Flattest star ever seen
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: June 14, 2003
To a first approximation, planets and stars are round. Think of the Earth we live on. Think of the Sun, the nearest star, and how it looks in the sky.
Recent observations with the VLT Interferometer (VLTI) at the ESO Paranal Observatory have allowed a group of astronomers to obtain by far the most detailed view of the general shape of a fast-spinning hot star, Achernar (Alpha Eridani), the brightest in the southern constellation Eridanus (The River).
They find that Achernar is much flatter than expected - its equatorial radius is more than 50% larger than the polar one! In other words, this star is shaped very much like the well-known spinning-top toy, so popular among young children.
The high degree of flattening measured for Achernar - a first in observational astrophysics - now poses an unprecedented challenge for theoretical astrophysics. The effect cannot be reproduced by common models of stellar interiors unless certain phenomena are incorporated, e.g. meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star.
As this example shows, interferometric techniques will ultimately provide very detailed information about the shapes, surface conditions and interior structure of stars.
VLTI observations of Achernar
One spectacular result, just announced, is based on a series of observations of the bright, southern star Achernar (Alpha Eridani; the name is derived from "Al Ahir al Nahr" -- "The End of the River"), carried out between September 11 and November 12, 2002. The two 40-cm siderostat test telescopes that served to obtain "First Light" with the VLT Interferometer in March 2001 were also used for these observations. They were placed at selected positions on the VLT Observing Platform at the top of Paranal to provide a "cross-shaped" configuration with two "baselines" of 66 m and 140 m, respectively, at 90 deg angle.
At regular time intervals, the two small telescopes were pointed towards Achernar and the two light beams were directed to a common focus in the VINCI test instrument in the centrally located VLT Interferometric Laboratory. Due to the Earth's rotation during the observations, it was possible to measure the angular size of the star (as seen in the sky) in different directions.
The apparent profile of Achernar, based on about 20,000 VLTI interferograms (in the K-band at wavelength 2.2) with a total integration time of over 20 hours, indicates a surprisingly high axial ratio of 1.56 +- 0.05. This is obviously a result of Achernar's rapid rotation.
Theoretical implications of the VLTI observations
The indicated ratio between the equatorial and polar radii of Achernar constitutes an unprecedented challenge for theoretical astrophysics, in particular concerning mass loss from the surface enhanced by the rapid rotation (the centrifugal effect) and also the distribution of internal angular momentum (the rotation velocity at different depths).
The astronomers conclude that Achernar must either rotate faster (and hence, closer to the "critical" (break-up) velocity of about 300 km/sec) than what the spectral observations show (about 225 km/sec from the widening of the spectral lines) or it must violate the rigid-body rotation.
The observed flattening cannot be reproduced by the "Roche-model" that implies solid-body rotation and mass concentration at the center of the star. The failure of that model is even more evident if the so-called "gravity darkening" effect is taken into account - this is a non-uniform temperature distribution on the surface which is certainly present on Achernar under such a strong geometrical deformation.
With the interferometric technique, new research fields are now opening which will ultimately provide much more detailed information about the shapes, surface conditions and interior structure of stars. And in a not too distant future, it will become possible to produce interferometric images of the disks of Achernar and other stars.
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