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Helioseismology, the Solar Standard Model and the study of the fundamental physics

Helioseismology has played a formidable role for the detailed comprehension of the structure and dynamics of the Sun, leading to the solution of the neutrino problem, and contributing to achieve a sensible improvement in the description of physical processes that govern the behaviour of matter and radiation in stellar interiors.

Unfortunately, despite the great successes achieved in realizing a standard solar model which highly reproduces the helioseismic observations of the Sun, a very recent discovery has spoiled any previous certainty of the solar modelists. In fact, recent analysis of photospheric spectra, based on NLTE models of atmospheres, brought to a new determination of abundances of elements present in the solar atmosphere and in particular of the solar metallicity, whose value seems to be sensibly lower than that known and adopted since today to built solar models (Asplund et al. 2004, A&A 417, 751). During the year 2004, M. P. Di Mauro has established a new collaboration with the stellar group of the University of Liège (Belgium), which have a deep experience in the field of solar structure modelling. With the aim to give an explanation to the present measured value of metallicity Montalbán et al. (2004) attempted to construct a new solar standard model, including the new metallicity. Unfortunately, helioseismic tests have shown that theoretical frequencies of oscillations of new solar models cannot reproduce observed oscillation frequencies of the Sun. The question, then, is still open.

Figure 1.4: The relative squared sound-speed differences between two standard solar models, differing for the chemical composition, and the Sun, as obtained by inversion of a set of MDI oscillation frequencies.
\begin{figure}\centerline{\psfig{file=sun/dimauro1.eps,width=8cm}}\end{figure}

Another important goal of helioseismology is to provide information about the basic physics and parameters which determine the structure of the solar interior. Recently, M.P. Di Mauro in collaboration with Christensen-Dalsgaard of the University of Aarhus and with H. Schlattl and A. Weiss of the Max Plank Institute of Garching have worked on the extension of the use of the tools of helioseismic inversions to provide significant constraints on physical fundamental parameters, such as, e.g., the value of Newton's gravitational constant $G$ (see Christensen-Dalsgaard et al. in press). The authors found that the effects of changes to $G$ are small compared with, and cannot in any obvious way be separated from, effects of other uncertainties in the assumed physics of the solar interior, such as nuclear reaction parameters or opacities. They concluded that the solar calibration and the present solar standard model are clearly not accurate enough to allow a determination of $G$ with a precision better than what can be reached with direct experimental measurements. The conclusion emphasizes the importance in helioseismic inferences of considering not only the accuracy with which solar oscillations are measured, but also the effect of uncertainties in other aspects of the model computation and helioseismic analysis.


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Next: Stellar physics Up: Solar Physics Previous: Irradiance variations of the   Contents   Index
Innocenza Busa' 2005-11-14