Stellar Structure - Stellar structure and evolution

 Asteroseismology

Plot
Figure 1:Power Spectrum of the oscillation frequencies of mu Herculis. The excess of power, centered at 1.2 mHz in agreement with the theoretical prediction, represents the clear detection of the pulsations in this star. The analysis of the Power Spectrum provides the value of the large frequency separation, which allows us to extract the oscillation frequencies.

Seismic investigations of stellar non-radial pulsations, based on the analysis of oscillation frequencies, provide the tool to probe the inner parts of stars, enabling us to study their internal structure, evolutionary state and the physical processes occurring in the stellar interiors. Furthermore star parameters inaccessible to observations like, for example, mass and angular momentum, can be determined with this technique. The oscillation frequencies are dependent on the internal structure and on the physical and thermodynamic properties of the stellar layers through the inverse dependence by the sound speed in the gas. Sound speed and masses are tied together because of hydrostatic equilibrium. Furthermore the sound speed underwent to a secondary dependence on hydrogen abundance which arises through the change in the mean atomic mass on which the sound speed depends. Thus if a sufficient number of modes is present and identified in the time series of an oscillating star, in principle it is possible to measure its mass, its core hydrogen abundance "age", and its rotation rate. The power of this analysis has been demonstrated in the solar case. The search for solar-like oscillations in stars has experienced a tremendous growth in recent years.

Plot
Figure 2:The figure shows the echelle diagram of mu Herculis, where the identified frequencies are plotted against the frequency modulo. It is possible to note the very good agreement with the expected values given by the asymptotic theory, represented by the vertical lines.

 Most of the results are obtained thanks to high-precision Doppler measurements using the spectrographs mainly used in the Extrasolar Planets search. Among these there is SARG, the high resolution spectrograph at the TNG. In particular, the recent determination of the mode amplitudes, lifetime and granulation noise obtained with SARG, has confirmed the p-mode nature of the oscillation spectrum in Procyon A (Claudi et al. 2005; Leccia et al. 2007). As a consequence of these measurements, coordinate observations (HARPS, UCLES and SARG) spanning in longitude were conducted in 2007 (Arentoft et al., 2008). In June 2006 we obtained more than 1100 SARG spectra (R=144,000, mean inner error=1.0 m/s) of the G5IV star mu Herculis with Iodine cell and we derived a relevant time series oh high precision radial velocities. The relative power spectrum is shown in figure 1 and the oscillation frequencies analysis is described in Bonanno et al., 2008. We determined a set of p-mode frequencies and the value of the "large separation", the separation between adjacent similar modes, for this star (see the echelle diagram in Figure 2). Both the frequencies set and the large separation are in full agreement with the asteroseismic theory for stochastic (solar-like) oscillations in stars. Observation of other several solar-like stars are envisaged in order to obtain radial velocity time series to reveal their oscillations spectra. These data will be used to add bricks to the inner stellar structure comprehension and improve theoretical stellar structure models.

People: R.U. Claudi, S. Benatti, R. Gratton, S. Desidera

Collaboration: A. Bonanno (INAF OA Catania), L. Paternò (Catania Univ.), M. Endl (Austin Univ.,USA), T. Beddings (Sydney Univ., Australia), H. Kjeldsen (Aarhus Univ., Denmark), S. Leccia (INAF OA Napoli)

Publications: Leccia et al. (2007),   A&A 464,1059; Leccia et al. (2007), A&A 464,1059; Bonanno et al. (2008), MmSAI 79,639; Benatti et al. (2008), MmSAI 79,666; Bonanno et al. (2008), ApJ 676,1248; Arentoft et al. (2008), ApJ 687,1180

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