Andrew Yee
January 10th 06, 03:14 PM
Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Tucson, Arizona
For More Information:
Douglas Isbell
Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Phone: 520/318-8230
John Allen
CHARA/Georgia State University
Phone: (404) 651-3576
EMBARGOED FOR RELEASE: 9:20 a.m. EST, Tuesday, January 10, 2006
RELEASE NO: NOAO 06-03
Rapidly Spinning Star Vega has Cool Dark Equator
Strong darkening observed around the equator of Vega suggests that the
fifth brightest star in Earth's sky has a huge temperature difference of
4,000 degrees Fahrenheit from its cool equatorial region to its hot poles.
Models of the star based on these observations suggest that Vega is
rotating at 92 percent of the angular velocity that would cause it to
physically break apart, an international team of astronomers announced
today in Washington, DC, at the 207th meeting of the American Astronomical
Society.
This result confirms the idea that very rapidly rotating stars are cooler
at their equators and hotter at their poles, and it indicates that the
dusty debris disk known to exist around Vega is significantly less
illuminated by the star's light than previously recognized.
"These findings are significant because they resolve some confusing
measurements of the star, and they should help us gain a much better
understanding of Vega's circumstellar debris disk," says Jason P.
Aufdenberg, the Michelson Postdoctoral Fellow at the National Optical
Astronomy Observatory in Tucson, Arizona.
This debris disk arises mainly from the collision of rocky asteroid-like
bodies. "The spectrum of Vega as viewed from its equatorial plane, the
same plane as the debris disk, should be about half as luminous as the
spectrum viewed from the pole, based on these new results," Aufdenberg
explains.
The team obtained high-precision interferometric measurements of the
bright standard star Vega using the Center for High Angular Resolution
Astronomy (CHARA) Array, a collection of six 1-meter telescopes located on
Mount Wilson, California, and operated by Georgia State University.
With a maximum baseline of 330 meters (1,083 feet), the CHARA Array is
capable of resolving details as small as 200 micro-arcseconds, equivalent
to the angular size of a nickel seen from a distance of 10,000 miles. The
CHARA Array fed the starlight of Vega to the Fiber Linked Unit for Optical
Recombination (FLUOR) instrument, developed by the Laboratoire d'Etudes
Spatiales et d'Instrumentation en Astrophysique of the Observatoire de
Paris.
One major consequence of Vega's rapid rotation is a significant drop in
the effective atmospheric temperature by approximately 2,300 Kelvin (4,000
degrees Fahrenheit) from the pole to the equator. This effect, known as
"gravity darkening," was first predicted by theoretical astronomer E. Hugo
von Zeipel in 1924.
The CHARA/FLUOR measurements of the brightness distribution of Vega's
surface also show it to be strongly "limb darkened." Limb darkening refers
to the diminishing brightness in the image of a star from the center of
the image to the edge or "limb" of the image.
The new measurements are consistent with the "pole-on" model for Vega
first proposed by Richard O. Gray of Appalachian State University, which
proposes that Vega's pole of rotation points toward Earth. The pole-on
view of Vega means that the relatively cool equator corresponds to the
limb of the star, such that the gravity-darkening effect further enhances
the limb-darkening effect.
The CHARA/FLUOR data support the pole-on, gravity darkened model for Vega
by showing that Vega's limb darkening is 2.5 times stronger at a
wavelength of 2.2 microns than expected for a star with a single effective
atmosphere temperature. Archival observations from the International
Ultraviolet Explorer indicate that this model for Vega is not complete. At
far ultraviolet wavelengths, below 140 nanometers, the model is generally
too bright.
Located at a distance of 25 light-years from Earth in the constellation
Lyra, Vega rotates about its axis once every 12.5 hours. For comparison,
the Sun's average rotation period is approximately 27 Earth days. Vega is
about 2.5 times more massive than the Sun, and 54 times brighter.
At Vega's rapid rate of rotation, the star's atmosphere is distorted,
bulging 23 percent wider at its equator compared to its poles. This type
of rotational distortion can be seen in images of the planet Saturn, where
the planet's equatorial diameter is roughly 10 percent wider than the
polar diameter. A direct measurement of Vega's rotational distortion is
hidden by its pole-on appearance. However, the accurate angular diameter
and darkening measured by CHARA/FLUOR are consistent with this distortion.
These results build upon recent measurements of Vega obtained by a team
lead by Deane M. Peterson of the State University of New York, Stony
Brook, using the Navy Prototype Optical Interferometer.
Co-authors of this result include Antoine Mérand, Vincent Coudé du
Foresto, Emmanuel Di Folco, and Pierre Kervella of the Observatoire de
Paris-Meudon, France; Olivier Absil of the University of Liège, Belgium;
Stephen T. Ridgway of the National Optical Astronomy Observatory, Tucson,
Arizona and NASA; Harold A. McAlister, Theo A. ten Brummelaar, Judit
Sturmann, Laszlo Sturmann, and Nils H. Turner of the Center for High
Angular Resolution Astronomy, Georgia State University, Atlanta, Georgia,
and Mount Wilson Observatory, California; and David H. Berger of the
University of Michigan, Ann Arbor, Michigan.
A color graphic to illustrate this result is available at
http://www.noao.edu/outreach/press/pr06/pr0603.html#images
This work was performed in part under contract with the Jet Propulsion
Laboratory (JPL) funded by NASA through the Michelson Fellowship Program.
JPL is managed for NASA by the California Institute of Technology. The
CHARA Array is operated by the Center for High Angular Resolution
Astronomy, Georgia State University, Atlanta, GA. Additional support comes
from the National Science Foundation, the Keck Foundation and the Packard
Foundation.
The National Optical Astronomy Observatory is operated by the Association
of Universities for Research in Astronomy Inc. (AURA), under a cooperative
agreement with the NSF.
National Optical Astronomy Observatory
Tucson, Arizona
For More Information:
Douglas Isbell
Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Phone: 520/318-8230
John Allen
CHARA/Georgia State University
Phone: (404) 651-3576
EMBARGOED FOR RELEASE: 9:20 a.m. EST, Tuesday, January 10, 2006
RELEASE NO: NOAO 06-03
Rapidly Spinning Star Vega has Cool Dark Equator
Strong darkening observed around the equator of Vega suggests that the
fifth brightest star in Earth's sky has a huge temperature difference of
4,000 degrees Fahrenheit from its cool equatorial region to its hot poles.
Models of the star based on these observations suggest that Vega is
rotating at 92 percent of the angular velocity that would cause it to
physically break apart, an international team of astronomers announced
today in Washington, DC, at the 207th meeting of the American Astronomical
Society.
This result confirms the idea that very rapidly rotating stars are cooler
at their equators and hotter at their poles, and it indicates that the
dusty debris disk known to exist around Vega is significantly less
illuminated by the star's light than previously recognized.
"These findings are significant because they resolve some confusing
measurements of the star, and they should help us gain a much better
understanding of Vega's circumstellar debris disk," says Jason P.
Aufdenberg, the Michelson Postdoctoral Fellow at the National Optical
Astronomy Observatory in Tucson, Arizona.
This debris disk arises mainly from the collision of rocky asteroid-like
bodies. "The spectrum of Vega as viewed from its equatorial plane, the
same plane as the debris disk, should be about half as luminous as the
spectrum viewed from the pole, based on these new results," Aufdenberg
explains.
The team obtained high-precision interferometric measurements of the
bright standard star Vega using the Center for High Angular Resolution
Astronomy (CHARA) Array, a collection of six 1-meter telescopes located on
Mount Wilson, California, and operated by Georgia State University.
With a maximum baseline of 330 meters (1,083 feet), the CHARA Array is
capable of resolving details as small as 200 micro-arcseconds, equivalent
to the angular size of a nickel seen from a distance of 10,000 miles. The
CHARA Array fed the starlight of Vega to the Fiber Linked Unit for Optical
Recombination (FLUOR) instrument, developed by the Laboratoire d'Etudes
Spatiales et d'Instrumentation en Astrophysique of the Observatoire de
Paris.
One major consequence of Vega's rapid rotation is a significant drop in
the effective atmospheric temperature by approximately 2,300 Kelvin (4,000
degrees Fahrenheit) from the pole to the equator. This effect, known as
"gravity darkening," was first predicted by theoretical astronomer E. Hugo
von Zeipel in 1924.
The CHARA/FLUOR measurements of the brightness distribution of Vega's
surface also show it to be strongly "limb darkened." Limb darkening refers
to the diminishing brightness in the image of a star from the center of
the image to the edge or "limb" of the image.
The new measurements are consistent with the "pole-on" model for Vega
first proposed by Richard O. Gray of Appalachian State University, which
proposes that Vega's pole of rotation points toward Earth. The pole-on
view of Vega means that the relatively cool equator corresponds to the
limb of the star, such that the gravity-darkening effect further enhances
the limb-darkening effect.
The CHARA/FLUOR data support the pole-on, gravity darkened model for Vega
by showing that Vega's limb darkening is 2.5 times stronger at a
wavelength of 2.2 microns than expected for a star with a single effective
atmosphere temperature. Archival observations from the International
Ultraviolet Explorer indicate that this model for Vega is not complete. At
far ultraviolet wavelengths, below 140 nanometers, the model is generally
too bright.
Located at a distance of 25 light-years from Earth in the constellation
Lyra, Vega rotates about its axis once every 12.5 hours. For comparison,
the Sun's average rotation period is approximately 27 Earth days. Vega is
about 2.5 times more massive than the Sun, and 54 times brighter.
At Vega's rapid rate of rotation, the star's atmosphere is distorted,
bulging 23 percent wider at its equator compared to its poles. This type
of rotational distortion can be seen in images of the planet Saturn, where
the planet's equatorial diameter is roughly 10 percent wider than the
polar diameter. A direct measurement of Vega's rotational distortion is
hidden by its pole-on appearance. However, the accurate angular diameter
and darkening measured by CHARA/FLUOR are consistent with this distortion.
These results build upon recent measurements of Vega obtained by a team
lead by Deane M. Peterson of the State University of New York, Stony
Brook, using the Navy Prototype Optical Interferometer.
Co-authors of this result include Antoine Mérand, Vincent Coudé du
Foresto, Emmanuel Di Folco, and Pierre Kervella of the Observatoire de
Paris-Meudon, France; Olivier Absil of the University of Liège, Belgium;
Stephen T. Ridgway of the National Optical Astronomy Observatory, Tucson,
Arizona and NASA; Harold A. McAlister, Theo A. ten Brummelaar, Judit
Sturmann, Laszlo Sturmann, and Nils H. Turner of the Center for High
Angular Resolution Astronomy, Georgia State University, Atlanta, Georgia,
and Mount Wilson Observatory, California; and David H. Berger of the
University of Michigan, Ann Arbor, Michigan.
A color graphic to illustrate this result is available at
http://www.noao.edu/outreach/press/pr06/pr0603.html#images
This work was performed in part under contract with the Jet Propulsion
Laboratory (JPL) funded by NASA through the Michelson Fellowship Program.
JPL is managed for NASA by the California Institute of Technology. The
CHARA Array is operated by the Center for High Angular Resolution
Astronomy, Georgia State University, Atlanta, GA. Additional support comes
from the National Science Foundation, the Keck Foundation and the Packard
Foundation.
The National Optical Astronomy Observatory is operated by the Association
of Universities for Research in Astronomy Inc. (AURA), under a cooperative
agreement with the NSF.