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Vampires and collisions give stellar stragglers new life
Posted: December 23, 2009

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In two separate studies published in the journal Nature this week, astronomers have revealed that stellar collisions and a process called vampirism are responsible for giving 'blue stragglers' a youthful appearance.

Blue stragglers shine brightly, are older than they appear, and, unusually, gain mass at a late stage of life. A likely explanation is that they originate from normal stars that increase their mass by collisions or stealing mass from a companion, but the results of the two studies show that both mechanisms appear to be at work.

M 30 formed 13 billion years ago and is located 28,000 light years away. Astronomers have identified two main populations of blue straggler: those that form in collisions and those in a binary system where one star siphons material from the other. Image: NASA/ESA.

“These blue, luminous stars should have used up their hydrogen fuel and flamed out long ago,” explains Robert Mathieu, a University of Wisconsin-Madison astronomer. “Yet they are still here. By some means or another, they have recently increased their mass, their fuel supply.”

Mathieu and colleague Aaron Geller report their observations of an old, open star cluster NGC 188, which hosts 21 blue stragglers. They used the 3.5-meter WIYN Telescope on Kitt Peak and the National Optical Astronomical Observatory to find that these stellar curiosities gain mass in three ways.

In the first scenario, one of the two stars in a close binary orbit puffs up into a red giant, dumping its outer envelope onto its companion star. The second scenario involves two binary star systems crossing paths, resulting in a greater chance of cosmic collisions, and the third scenario describes a star approaching a binary system, exerting enough pull for the binary stars to merge with each other to create one massive star.

Top: the collision model, where two low-mass stars in a crowded environment combine to form a single hot star. Bottom: the vampire model, where the lower mass star drains its companion of live-giving fuel. Image: NASA/ESA.

At least three-quarters of the blue stragglers in NGC 188 were found in binary systems. "These aren’t just normal stars that are straggling behind in their evolution," says Mathieu. "There is something unusual going on with their companions."

One binary system was even found to comprise two blue stragglers. “Almost certainly these blue stragglers formed separately, and then the two binaries that each were in encountered one another, ejecting two of the stars and leaving behind this truly unique object,” says Geller. The blue stragglers were also found to be spinning much faster than average stars.

In a separate study, lead author Francesco Ferraro from the University of Bologna in Italy, describes his team's observations of blue stragglers in globular cluster M 30, using the now retired Wide Field Planetary Camera 2 on the Hubble Space Telescope. M 30 formed some 13 billion years ago, but the new study presents evidence that the cluster's blue stragglers were spawned from a dynamical event just a few billion years ago.

“It’s like seeing a few kids in the group picture of a rest-home for retired people. It is natural to wonder why they are there,” says Ferraro.

These blue stragglers were thought to have arisen in a tight binary system where the less massive star acts as a vampire, siphoning fresh hydrogen from its more massive companion star that causes it to heat up, growing bluer, like a star at an earlier stage in its evolution. But the new study shows that some of the blue stragglers have instead been rejuvenated by cosmic collisions. In this scenario, the star's nuclear fuel becomes mixed and replenished, increasing its mass to around twice that of other, individual stars in the cluster.

"Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism," says team member Giacomo Beccari from ESA. "This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster.”

The team also found that the blue stragglers are more concentrated towards the centre of the cluster than the average star. “This indicates that blue stragglers are more massive than the average star in this cluster,” says Ferraro. “More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey.”

The team suggest that one or two billion years ago, M 30 underwent a major “core collapse” that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars located there. This increased both the number of collisions and the chances of vampirism, leading to the two main populations of blue stragglers.

“Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population,” says Barbara Lanzoni of the University of Bologna. The team hope to study other globular clusters to see if this double population of blue stragglers is common.

The two papers are Two distinct sequences of blue straggler stars in the globular cluster M 30 by Ferraro et al, and A binary star fraction of 76 percent and unusual orbit parameters for the blue stragglers of NGC 188 by Mathieu and Geller.