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Lonely Old Stars

Many of the stars in the Universe live with companions by their sides in so-called “binary systems”.  In these binary systems, two stars orbit around their common center of gravity.  Suitable binary systems are extremely important in astrophysics, since their properties can be inferred with unparalleled accuracy from detailed analysis of their orbital properties.  Oddly enough, however, a huge majority of the known members of the RR Lyrae variable star family have lived their lives alone.  These stars, the oldest known stars in the cosmos, contain vital information about the origin and evolution of the stellar systems that host them, including the Milky Way itself.  However, a lack of RR Lyrae stars in binary systems has made directly Carlos Arias Delgado RR Lyraeassessing some of their key properties extremely difficult, and theory frequently has to be invoked to fill the gap.

The solitude of these stars has always puzzled astronomers.  Now, however, an international research team led by the Millennium Institute of Astrophysics (MAS) and the Pontificia Universidad Catolica de Chile’s Institute of Astrophysics (IA-PUC) have discovered that these stars may not hate companionship as much as previously thought.  The team reported on identification of as many as 20 candidate binaries for RR Lyrae, an increase of up to 2000% from previous tallies.  Twelve of these candidates have enough measurements to conclude that they do indeed consist of two stars orbiting each other.  In their paper, the authors used a method that astronomers call the “light-travel time effect”, which exploits subtle differences in time that it takes starlight to reach us.  The RR Lyrae stars pulsate regularly, significantly increasing and then decreasing their sizes, temperatures and brightness in just a few hours.  When a pulsating star is in a binary system, the changes in brightness that we perceive can be affected by where the star is in the course of its orbit around the companion.  This means that starlight takes longer to reach us when it’s at the farthest point along its orbit, and vice-versa.

The researchers’ measurements were based on data published by the Polish OGLE Project, who has obtained their data using the 1.3m Warsaw telescope in Las Campanas Observatory in northern Chile, repeatedly observing the same patches of the sky for many years.  The 20 binary candidates were found analyzing the roughly 2000 best observed RR Lyrae stars towards the central parts of the Milky Way, making up about 5% of the known ones.  Only thanks to the high quality of the OGLE data and long timespan of these observations that the researchers could finally find signs of companions around so many of these stars.

Indeed, the systems detected by researchers have orbital periods of several years, indicating that the companions aren’t very close to one another, even though they’re bound together by gravity.  Binaries with even longer periods could also exist, but the current data doesn’t extend long enough for researchers to reach strong conclusions about them.  The researchers have pointed out that these results have significant implications for astrophysics; these are extremely old stars that have witnessed the formation of galaxies.  They’re also easy to identify, since they show characteristic, cyclical brightness variations that make them excellent distance indicators for the nearby Universe.  Now, researchers can exploit the orbital information contained in these binary systems in order to directly measure their physical properties, especially their masses but maybe their diameters as well.  This could open new doors to discoveries that had previously seemed impossible.

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