H2 Line Emission Associated with the Formation of the First Stars

Abstract

Abstract Molecular hydrogen ($\mathrm{H}_2$) line radiation emitted in the formation events of first-generation stars is evaluated in a discussion of its detectability by future observational facilities. $\mathrm{H}_2$ luminosity evolution from the onset of prestellar collapse until the formation of a $\sim 100 \,{{{M}_{\odot}}}$ protostar is followed. Calculations are extended not only to the early phase of the runaway collapse, but also to the later phase of accretion, whose observational features have not been studied before. Contrary to the runaway collapse phase, where the pure-rotational lines are always dominant, in the accretion phase rovibrational line emission becomes prominent. The maximum luminosity is also attained in the accretion phase for strong emission lines. The peak intensity of the strongest rovibrational line reaches $\sim 10^{-29} \,\mathrm{W} \,\mathrm{m}^{-2}$, corresponding to the flux density of $10^{-5} \,\mu\mathrm{Jy}$, for a source at the typical redshift of first-generation star formation, $1+z = 20$. Although the redshifted rovibrational $\mathrm{H}_2$ emission from such an epoch falls in the wavelength range of the next-generation infrared satellite, Space Infrared Telescope for Cosmology and Astrophysics, for exceeding the detection threshold, $10^7$ such protostars are required to reach the maximum luminosity simultaneously in a pregalactic cloud. It is improbable that this condition is satisfied in a realistic scenario of early structure formation.