Asteroseismology: sonic probing of stellar interiors

The proposed research programme addresses unsolved problems in stellar physics, using helioseismological and asteroseismological techniques. Helioseismology is the science of inferring the nature of the inside of the Sun from a study of the properties of sound waves that are trapped inside. Asteroseismology is the same applied to other stars. The techniques resemble those used by geophysicists for determining the internal state of the Earth. The purpose of this proposal is to combine expertise in stellar evolution and asteroseismology with the objectives developing, testing and applying diagnostic tools that are necessary to exploit fully the asteroseismic data from stars with solar-like oscillations. There is no better time than now to make rapid progress. The range of stars in which oscillations can be observed is expanding enormously, particularly as a result of the advent of new high- quality data from the very successful French space mission CoRoT, NASA`s up-coming Kepler mission, the Austro-Canadian satellite BRITE-constellation and ground-based observations such as those from the new Danish Stellar Oscillation Network Group (SONG). We shall interpret the enormous amount of these high-quality seismic stellar data, The research plans are based around the following highly topical problems: (i) the development of a cleaner separation of the contributions to the mode frequencies that relate to different physical attributes of a star, (ii) the formalism of the interaction between convection and pulsation, and (iii) the understanding of the near- surface layers of the Sun, and thence of solar-type stars and red giants too. For example, the results of this study will lead to an independent and more accurate determination of stellar ages and stellar chemical abundances, both of which are of great importance to other fields in astronomy, such as the chemical evolution of the Galaxy, and hence of the Universe. The consequences are bound to elevate to new heights the sophistication of the theory of the evolution of stars, the backbone of theoretical astrophysics. In addition to extant stellar-oscillation data from ground-based and space-born observations, NASA`s up-coming Kepler mission will also hunt for planets outside our solar system. This work will help to determine some key properties of planet-hosting stars, and will provide crucial information, such as the planetary radii, and help to determine whether or not the planets are capable of sustaining life.

Expertise in stellar evolution and asteroseismology was combined to improve the understanding of the physics in stars, such as our Sun, using data from space-born and ground-based instruments. Seismic diagnostic techniques were used to analyze data from sound waves travelling throughout a star, in a similar way as geophysicists use data from earthquakes to get information about the internal state of the Earth. New seismic diagnostic techniques were developed for measuring stellar properties with high accuracy, such as the depth of the surface convection zone, stellar age and chemical abundance, of the Sun and of stars similar to the Sun. One of the main results of this research work is that, with the help of our newly developed seismic diagnostic technique, the determined seismic solar age is 4.60 0.04 billion years, which is similar to, although somewhat greater than, today`s commonly adopted values. We also determined seismically the solar surface heavy-element abundance by mass, 0.0142 0.0005, which is substantially smaller than the value used in standard solar modelling, but it is similar to, though by about 6% larger than, the value reported by other research groups using numerical simulations of the solar atmosphere. The results of this study are of great importance to other fields in astronomy, such as the chemical evolution of the Galaxy, and hence of the Universe. The consequences are bound to elevate to new heights the sophistication of the theory of the evolution of stars, the backbone of theoretical astrophysics. Furthermore, a model for stellar convection was generalized to a nonzero background flow, with the principal motivation of studying nonradial oscillations of stars with turbulent convection zones. The most important outcome of our extended convection model is that the nonzero background flow reduces the magnitude of the heat flux, generating a component perpendicular to the temperature gradient in the direction of the flow; it also modifies the diagonal components of the Reynolds stress, and generates off-diagonal components. This result is of immediate importance to stellar pulsation mode physics. The reduction of the heat flux in a background shearing flow will affect, for example, the oscillation amplitudes in solar-type stars, which are now measured with high accuracy using NASA`s Kepler satellite. The outcome of this project also helps to determine with higher accuracy some of the key properties of planets around stars outside our solar system, now detected with NASA`s exo-planet mission Kepler, such as the planetary radii, and whether or not the planets are capable of sustaining life.