A failed star that was swallowed by its bloated, dying companion survived the ordeal unscathed, a new study reports. But it is not in the clear yet: in another billion years or so its dead companion will begin consuming it.
When stars like the Sun run out of hydrogen fuel in their cores, they bloat up into colossal 鈥渞ed giants鈥� before shedding their outer layers entirely and becoming white dwarfs. But it was not clear what would happen to any smaller companions when the stars expanded into red giants.
New observations with the European Southern Observatory鈥檚 Very Large Telescope (VLT) in Chile suggest that at least some companions make it out of this phase in one piece.
Advertisement
Astronomers led by Pierre Maxted at Keele University in Staffordshire, UK, studied a pair of stars labelled WD 0137-349, in which the more massive star is a white dwarf with 40% the mass of the Sun. Its companion is a brown dwarf 鈥� a so-called 鈥渇ailed star鈥� because it is not massive enough to burn hydrogen like true stars.
Troubled history
The VLT observations suggest the brown dwarf is about 5% the mass of the Sun, or 55 times the mass of Jupiter. The white and brown dwarves orbit each other about every two hours at a distance of less than 700,000 kilometres 鈥� just half the diameter of the Sun. That suggests that about 250 million years ago, the white dwarf enveloped its partner as it swelled into a red giant.
鈥淪uch a system must have had a very troubled history,鈥� says Maxted. 鈥淚ts existence proves that the brown dwarf came out almost unaltered from an episode in which it was swallowed by a red giant.鈥�
The relatively low mass of the brown dwarf means it does not have a great deal of gravity to hold itself together. So 鈥渙ne might expect it would not survive being swallowed 鈥� it would simply be digested鈥�, says James Liebert, at the University of Arizona in Tucson, US, in an accompanying article in Nature.
Indeed, the team believes a less massive brown dwarf would have been destroyed. 鈥淗ad the companion been less than about 20 Jupiter masses, it would have evaporated during this phase,鈥� says Maxted.
Giant vacuum cleaner
In fact, the brown dwarf may actually have gained mass during its run-in with the red giant, and the research team says future observations at infrared wavelengths could look for this effect.
The white dwarf looks set to gain back whatever material it lost to the brown dwarf, however. The two stars will grow closer in time as they radiate away energy by emitting gravitational waves. In about 1.4 billion years, they will orbit each other every hour or so and will be near enough for the white dwarf to start pulling matter from the brown dwarf.
鈥淎t 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 cannibalism act,鈥� says team member Ralf Napiwotzki at the University of Hertfordshire, UK.
Journal reference: Nature (vol 442, p 520 and p 543)
