How ‘blue stragglers’ stay forever young

NORTHWESTERN (US) — To  maintain their youthful bright appearance, old stars called blue stragglers eat up the outer envelope of giant-star companions, stripping them down to their white dwarf core in a process called mass transfer.

Several theories have attempted to explain why blue stragglers appear younger than they actually are, but, until now, scientists have lacked the crucial observations with which to test each hypothesis. Armed with such observational data, two astronomers report in the journal Nature that this mechanism of mass transfer explains the origins of blue stragglers.

The majority of blue stragglers in the study are binaries—they have a companion star. “It’s really the companion star that helped us determine where the blue straggler comes from,” says Aaron M. Geller, an astrophysicist at Northwestern University and first author of a new study.

“The companion stars orbit at periods of about 1,000 days, and we have evidence that the companions are white dwarfs. Both point directly to an origin from mass transfer.”

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For the study, Geller and Robert Mathieu, professor of astronomy at the University of Wisconsin-Madison, studied the NGC 188 open cluster, which is in the constellation Cepheus, situated in the sky near Polaris, the North Star. The cluster is one of the most ancient open star clusters, yet it features the mysterious young blue stragglers.

The cluster has around 3,000 stars, all about the same age, and has 21 blue stragglers. Geller and Mathieu are the first to use detailed observational data from the WIYN Observatory in Tucson, Ariz., of the blue stragglers in NGC 188.

They used the information to analyze and compare the three main theories of blue straggler formation: collisions between stars, mergers of stars, and mass transfer from one star to another. The only one left standing when their analysis was complete was the theory of mass transfer.

The light from the blue stragglers’ companion stars is not actually visible in observations. While the companions haven’t been seen directly, their effect on the blue stragglers is evident: each companion pulls gravitationally on its blue straggler and creates a “wobble” as it orbits, allowing astronomers to measure the mass of the companion stars. The WIYN data show that each companion star is about half the mass of the sun, which is consistent with a white dwarf.

The other two origin theories—collisions and mergers—require the companion stars to be more massive than what is observed. In fact, in both scenarios, some of the companion stars could be bright enough to be visible in the WIYN data, which is not the case.

“We think we have a good understanding of stellar evolution, but it doesn’t predict blue stragglers,” Geller says. “People have been trying to explain the origin of blue stragglers since their discovery in 1953, and now we have the detailed observations needed to identify how they were created.”

“As so often happens in astronomy, it is the objects that you don’t see that provide the critical clues,” says Mathieu, an expert on binary stars. “Now we will use the Hubble Space Telescope to search for the ultraviolet light in which white dwarf secondary stars shine.”

The next step will be to collect observations in about a year from Hubble that will confirm whether or not the blue stragglers’ companions are indeed white dwarfs.

The NGC 188 data set was collected during the last decade by the 3.5-meter WIYN Telescope on Kitt Peak, Ariz. The observatory is operated by Indiana University, Yale University, UW-Madison, and the National Optical Astronomical Observatory (NOAO).

The National Science Foundation, the Wisconsin Space Grant Consortium and the Lindheimer Fellowship at Northwestern University supported the research.

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