New class of 'hypervelocity stars' discovered escaping the Milky Way

Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. 

These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. 

The general directions from which the stars have come are shown by the colored bands. Image courtesy Julie Turner, Vanderbilt University. 

The top view of the galaxy comes from the National Aeronautics and Space Administration and the side view comes from the European Southern Observatory.

An international team of astronomers has discovered a surprising new class of "hypervelocity stars" - solitary stars moving fast enough to escape the gravitational grasp of the Milky Way galaxy.

The discovery of this new set of "hypervelocity" stars was described at the annual meeting of the American Astronomical Society this week in Washington, D.C., and is published in the Jan. 1 issue of the Astrophysical Journal.

Lauren Palladino

"These new hypervelocity stars are very different from the ones that have been discovered previously," said Vanderbilt University graduate student Lauren Palladino, lead author on the study.

"The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small - about the size of the sun - and the surprising part is that none of them appear to come from the galactic core."

The discovery came as Palladino, working under the supervision of Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey, a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.

"It's very hard to kick a star out of the galaxy," said Holley-Bockelmann. "The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core."

"That means when you trace the star back to its birthplace, it comes from the center of our galaxy. 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."

Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity.

They also estimate that the Milky Way's central black hole has a mass equivalent to four million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.

The typical scenario involves a binary pair of stars that get caught in the black hole's grip.

As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity.

So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Palladino and her colleagues have discovered an additional 20 sun-sized stars that they characterize as possible hypervelocity stars.

"One caveat concerns the known errors in measuring stellar motions," she said.

"To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does."

"We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real."

The astronomers are following up with additional observations.

Supernova Legacy Survey: Powerful ancient explosions explain new class of supernovae

A small portion of one of the fields from the Supernova Legacy Survey showing SNLS-06D4eu and its host galaxy (arrow). 

The supernova and its host galaxy are so far away that both are a tiny point of light that cannot be clearly differentiated in this image. 

The large, bright objects with spikes are stars in our own galaxy. 

Every other point of light is a distant galaxy. 

Credit: UCSB

Astronomers affiliated with the Supernova Legacy Survey (SNLS) have discovered two of the brightest and most distant supernovae ever recorded, 10 billion light-years away and a hundred times more luminous than a normal supernova. Their findings appear in the Dec. 20 issue of the Astrophysical Journal.

These newly discovered supernovae are especially puzzling because the mechanism that powers most of them—the collapse of a giant star to a black hole or normal neutron star—cannot explain their extreme luminosity.

Discovered in 2006 and 2007, the supernovae were so unusual that astronomers initially could not figure out what they were or even determine their distances from Earth.

"At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one," said lead author D. Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT) and adjunct faculty at UC Santa Barbara.

"I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."

One of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and possibly the most luminous member of an emerging class of explosions called superluminous supernovae.

These new discoveries belong to a special subclass of superluminous supernovae that have no hydrogen.

The new study finds that the supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second.

Magnetars have the mass of the sun packed into a star the size of a city and have magnetic fields a hundred trillion times that of the Earth.

While a handful of these superluminous supernovae have been seen since they were first announced in 2009, and the creation of a magnetar had been postulated as a possible energy source, the work of Howell and his colleagues is the first to match detailed observations to models of what such an explosion might look like.

Co-author Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen.

The star likely was initially much bigger but apparently shed its outer layers long before exploding, leaving only a smallish, naked core.

More information: dx.doi.org/10.1088/0004-637X/779/2/98