![]() Such a stellar seed with 1–5 R ⊙ and 10 –3 − 10 –2 M ⊙ ( Larson, 1969 Bhandare et al., 2018), constitutes the first stage of the pre-main sequence evolution, and continues to accrete material from its surrounding cloud or disk. This is a strongly endothermic process and leads to a second collapse, ending in the formation of the second hydrostatic core (see e.g. The increase in density and temperature leads to the formation of a first hydrostatic core which will further heat up until molecular hydrogen dissociates at ∼ 2000 K. Such an optically thin cloud collapses under its own gravity. While the latter is taken as a huge ( ∼ 55 R ⊙ for a 2 M ⊙ star) fully convective star at ZAMS mass, real stellar seeds are produced in the collapse of molecular clouds. Iben (1965) refined the picture of pre-main sequence evolution (classical pre-main sequence model from here on) by following the C 12-depletion in more detail.Ĭompared to the real star formation process, however, this classical view of the pre-main sequence evolution suffers from a crude approximation: the initial model. Because of the forbidden zone in the Hertzsprung-Russell diagram, the models by Hayashi (1961) follow the ‘Hayashi track’ before joining the ‘Henyey track’ on their contraction towards the ZAMS. Hayashi (1961) discussed the forbidden zone in the Hertzsprung-Russell diagram–an area in which no star can be in the hydrostatic equilibrium as the needed temperature gradient would immediately be brought down by rapid convection–and provided calculation after which stars follow a fully convective ‘Hayashi track’ before joining the ‘Henyey track’ on their contraction towards the ZAMS. Once it was evident that convection plays a major part in the evolution of stars, Hayashi (1961) delivered improved theoretical models for the pre-main sequence phase, achieving good agreement with the observational data of NGC 2264 ( Walker, 1956). Their models described the gravitational contraction of radiative stars and the respective evolution of the spectroscopic parameters is still referred to as the ‘Henyey track’ today. (1955) provided the first calculations of stars before their main sequence phase. The study of pre-main sequence stars was initiated in the 1950s when what appear to be recently formed groups of stars ( Henyey et al., 1955) drew interest from the astronomical community. In this work we discuss its potential for an advancement of our understanding of stellar structure and evolution. The increasing interest of stellar astrophysics in general to investigate the formation and early evolution of stars and planets illustrates the growing importance of pre-main sequence asteroseismology. While gyrochronology, for example, struggles to determine the ages of the youngest clusters, pulsations in pre-main sequence stars can function as an independent age indicator yielding higher precision for single stars. An improved understanding of the structure of young stellar objects has the potential to answer some of the open questions of stellar evolution, including angular momentum transport and the formation of magnetic fields. Keeping all this in mind, the prospects for pre-main sequence asteroseismology are manifold. Theoretical models of pre-main sequence stars include several assumptions and simplifications that influence the calculation of pulsation frequencies and excitation properties of pulsation modes. The lack of long time-base satellite observations in addition limits the applications of the method. The remnants of their birth environment which is often still surrounding the young stars causes variability that can interfere with the signal of pulsations. Asteroseismology of pre-main sequence stars faces observational and theoretical challenges. ![]() While asteroseismology offers a great tool to investigate these physical processes, studying pre-MS oscillations in turn has the potential to further advance the field. Although this evolutionary phase lasts a relatively short time, it is the imprint of these important physical processes that is often ignored by simplified assumptions. Before the stars arrive on the zero-age main sequence, they form in the collapses of molecular clouds, gain matter through accretion processes, and compress their cores until hydrogen can burn in full equilibrium. Stars do not simply pop up on the main sequence. Institute for Astro- and Particle Physics, University of Innsbruck, Innsbruck, Austria.
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