Turbulence to make stars vibrate
A recent theoretical work of RÈza Samadi and its co-authors at Paris
Observatory supplements and reinforces the theory of stochastic excitation
of star vibrations. This process was generalized to a global description
of the turbulent state of their convective zone. The comparison between
observation and theory, thus generalized, will allow to better know the
turbulent spectrum of stars, and this in particular thanks to the COROT
mission.
Stellar seismology
Some stars are prone to periodic and weak brightness and stellar matter
surface velocity variations. These variations, measured on the stellar
surface, result from the propagation of oscillating waves which naturally
establish in the resonant cavity constituted by the star. Oscillation
periods are forced to discrete values by surface reflexion and center
refraction conditions. One then speaks of eigen modes of oscillation,
very similar to the proper modes of a musical instrument (Figure 1).
These discrete values result from the properties of the crossed medium.
The analysis of their frequencies allows then to determine the stellar
structure. Stellar seismology (or asteroseismology) consists in studying
these oscillations properties and thus constitutes a formidable tool to
probe stellar interiors.
The excitation of the vibrations
Where asteroseismology predicts stellar oscillations frequencies with a
high degree of accuracy, it fails in determing their amplitude. The mode
amplitude results from a balance between excitation and damping, and the
physics of these processes still remains badly known. Because of its
proximity, it was possible to detect and measure on the Sun a very
great number of oscillation modes (more than 1 million), characterized
by extremely low amplitudes (about 10**-6 in relative luminosity
fluctuation). Since more than about thirty years, solar oscillations are
thought to be excited by the turbulent motions in the solar convective
zone.
Convection in stars
Stars lower than 2 solar masses possess an upper convective zone. In this
zone, convection mainly drives the transport of the energy radiated by
the star. This extremely hot and unstable medium is the site of incoherent
motions of a great number of elements matter. These elements called
eddies confer to the convective zone a strongly turbulent character
(Figure 2). These eddies force the proper oscillation modes and make the
excitation process random: it is named “stochastic excitation”.
Stochastic excitation is very close to the action of striking a drum with
rod (Figure 3). In this analogy the rod would play the role of the eddies
while the skin of the drum would be the stellar medium.
And tomorrow?
The COROT mission (Figure 4), whose launch is planned for 2004, is
interested particularly in these types of stars. Thus, objects of mass
lower than 2 solar masses (stars of F and G types) are selected.
References:
1.Samadi R. & Goupil M.-J., “Excitation of stellar p-modes by turbulent
convection. I. Theoretical formulation”, Astronomy & Astrophysics, 2001,
http://arXiv.org/abs/astro-ph/0101109
2.Samadi R., Goupil M.-J., Lebreton Y., “Excitation of stellar p-modes by
turbulent convection. II. The Sun “, Astronomy & Astrophysics, 2001,
http://arXiv.org/abs/astro-ph/0101111
3.Samadi R., Goupil M.-J., Lebreton Y., Baglin A., “Oscillation power as
a test of stellar turbulence : scanning the HR diagram “, proceedings
of the SOHO-10/GONG-2000 workshop, October 2000
http://xxx.lpthe.jussieu.fr/abs/astro-ph/0101129
[NOTE: Images supporting this release are available at
http://www.obspm.fr/actual/nouvelle/solar-like.en.html]
