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

We study the effects of the fully ionized initial state, or pre-ionization, on the subsequent thermal evolution of low-metallicity clouds under various intensities of external far-ultraviolet (FUV) and cosmic-ray (CR) fields. Preionization 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 thermal evolution is limited in the very low-density regime for more metal-enriched clouds ([Z=H] ≳ -4), or those under a modest FUV (≳ 10^-3) field or CR field (≳ 0.1) of the present-day galactic disk levels. In any case, for ≳ 10 ⁸cm^-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 dust cooling at ≳ 10¹⁰ cm^-3, this process is not vulnerable either to the initial ionization state or external radiation.