We use hydrodynamical simulations in a (256 pc) 3 periodic box to model the impact of supernova (SN) explosions on the multi-phase interstellar medium (ISM) for initial densities n = 0.5-30 cm −3 and SN rates 1-720 Myr −1 . We include radiative cooling, diffuse heating, and the formation of molecular gas using a chemical network. The SNe explode either at random positions, at density peaks, or both. We further present a model combining thermal energy for resolved and momentum input for unresolved SNe. Random driving at high SN rates results in hot gas (T 10 6 K) filling > 90% of the volume. This gas reaches high pressures (10 4 < P/k B < 10 7 K cm −3 ) due to the combination of SN explosions in the hot, low density medium and confinement in the periodic box. These pressures move the gas from a two-phase equilibrium to the single-phase, cold branch of the cooling curve. The molecular hydrogen dominates the mass (> 50%), residing in small, dense clumps. Such a model might resemble the dense ISM in high-redshift galaxies. Peak driving results in huge radiative losses, producing a filamentary ISM with virtually no hot gas, and a small molecular hydrogen mass fraction ( 1%). Varying the ratio of peak to random SNe yields ISM properties in between the two extremes, with a sharp transition for equal contributions. The velocity dispersion in Hi remains 10 km s −1 in all cases. For peak driving the velocity dispersion in H α can be as high as 70 km s −1 due to the contribution from young, embedded SN remnants.

Photoionizing radiation and stellar winds from massive stars deposit energy and momentum into the interstellar medium (ISM). They might disperse the local ISM, change its turbulent multi-phase structure, and even regulate star formation. Ionizing radiation dominates the massive stars' energy output, but the relative effect of winds might change with stellar mass and the properties of the ambient ISM. We present simulations of the interaction of stellar winds and ionizing radiation of 12, 23, and 60 M stars within a cold neutral (CNM, n 0 = 100 cm −3 ), warm neutral (WNM, n 0 = 1, 10 cm −3 ) or warm ionized (WIM, n 0 = 0.1 cm −3 ) medium. The FLASH simulations adopt the novel tree-based radiation transfer algorithm TreeRay. With the On-the-Spot approximation and a temperature-dependent recombination coefficient, it is coupled to a chemical network with radiative heating and cooling. In the homogeneous CNM, the total momentum injection ranges from 1.6×10 4 to 4×10 5 M km s −1 and is always dominated by the expansion of the ionized H II region. In the WIM, stellar winds dominate (2×10 2 to 5×10 3 M km s −1 ), while the input from radiation is small (∼ 10 2 M km s −1 ). The WNM (n 0 = 1 cm −3 ) is a transition regime. Energetically, stellar winds couple more efficiently to the ISM (∼ 0.1 percent of wind luminosity) than radiation (< 0.001 percent of ionizing luminosity). For estimating the impact of massive stars, the strongly mass-dependent ratios of wind to ionizing luminosity and the properties of the ambient medium have to be considered.

Supernova (SN) blast waves inject energy and momentum into the interstellar medium (ISM), control its turbulent multiphase structure and the launching of galactic outflows. Accurate modelling of the blast wave evolution is therefore essential for ISM and galaxy formation simulations. We present an efficient method to compute the input of momentum, thermal energy, and the velocity distribution of the shock-accelerated gas for ambient media (densities of 0.1 n 0 [cm −3 ] 100) with uniform (and with stellar wind blown bubbles), power-law, and turbulent (Mach numbers M from 1 − 100) density distributions. Assuming solar metallicity cooling, the blast wave evolution is followed to the beginning of the momentum conserving snowplough phase. The model recovers previous results for uniform ambient media. The momentum injection in wind-blown bubbles depend on the swept-up mass and the efficiency of cooling, when the blast wave hits the wind shell. For power-law density distributions with n(r) ∼ r −2 (for n(r) > n floor ) the amount of momentum injection is solely regulated by the background density n floor and compares to n uni = n floor . However, in turbulent ambient media with log-normal density distributions the momentum input can increase by a factor of 2 (compared to the homogeneous case) for high Mach numbers. The average momentum boost can be approximated as . The velocity distributions are broad as gas can be accelerated to high velocities in low-density channels. The model values agree with results from recent, computationally expensive, three-dimensional simulations of SN explosions in turbulent media.

We present high-resolution (∼ 0.1 pc), hydrodynamical and magnetohydrodynamical simulations to investigate whether the observed level of molecular cloud (MC) turbulence can be generated and maintained by external supernova (SN) explosions. The MCs are formed self-consistently within their large-scale galactic environment following the non-equilibrium formation of H 2 and CO including (self-) shielding and important heating and cooling processes. The MCs inherit their initial level of turbulence from the diffuse ISM, where turbulence is injected by SN explosions. However, by systematically exploring the effect of individual SNe going off outside the clouds, we show that at later stages the importance of SN-driven turbulence is decreased significantly. This holds for different MC masses as well as for MCs with and without magnetic fields. The SN impact also decreases rapidly with larger distances. Nearby SNe (d ∼ 25 pc) boost the turbulent velocity dispersions of the MC by up to 70 per cent (up to a few km s −1 ). For d > 50 pc, however, their impact decreases fast with increasing d and is almost negligible. For all probed distances the gain in velocity dispersion decays rapidly within a few 100 kyr. This is significantly shorter than the average timescale for an MC to be hit by a nearby SN under solar neighbourhood conditions (∼ 2 Myr). Hence, at these conditions SNe are not able to sustain the observed level of MC turbulence. However, in environments with high gas surface densities and SN rates ,like the Central Molecular Zone, observed elevated MC dispersions could be triggered by external SNe.

As part of the SILCC-ZOOM project we present our first sub-parsec resolution radiation-hydrodynamic simulations of two molecular clouds self-consistently forming from a turbulent, multi-phase ISM. The clouds have similar initial masses of few 10 4 M , escape velocities of ∼5 km s −1 , and a similar initial energy budget. We follow the formation of star clusters with a sink based model and the impact of radiation from individual massive stars with the tree-based radiation transfer module TreeRay. Photo-ionizing radiation is coupled to a chemical network to follow gas heating, cooling and molecule formation and dissociation. For the first 3 Myr of cloud evolution we find that the overall star formation efficiency is considerably reduced by a factor of ∼4 to global cloud values of < 10 % as the mass accretion of sinks that host massive stars is terminated after 1 Myr. Despite the low efficiency, star formation is triggered across the clouds. Therefore, a much larger region of the cloud is affected by radiation and the clouds begin to disperse. The time scale on which the clouds are dispersed sensitively depends on the cloud substructure and in particular on the amount of gas at high visual extinction. The damage of radiation done to the highly shielded cloud (MC 1 ) is delayed. We also show that the radiation input can sustain the thermal and kinetic energy of the clouds at a constant level. Our results strongly support the importance of ionizing radiation from massive stars for explaining the low observed star formation efficiency of molecular clouds.