How seismology can distinguish a pre-main-sequence star from a main-sequence star ?
1er juin 2001
How seismology can distinguish a pre-main-sequence star from a main-sequence star ? A star can be seen as a resonant cavity within which (acoustic and gravity) waves are trapped and act as normal or oscillation modes. This leads to stellar oscillations which are detected at the stellar surface. The frequencies associated with the oscillation modes are therefore easily and precisely measured in light or radial velocity temporal variations. The frequency of a given mode is strongly dependent on the properties of the propagation cavity for that particular mode. When several modes are detected, they can serve to probe the interior of the star as they propagate in slightly different cavities within the star. This is particularly interesting for intermediate mass stars (1.6-2.5 solar masses) on the main sequence. The outer layers of these stars have dynamical and thermal properties which (vibrationnally) excite particular modes. These modes propagate both in the outer layers where they can be excited and in the deep interior where they are sensitive to the properties of the core. The wave propagation properties, hence the characteristic time scales (i.e frequencies) are therefore different when the core is convective or radiative. This property can be used to discriminate between a pre-main-sequence star (on its way toward the ZAMS (Zero Age Main Sequence) in a HR (Hertzprung-Russel) diagram, as that of figure 1) and a main sequence star (moving from the ZAMS toward the giant branch) which lie at the same location in the HR diagram. These two stars would indeed have the same properties of the outer layers but different cores : a pre-mainsequence star has a radiative core where no major nuclear reaction rates have yet taken place, whereas a main sequence star develops a convective instability in its central region with nuclear reactions transforming hydrogen into helium. rays2.gif Figure 2 : Representation of the various modes occuring inside a star, according to their wave-length and depth. A few modes, sensitive to the central regions (the blue one in the figure 2) have a different behavior in the two types of stars while most of the modes (e.g. red and green in the figure 2) propagate in the outer layers and do not feel the deep internal structure. This effect causes differences in the frequency patterns between pre-main sequence and main sequence stars which will be detectable with the future asteroseismology missions in space, like COROT (2004) and possibly EDDINGTON (2007), both missions to which the Paris Observatory participates actively.
Reference : "Comparative seismology of pre- and main sequence stars in the instability strip" Marian Suran, Marie-Jo Goupil, Annie Baglin, Yveline Lebreton, Claude Catala : 2001 Astronomy and Astrophysics in press astro-ph/0104319. Contact : Marie-Jo Goupil (Département DASGAL, Observatoire de Paris)
Dernière modification le 4 mars 2013