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Register a new userProbing the interior physics of stars through asteroseismology
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C. Aerts
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Yearslong time series of high-precision brightness measurements have been assembled for thousands of stars with telescopes operating in space. Such data have allowed astronomers to measure the physics of stellar interiors via nonradial oscillations, opening a new avenue to study the stars in the Universe. Asteroseismology, the interpretation of the characteristics of oscillation modes in terms of the physical properties of the stellar interior, brought entirely new insights in how stars rotate and how they build up their chemistry throughout their evolution. Data-driven space asteroseismology delivered a drastic increase in the reliability of computer models mimicking the evolution of stars born with a variety of masses and metallicities. Such models are critical ingredients for modern physics as a whole, because they are used throughout various contemporary and multidisciplinary research fields in space science, including the search for life outside the solar system, archaeological studies of the Milky Way, and the study of single and binary supernova progenitors, among which are future gravitational wave sources. The specific role and potential of asteroseismology for those modern research fields are illustrated. The review concludes with current limitations of asteroseismology and highlights how they can be overcome with ongoing and future large infrastructures for survey astronomy combined with new theoretical research in the era of high-performance computing. This review presents results obtained through major community efforts over the past decade. These breakthroughs were achieved in a collaborative and inclusive spirit that is characteristic of the asteroseismology community. The reviews aim is to make this research field accessible to graduate students and readers coming from other fields of physics, with incentives to join future applications in this domain of astrophysics.
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We provide a status report on the determination of stellar ages from asteroseismology for stars of various masses and evolutionary stages. The ability to deduce the ages of stars with a relative precision of typically 10 to 20% is a unique opportunity for stellar evolution and also of great value for both galactic and exoplanet studies. Further, a major uncalibrated ingredient that makes stellar evolution models uncertain, is the stellar interior rotation frequency $Omega(r)$ and its evolution during stellar life. We summarize the recent achievements in the derivation of $Omega(r)$ for different types stars, offering stringent observational constraints on theoretical models. Core-to-envelope rotation rates during the red giant stage are far lower than theoretical predictions, pointing towards the need to include new physical ingredients that allow strong and efficient coupling between the core and the envelope in the models of low-mass stars in the evolutionary phase prior to the core helium burning. Stars are subject to efficient mixing phenomena, even at low rotation rates. Young massive stars with seismically determined interior rotation frequency reveal low core-to-envelope rotation values.
Our knowledge of the dynamics of stars has undergone a revolution thanks to the simultaneous large amount of high-quality photometric observations collected by space-based asteroseismology and ground-based high-precision spectropolarimetry. They allowed us to probe the internal rotation of stars and their surface magnetism in the whole Hertzsprung-Russell diagram. However, new methods should still be developed to probe the deep magnetic fields in those stars. Our goal is to provide seismic diagnoses that allow us to sound the internal magnetism of stars. Here, we focus on asymptotic low-frequency gravity modes and high-frequency acoustic modes. Using a first-order perturbative theory, we derive magnetic splittings of their frequencies as explicit functions of stellar parameters. As in the case of rotation, we show how asymptotic gravity and acoustic modes can allow us to probe the different components of the magnetic field in the cavities where they propagate. This demonstrates again the high potential of using mixed-modes when this is possible.
We present an asteroseismological analysis of four ZZ Ceti stars observed with emph{Kepler}: GD 1212, SDSS J113655.17+040952.6, KIC 11911480 and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf stars. We employ a grid of carbon-oxygen core white dwarfs models, characterized by a detailed and consistent chemical inner profile for the core and the envelope. In addition to the observed periods, we take into account other information from the observational data, as amplitudes, rotational splittings and period spacing, as well as photometry and spectroscopy. For each star, we present an asteroseismological model that closely reproduce their observed properties. The asteroseismological stellar mass and effective temperature of the target stars are (0.632 +/- 0.027 Msun, 10737 +/- 73 K) for GD 1212, (0.745 +/- 0.007 Msun, 11110 +/- 69 K) for KIC 4552982, (0.5480 +/- 0.01 Msun, 12721 +/- 228 K) for KIC1191480 and (0.570 +/- 0.01 Msun, 12060 +/- 300 K) for SDSS J113655.17+040952.6. In general, the asteroseismological values are in good agreement with the spectroscopy. For KIC 11911480 and SDSS J113655.17+040952.6 we derive a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6, that is part of a binary system and went through a common envelope phase.
Until the last few decades, investigations of stellar interiors had been restricted to theoretical studies only constrained by observations of their global properties and external characteristics. However, in the last thirty years the field has been revolutionized by the ability to perform seismic investigations of stellar interiors. This revolution begun with the Sun, where helioseismology has been yielding information competing with what can be inferred about the Earths interior from geoseismology. The last two decades have witnessed the advent of asteroseismology of solar-like stars, thanks to a dramatic development of new observing facilities providing the first reliable results on the interiors of distant stars. The coming years will see a huge development in this field. In this review we focus on solar-type stars, i.e., cool main-sequence stars where oscillations are stochastically excited by surface convection. After a short introduction and a historical overview of the discipline, we review the observational techniques generally used, and we describe the theory behind stellar oscillations in cool main-sequence stars. We continue with a complete description of the normal mode analyses through which it is possible to extract the physical information about the structure and dynamics of the stars. We then summarize the lessons that we have learned and discuss unsolved issues and questions that are still unanswered.
Aims: We investigate from a theoretical perspective if space asteroseismology can be used to distinguish between different thermal structures and shapes of the near-core mixing profiles for different types of coherent oscillation modes in massive stars with convective cores, and if this capacity depends on the evolutionary stage of the models along the main sequence. Methods: We compute 1D stellar structure and evolution models for four different prescriptions of the mixing and temperature gradient in the near-core region. Their effect on the frequencies of dipole prograde gravity modes in both Slowly Pulsating B and $beta$ Cep stars is investigated, as well as for pressure modes in $beta$ Cep stars. Results: A comparison between the mode frequencies of the different models at various stages during the main sequence evolution reveals that they are more sensitive to a change in temperature gradient than to the exact shape of the mixing profile in the near-core region. Depending on the duration of the observed light curve, one can distinguish between either just the temperature gradient, or also between the shapes of the mixing coefficient. The relative frequency differences are in general larger for more evolved models, and are largest for the higher-frequency pressure modes in $beta$ Cep stars. Conclusions:In order to unravel the core boundary mixing and thermal structure of the near-core region, one must have asteroseismic masses and radii with $sim 1%$ relative precision for hundreds of stars.
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