Is H3+ cooling ever important in primordial gas?

Abstract: Studies of the formation of metal-free Population III stars usually focus primarily on the role played by H2 cooling, on account of its large chemical abundance relative to other possible molecular or ionic coolants. However, while H2 is generally the most important coolant at low gas densities, it is not an effective coolant at high gas densities, owing to the low critical density at which it reaches local thermodynamic equilibrium (LTE) and to the large opacities that develop in its emission lines. It is therefore possible that emission from other chemical species may play an important role in cooling high density primordial gas. A particularly interesting candidate is the H3+ molecular ion. This ion has an LTE cooling rate that is roughly a billion times larger than that of H2, and unlike other primordial molecular ions such as H2+ or HeH+, it is not easily removed from the gas by collisions with H or H2. It is already known to be an important coolant in at least one astrophysical context -- the upper atmospheres of gas giants -- but its role in the cooling of primordial gas has received little previous study. In this paper, we investigate the potential importance of H3+ cooling in primordial gas using a newly-developed H3+ cooling function and the most detailed model of primordial chemistry published to date. We show that although H3+ is, in most circumstances, the third most important coolant in dense primordial gas (after H2 and HD), it is nevertheless unimportant, as it contributes no more than a few percent of the total cooling. We also show that in gas irradiated by a sufficiently strong flux of cosmic rays or X-rays, H3+ can become the dominant coolant in the gas, although the size of the flux required renders this scenario unlikely to occur.

Insights into the Role of $H_{3}^{+}$ Cooling in Primordial Gas

The research paper titled "Is $H_{3}^{+}$ cooling ever important in primordial gas?" by Glover and Savin explores the intricate dynamics of primordial gas cooling, emphasizing the potential role of $H_{3}^{+}$ ions. Historically, studies of Population III stars have prioritized molecular hydrogen ($H_2$) due to its abundance and effectiveness in cooling low-density gas. However, at high densities, $H_2$ loses efficacy owing to its transition to local thermodynamic equilibrium and increased opacity. This opens avenues for other species like $H_{3}^{+}$, which boasts a significantly higher LTE cooling rate compared to $H_2$, to potentially contribute to the cooling process.

Numerical and Chemical Analysis

Using a sophisticated one-zone model incorporating comprehensive chemical networks and cooling functions, the authors analyze the contribution of $H_{3}^{+}$ to primordial gas cooling. The model integrates 392 chemical reactions across 30 species, reflecting the most detailed primordial chemistry model thus far. Key findings reveal that $H_{3}^{+}$ is typically the third most significant coolant after $H_2$ and HD but contributes only modestly to the cooling rate in most scenarios. It becomes dominant only under extreme conditions, such as high cosmic-ray or X-ray fluxes—situations predicted to be rare given the required intensity of these fluxes.

Implications and Key Findings

A significant contribution of this study is the detailed breakdown of chemical processes governing the formation and destruction of $H_{3}^{+}$ under varying conditions. The analysis elucidates the reasons behind $H_{3}^{+}$'s limited role, primarily its rapid destruction through reactions in dense environments despite its high cooling potential. Key insights reveal that while $H_{3}^{+}$ can be critical in specific astrophysical contexts, such as cooling in gas giant atmospheres, its primordial gas role remains marginal.

The implications extend to both theoretical models and computational workflows used in simulating early universe conditions. The findings suggest that while $H_{3}^{+}$ presents an intriguing alternative under specific conditions, its broad applicability in primordial star formation is constrained. These insights challenge the necessity of including $H_{3}^{+}$ in extensive cooling models unless high-density cosmic-ray environments are definitively established.

Future Directions

Glover and Savin's work lays the groundwork for future explorations into the nuanced chemistry of early star formation, highlighting areas where observational or theoretical advancements could further elucidate the conditions conducive to $H_{3}^{+}$'s cooling dominance. As cosmic ray and X-ray backgrounds are central to its significance, more refined studies on early cosmic environments and localized high-energy processes could redefine the boundaries of its importance.

In summary, while $H_{3}^{+}$ cooling in primordial gas is substantial under specific high-energy conditions, its contribution is minimal in typical scenarios anticipated in early star-forming regions. The paper encourages a more nuanced critique of its role, promoting focused research into conditions where $H_{3}^{+}$ may alter the landscape of primordial star formation.