The supernova remnant Cassiopeia A, one of the youngest in
our galaxy and one that has long puzzled astronomers, is likely a dense type of
star called a neutron star swathed in a carbon atmosphere, a new study finds.
Cassiopeia A, a remnant of the explosion of a star that once
shone brightly, is thought to be just 330 years old based on observations of
the constellation in which it sits by Britain's first Astronomer Royal, John
Flamsteed, in 1680.
Astronomers didn't get their first actual up-close glimpse
of the core remnant, which lies about 11,000 light-years away, until 1999, when
the Chandra
X-ray Observatory imaged the
collapsed star.
"Before then it was thought there's probably going to
be either a neutron star or a black hole in the center of this object, but it
wasn't sure what — nobody had seen it," said Craig Heinke of the
University of Alberta in Canada and a co-author of the new study. With Chandra
"we were actually able to pick out something at the center."
But even with a closer view of the object, it still puzzled
astronomers: "The properties of this object were a little strange,"
Heinke said.
Puzzling properties
In particular, the star's spectrum — the amount of energy it
radiates out at each wavelength of light — implied that the star's radius was
either much too small for a neutron
star (only 0.1 miles [0.2 km] in radius, instead of the accepted 12 miles [20
km]) or that the high-energy emission seen from it was coming from hotspots on
the surface, not from the entire face of the star. But radiation from a hotspot
would appear as a pulse as the star rotated, and no pulsations were seen in the
star's radiation. The star also had a low magnetic field, which would be
unlikely to drive any pulsing behavior.
Since a pulsating
star with surface hotspots seemed to be out of the equation, Heinke and his
colleague Wynn Ho of the University of Southampton in the U.K. tried to find a
way to tackle the size problem.
To do this, they first added an atmosphere to models of the
star. They first tried a hydrogen atmosphere, as it was thought that in the
extreme gravitational field of a neutron star, the star's layers would quickly
stratify, with the heaviest elements relegated to the interior and the lightest
to the outermost layer. Hydrogen, of course, is the lightest element in the
universe.
A hydrogen atmosphere ballooned the star's size up to 2.5
miles (4 km) in radius — better, but still not big enough. Trying a helium
atmosphere also "helped, but not very much," Heinke said.
Next on the list was carbon, and sure enough, that gave a
radius in the models "that was in the right ballpark for neutron stars,"
Heinke said.
But that left the researchers with another question: How did
the star end up with an atmosphere made up entirely of carbon?
Carbon atmosphere
That's where the star's youth comes into play.
"This is the youngest neutron star that we have ever
observed," Heinke said. "The fact that this is so young means that
it's been really, really hot the most recently of any neutron stars."
In this case, "hot" means temperatures up to 1
billion degrees Kelvin (2 billion Fahrenheit). Ho and Heinke think that the
star "was actually able to conduct nuclear fusion on its surface and burn
the hydrogen and helium into carbon," Heinke explained. (The hydrogen and
helium came from a continuous rain falling onto the star's surface from the supernova
debris.)
As the star gets older, it will cool substantially and
eventually stop burning the hydrogen and helium into carbon and develop a
hydrogen atmosphere, Heinke said.
He and Ho plan to test this model on other known young
neutron stars to see how well it holds up. Their findings are detailed in the
Nov. 5 issue of the journal Nature.
advertisement
