Brown dwarf survives being 'swallowed' by nearby star
Astronomers have discovered a situation where a brown dwarf appears to have survived being swallowed by a nearby red giant. The binary system was observed by a team of British researchers using the European Southern Observatory's Very Large Telescope (VLT). Their findings are reported in the journal Nature. In the system, a hot white dwarf and a cooler brown dwarf orbit one another at a distance of less than 2/3 of the radius of the sun. Brown dwarves are objects that would be stars, except their mass is too small to sustain the nuclear fusion reaction. White dwarves are old stars; when stars run out of hydrogen fuel, they increase rapidly in size and become a red giant. They then expel huge quantities of gas, which steadily dissolve. What is left behind is a white dwarf. The researchers realised that the brown dwarf must have been completely engulfed by the other star when it expanded to the red giant stage. While enclosed in the red giant's envelope, the brown dwarf would have been dragged in towards the centre of the other star; many objects which are engulfed in this way are destroyed. In fact, when the envelope of the red giant dissipated, the brown dwarf emerged largely unscathed. 'The significance of this finding is that a body of substellar mass, such as a brown dwarf, can survive the evolutionary process that created the white dwarf,' commented James Liebert of the Steward Observatory at the University of Arizona in an accompanying article. According to Pierre Maxted of Keele University, who lead the research, what saved the brown dwarf was its size, which has been measured as 55 Jupiter masses. 'Had the companion been less than 20 Jupiter masses, it would have evaporated during this phase,' he said. However, the brown dwarf is not out of the woods yet. Currently the orbital period of the two stars is around two hours, but Einstein's Theory of Relativity predicts that the distance between the two starts will slowly decrease. 'Thus, in about 1.4 billion years, the orbital period will have decreased to slightly more than one hour,' explained Ralf Napiwotzki of 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.'