Largest collection of anomalous white dwarfs observed in
new Hubble images
Thursday, April 23, 2009
Twenty-four unusual stars have
been spotted in an ancient swarm of stars in the Milky Way. These
white dwarfs, a type of dead star, are made of helium rather than
the usual carbon and oxygen, and appear as faint, pale blue stars.
This collection of helium-core white dwarfs was discovered by a
team including Professor Adrienne Cool of San Francisco State
University and Rachel Strickler of UC Santa Cruz. They suggest that
thousands of stars in star cluster NGC 6397 are coupled with close
companions that interfere with their normal delevopment, resulting
in these odd white dwarfs made of helium. Credit: Jay Anderson /
Space Telescope Science Institute
Twenty-four unusual stars, 18 of them newly discovered, have
been observed in new Hubble telescope images. The stars are white
dwarfs, a common type of dead star, but they are odd because they
are made of helium rather than the usual carbon and oxygen.
A study, accepted for publication in the Astrophysical Journal,
suggests that these helium-core white dwarfs have had their lives
cut short because of their orbital dance around a partner
star.
"Helium-core white dwarfs have only about half the mass of typical
white dwarfs, but they are found concentrated in the center of the
cluster" said Adrienne Cool, professor of physics and astronomy at
San Francisco State University, who co-authored the study with
graduate student Rachel R. Strickler. "With such low masses, the
helium-core white dwarfs ought to be floating all around the
cluster, according to theory. The fact that we find them only in
the central regions suggests that they have heavy companions --
partner stars that anchor them to the cluster center."
Being coupled with companions also helps explain the stars'
atypical chemical make up. White dwarfs are stars that have reached
the end of their lives and have run out of fuel. Most stars burn
their fuel leaving behind a dense ball of carbon and oxygen, but
these white dwarfs are made of helium. Cool suggests that a star
that goes on to become a helium-core white dwarf must have a close
companion so that when the star became a red giant and expanded,
its outer layers spilled onto the companion. The star never had the
chance to reach maturity and burn its helium into carbon and
oxygen.
The study focused on star cluster NGC 6397, one of the globular
clusters closest to Earth at approximately 7,200 light years away.
Six helium-core white dwarfs have been observed before in this
cluster. Cool and colleagues discovered the first three in
1998.
"This is the first time that helium-core white dwarf stars have
been discovered in partnerships with other white dwarfs in a
globular cluster," Cool said. "This large sample allows us to
answer questions about the mass and nature of the partner stars,
and the prevalence of these kinds of binaries in the globular
cluster."
Binary stars play an important role in the evolution of star
clusters. Their continual dance around each other provides energy
to the cluster that astronomers believe can help prevent black
holes from forming. From the data, Cool and her team are able to
infer that one to five percent of stars in this globular cluster
will end their lives as helium-core white dwarfs with companion
stars, a finding that will help improve theoretical models of
cluster dynamics. "It may not sound like a lot but it doesn't take
many binaries to stir things up," Cool said.
One finding remains a mystery. Despite the Hubble camera's high
level of sensitivity, the faintest helium-core white dwarfs appear
to be missing.
"It's possible that these helium-core white dwarfs cool so slowly
that they haven't had time to get very faint yet," Cool said.
Another possibility is that the oldest binaries containing
helium-core white dwarfs have been destroyed by interactions with
other stars in the cluster.
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San Francisco State University
Thanks to San Francisco State University for this article.