Dwarf survives in stellar furnace

From: http://news.bbc.co.uk/2/hi/science/nature/5235306.stm

By Helen Briggs Science reporter, BBC News, Aug 1, 2006

The brown dwarf (right) came out almost unaltered after being swallowed by a red giant (left)

A distant world that escaped the likely fate of the Earth - being fried when the Sun grows old and dies - has been discovered by UK astronomers.

The brown dwarf withstood being swallowed by a red giant and is now locked in a perpetual dance with the remains of the larger star.

The two objects, of different colours, rotate around each other in two hours.

The findings, reported in Nature, suggest some planets might also survive the natural death throes of stars.

The fate of most stars is to grow old, run out of hydrogen fuel, then collapse under its own gravity.

The atmosphere becomes unstable and starts to expand, transforming the star into what is known as a red giant.

The dying core eventually turns into a white dwarf - a spherical body the size of the Earth, made up of carbon and oxygen.

The star then gradually fades away, becoming dimmer and dimmer until its light is finally extinguished.

If a star fails early its history, before it is fully born, it is known as a brown dwarf; a cold, dim body intermediate in size between a very large planet and a Sun-like star.


Computer simulation of the merger process of a red giant with a brown dwarf (shown as a black dot)

Until now, astronomers believed something as small, in relative terms, as a brown dwarf could not emerge unscathed from immersion in the fiery furnace of a dying star.

"We've discovered a small failed star called a brown dwarf lying next to another star called a white dwarf and the two are orbiting each other in a tiny orbit of two hours," Dr Matt Burleigh, a co-author of the paper and an astronomer at the University of Leicester, UK, told BBC News.

"We've found something 55 times more massive than Jupiter that has survived being swallowed by a red giant; can something smaller, like a known extrasolar planet, also survive?"

Extra-solar planets

Dr Burleigh believes it is unlikely that something as small and rocky as Earth could escape the fate of incineration some four billion years from now.

But he believes one of the 200 or so extra-solar planets - a planet which orbits a star other than the Sun, and therefore belongs to a planetary system other than the Solar System - might just make it.

"My guess is that the Earth probably couldn't survive because it is so much less massive than a brown dwarf," he said.

"But whether other big planets, Jupiter-size or bigger, could is an interesting question."

The Leicester team, and colleagues at the Universities of Hertfordshire and Keele, think the brown dwarf survived being engulfed by the red giant because the envelope of the giant was ejected very rapidly.

"Prior to this discovery, you would expect the brown dwarf to be swallowed and crash into the core of the red giant but it seems it was able to eject the atmosphere of the red giant before it crashed into the core," said Dr Burleigh.

The two objects have ended up separated by less than two-thirds of the radius of the Sun or only a few thousandths of the distance between the Earth and the Sun.

They rotate around each other in about two hours, the brown dwarf moving on its orbit at a speed of 800,000km/h (497,097mph).

Cosmic cannibal

Despite this temporary reprieve from destruction, it is not all good news for the brown dwarf. Einstein's General Theory of Relativity predicts that the gap between the two stars will slowly decrease.

"Thus, in about 1.4bn years, the orbital period will have decreased to slightly more than one hour," said Ralf Napiwotzki, from the University of Hertfordshire.

"At that stage, the two objects will be so close that the white dwarf will work as a giant 'vacuum cleaner', drawing gas off its companion, in a cosmic cannibal act."

The companion to the white dwarf goes under the official name of WD0137-349. It was found using the European Southern Observatory's (ESO's) New Technology Telescope at La Silla in Chile.