Astronomers Solve 40-year Mystery Of Nearby Supernova
January 12, 2012

Astronomers Solve 40-year Mystery Of Nearby Supernova

NASA's Hubble Space Telescope has helped astronomers solve a 40-year mystery over what type of star, or so-called progenitor, caused a supernova in a nearby galaxy.

The powerful telescope´s findings have provided new observational data for identifying one of several scenarios that trigger such outbursts.

Based on previous observations from ground-based telescopes, astronomers knew the supernova class, called a Type Ia, created a remnant named SNR 0509-67.5, which lies 170,000 light-years away in the Large Magellanic Cloud galaxy.

Type Ia supernovae release vast amounts of energy, in which the light produced is often brighter than an entire galaxy of stars.

In theory, this type of supernova explosion is caused by a star spilling material onto a white dwarf companion -- the compact remnant of a normal star -- until it sets off one of the most powerful explosions in the universe.

However, astronomers failed to find any remnant of the companion star, leading them to conclude this common scenario did not apply in this case, although it is still a possible theory for other Type Ia supernovae.

"We know Hubble has the sensitivity necessary to detect the faintest white dwarf remnants that could have caused such explosions," said lead investigator Bradley Schaefer of Louisiana State University (LSU) in Baton Rouge.

"The logic here is the same as the famous quote from Sherlock Holmes: 'when you have eliminated the impossible, whatever remains, however improbable, must be the truth.'"

The astronomers said SNR 0509-67.5 can be best explained by two tightly orbiting white dwarf stars spiraling closer and closer until they finally collided and exploded.

The longstanding search for Type Ia supernovae progenitors has been a critical question in astrophysics, taking on particularly importance during the last decade with Type Ia supernovae being the main tools for measuring the accelerating universe.

The challenge for scientists has been to correctly identify the type of star system that pushes the white dwarf's mass over the edge and triggers this type of explosion.

Many possibilities have been theorized, although most require that a companion star near the exploding white dwarf be left behind after the explosion.

One possible way to distinguish between the various progenitor models has been to look deep in the center of an old supernova remnant to search for the ex-companion star.

In 2010, Schaefer and Ashley Pagnotta of LSU were preparing a proposal to look for any faint ex-companion stars in the center of four supernova remnants in the Large Magellanic Cloud when they discovered the Hubble Space Telescope already had taken the desired image of one of their target remnants, SNR 0509-67.5, for the Hubble Heritage program, which collects images of particularly photogenic astronomical targets.

The astronomers analyzed the central region, and found it to be completely empty of stars down to the limit of the faintest objects Hubble can detect in the photos.

Schaefer suggests the best explanation remaining is the "double degenerate model” in which two white dwarfs collide.

Although there are no recorded observations of the star exploding, researchers at the Space Telescope Science Institute in Baltimore have identified light from the supernova that was reflected off of interstellar dust, delaying its arrival at Earth by 400 years.

This delay, known as a light echo, of the supernova explosion also allowed the scientists to measure the spectral signature of the light from the explosion, and to use the color signature to deduce it was a Type Ia supernova.

Because the remnant appears as a nice symmetric shell or bubble, the geometric center can be determined accurately, properties that make SNR 0509-67.5 an ideal target to search for ex-companions.

Furthermore, the young age means that any surviving stars have not moved far from the site of the explosion.

The scientists plan to look at other supernova remnants in the Large Magellenic Cloud to further test their observations.

A paper on the results was published in the January 12 issue of the journal Nature. The researchers also presented their findings at the American Astronomical Society meeting on January 11.


Image Caption: This image of Type Ia Supernova Remnant 0509-67.5 was made by combining data from two of NASA´s Great Observatories. The result shows soft green and blue hues of heated material from the X-ray data surrounded by the glowing pink optical shell, which shows the ambient gas being shocked by the expanding blast wave from the supernova. Credit: NASA, ESA, and B. Schaefer and A. Pagnotta (Louisiana State University, Baton Rouge); Image Credit: NASA, ESA, CXC, SAO, the Hubble Heritage Team (STScI/AURA), J. Hughes (Rutgers University)


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