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.
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