X-ray Evidence Supports Possible New Class Of Supernova (Forwarded)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-6535)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)

For Release: January 4, 2007

CXC RELEASE 01-07

X-ray Evidence Supports Possible New Class Of Supernova

Evidence for a significant new class of supernova has been found with
NASA's Chandra X-ray Observatory and the European Space Agency's
XMM-Newton. These results strengthen the case for a population of stars
that evolve rapidly and are destroyed by thermonuclear explosions. Such
"prompt" supernovas could be valuable tools for probing the early history
of the cosmos.

A team of astronomers uncovered a puzzling situation when they examined
X-ray data from DEM L238 and DEM L249, the remnants of two supernovas in a
nearby galaxy. On the one hand, the unusually high concentration of iron
atoms implied that the remnants are the products of thermonuclear
explosions of white dwarf stars, a well-known type of supernova known as
Type Ia. On the other hand, the hot gas in the remnants was much denser
and brighter in X-rays than typical Type Ia remnants.

A white dwarf, the dense final stage in the evolution of a sun-like star,
is a very stable object and will not explode on its own. However, if a
white dwarf has a close companion star it can grow beyond a critical mass
by pulling gas off the companion and explode.

Computer simulations of Type Ia supernova remnants showed that the most
likely explanation for the X-ray data is that the white dwarfs exploded
into very dense environments. This suggests that the stars which evolved
into these white dwarfs were more massive than usual, because heavier
stars are known to expel more gas into their surroundings.

"We know that the more massive a star is, the shorter its lifetime," said
Kazimierz Borkowski of North Carolina State University, Raleigh. "If such
a star could also begin to pull matter from its companion at an early
stage, then this star would have a much shorter fuse and explode in only
about 100 million years -- much less than other Type Ia supernovas."

Other teams have independently found evidence for prompt Type Ia
explosions using optical observations, but at much greater distances where
the environment of the stellar explosion cannot be probed. The X-ray data
of DEM L238 and DEM L249 represent nearby examples of prompt Type Ia
supernovas.

"We still need to know more about the details of these explosions since
they are such an important tool for studying cosmology," said Stephen
Reynolds also of North Carolina State University. "So, it's exciting to
discover we have some really nearby examples, astronomically speaking, of
this different class of explosion."

The luminosity of Type Ia explosions is thought to be very consistent from
star to star, and astronomers have used observations of Type Ia supernovas
in optical light as cosmic mile markers to study the accelerating
expansion of the cosmos caused by dark energy.

If Type Ia supernovas can occur so quickly, they can exist much earlier in
the Universe's history than generally believed, allowing them to probe the
expansion at these epochs. Another possibility is that the prompt Type
Ia's may also differ in other properties. If so, the assumption that Type
Ia's are standard candles may be compromised, complicating attempts to
study dark energy.

"We weren't around to see these stars before they exploded," said Sean
Hendrick of Millersville University, Penn., "but these X-ray clues tell us
that something unusual happened in the case of these two."

After finding this evidence for prompt Type Ia explosions in the Large
Magellanic Cloud, a nearby galaxy, the researchers are looking at other
supernova remnants within the Milky Way to see if they might be examples
of this potential new class. For example, the famous supernova observed by
Johannes Kepler in 1604 might have been a prompt Type Ia supernova.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra
program for the agency's Science Mission Directorate. The Smithsonian
Astrophysical Observatory controls science and flight operations from the
Chandra X-ray Center, Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2007/deml238/
and

http://www.nasa.gov/mission_pages/ch...tos07-001.html