Sunday, October 22, 2017

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An artistic representation of an exploding star early in its outburst. (Image courtesy STFC and David Hardy) An artistic representation of an exploding star early in its outburst. (Image courtesy STFC and David Hardy)

Astronomers Find Fluctuations in Exploding Stars

New observations reveal pauses, flickers and flares not reliably seen before.

Astronomers have traced the waxing and waning light of exploding stars more closely than ever before and seen patterns that aren’t yet accounted for in our current understanding of how these eruptions occur.

Using data from a sensitive instrument aboard a satellite that images the entire sky every 102 minutes, they studied four of these stars, or novae, which exploded so violently their light would have been visible without a telescope and measured their brightness over the course of the outburst.

Three of the novae stalled before reaching a peak, and all flickered or flared as the explosions ran their course, according to a new article in the Astrophysical Journal co-authored by San Diego State University astronomy professor Allen Shafter.

Rebekah Hounsell, a graduate student at Liverpool John Moores University in Britain made the measurements while visiting SDSU and University of California San Diego.

Because starlight is a distraction, researchers must subtract it from their data so that they can focus on the sun’s outer corona and the heliosphere. They make detailed maps of stellar light, including its brightness.

In those maps, Hounsell identified the four novae by finding points of light that rapidly brightened and dimmed over the course of days.

Wavering light

Other astronomers had observed a pause in brightening of novae, or “pre-maximum halt” before, but some thought it an anomaly. According to the authors, the precise time-scale and repeated observations of the current study confirms it.

“The reality of this halt as found in all three of the fast-declining novae observed is a challenge to detailed models of the nova outburst," said one of the authors, astrophysicist Mike Bode of Liverpool John Moores University.

Two independent teams of theorists have already begun to refine their models of how novae explode in response.

Astronomers typically characterize a nova’s changing light with curves smoothly fit to more sporadic observations, but the rapid cadence of the solar imager captured glimmers that hadn’t been observed before. All flickered as their light dimmed and one nova, the slowest of the four to dim, flared brightly twice after reaching its peak luminosity.

These novae are white dwarf stars that steal matter, in the form of hydrogen, from a companion star, often an aging, expanding red giant. As hydrogen accumulates, the white dwarf’s gravity pulls it in and condenses it until it ignites, setting off a runaway nuclear fusion reaction.

The team speculates that the post-peak flares may correspond to changes in the dynamics of that reaction that still need to be explained.

Catching missing stars

“Before Hounsell looked through the data, most novae were observed only after their peak luminance,” said astrophysicist Bernard Jackson at the Center for Astrophysics and Space Sciences at the University of California San Diego. “The instrument's very even cadences and uniformly exposed images allow us to trace the entire evolution of these explosions as they brighten and dim.”

Data from the Solar Mass Ejection Imager (SMEI) developed by Jackson and his team, which has been in operation aboard the Coriolis satellite since January 2003, allows astronomers to measure novae that they initially missed.

"Even today novae are mainly discovered by amateur astronomers around the world who then alert their professional counterparts to conduct observations,” Hounsell said.

As many as five such novae explode in our galaxy each year Shafter has previously estimated, but more than half went undetected. “Now, the SMEI assures that the brightest and most rapidly evolving novae — ones that brighten and then fade within a few days — are not overlooked,” Shafter said. "The high time resolution of the SMEI observations has opened up a new window into the study of novae in our galaxy.”

The U.S. Air Force and NASA jointly operate the Coriolis satellite and SMEI. Bernard Jackson’s research at UCSD is supported by the National Science Foundation and NASA. Allen Shafter’s work at SDSU is supported by the National Science Foundation.

About the SDSU Department of Astronomy

San Diego State University has the only astronomy department in the California State University system, and is the only campus to offer bachelor's and master's degrees in astronomy.