Low-mass Pop III star formation due to the HD-cooling induced by weak Lyman-Werner radiation

TL;DR

This study explores how weak Lyman-Werner radiation influences primordial star formation, revealing that HD-cooling can lead to low-mass Pop III stars even under low radiation intensities, contrasting with previous assumptions.

Contribution

It demonstrates that at specific LW intensities, HD-cooling induces low-mass Pop III star formation, a novel insight into early universe star formation processes.

Findings

HD-cooling maintains low gas temperatures below 200 K.

Low-mass Pop III stars (≤0.8 M_sun) can form via HD-cooling.

Star formation efficiency decreases with increasing LW radiation intensity.

Abstract

Lyman-Werner (LW) radiation photodissociating molecular hydrogen (H$_2$) influences the thermal and dynamical evolution of the Population III (Pop III) star-forming gas cloud. The effect of powerful LW radiation has been well investigated in the context of supermassive black hole formation in the early universe. However, the average intensity in the early universe is several orders of magnitude lower. For a comprehensive study, we investigate the effects of LW radiation at $18$ different intensities, ranging from $J_{\rm LW}/J_{21}=0$ (no radiation) to $30$ (H-cooling cloud), on the primordial star-forming gas cloud obtained from a three-dimensional cosmological simulation. The overall trend with increasing radiation intensity is a gradual increase in the gas cloud temperature, consistent with previous works. Due to the HD-cooling, on the other hand, the dependence of gas cloud temperature on $J_{\rm LW}$ deviates from the aforementioned increasing trend for a specific range of intensities ($J_{\rm LW}/J_{21}=0.025-0.09$). In HD-cooling clouds, the temperature remained below $200$ K during $10^5$ yr after the first formation of the high-density region, maintaining a low accretion rate. Finally, the HD-cooling clouds have only a low-mass dense core (above $10^8\,{\rm cm^{-3}}$) with about $1-16\, M_{\odot}$, inside which a low-mass Pop III star with $\leq\!0.8\,M_{\odot}$ (so-called "surviving star") could form. The upper limit of star formation efficiency $M_{\rm core}/M_{\rm vir, gas}$ significantly decreases from $10^{-3}$ to $10^{-5}$ as HD-cooling becomes effective. This tendency indicates that, whereas the total gas mass in the host halo increases with the LW radiation intensity, the total Pop III stellar mass does not increase similarly.