Wild Orbits: Stars Not From Around Here
A view of the wild orbits of ultracool subdwarfs from 150,000 light years above Galactic plane.
Credit: Burgasser/AMNH

PASADENA, CALIF. — A recently discovered type of cold, lightweight star follows some wild orbits around the Milky Way, a team of astronomers have found.

Some of these so-called "ultracool subdwarfs" plunge almost through the center of the Milky Way, while others venture so far beyond the galaxy that they might be visitors from another galaxy, astronomers said here this week at the 214th meeting of the American Astronomical Society.

Ultracool subdwarfs were first recognized as a unique class of stars in 2003. They are distinguished by their low temperatures and low concentrations of elements other than hydrogen and helium.

They are at the bottom end of the size range of stars, with some being so small that they are closer to objects called brown dwarfs. They are up to 10,000 times fainter than the sun and are extremely rare — only a few dozen ultracool subdwarfs are known today.

These small stars also move surprisingly swiftly through the galaxy.

"Most nearby stars travel more or less in tandem with the sun tracing circular orbits around the center of the Milky Way once every 250 million years," said team member Adam Burgasser of MIT. The sun moves at about 485,000 mph (782,000 kilometers per hour).

The ultracool subdwarfs, on the other hand, zip along at more than 1 million mph (1.8 million kph).

"If there are interstellar cops out there, these stars would surely lose their driver's licenses," Burgasser said.

Wild orbits

Burgasser's team wanted to find out what kind of orbits these speedy stars followed. To do so, they assembled measurements of the positions, distances and motions of about two dozen of the stars and used model developed to study galactic collisions to chart the orbits.

The results were surprising indeed: "These orbits were like nothing I'd ever seen before," said team member Robyn Anderson, an MIT graduate student.

Some of these stars follow eccentric comet-like tracks that take them deep into the center of the Milky Way. Others swoop far beyond the sun's own orbit. In fact, most spend most of their time thousands of light-years above or below the disk of the Milky Way.

"Someone living on a planet around one of these subdwarfs would have an incredible nighttime view of a beautiful spiral galaxy — our Milky Way — spread across the sky," Burgasser said.

The orbital results confirm that all of the ultracool subdwarfs are part of the Milky Way's halo, a widely dispersed population that likely formed in the distant past.

Out of this galaxy

But one of these subdwarfs, a star dubbed 2MAS 1227-0447 in the constellation Virgo, has an orbit that is so far out that it suggests it has an extragalactic origin.

Our calculations show that this subdwarf travels up to 200,000 light years away from the center of the galaxy, almost 10 times farther than the sun," said team member John Bochanski, also of MIT. In fact, it's further even than the Milky Way's nearest galactic neighbors.

"Based on the size of its one billion-year orbit and direction of motion, we speculate that 2MAS 1227-0447 might have come from another, smaller galaxy that at some point got too close to the Milky Way and was ripped apart by gravitational forces," Bochanski said.

Astronomers have found other stars that came from neighboring galaxies, but these have all been massive red giants. The star mapped by Burgasser's team is the first alien low-mass star.

"If we can identify what stream this star is associated with, or which dwarf galaxy it came from, we could learn more about the types of stars that have built up in the Milky Way's halo over the past 10 billion years," Burgasser said.

The team's results were detailed in the Astrophysical Journal.