The First Million Years of the Sun: A Calculation of the Formation and Early Evolution of a Solar Mass Star

Abstract

We present the first coherent dynamical study of the cloud fragmentation phase, collapse, and early stellar evolution of a solar mass star. We determine young star properties as the consequence of the parent cloud evolution. Mass, luminosity, and effective temperature in the first million years of the proto-Sun result from gravitational fragmentation of a molecular cloud region that produces a cluster of prestellar clumps. We calculate the global dynamical behavior of the cloud using isothermal three-dimensional hydrodynamics and follow the evolution of individual protostars in detail using a one-dimensional radiation hydrodynamic system of equations that comprises a correct standard solar model solution, as a limiting case. We calculate the pre-main-sequence (PMS) evolutionary tracks of a solar mass star in a dense stellar cluster environment and compare it to one that forms in isolation. Up to an age of 950,000 yr, differences in the accretion history lead to significantly different temperature and luminosity evolution. As accretion fades and the stars approach their final masses, the two dynamic PMS tracks converge. After that, the contraction of the quasi-hydrostatic stellar interiors dominate the overall stellar properties and proceed in very similar ways. Hence, the position of a star in the Hertzsprung-Russell diagram becomes a function of age and mass only. However, our quantitative description of cloud fragmentation, star formation, and early stellar evolution predicts substantial corrections to the classical, i.e., hydrostatic and initially fully convective models: at an age of 1 million yr, the proto-Sun is twice as bright and 500 K hotter than according to calculations that neglect the star formation process.