Observatoire de Paris http://www.obspm.fr/turbulence-to-make-stars-vibrate.html

Turbulence to make stars vibrate

1er juin 2001

A recent theoretical work of Réza 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. Figure 1

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. 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.

Figure 2 Figure 3

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.

Figure 4

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.

Figure 3Stochastic 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.

References : Samadi R. & Goupil M.-J., ``Excitation of stellar p-modes by turbulent convection. I. Theoretical formulation``, Astronomy & Astrophysics, 2001, astro-ph/0101109. Samadi R., Goupil M.-J., Lebreton Y., ``Excitation of stellar p-modes by turbulent convection. II. The Sun ``, Astronomy & Astrophysics, 2001, astro-ph/0 101111. 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, octobre 2000, astro-ph/0101129. Contact : Réza Samadi (Département DESPA, Observatoire de Paris)

Dernière modification le 4 mars 2013