Do the environmental conditions affect the dust-induced fragmentation in low-metallicity clouds ?: Effect of pre-ionization and far-ultraviolet/cosmic-ray fields

TL;DR

This study investigates how pre-ionization and external radiation fields influence the thermal evolution and dust-induced fragmentation of low-metallicity clouds, revealing that dust cooling remains effective regardless of initial ionization or radiation conditions.

Contribution

It provides new insights into the roles of pre-ionization and external radiation in the thermal evolution and fragmentation of low-metallicity clouds, emphasizing the robustness of dust cooling.

Findings

Pre-ionization significantly affects metal-free clouds without external fields.

Dust cooling drives sub-solar mass fragmentation at high densities.

External radiation fields have limited impact on thermal evolution at high densities.

Abstract

We study effects of the fully ionized initial state, or pre-ionization, on the subsequent thermal evolution of low-metallicity clouds under various intensities of the external far-ultraviolet(FUV) and cosmic-ray(CR) fields. The pre-ionization significantly affects the thermal and dynamical evolution of metal-free clouds without FUV/CRs by way of efficient HD formation. On the other hand, the pre-ionization effect on the thermal evolution is limited in very low-density regime for more metal-enriched clouds ([Z/H] >~ -4) or those under modest FUV (>10^{-3}) or CR field (>0.1 of the present-day Galactic disk levels). In any case, for >10^8 cm^{-3}, neither the initial ionization state nor the irradiating FUV strength affect the thermal evolution. The dust cooling is an important mechanism for making sub-solar mass fragments in low-metallicity gas. Since this fragmentation occurs at the temperature minimum by the dust cooling at >10^{10} cm^{-3}, this process is not vulnerable either to initial ionization state or external radiation.