Other
Scientific paper
Mar 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eso..pres....5.&link_type=abstract
ESO Press Release, 03/2003
Other
Scientific paper
VLT Interferometer Studies the Nearest Stars
Summary
Observations with the Very Large Telescope Interferometer (VLTI) at the ESO Paranal Observatory (Chile) have provided the first-ever direct determination of the angular sizes of the disks of the solar-type stars Alpha Centauri A and Alpha Centauri B. As the two largest members of this triple stellar system that also includes the much smaller Proxima Centauri, they are the Sun's nearest neighbours in space at a distance of just over 4 light-years.
Together with photometric and asteroseismic observations, this fundamental measurement with the VLTI has lead to a complete characterization of Alpha Centauri A and Alpha Centauri B - they are now the "best known" stars.
This has also allowed a unique and very detailed comparison between "real nature" and current stellar theory for solar-type stars. There is clearly very good agreement, indicating that the structure and evolution of stars like our Sun are well understood.
The new observations of the nearest stars have therefore contributed to raise the astronomers' confidence in their solar models as well. We can now be more sure about the conditions inside the Sun, our central energy source, and also about the way it will change during the next hundreds of millions of years.
PR Photo 07a/02: The Alpha Centauri stellar system in the sky. PR Photo 07b/02: Relative sizes of Alpha Centauri A and B, Proxima and other stars. PR Photo 07c/02: VLTI configurations used for observations of Alpha Centauri A and B. PR Photo 07d/02: Observed "visibility curves". PR Photo 07e/02: HR-diagram with positions of the stars in the triple Alpha Centauri system. The Alpha Centauri Triple System
ESO PR Photo 07a/03
ESO PR Photo 07a/03
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ESO PR Photo 07b/03
ESO PR Photo 07b/03
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Caption: PR Photo 07a/03 shows the location of the Alpha Centauri triple stellar system in the sky. The brighter stars (Alpha Centauri Aand B) are strongly overexposed, with the outlying member, Proxima lying approx. 2.2° to the south-west (arrow). Smaller areas around the stars are shown in the inserts to the right. The photo has been reproduced from a blue-sensitive photographic plate obtained by the ESO 1-m Schmidt Telescope, a wide-angle telescope at the La Silla observatory in Chile that has now been decommissioned. PR Photo 07b/03 shows the relative sizes of a number of objects, including the three (known) members of Alpha Centauri triple system and some other stars for which the angular sizes have also been measured with Very Large Telescope Interferometer (VLTI) at the ESO Paranal Observatory (Chile) during the past year, cf. ESO PR 22/02. The Sun and planet Jupiter are shown for comparison.
The Alpha Centauri triple stellar system is our closest neighbour in space. It is located at a distance of 4.36 light-years, or 41 million million km, in the direction of the southern constellation Centaurus (The Centaur) [1]. The two main stars in the system, Alpha Centauri A and Alpha Centauri B, are rather similar to the Sun; their stellar spectral types are "G2V" and "K1V", respectively. The third star is a "red dwarf" known as Proxima. It is much cooler and smaller than the other two, cf. PR Photo 07b/03. It was observed in 2002 with the VLT Interferometer, see ESO PR 22/02.
Alpha Centauri A and B orbit each other at a distance of about 3600 million km, or somewhat more than the distance of planet Uranus from the Sun [2]. The orbital period is almost exactly 80 years. Their smaller companion, Proxima, is about 1.5 million million km (10,000 Astronomical Units) nearer to the solar system than A and B. It is possibly orbiting that pair with a period of millions of years.
The A and B pair offers a unique possibility to study stellar physics in stars that are only slightly different from our own Sun. Their masses nicely bracket that of their neighbour star, and they are only slightly older than the Sun.
In addition to providing general information about stellar evolution, the detailed study of Alpha Centauri A and B is particular interesting as it allows verification of our current knowledge about the composition, structure and indeed, future development, of our own main energy source, the Sun. The VLTI observations
ESO PR Photo 07c/03
ESO PR Photo 07c/03
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ESO PR Photo 07d/03
ESO PR Photo 07d/03
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Caption: PR Photo 07c/03 shows the layout of the Very Large Telescope Interferometer (VLTI) on the observing platform at the summit of Cerro Paranal, with the locations of the main components (8.2-m Unit Telescopes; Auxiliary Telescope (AT) rail tracks and observing stations; Delay Line tunnel; Beam Combination (Interferometric) Laboratory) indicated. The E0-G0 and E0-G1 configurations (baselines) used for observations of Alpha Centauri A and Alpha Centauri B are shown. PR Photo 07d/03 displays the so-called "visibility curves" of these two stars (with a "calibration" star, Theta Centauri) from which the angular diameters, i.e. the angle subtended by their disks, can be deduced. The data from observations at different baselines, with the corresponding uncertainties, are indicated by red points.
An international group of astronomers [3] has now used observations of Alpha Centauri A and Alpha Centauri B obtained with the ESO VLTI/Paranal team by the ESO Very Large Telescope Interferometer (VLTI) to measure the sizes of these two stars. Despite their proximity and brightness, these two southern stars have never before been resolved by long-baseline stellar interferometry, and the VINCI/VLTI observations are the first direct measurement of their angular diameters.
For the observations of the A and B pair, the 0.35-m VLTI siderostats on the observing platform at the Paranal summit were used. These two small test telescopes were placed at distances of 16 and 66 metres, respectively (PR Photo 07c/03). They captured the light from the two stars and sent it on via a series of reflecting mirrors to the common focus in the commissioning instrument VINCI.
Although they were obtained only a few days after the successful accomplishment of "First Fringes" with the VLTI (ESO PR 06/01), the 16-m measurements were found to be scientifically very useful and helped to improve the measurement of the angular diameter of Alpha Centauri A. The 66-m baseline measurements provided the most accurate values of "calibrated visibilities" (PR Photo 07d/03) - from these, the angular diameters were then derived.
The VLTI measurements provided high-quality angular diameter values for both stars, 8.512 ± 0.022 milliarcsec and 6.002 ± 0.048 milliarcsec for A and B, respectively. With the distance measured earlier by the Hipparcos satellite of the European Space Agency (ESA), 4.36 light-years or 41 million million km, the true radii were then found to be 854,000 km and 602,000 km, or 1.227 ± 0.005 and 0.865 ± 0.007 times the radius of the Sun, respectively. Stellar models
ESO PR Photo 07e/03
ESO PR Photo 07e/03
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Caption: PR Photo 07e/03 shows the location of Alpha Centauri A and B, Proxima Centauri and the Sun in the Hertzsprung-Russell (HR) diagram [4].
During the past years, a number of more distant binary stellar systems like Alpha Centauri A and B have been observed with different methods, including spectrophotometry (emission at different wavelengths) and astrometry (position in the sky; motions). When compared with theoretical models of the stars, such measurements determine the main stellar parameters, including the masses of each component, their ages, their luminosities, effective temperatures and content of various chemical elements. At the same time, these models predict the evolution of the stars with time [4], in particular how their
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