New observations from the Very Large Array have turned up a system with two stars that are so close they’re actually touching. Named VFTS 352, this isn’t the first time we've observed a system of this type, known as an overcontact binary. But this is the most massive one discovered to date and the hottest one yet discovered, with both stars having temperatures over 40,000ºC.
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VFTS 352’s stars are continuing to orbit one another despite their close proximity of 12 million kilometers. They complete an orbit of their common center of mass in just over a day. By contrast, Mercury’s tight orbit of the Sun, though much faster than Earth’s, still takes a full 88 days more or less. They’re so close that they’re actually sharing a bridge of gas that connects them (as illustrated in this animation).
In other close binary systems, one star might be siphoning material off its neighbor, growing its mass at the expense of the other’s. That's why they are nicknamed "vampire stars." But that’s not the case here, since the two stars have almost the same mass and thus nearly equal gravity.
This sharing of material allows the stars to become mixed, their chemical compositions gradually reaching an equilibrium. Due to the strong tidal forces that the stars exert on each other, this mixing process is likely enhanced, and the researchers estimate a full 30 percent of each individual star’s material is being shared.
“The VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing,” said lead author Leonardo A. Almeida of the University of São Paulo, Brazil. “As such, it’s a fascinating and important discovery.”
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