Ancient variable stars may not be isolated after all

Chuck Bednar for – @BednarChuck

Unlike most types of stars, which are just as likely as not to have companions and be part of a binary system, an ancient type of variable stars known as RR Lyrae stars seemed to always live out their lives on their own – something that has long puzzles astronomers.

Now, however, new research now appearing online in the journal Monthly Notices of the Royal Astronomical Society Letters suggests that some of these lonesome old stars may not be all alone after all, identifying as many as 20 candidate RR Lyrae binary systems (an increase of up to 2000 percent over previous observations, and including 12 identified with high confidence).

Sadly, “light-travel time effect” does not involve a Tardis

Binary stars are essential when it comes to studying astrophysics, because their properties can be accurately inferred from detailed analysis of their orbital properties. The apparent isolation of RR Lyrae variables has made them difficult to study directly, forcing scientists to fall back on theory to explain some of their key properties.

The study, which was led by scientists at the Millennium Institute of Astrophysics (MAS) and the Pontificia Universidad Católica de Chile’s Institute of Astrophysics (IA-PUC), used a method they call the “light-travel time effect,” which takes advantage of subtle differences in the amount of time that starlight takes to reach us here on Earth.

“In the solar neighborhood, about every second star is in a binary,” lead author Gergely Hajdu, an Ph. D. student at IA-PUC and a researcher at the MAS, said in a statement. “The problem with RR Lyrae variables is that for a long time only one of them was known to be in a long-period binary system. The fact that among 100,000 known RR Lyrae stars only one of them had been seen to have such a companion was something really intriguing for astronomers.”

Observations may allow for direct measurements of RR Lyrae stars

Hajdu explained that RR Lyrae stars pulsate on a regular basis, with their size, temperature and brightness levels all significantly increasing and decreasing in a matter of hours. When a star in a binary system pulsates, the perceived changes in brightness can be affected by the star’s position in its orbit around its companion star, meaning that the light takes longer to reach Earth if it is at the furthest point in its orbit and less time when it is at the closest point.

“This subtle effect is what we have detected in our candidates,” he explained, adding that his team’s measurements were based on data collected and published by the Polish OGLE Project team using the 1.3m Warsaw telescope at Las Campanas Observatory in northern Chile.

“Our 20 candidates were found analyzing the roughly 2000 best observed RR Lyrae stars towards the central parts of the Milky Way,” Hajdu said. “That’s about 5 percent of the known ones. It was only thanks to the high quality of the OGLE data and the long timespan of these observations that we could finally find signs of companions around so many of these stars.”

In fact, the systems detected by he and his fellow researchers reportedly have orbital periods that last several years, suggesting that the even though the companion stars are linked gravitationally, they are not very close to one another. They now plan to try and directly measure their mass and other physical properties – something that, until now, seemed all but impossible.


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