The Runaway

In the news recently: a “runaway” star.

By “Runaway Star”, I mean a star that’s going so fast it’s not gravitationally bound to the Galaxy any more, and will fly off into intergalactic space. I’ve been interested in these for a while, because pretty much every simulation of a galaxy I’ve ever seen has had stars being flung out into intergalactic space… so we’d expect to see at least a few of them.

Given that the Galaxy is tens of thousands of light years across and contains a hundred billion stars, that’s a lot of mass to escape and a very high velocity to build up. Usually this is thought to be the result of the supermassive black hole at the center of the Galaxy – think of the gravity boosts that our interplanetary probes get by passing Earth or Jupiter, and now imagine what you could do with a 4 billion solar mass black hole instead…

In this case, the star was going 1200 kilometers per second. For comparison, circular orbital velocity out in the Sun’s neck of the woods is about 250 kilometers per second, and any time someone gets a number above 400 km/sec or so, eyebrows go up. The strange thing about this particular case is that THIS star doesn’t seem to have come from the galactic center at all. The authors traced its current motions back in time and found it never got close to the galactic center. Instead, it was probably tossed out by a massive kick from a supernova.

The neat part of this is that US 708 is an odd kind of star called a helium-rich subdwarf, which is either
1.) a once-massive star that quickly fused all the hydrogen in its core into helium and then had its outer layers removed, exposing the helium core.
2.) the collision of two white dwarfs that had just enough non-compacted material to restart helium fusion for a few million years.

Either way, US 708 was a massive star that isn’t going to last much longer before it turns into a white dwarf. So finding out that it didn’t come from the Galactic center is actually kind of reassuring. What are the chances that it, an unusual star, took a slingshot around the Galactic center right as it was about to die? Not high. (Most runaways to date have been ordinary stars). And massive stars usually come in pairs or triples, so it’s reasonable to consider a scenario where US 708 USED to have a buddy.

In this case, the authors say US 708 was in a very tight orbit with a white dwarf that stripped off and absorbed almost all of its outer hydrogen-rich layers. The new material transferred to the white dwarf started fusing, which set off such a catastrophic explosion that it destroyed the white dwarf entirely in a Supernova Type 1a. US 708 shoots off into space with whatever orbital velocity it had before the thing it was orbiting went boom, plus a roughly 200 km/s sideways kick contribution from the supernova itself. To get to the velocity US 708 has now, the original binary must have had an orbital period of 10 minutes! Yes, minutes. Those short periods really do exist.

This was another case where kinematics were used to determine the origins of objects, which is something I am very much interested in. Unlike the Death Star earlier this year (which has now been thoroughly refuted by Dr. Eric Mamajek), this paper looks much more solid. They have good evidence that US 708 really is this specific and unusual type of star, and that it is the distance away it’s said to be (and even if they’re wrong about that, they have a direct measurement of its motion away from us at 900 km/s, which still means it’s a galactic escapee.

Space is full of outrageous things.


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