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CfA Press Release
 
 Release No.: 99-03

EMBARGOED FOR RELEASE: 9:20 a.m. JANUARY 7, 1999

Faint, blue, and non-flickering, odd-ball stars may help test star cluster dynamics

AUSTIN, TX--A new class of non-flickering, faint, blue stars has been found near the center of a globular cluster, thus adding to the list of exotic objects found near the centers of these clusters as AUSTIN, TX--A new class of non-flickering, faint, blue stars has been found near the center of a globular cluster, thus adding to the list of exotic objects found near the centers of these clusters as well as offering possible new tests of both globular cluster dynamics and stellar evolution.well as offering possible new tests of both globular cluster dynamics and stellar evolution.

The discovery, made with the Hubble Space Telescope (HST) by Adrienne Cool of San Francisco State University and Peter Edmonds and Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics, was announced today at the American Astronomical Society meeting in Austin, Texas.The discovery, made with the Hubble Space Telescope (HST) by Adrienne Cool of San Francisco State University and Peter Edmonds and Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics, was announced today at the American Astronomical Society meeting in Austin, Texas.

The new type of stars, tentatively identified as "low-mass white dwarfs," were seen in the globular cluster NGC 6397, a massive collection of stars about 6,000 light years from Earth. This cluster is already known to contain two classes of stars thought to have been formed by extreme stellar interactions. The new type of stars, tentatively identified as "low-mass white dwarfs," were seen in the globular cluster NGC 6397, a massive collection of stars about 6,000 light years from Earth. This cluster is already known to contain two classes of stars thought to have been formed by extreme stellar interactions.

The first, "blue stragglers," are bright blue stars that are thought to form via the merger of two "normal" globular cluster stars (stars similar to the Sun but slightly less massive). The second, even more unusual, objects are "cataclysmic variables," or CVs, compact binary systems consisting of a normal star and a white dwarf, in which the burned-out stellar core containing most of the mass of the original star has collapsed to an extremely dense object about the size of Earth. Gas flows from the normal star onto the white dwarf via a pancake-shaped disk, which gives CVs many of their distinguishing features, including variability (flickering), emission lines from the hot gas, and a blue color. Since CVs and blue stragglers are heavier than normal stars, they are found concentrated near the center of NGC 6397.The first, "blue stragglers," are bright blue stars that are thought to form via the merger of two "normal" globular cluster stars (stars similar to the Sun but slightly less massive). The second, even more unusual, objects are "cataclysmic variables," or CVs, compact binary systems consisting of a normal star and a white dwarf, in which the burned-out stellar core containing most of the mass of the original star has collapsed to an extremely dense object about the size of Earth. Gas flows from the normal star onto the white dwarf via a pancake-shaped disk, which gives CVs many of their distinguishing features, including variability (flickering), emission lines from the hot gas, and a blue color. Since CVs and blue stragglers are heavier than normal stars, they are found concentrated near the center of NGC 6397.

Along with new observations of CVs, Cool, Edmonds, and Grindlay found three faint blue stars close to the center of NGC 6397 that, unlike CVs, do not flicker. "These 'non-flickerers' took us by surprise," says Cool, who led the team. "With their blue colors, we had assumed they were cataclysmic variables, and that we'd see them flickering away, like all the others do. But the Hubble observations show that they really are a different class of stars."Along with new observations of CVs, Cool, Edmonds, and Grindlay found three faint blue stars close to the center of NGC 6397 that, unlike CVs, do not flicker. "These 'non-flickerers' took us by surprise," says Cool, who led the team. "With their blue colors, we had assumed they were cataclysmic variables, and that we'd see them flickering away, like all the others do. But the Hubble observations show that they really are a different class of stars."

The unusual color and brightness of these stars is difficult to explain, but the scientists say there is a good possibility they are white dwarfs with unusually low masses. "Low-mass white dwarfs are believed to form when the normal process of single star evolution is interrupted by interactions with other stars," explains Edmonds.The unusual color and brightness of these stars is difficult to explain, but the scientists say there is a good possibility they are white dwarfs with unusually low masses. "Low-mass white dwarfs are believed to form when the normal process of single star evolution is interrupted by interactions with other stars," explains Edmonds.

For example, a red giant (an older bloated version of the Sun) may collide with a white dwarf, stripping away layers of gas and leaving the core of the red giant exposed while it is still growing. This aborted white dwarf is therefore less massive than the average cluster white dwarf.For example, a red giant (an older bloated version of the Sun) may collide with a white dwarf, stripping away layers of gas and leaving the core of the red giant exposed while it is still growing. This aborted white dwarf is therefore less massive than the average cluster white dwarf.

If the non-flickerers are indeed low-mass white dwarfs, there is a good chance they are part of binary systems. According to the researchers, tantalizing evidence has been found for the presence of a faint companion (like a more massive white dwarf) to one of the non-flickerers.If the non-flickerers are indeed low-mass white dwarfs, there is a good chance they are part of binary systems. According to the researchers, tantalizing evidence has been found for the presence of a faint companion (like a more massive white dwarf) to one of the non-flickerers.

An observation made with the HST's Faint Object Spectrograph shows a broad spectral line formed by absorption in the star's atmosphere. The wavelength of the spectral line is different from the expected value, possibly because orbital motion of the star in a binary system is causing the line to be Doppler shifted. While this observation needs confirmation, further study of these stars holds great promise for studies of stellar evolution and stellar dynamics.An observation made with the HST's Faint Object Spectrograph shows a broad spectral line formed by absorption in the star's atmosphere. The wavelength of the spectral line is different from the expected value, possibly because orbital motion of the star in a binary system is causing the line to be Doppler shifted. While this observation needs confirmation, further study of these stars holds great promise for studies of stellar evolution and stellar dynamics.

One possible problem with the low-mass white dwarf explanation is the lack of fainter stars with similar colors. Once formed, low-mass white dwarfs are expected to cool and dim just like their more massive cousins. "It is possible that some of the objects are removed by further interactions, or it could be some quirk in the evolution of these objects," says Edmonds.One possible problem with the low-mass white dwarf explanation is the lack of fainter stars with similar colors. Once formed, low-mass white dwarfs are expected to cool and dim just like their more massive cousins. "It is possible that some of the objects are removed by further interactions, or it could be some quirk in the evolution of these objects," says Edmonds.

"Globular clusters have long been used to study the evolution of single stars because they contain large numbers of stars with the same age, chemical abundance, and distance from the Earth," adds Edmonds. "With the unique ability of HST to resolve stars in crowded fields, globular clusters are becoming wonderful tools for the study of stellar interactions in dense stellar environments, and the exotic binary stars that form in them.""Globular clusters have long been used to study the evolution of single stars because they contain large numbers of stars with the same age, chemical abundance, and distance from the Earth," adds Edmonds. "With the unique ability of HST to resolve stars in crowded fields, globular clusters are becoming wonderful tools for the study of stellar interactions in dense stellar environments, and the exotic binary stars that form in them."

"Overall, it is remarkable how concentrated the blue stragglers, CVs, and non-flickerers are towards the center of the cluster," says Grindlay. "For example, no evidence has been found for blue stragglers or non-flickerers in the outer region of this cluster. The CVs and these possible low-mass white dwarfs are particularly important because compact binaries may prevent or delay the center of a globular cluster from collapsing down on itself. Further study of these objects should help us understand the formation and survival of globular clusters, the oldest collections of stars in our Galaxy.""Overall, it is remarkable how concentrated the blue stragglers, CVs, and non-flickerers are towards the center of the cluster," says Grindlay. "For example, no evidence has been found for blue stragglers or non-flickerers in the outer region of this cluster. The CVs and these possible low-mass white dwarfs are particularly important because compact binaries may prevent or delay the center of a globular cluster from collapsing down on itself. Further study of these objects should help us understand the formation and survival of globular clusters, the oldest collections of stars in our Galaxy."

The research was based on observations with the NASA/ESA Hubble Space Telescope and obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract No. NAS5-26555.The research was based on observations with the NASA/ESA Hubble Space Telescope and obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract No. NAS5-26555.

For more information contact:For more information contact:

Prof. Adrienne Cool (415-338-6450, cool@stars.sfsu.edu)
Dr. Peter Edmonds (617-496-7720, pedmonds@cfa.harvard.edu)
Prof. Jonathan Grindlay (617-495-7204, josh@head-cfa.harvard.edu)

A color image of the new class of stars in globular cluster NGC 6397 can be found at:
http://hea-www.harvard.edu/~edmonds/pr.html

 
 
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