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The regulation of star formation by cosmic ray heating in the low–metallicity interstellar medium

Figure 1. Temperature–density and pressure–density phase plots for the four runs. The red dotted line is the equilibrium curve computed for G0 = 1.7 and ζ = 3 × 10−17 s−1. The orange dashed line indicates T = 104 K. We note that the structure of the interstellar medium changes dramatically depending on the prescription of the two heating mechanisms.

Vittoria Brugaletta

Low-metallicity environments experience a more inefficient cooling due to the lack of coolants. Therefore, appropriately modelling of the main heating rates is essential to describe the evolution of the multi-phase interstellar medium in metal-poor conditions. Since the photoelectric heating is dependent on the dust-to-gas-ratio, which in turn decreases for low metallicity, its efficiency is lowered. On the other side, the heating due to low-energy cosmic rays is independent on metallicity. Therefore, at a metallicity threshold of 0.02 Z the photoelectric heating is comparable, if not lower, than the cosmic-ray heating (Brugaletta et al. in prep.). In our work we describe the evolution of the interstellar medium with a metallicity of 0.02 Z using two new models for the characterization of the photoelectric heating and the cosmic-ray heating. In previous works, we assumed both of them to depend on a constant interstellar radiation field strength in Habing units, G0, and a constant cosmic-ray ionization rate, ζ, respectively. In this work (Brugaletta et al. in prep.) we include the AdaptiveG0 module from Rathjen et al. in prep. , which computes the parameter G0 self-consistently with the present stellar population, and a newly-implemented method to compute ζ from the energy density of cosmic rays.

The different impact that these two methods have on the metal-poor gas can be seen in Fig. 1, where we show the temperature-density and pressure-density phase plots. If we assume constant G0 = 1.7 and ζ = 3 x 10-17 s-1 (run Z0.02), the heating rates are so high that the gas is unable to cool down, but it exists in a warm phase. If we assume a variable G0 and a constant ζ (run Z0.02-vG0) the interstellar medium is able to cool down to a certain temperature, however it is not sufficient to form stars. Assuming both G0 and ζ to be variable (runs Z0.02-vG0-vζ and Z0.02-vG0-vζ-BS) lets the gas cool down and form stars. The difference between the last two simulations is a different scaling of the dust-to-gas ratio with metallicity.