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Hybrid clusters shake up stellar evolution theories
Posted: 02 June 2011

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In two separate reports, astronomers reveal ancient globular clusters to be breeding much younger blue-straggler members, and a relatively young open cluster hosting an aging stellar population, calling into question theories of stellar evolution.

Globular clusters, densely populated hubs of stars, lying in the outskirts of our Milky Way Galaxy are around the same age as the Milky Way itself, about 10 billion years. It is strongly believed that all of the cluster's inhabitants formed at the same time, with individual stars evolving at their own pace, but with no new additions to the group.

A close binary pair of stars merge to form a blue-straggler star, stirring up both stars' fuel to make it burn hotter and bluer, as this artist's impression depicts. Image: NASA, ESA, W. Clark (Indiana University and UCLA), and K. Sahu (STScI).

Then, "blue stragglers" were discovered. Hotter, brighter and more massive than typical globular cluster stars, their apparent youth called into question how they got there. Collisions and interactions seemed the logical solution. "Astronomers expect that the stars get too close to each other because of the complicated dance that stars perform in these dense clusters, where thousands of stars are packed into a relatively small space, and each star is moving through this cluster under the influence of the gravity of all the other stars," explains Alison Sills of McMaster University in Hamilton, Ontario, who is presenting the research at the Canadian Astronomical Society Conference this week. "Somewhat like a traffic system with no stop lights, there are a lot of close encounters and collisions."

But new Hubble Space Telescope observations of a dozen clusters revealed that not only are blue stragglers present in all of the clusters studied, but there are two distinct generations of star formation, separated by between 30 and 100 million years, with the second generation apparently forming from the material shed by the first generation of stars.

"There are data for about a dozen clusters with both blue stragglers and good information about the multiple populations," Sills tells Astronomy Now. "About 25 clusters have been surveyed of the ~150 globular clusters in our Galaxy by different groups, and all of them show one or more of these hints [of multiple populations]."

Sills speculates that the stars of the first generation put gas back into the cluster when they are in their late phase of evolution, that is, at the end of their lives and are about to become a white dwarf – a phase on the Hertzprung-Russell diagram of stellar evolution referred to as the Asymptotic Giant Branch. "These stars have really strong winds, and if the material from the wind can collect in the centre of the cluster, the second generation can form from this 'stuff'," she says. "However, blue stragglers are thought to be a completely separate problem and aren't part of this picture." Sills says that even though the blue stragglers and the second generation of stars share similar characteristics, there is a long way to go before finding the missing link between these two unexpected members of globular clusters.

Forty-two blue straggler candidates (circled) were found in the Milky Way's bulge during the SWEEPS survey, of which astronomers believe 18-37 are true blue stars much younger than ordinary bulge stars. Image: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI).

Meanwhile, the seemingly open cluster NGC 6791 in Lyra has some of the traits of an older globular cluster. “This cluster is about twice the age of the Sun and is unusually metal rich (at least twice the Sun’s metallicity),” says Imants Platais of the Johns Hopkins University in Baltimore. “A couple of decades ago, it was also found that NGC 6791 contains a handful of very hot but somewhat dim stars, called hot subdwarfs. The presence of such stars in an open cluster is rare, though not unique.”

Platais and colleague's new census of some 60 thousand of the cluster's stars revealed that NGC 6791 contains several luminous stars belonging to the so-called classical horizontal branch. These are stars which are normally found only in globular clusters which are much older than NGC 6791. Unlike a typical globular cluster, however, NGC 6791 contains simultaneously both red and very blue horizontal branch stars, adding to the strange observation that it is both metal rich and old.

The team say that NGC 6791 is the first known star cluster juxtaposing the properties of open and globular clusters and, as such, represents a new class of star clusters, which could have originated in the central bulge region of the Milky Way.

The research was presented at the 218th meeting of the American Astronomical Society in Boston, where the first announcement of blue stragglers within the central hub of the Milky Way was also made. A team lead by Will Clarkson of Indiana University, used data from the 2006 Hubble Space Telescope survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) to study stars in the central bulge of our Galaxy, a survey intended to find hot Jupiter-class planets that orbit very close to their stars, but which uncovered 42 oddball blue stars with brightness and temperatures typical for stars much younger than ordinary bulge stars.

“Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood,” says Clarkson. “Many details of its star-formation history remain controversial."

The observations clearly indicate that if there is a young star population in the bulge, it is very small, and that the central bulge stopped producing stars billions of years ago, and is now home to aging Sun-like stars.

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