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Dependence of HII region temperature on metallicity and surface density

Figure 1: Mass-weighted mean temperature of HII regions in our runs (Brugaletta et al. in prep.) as a function of time. Here t is the simulation time, whereas tSF is the moment in time corresponding to the first episode of star formation. The names of the simulations follow the rule: ΣXXX stands for the adopted gas surface density, where XXX is a value in M pc-2; Z indicates the initial metallicity, and the following numbers are values in units of Z. The Σ010 run is taken from Rathjen et al. (2022). We note that at a fixed gas surface density (10 M pc-2), the HII regions become progressively hotter as the metallicity decreases. Moreover, considering only the runs with fixed metallicity (Z = 0.02 Z), we observe that the mean temperature falls in the same range, regardless of the gas surface density.

Vittoria Brugaletta

HII regions are bubbles of ionized gas surrounding regions where star formation has taken place. These regions are formed through ionizing radiation from massive stars and are thought to be one of the primary mechanisms for slowing down or even stopping star formation. The region's high temperature and, therefore high pressure, counteract gravity and the inflow of new gas. It is important to learn about the temperature of HII regions in different interstellar medium (ISM) conditions because it provides an understanding on how effective HII regions are as a feedback mechanism.
The HII region temperature is determined by the balance between heating processes, e.g. photoionization, and cooling processes such as emission from recombinations and from collisionally-excited particles. Both types of processes depend on the metallicity of the gas. For instance, in low-metallicity environments stars are usually more luminous and have a higher effective temperature, thus providing ionizing photons with higher energies. Also, metal-line cooling depends on the presence of metals, therefore, the low-metallicity gas tends to cool down less efficiently than in solar neighborhood conditions. These two effects together cause a higher temperature in HII regions in metal-poor environments.

We investigate these effects by running a set of simulations within the SILCC project using the same setup as in Rathjen et al. (2022), where we vary the initial metallicity and gas surface density (Brugaletta et al. in prep.). In Fig. 1 we compute the mass-weighted mean temperature of all HII regions that are being formed in every run, and we do it for every snapshot of the simulations to obtain a time evolution. The Σ010 simulation is taken from Rathjen et al. (2022), and it has the same initial conditions as the Σ010-Z1 run, with the only difference that the latter employs the solar BoOST stellar models, whereas the former adopts the solar-metallicity non-rotating Geneva models. For some runs, the time evolution of the mean temperature does not cover the entire interval of 100 Myr (after the first stars are born) for which the simulations are run. This is because there are times in which no HII regions exist, as no new stars are forming due to stellar feedback inhibiting star formation.

As expected from the discussion above, we note from Fig. 1 that the mean temperature of HII regions increases with decreasing metallicity. We also notice that for very low metallicity (Z = 0.02 Z) the mean temperature is similar in all three simulations with the same initial metallicity. Therefore, we conclude that metallicity is the primary factor in determining the mean temperature of HII regions, and that the influence from the gas surface density on the mean temperature is not evident.