Lehigh University
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A brighter method for measuring the surface gravity of distant stars

Astronomers have found a clever new way to slice and dice the flickering light from a distant star in a way that reveals the strength of gravity at its surface.

That is important because a star’s surface gravity is one of the key properties that astronomers use to calculate a star’s physical properties and assess its evolutionary state.

The new technique can also be used to significantly improve estimates of the sizes of the hundreds of exoplanets that have been discovered in the last 20 years. Current estimates have uncertainties ranging from 50 percent to 200 percent. Using the improved figures for the surface gravity of the host stars calculated by the new method should cut these uncertainties at least in half.

The technique was developed by a team of astronomers including Joshua Pepper, assistant professor of physics at Lehigh University and headed by Vanderbilt University graduate student Fabienne Bastien (the first black astronomer to publish as a first author in Nature), Keivan Stassun, Vanderbilt Professor of Physics and Astronomy and Gibor Basri, professor of astronomy at the University of California, Berkeley. The full study appears in the current issue of the journal Nature along with a separate commentary.
 
"Once you know a star's surface gravity then you only need one other measurement, its temperature, which is pretty easy to obtain, to determine its mass, size and other important physical properties," said Stassun.

“This actually could be the breakthrough we’ve needed to pin down the sizes of hundreds more stars and exoplanets,” said Maria Womack, the program director at the National Science Foundation which funded the research. “Getting accurate sizes is critical to measuring exoplanet density, which has been a missing puzzle piece for so many planets. So, in addition to having implications for stellar evolution, this innovative work will be invaluable for identifying hundreds of exoplanets as either rocky or gaseous.”

Exquisitely simple

The new method is remarkably simple – requiring only five lines of computer code to make the basic measurement – substantially reducing the cost and effort required to calculate the surface gravities of thousands of stars.

To determine the accuracy of the flicker method, they used it to calculate the surface gravity of stars that have been analyzed using asteroseismology. They found that it has an uncertainty of less than 25 percent, which is better than both the photometric and spectroscopic methods. Its major limitation is that it requires extremely high quality data taken over long time periods. But this is precisely the type of observations made by Kepler while it was searching for periodic dips in light caused when exoplanets cross the face of a star. So the Flicker method can be applied to the tens of thousands of stars already being monitored by Kepler.

“The exquisite precision of the data from Kepler allows us to monitor the churning and waves on the surfaces of stars,” said team member Joshua Pepper, assistant professor of physics at Lehigh University. “This behavior causes subtle changes to a star's brightness on the time scale of a few hours and tells us in great detail how far along these stars are in their evolutionary lifetimes.”

The research was funded by the National Science Foundation and the Vanderbilt Initiative in Data-intensive Astrophysics (VIDA).

For the complete text of the press release, visit http://news.vanderbilt.edu/2013/08/surface-gravity-of-stars/

For NSF coverage, visit http://www.nsf.gov/news/news_summ.jsp?cntn_id=128899&org=AST&from=news

Story by David F. Salisbury

Posted on Friday, August 23, 2013

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