carbon, consistent with Earth's composition, which has little carbon compared to the Sun. The results were presented on Monday at the American Astronomical Society meeting in Long Beach, California.

Two previously studied white dwarfs have dust of a similar composition, bringing the tally of such stellar gluttons up to eight. "What was once kind of a freak is now a systematic pattern," Jura said.

Since asteroids form in the same way as planets, by bulking up through collisions between smaller rocky objects, they have a similar composition to their larger brethren. That suggests terrestrial planets might have once existed in these systems. "This strengthens suspicions that Earth-like planets are common," Jura said.

Many white dwarfs may host rocky discs, but they may be impossible to detect because asteroids were not jostled out of position and sent careening towards the star, leaving traces of their existence in the star's atmosphere and in surrounding dust.

And even when an asteroid has plunged into a star, the evidence of its violent end does not last long. Single asteroids no larger than 200 kilometres across could explain the dust around each of the newly studied white dwarfs, and their remains could be gobbled up and 'digested' - sinking to the star's centre - in as little as 10,000 years. "In a way it's amazing that any dust at all survives," Jura told New Scientist.

UCLAのMichael Juraと共同研究者たちは、これら[ダスト円盤を持つ6つの白色矮星]からの赤外線をNASAのスピッツァー宇宙望遠鏡で観測した。研究チームはダストが、地球にありふれていて、月や火星でも見られる橄欖石に似たガラス質のケイ酸塩を含んでいることを発見した。

ダストには炭素はないようである。これは、太陽と比べると、炭素がほとんど無きに等しい地球の成分と似ている。この研究結果は月曜日にカリフォルニア州Long Beachで開催されている米国天文学会で発表された。




[The Spitzer Space Telescope]

The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched into space by a Delta rocket from Cape Canaveral, Florida on 25 August 2003. During its mission, Spitzer will obtain images and spectra by detecting the infrared energy, or heat, radiated by objects in space between wavelengths of 3 and 180 microns (1 micron is one-millionth of a meter). Most of this infrared radiation is blocked by the Earth's atmosphere and cannot be observed from the ground.



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