News and Events
Stars Caught in Bizarre Death-Dance
(Originally published by Massachusetts Institute of Technology News Office)
September 13, 2007 Cambridge, Mass.
MIT astronomers played a key role in discovering what NASA
calls one of the most bizarre objects in space: a star "skeleton" of
very low mass that is orbiting and being slowly consumed by a pulsar,
or remains of a second massive star, that is itself spinning faster
than a kitchen blender.
 |
| In this artist depiction of the SWIFT J1756.9-2508 system, the foreground object is the planet-mass object. The pulsar, located at the upper right, is tidally distorting the companion into a teardrop-shaped object, and ripping gas from it. This material flows in a stream toward the pulsar and forms a disk around it. Eventually, enough gas builds up in the disk to produce an outburst bright enough to make the system visible from Earth. (Image: Aurore Simonnet, Sonoma State University) |
A NASA team led by Hans Krimm and Craig
Markwardt at Goddard Space Flight Center and an MIT team led by Deepto
Chakrabarty, an associate professor of physics in MIT's Kavli Institute for Astrophysics and Space Research,
described the overall system (known by its abbreviation SWIFT J1756.9)
in an article accepted for publication in the Astrophysical Journal
Letters.
"While we already know of several cases of pulsars that
have consumed or vaporized most of the mass in their companion star,
SWIFT J1756.9 is possibly the most extreme example," said Chakrabarty.
Systems
like SWIFT J1756.9 provide a rare opportunity for astronomers to
examine how millisecond pulsars are spun up to such incredibly rapid
speeds, and to determine their eventual fate, he added.
SWIFT
J1756.9 was discovered earlier this year using NASA's Swift and Rossi
X-Ray Timing Explorer (RXTE) satellites. The RXTE observations indicate
that the pulsar, a type of neutron star, is spinning 182.07 times per
second, even though it is believed to contain at least 1.4 times the
mass of the sun but is only about 10 miles across. "This means that the
surface of the star is moving at about 7000 miles per second, or
roughly 4 percent the speed of light," Chakrabarty said.
The
companion object was found to orbit the pulsar every 54.7 minutes at an
average distance of only about 230,000 miles (slightly less than the
distance from Earth to the moon). It has what astronomers consider a
very low mass: about seven times that of Jupiter. For comparison, the
sun is over 1000 times more massive than Jupiter.
 |
| The low-mass companion in SWIFT J1756.9-2508 may have a mass just a few times greater than Jupiter, but up close, it would probably look nothing like a planet. The object is probably dominated by helium gas. Even though it is much larger than the pulsar, the pulsar is at least 100 times more massive. (Image: Aurore Simonnet, Sonoma State University) |
"This object is
merely the skeleton of a star," says Markwardt. "The pulsar has eaten
away the star's outer envelope, and all that remains is its helium-rich
core."
The system is only the eighth millisecond pulsar observed
to be pulling mass from a companion, and only one other such system has
a companion with such a low mass. The companion in that system also has
a minimum mass of about seven Jupiters.
The system probably
formed several billion years ago, when it consisted of a very massive
star and a smaller star. The more massive star evolved quickly and
exploded as a supernova, leaving behind a pulsar. The smaller star
eventually started to puff up as it aged, and the two objects became
embedded in the extended stellar envelope. This drained orbital energy,
causing the two stars to draw ever nearer.
Today, the two objects
are so close to each other that the pulsar's powerful gravity produces
a tidal bulge on its companion, siphoning off gas that flows into a
disk that surrounds the pulsar. The flow eventually becomes unstable
and dumps large quantities of gas onto the pulsar, causing an outburst
like the one observed in June.
With an estimated distance of
roughly 25,000 light-years, the system is normally too faint to be
detected, and is only visible during an outburst. SWIFT J1756.9 has
never been seen to erupt until this June, so as Markwardt points out,
"We don't know how long it will slumber before it wakes up again."
In
addition to Chakrabarty, the MIT team includes Jacob Hartman, a
graduate student in physics who defended his Ph.D. thesis on August 2.
MEDIA CONTACT:
Elizabeth A. Thomson
MIT News Office
Phone:
617-258-5402
E-mail:
thomson@mit.edu
Original press release located here.
