Heavy in the Halo

If you've been following my blog a long time, you may remember a post on stellar evolution that was responding to some rather silly claims made by some Creationists. One of the claims was that the universe must be young, because we see young, massive stars where there's no gas and dust to form them. The example he used was near the galactic center. My response then (and still is) that this makes the rather ignorant assumption that stars stay where they're formed.

The center of the galaxy is a very messy place and this is certainly not true. The massive black hole swings stars far and wide.

Meanwhile, the center of the galaxy isn't the only place we find these peculiar stars. They're in the halo of the galaxy too, far removed from the disk of the galaxy. In many instances, this isn't that much of a problem. The stars we find way out there have velocities that are too high for them to have remained in the disk, indicating they were indeed flung out by either a close binary reaction with another star, or in the case of some of the exceedingly fast ones, by our galactic black hole.

However, for some of the most massive stars, their lifetimes are shorter than the amount of time since they were thrown from the galaxy. In other words, if we track backwards, the stars didn't exist when they were last in the galaxy!

Uh oh.

Is is possible that these stars actually came from a satellite galaxy like the Large or Small Magellanic Clouds?

Not likely. Their spectra show their chemical composition is more likely related to our Milky Way than any of our neighbors.

So what's the story?

The outer reaches and inner disk aren't the only place we've seen stars of masses and lifespans that don't fit. Nearly three years ago, I mentioned another one: blue stragglers in globular clusters.

In globular clusters, all the stars tend to form in one big burst from one big cloud of gas and dust. This means that their chemical composition and age are effectively the same. The only thing that will determine their evolution from their on out, is their mass. The massive ones peel off the main sequence first and slowly die towards the less massive ones (see this post for more on this topic).

Yet in several globular clusters, there are massive stars remaining on the main sequence when there shouldn't be. The notion here was that normal stars interacted, either by direct collision or mass transfer in a post main-sequence phase, which rejuvenated a star by giving it more mass and dropping it back on the main sequence.

Could such an explanation be possible for these odd runaway and hypervelocity stars?

The first question is whether or not there are any known runaway stars exist in binary systems in the first place. It turns out there are, but not many. Only about 1% of ejected stars appear to be binaries. However, many of these binary systems have very short periods (<5 days) which indicate a very close pair. This is perfect for mass transfer or instability such as predicted by the theory.

Additionally, many individual high mass stars like these have strong cases to make for their binary birth. Some are still binaries where one star is significantly more massive than the other with periods of closer to 10 days. More need to be observed to get a really good feeling on whether or not this possibility is right, but statistically, it works out pretty well on the theoretical level.

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Perets, H. (2009). RUNAWAY AND HYPERVELOCITY STARS IN THE GALACTIC HALO: BINARY REJUVENATION AND TRIPLE DISRUPTION The Astrophysical Journal, 698 (2), 1330-1340 DOI: 10.1088/0004-637X/698/2/1330