Summary

Sound waves running through a star can help astronomers reveal its inner
properties. This particular branch of modern astrophysics is known as
“asteroseismology”.

In the case of our Sun, the brightest star in the sky, such waves have
been observed since some time, and have greatly improved our knowledge
about what is going on inside. However, because they are much fainter,
it has turned out to be very difficult to detect similar waves in other
stars.

Nevertheless, tiny oscillations in a solar-twin star have now been
unambiguously detected by Swiss astronomers Francois Bouchy and Fabien
Carrier from the Geneva Observatory, using the CORALIE spectrometer on
the Swiss 1.2-m Leonard Euler telescope at the ESO La Silla Observatory.
This telescope is mostly used for discovering exoplanets (see ESO PR
07/01).

The star Alpha Centauri A is the nearest star visible to the naked eye, at
a distance of a little more than 4 light-years. The new measurements show
that it pulsates with a 7-minute cycle, very similar to what is observed
in the Sun.

Asteroseismology for Sun-like stars is likely to become an important probe
of stellar theory in the near future. The state-of-the-art HARPS
spectrograph, to be mounted on the ESO 3.6-m telescope at La Silla, will
be able to search for oscillations in stars that are 100 times fainter
than those for which such demanding observations are possible with
CORALIE.

PR Photo 23a/01: Oscillations in a solar-like star (schematic picture).

PR Photo 23b/01: Acoustic spectrum of Alpha Centauri A, as observed with
CORALIE.

Asteroseismology: listening to the stars

ESO PR Photo 23a/01

Caption: PR Photo 23a/01 is a graphical representation of resonating
acoustic waves in the interior of a solar-like star. Red and blue colours
show element displacements in opposite directions.

Geologists monitor how seismic waves generated by earthquakes propagate
through the Earth, and thus learn about the inner structure of our planet.
The same technique works for stars. The Sun, our nearest star and a typical
middle-age member of its class, has been investigated in this way since the
1960’s. With “solar seismology”, astronomers have been able to learn much
about the inner parts of the star, and not only the outer layers normally
visible to the telescopes.

In the Sun, heat is bubbling up from the central regions where enormous
amount of energy is created by nuclear reactions. In the so-called
convective zone, the gas is virtually boiling, and hot gas-bubbles are
rising with a speed that is close to that of sound.

Much like you can hear when water starts to boil, the turbulent convection
in the Sun creates noise. These sound waves then propagate through the
solar interior and are reflected on the surface, making it oscillate. This
“ringing” is well observed in the Sun, where the amplitude and frequency of
the oscillations provide astronomers with plenty of information about the
physical conditions in the solar interior.

From the Sun to the stars

There is every reason to believe that our Sun is a quite normal star of
its type. Other stars that are similar to the Sun are therefore likely to
pulsate in much the same way as the Sun. The search for such oscillations
in other solar-like stars has, however, been a long and difficult one. The
problem is simply that the pulsations are tiny, so very great precision is
needed in the measurements.

However, the last few years have seen considerable progress in
asteroseismology, and Francois Bouchy and Fabien Carrier from the Geneva
Observatory have now been able to detect unambiguous acoustic oscillations
in the solar-twin star, Alpha Centauri A.

The bright and nearby star Alpha Centauri

Alpha Centauri (Alpha Cen) [1] is the brightest star in the constellation
Centaurus in the southern hemisphere. It is actually a double star,
consisting of the solar twin Alpha Cen A and its fainter companion Alpha
Cen B.

A third star, Proxima Centauri, seems to be loosely connected with the
binary. Proxima is, after the Sun, the nearest star we know now, only 4.3
light years away. However, Proxima is such a faint star that it can only
be seen with a good telescope.

Alpha Centauri A is one of the brightest stars on the southern sky, and the
closest star observable with the naked eye. Being a near twin to the Sun and
very nearby — in astronomical terms — made this star the ideal candidate
in the search for tiny oscillations.

The observational technique

The observations of Alpha Cen A were conducted in May 2001 during five
nights at the Swiss 1.2-m Leonard Euler telescope at the ESO La Silla
Observatory.

The CORALIE spectrograph at this telescope is well known for its very
successful programme of detecting exoplanets, cf. PR 07/01. In searching
for exoplanets close to nearby stars, the spectrograph identifies the tiny
wobbling motion of the star, induced by the gravitational pull of the
unseen planetary companion.

The same technique was used for the investigation of oscillations in Alpha
Cen A. The acoustic waves make the surface of the star periodically pulsate
in and out, and the spectra of the stellar surface will show corresponding
(very) small velocity shifts.

The detected oscillations only move with speeds of up to 35 cm per second.
This means that the perturbations on the stellar surface only amount to some
40 metres up and down on a star with a radius of 875,000 kilometres, see
also the illustration (PR Photo 23a/01). A very small change indeed, and
not easy to detect with current astronomical instruments!

Acoustic waves in Alpha Centauri A

ESO PR Photo 23b/01

Caption: PR Photo 23b/01 represents the spectrum of acoustic modes, as
observed in Alpha Cen A with the CORALIE spectrograph on the 1.2-m Swiss
telescope at the ESO La Silla observatory. Several “eigenmodes” appear
as high peaks in the frequency interval between 1.7 and 3 mHz; they
correspond to oscillation periods in the range from 5 to 10 min. A very
similar pattern is observed in the Sun.

The data from five nights of observations were then searched to detect any
changes of velocity and hence, oscillations. Astronomers use sophisticated
mathematical methods for this kind of analysis, and normally present their
results in terms of a “power spectrum” (PR Photo 23b/01). It displays the
“intensity” of oscillations at different frequencies, that is, of different
periods; particularly high “peaks” indicate a “real” oscillation of that
frequency.

The comb of peaks visible between 2-3 mHz is the unambiguous and typical
signature of solar-like oscillations. This frequency corresponds to a period
of about 7 minutes, close to the well-known 5-minute oscillations of our
Sun. This is in full agreement with expectations from theoretical models of
the two stars. Continued detailed modeling of these new results will further
improve the associated determination of the mass, radius, age, chemical
composition and other properties of Alpha Cen A.

This result is another proof of the excellent performance of the CORALIE
spectrograph, providing extremely accurate measurements without the present
investigation would have been impossible.

Models of stellar interiors

Our current understanding of stellar interiors is severely limited by lack
of detailed and accurate observations of stars other than the Sun. In
technical terms, for a complete description of the conditions inside a star,
we need detailed knowledge of at least five stellar parameters (mass, age,
initial content of helium and heavier elements, and a parameter describing
the convection). However, in most cases, only two stellar properties can be
measured directly (the temperature and the luminosity), so the models are
necessarily quite uncertain (i.e., they are not well “constrained”).

It is therefore imperative to enlarge the number of observables and this is
possible with asteroseismology. Helioseismology has opened up the way. These
observations severely constrain the possible models of the Sun’s internal
structure. But, depending on their mass and age, stars have very different
internal structures, and may also harbour physical processes that are quite
different from those in the Sun.

Asteroseismological observations of stellar oscillations add crucial
information that constrain the models of their inner structure, since the
measured frequencies may be compared directly with those computed for the
models. The observation of the full stellar disk allows to characterize
certain (low degree) oscillation modes which penetrate deep inside the star
and it is not necessary to resolve the stellar disk (as we can do for the
Sun) in order to obtain useful seismological information.

More stars to be observed

Observations of bright solar-like stars are already planned with the CORALIE
spectrograph. Even fainter stars can be observed with the HARPS spectrograph
which will be installed on the 3.6-m telescope at La Silla Observatory at
the end of 2002. It will be able to observe stars that are one hundred times
fainter than those now reachable with CORALIE and with even better accuracy
of the velocity measurements. While it will be mostly dedicated to the
search of exoplanets, HARPS will be able to conduct an asteroseismological
study of about 100 solar-like stars.

More information

The research reported in this Press Release is described in a scientific
article (“P-mode observations on Alpha Cen A” by Francois Bouchy and Fabien
Carrier) that has been accepted for publication as a Letter in the European
journal “Astronomy & Astrophysics”.

Note

[1]: Alpha Centauri was earlier known as Rigil Centauri, but that name is
not much used because of the similarity with the name of the bright star
Rigel in Orion. Alpha Centauri is one of the brightest stars in the sky
(visual magnitude 0) and is a splendid view in the southern Milky Way, next
to Beta Centauri. It was an object of worship on the Nile and the first
visible emergence in the morning sky at the autumn equinox has been
connected with the orientation of several temples in Northern and Southern
Egypt from the fourth millenium B.C.

Contacts

Francois Bouchy

Geneva Observatory

Sauverny, Switzerland

Tel.: +41-22-7552611

E-Mail: Francois.Bouchy@obs.unige.ch

Fabien Carrier

Geneva Observatory

Sauverny, Switzerland

Tel.: +41-22-7552611

E-Mail: Fabien.Carrier@obs.unige.ch

Text with all links and the photo is available on the ESO Website at :
http://www.eso.org/outreach/press-rel/pr-2001/pr-15-01.html