AMBER ALPHA STAR CESAM stellar model

ESO Press Release 05/03

For immediate release: 15 March 2003

A Family Portrait of the Alpha Centauri System

VLT Interferometer Studies the Nearest Stars

Summary

Observations with the Very Large AMBER ALPHA STAR 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 AMBER ALPHA STAR Centauri A and AMBER ALPHA STAR
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 AMBER ALPHA STAR Centauri A and AMBER ALPHA STAR
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 AMBER ALPHA STAR 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 07b/03

Caption: PR Photo 07a/03 shows the location of the Alpha Centauri
triple stellar system in the sky. The brighter stars (Alpha Centauri
A and B) are strongly overexposed, with the outlying member, Proxima
lying approx. 2.2 deg 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)AMBER ALPHA STAR 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 AMBER ALPHA STAR 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 07d/03

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. 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

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
luminosity and temperature gradually changes.

In addition to these observational tools, and four decades after
the discovery of the solar seismic frequencies in 1962, solar-like
oscillations were recently detected in Alpha Centauri A, by means of
observations with the CORALIE fiber-fed spectrograph installed at the
ESO La Silla Observatory, cf. ESO PR 15/01 [5].

Since then, "asteroseismic oscillations" have been detected in five
more solar-like stars -- they provide crucial information about the
interiors of those stars and also about their masses and radii. For
instance, the comparison between the spacing of the observed
frequencies in Alpha Centauri A in the high frequency range and
those predicted by theory provides information about the size of
the star.

Some time ago, French astronomer Pierre Morel of the Observatoire de
la
Côte d'Azur (Nice, France) developed a very powerful computer code to
simulate stellar interior physics. Recently, his group used this code
to produce a new "model" of Alpha Centauri A, based on the best
available photometric, astrometric, spectroscopic and asteroseismic
observations [6]. From this multi-technique calibration, the mass of
Alpha Centauri B component was also deduced and, among other stellar
parameters, the radii of both stars were predicted with high
precision,
as 1.230 +/- 0.003 (star "A") and 0.857 +/- 0.007 ("B") times that of
the Sun, respectively.

Comparison of the Alpha Centauri stars and the Sun

ESO astronomer Pierre Kervalla is happy: "The agreement between the
VLTI measurements and the theory is very satisfactory for both stars.
This confirms the validity of the multi-technique approach and with
the interferometric measurement of Proxima obtained with the two
8.2-m VLT ANTU and MELIPAL telescopes, we now have a rather complete
view of this famous triple system".

His colleague Fréderic Thévenin at the Nice Observatory adds: "Alpha
Centauri is not only the nearest stellar system -- thanks to these
studies, it is now also the best known one!".

The following table summarizes the main characteristics of the stars
in
the Alpha Centauri triple system and our Sun. The similarities between
the "A" and "B" stars and the Sun are evident:

Parameter Alpha Alpha Proxima Sun Unit
Cen A Cen B

Age 4850 4850 4850 4650 million years

Mass 1.100 0.907 0.123 1.000 solar mass

Radius 1.227 0.865 0.145 1.000 solar radius

Temperature 5790 5260 3040 5770 Kelvin

Luminosity 1.519 0.500 0.000138 1.000 solar luminosity

Hydrogen 71.5 69.4 69.5 73.7 percent

Helium 25.8 27.7 27.8 24.5 percent

Heavier
elements 2.74 2.89 2.90 1.81 percent

1 solar mass = 1.989 x 10^33 g
1 solar radius = 6.96 x 105 km
1 solar luminosity = 3.827 x 10^26 watts

The indicated chemical composition is that at the surface
of the star.

Observations of other stars

The team will now extend these fundamental interferometric studies to
other stars for which asteroseismic observations are available. For
this, they will continue working at the VLT Interferometer, first
with the VINCI test instrument, and later with the AMBER instrument
that will combine the light beams from three telescopes at a time.
The first of the four 1.8-m Auxiliary Telescopes for the VLTI will
arrive at Paranal in the course of 2003.

There is little doubt that the unprecedented precision that is
achievable with the VLTI, together with highly sensitive asterosismic
measurements from the new HARPS spectrometer now being installed at
the ESO 3.6-m telescope on La Silla and powerful computer software
now available for stellar modelling will greatly improve our knowledge
of these stars. This in turn will help us to better understand the
structure and evolution of our own star, the Sun.

More information

The information presented in this communication is based on a
research article soon to appear in the European journal Astronomy &
Astrophysics ("The diameters of Alpha Centauri A and B" by Pierre
Kervella with the other members of the team as co-authors).

Notes

[1]: An instructive view of the spatial distribution of the stars
nearest to the solar system is available as ESO PR Photo 03c/03 in
different formats (Preview; Normal; Full-Res; Java applet).

[2]: Local astronomical distances are often indicated in Astronomical
Units (AU), the mean distance between the Earth and the Sun (1 AU =
149,600,000 km). Another unit, the light-year -- the distance that
light, travelling at 300,000 km/sec, covers in one year -- is useful
for distances outside the solar system: 1 light-year = 9.5 million
million km.

[3]: The team consists of Pierre Kervella (ESO Chile), Fréderic
Thévenin, Gabriele Berthomieu, Bruno Lopez, Pierre Morel and Janine
Provost (Observatoire de la Côte d'Azur, Nice France) and Damien
Ségransan (Observatoire de Genève, Switzerland).

[4]: The diagram is named after Danish astronomer Einar Hertzsprung
(1873-1967) and American astronomer Henry Norris Russell (1877-1957).
At the beginning of the 20th century they independently noticed that
red stars come in very different sizes, pioneering subsequent studies
of stellar diameters. This diagram plots stellar temperature (or
colour) against brightness (or magnitude) and is therefore also known
as the "colour-magnitude diagram".

[5]: The discovery of solar seismic frequencies is described in a
research article by Evans & Michard in the Astrophysical Journal
(Vol. 136, page 493, 1962). Solar-like p-oscillations in Alpha
Centauri A were announced by Bouchy & Carrier in Astronomy &
Astrophysics (Vol. 374, page L5, 2001).

[6]: The calibration AMBER ALPHA STAR was done using the CESAM
stellar model code
(Morel, 1997, A&AS 124, 597), developed at the Nice Observatory.
Among other parameters, the radii of the stars A and B were predicted
with high precision (Thévenin et al., 2002, A&A, 392, L9).

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--
Andrew Yee