Driving Conditions of Protostellar Outflows in Different Star-Forming Environments

Abstract

The evolution of collapsing clouds embedded in different star-forming environments is investigated using three-dimensional non-ideal magnetohydrodynamics simulations considering different cloud metallicities ($Z/\thinspace Z_\odot$ = 0, 10$^{-5}$, 10$^{-4}$, 10$^{-3}$, 10$^{-2}$, 10$^{-1}$ and 1) and ionisation strengths ($C_\zeta$=0, 0.01, 1 and 10, where $C_\zeta$ is a coefficient controlling the ionisation intensity and $C_\zeta=1$ corresponds to the ionisation strength of nearby star-forming regions). With all combinations of these considered values of $Z/\thinspace Z_\odot$ and $C_\zeta$, 28 different star-forming environments are prepared and simulated. The cloud evolution in each environment is calculated until the central density reaches $n\approx10^{16}\,{\rm cm}^{-3}$ just before protostar formation, and the outflow driving conditions are derived. An outflow appears when the (first) adiabatic core forms in a magnetically active region where the magnetic field is well coupled with the neutral gas. In cases where outflows are driven, their momentum fluxes are always comparable to the observations of nearby star-forming regions. Thus, these outflows should control the mass growth of the protostars as in the local universe. Roughly, an outflow appears when $Z/\thinspace Z_\odot>10^{-4}$ and $C_\zeta \ge 0.01$. It is expected that the transition of the star formation mode from massive stars to normal solar-type stars occurs when the cloud metallicity is enhanced to the range of $Z/\thinspace Z_\odot\approx 10^{-4}$--$10^{-3}$, above which relatively low-mass stars would preferentially appear as a result of strong mass ejection.