Astronomers
have found what they say is the strongest evidence yet that a mysterious class
of stars known as "blue stragglers" are the result of stellar
cannibalism.
Blue
stragglers are found throughout the universe in globular
clusters — which typically are collections of about 100,000 stars, tightly
bound by gravity. Because all the stars in these clusters are thought to have
been born at the same time, they should all be the same age, but blue
stragglers appear to be younger than their cluster peers.
The origin
of these strange, massive stars has been a longstanding mystery, said study
leader Christian Knigge of Southampton University in England.
"The
only thing that was clear is that at least two stars must be involved in the
creation of every single blue straggler, because isolated stars this massive
simply should not exist in these clusters," Knigge added.
Since these
oddball stars were first discovered more than half a century ago, two competing
explanations for their formation emerged: "that blue stragglers were
created through collisions
with other stars; or that one star in a binary system was 'reborn' by
pulling matter off its companion," said study team member Alison Sills of
the McMaster University in Canada.
A 2006
study examined the chemical signatures of 43 blue stragglers in the
globular cluster 47
Tucanae, and found that six of the unusual stars had less carbon and oxygen
than the others. The anomaly indicated that their surface material had been
sucked from the deep interior of a parent star in a binary system.
The new
study, detailed in the Jan. 15 issue of the journal Nature, provides
even more evidence in favor of the stellar cannibalism idea.
The
researchers looked at blue stragglers in 56 globular clusters and found that
the total number of blue stragglers in a given cluster didn't match the
predicted collision rate – dispelling the theory that blue stragglers are
created through collisions with other stars.
But there
was a connection between the total mass contained in the core of the globular
cluster and the number of blue stragglers observed within it. Since more
massive cores also contain more binary stars, the researchers could infer a
relationship between blue stragglers and binaries in globular clusters. This
conclusion is also supported by preliminary observations that directly measured
the abundance of binary stars in cluster cores.
"This
is the strongest and most direct evidence to date that most blue stragglers,
even those found in the cluster cores, are the offspring of two binary
stars," Knigge said. Though there is still plenty of research on these
stars to do.
"In
our future work we will want to determine whether the binary parents of blue
stragglers evolve mostly in isolation, or whether dynamical encounters with
other stars in the clusters are required somewhere along the line in order to
explain our results," Knigge said.
The study
was funded in part by the United Kingdom's Science and Technology Facilities
Council.
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