Conditions for HD Cooling in the First Galaxies Revisited: Interplay between Far-Ultraviolet and Cosmic Ray Feedback in Population III Star Formation

Abstract

HD dominates the cooling of primordial clouds with enhanced ionization, e.g. shock-heated clouds in structure formation or supernova remnants, relic HII regions of Pop III stars, and clouds with cosmic-ray (CR) irradiation. There, the temperature decreases to several 10 K and the characteristic stellar mass decreases to $\sim 10{\mathrm{M}}_{\odot }$, in contrast with first stars formed from undisturbed pristine clouds ($\sim 100{\mathrm{M}}_{\odot }$). However, without CR irradiation, even weak far ultra-violet (FUV) irradiation suppresses HD formation/cooling. Here, we examine conditions for HD cooling in primordial clouds including both FUV and CR feedback. At the beginning of collapse, the shock-compressed gas cools with its density increasing, while the relic HII region gas cools at a constant density. Moreover, shocks tend to occur in denser environments than HII regions. Owing to the higher column density and the more effective shielding, the critical FUV intensity for HD cooling in a shock-compressed gas becomes $\sim 10$ times higher than in relic HII regions. Consequently, in the shock-compressed gas, the critical FUV intensity exceeds the background level for most of the redshift we consider ($6\lesssim z\lesssim 15$), while in relic HII regions, HD cooling becomes effective after the CR intensity increases enough at $z\lesssim 10$. Our result suggests that less massive ($\sim 10{\mathrm{M}}_{\odot }$) Pop III stars may be more common than previously considered and could be the dominant population of Pop III stars.

15 pages, 12 figures, 1 table, accepted for publication in MNRAS

References in corpus (22)

• Protostar Formation in the Early Universe
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Uncertainties in H2 and HD Chemistry and Cooling and their Role in Early Structure Formation
• Fluctuations in the High-Redshift Lyman-Werner Background: Close Halo Pairs as the Origin of Supermassive Black Holes
• The Formation of the First Stars II. Radiative Feedback Processes and Implications for the Initial Mass Function
• Population III stars: hidden or disappeared ?
• The First Galaxies: Assembly, Cooling and the Onset of Turbulence
• Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
• Resolving the Formation of Protogalaxies. II. Central Gravitational Collapse
• Early Cosmological HII/HeIII Regions and Their Impact on Second-Generation Star Formation
• Star formation at very low metallicity. I: Chemistry and cooling at low densities
• Two populations of metal-free stars in the early Universe
• The HII Region of a Primordial Star
• Resolving the Formation of Protogalaxies. I. Virialization
• Formation of Massive Primordial Stars in a Reionized Gas
• Extremely metal-poor gas at a redshift of 7
• The minimum stellar metallicity observable in the Galaxy
• The Impact of HD Cooling on the Formation of the First Stars
• Impact of Cosmic Rays on Population III Star Formation
• Low-mass star formation triggered by early supernova explosions
• Cosmic rays and the primordial gas
• Cosmic Ray production of Beryllium and Boron at high redshift

Cited by in corpus (5)

• The critical radiation intensity for direct collapse black hole formation: dependence on the radiation spectral shape
• The suppression of direct collapse black hole formation by soft X-ray irradiation
• Simulating the growth of Intermediate Mass Black Holes
• Dissipation of magnetic fields in star-forming clouds with different metallicities
• Ionization degree and magnetic diffusivity in the primordial star-forming clouds