A Family Portrait of the Alpha Centauri System – 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.

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 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) 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.

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.

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

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 Cote
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 Fr=E9deric Th=E9venin 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:

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 =3D 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 =3D 9.5 million million km.

[3]: The team consists of Pierre Kervella (ESO Chile), Frederic Thevenin,
Gabriele Berthomieu, Bruno Lopez, Pierre Morel and Janine Provost
(Observatoire de la Cote d’Azur, Nice France) and Damien Segransan
(Observatoire de Geneve, 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 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 (Thevenin et al., 2002, A&A, 392, L9).

Contacts

Pierre Kervella
ESO Chile
Tel: +56 (2) 463 3082
email: pkervell@eso.org

Frederic Thevenin
Observatoire de la Cote d’Azur
Nice, France
Tel: +33 (0)4 9200 3026
email: thevenin@obs-nice.fr

Images online at http://www.eso.org/outreach/press-rel/pr-2003/pr-05-03.html