Speeding stars pose puzzle to scientists: What got them moving?

Astronomers say they've spotted a new class of stars speeding to the galactic exit, but not flung out from the galactic core by a black hole. Their origin and the reason for their haste are mysteries.

Courtesy via Reuters/S. Brunier/ESO/ ATG medialab/ESA/Reuters
An artist's impression shows Gaia mapping the stars of the Milky Way in this image received by Reuters December 19, 2013.

Astronomers say they have discovered a new class of stars traveling so fast through our galaxy that the stars may well be headed for intergalactic space.

The origin of these stellar speedsters is unclear. They do not appear to have originated near the Milky Way's central supermassive black hole, where other hypervelocity stars have been ejected. Instead, they appear to be coming from all directions, according to a research team reporting the discovery in the latest issue of the Astrophysical Journal.

"None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism,” Kelly Holly-Bockelmann, an astrophysicist at Vanderbilt University in Nashville, Tenn., said in a statement. She oversees the project that led to the discovery.

Hypervelocity stars move at speeds that allow them to overcome the tug from the Milky Way's gravity.

Their existence was first posited by Los Alamos National Laboratory astrophysicist Jack Hill in 1988, who calculated that the gravitational interaction between a supermassive black hole and a binary star system would split the pair of stars, sending one spiraling in to its doom and kicking the other out of the galaxy at speeds of more than 2 million miles an hour.

If found, such stars would represent "nearly definitive" evidence for a supermassive black hole in the center of the Milky Way, he wrote at the time.

Seventeen years later, a team led by Warren Brown at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., discovered the first hypervelocity star and traced its source to a region close to that of the the galaxy's supermassive black hole, whose mass is some 4 million times the mass of the sun.

Since then, the number of hypervelocity stars originating from the center of the galaxy has risen to 18, leading to estimates that one is produced about every 10,000 years. They tend to be stars with several times the sun's mass.

Researchers are interested in these objects for a range of reasons, Dr. Holly-Bockelmann says in an interview. Such stars, hurtling from the galactic center like kernels of popcorn, provide a direct look at the kinds of stars that make up the region around the galaxy's supermassive black hole.

In addition, stars that get ejected can serve as useful tools for mapping the distribution of dark matter around the galaxy. Dark matter emits no detectable radiation, so astronomers have inferred its presence by the influence its gravity has on the matter astronomers can see.

Dark matter is widely seen as the material that shepherded visible matter into the seeds from which galaxies formed early in the history of the universe and keeps those galaxies from flying apart now. It's also the scaffolding along which galaxies and clusters are distributed throughout the cosmos.

Beyond the serious astrophysics, however, lies a bit of the gee-whiz factor, she acknowledges.

"It's amazing to think about stars that can be flung out of our galaxy," she says.

But the candidates for hypervelocity stars that Vanderbilt graduate student Lauren Palladino uncovered failed to fit the mold of the others found to date.

Unlike their predecessors – massive, hot, blue stars that form in the galactic bulge that harbors the black hole – the composition and masses of the new group were more like those of the sun and other stars in its neighborhood.

Moreover, calculations of the stars' orbits showed that they didn't originate in the galactic center, but rather seemed to come from all directions.

The team examined the likelihood that the stars could have come from outside the galaxy, perhaps ejected from the Milky Way's nearest big neighbor, the Andromeda Galaxy, some 2.5 million light-years away.

That proposition failed to hold up, although based on motions alone, the team couldn't rule out the stars' arrival from globular clusters orbiting the 100,000-light-year-in-diameter Milky Way, any of the galaxy's smaller, satellite galaxies, or even other galaxies within a 32-million light-year radius.

That led them to try to use the chemical composition to figure out their points of origin. They found that the composition of the stars in their sample was similar to the make-up of sun-like and somewhat smaller stars in the Milky Way's disk.

The team began with a sample of 42 stars that looked to be potential hypervelocity candidates, and after various tests whittled the number down to 20. Of the 20, the vast majority are likely to prove out to be hypervelocity stars, the researchers say, although for now, the team has identified seven for which they are 98 percent confident that intergalactic space is their destination.

Not everyone is convinced yet that the detections represent hypervelocity stars.

The Center for Astrophysics' Dr. Brown says he suspects that these will turn out to be fast-moving stars for sure, but without the velocity to propel them beyond the galaxy – a type of star known as a runaway star, which is moving much faster than material surrounding it as it orbits the galactic center, but not fast enough for the Great Escape.

Even if the new stars merely turn out to be low-mass runaways, that will be valuable, he adds. They can shed light on the distribution and traits of binary-star systems, from which runaways are thought to emerge, and they may play a role in enriching regions of the galaxy with elements heavier than hydrogen and helium, he suggests.

Holly-Bockelmann notes that additional observations are needed to truly nail down the velocities of the stars in the team's sample. The effort requires the use of two ways of measuring the velocity, one of which can introduce significant errors in the estimates if too few observations are used.

Yet even if only one of the 20 turns out to be a hypervelocity star, "that's still a pretty big discovery," she says, noting that it would raise interesting questions about how these cooler, lower-mass hypervelocity stars initially got the boot.

And what would happen should any of the stars host planets? It depends on how close the planet or planets are to the star. The closer they are, the more tightly the star's gravity binds them as companions.

"It's easy to think that tightly bound planets will stay with the star and be flung out as well," she says.

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