Primordial stellar evolution - The pre-main-sequence phase

Abstract

We consider the quasi-static contraction of primordial stars composed of pure hydrogen and helium gas. The stars begin their contraction as the protostellar cores obtained in our earlier study. We follow numerically the evolution of a star of mass M * = 5 M 0 from the end of protostellar accretion to the onset of hydrogen burning. Although the protostellar core of this mass is radiatively stable and undergoing nonhomologous con- traction, its large surface area and luminosity force the star to a partially convective, homologously contract- ing state within only 100 yr. Deuterium later ignites at an off-center temperature maximum but fails to produce interior convection. The star follows a conventional pre-main-sequence track in the H-R diagram, reaching the zero-age main sequence after 1.2 x 106 yr, with a luminosity of 880 L0 and a radius of 1.2 R0 . All stars with M * < 7 M 0 undergo a similar rapid relaxation to homologous contraction because of their large initial radii and surface luminosities. Stars with mass between 7 and 9 M 0 exhibit an initial jump in luminosity from the eruption of an internal luminosity wave. Heavier stars are homologously contracting prior to the end of protostellar accretion. If the dark halos of spiral galaxies consist of very low-mass primor- dial stars, the radiation from their contraction could be detectable at moderate redshifts.