Non-equilibrium (time-dependent) cooling rates and ionization state calculations are presented for low-density gas enriched with heavy elements (meta

We present self-consistent calculations of non-equilibrium (time-dependent) cooling rates for a dust-free collisionally controlled gas in wide ranges of temperature (10 ≤ T ≤ 10⁸ K) and metallicity (10⁻⁴ ≤ Z ≤ 2 Z_⊙). We confirm that molecular hydrogen dominates cooling at 10² ≲ T ≲ 10⁴ K and Z ≲ 10^{− 3} Z_⊙. We find that the contribution from H₂ into the cooling rate around T ∼ (4–5) × 10³ K stimulates thermal instability in the metallicity range Z ≲ 10^{− 2} Z_⊙. Isobaric cooling rates are generally lower than isochoric cooling rates, because the associated increase of gas density leads to both more efficient hydrogen recombination and equilibration of the fine-structure level populations. Isochoric cooling means that the ionization fraction remains quite high at T ≲ 10⁴ K – up to ∼0.01 at T ≃ 10³ K and Z ≲ 0.1 Z_⊙, and even higher at higher metallicity – unlike isobaric cooling, where it is at least an order of magnitude lower. Despite this increase in ionization fraction, the gas-phase formation rate of molecular hydrogen (via H⁻) decreases with metallicity, because higher metallicity shortens the evolution time. We implement our self-consistent cooling rates into the multidimensional parallel code zeus-mp in order to simulate the evolution of a supernova remnant. We compare it to an analogous model with tabulated cooling rates published in previous studies. We find significant differences between the two descriptions, which might appear, for example, in the mixing of the ejected metals in the circumstellar medium.

Heating of the interstellar medium (ISM) by multiple supernova (SN) explosions is at the heart of producing galaxy-scale outflows in starburst galaxies. Standard models of outflows assume a high efficiency of SNe in heating the gas to X-ray emitting temperatures and filling the central region of starburst with hot gas, in order to launch vigorous outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the ISM and filling the volume with gas of high temperatures. We argue that it is important for SN remnants to have a large filling factor and a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas (T ≥ 3 × 10⁶ K) can percolate through the whole region after the total volume filling factor reaches a threshold of ∼0.3. The efficiency of heating the gas to X-ray temperatures can be ≥0.1 after this percolation epoch, which occurs after a period of ≈10 Myr for a typical starburst SN rate density of ν_SN ≈ 10⁻⁹ pc⁻³ yr⁻¹ and gas density of n ≈ 10 cm⁻³ in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures (≥10^6.5 K) roughly scales as $$\nu _{\rm SN}^{0.2} n^{-0.6}$$. For a typical SN rate density and gas density in starburst nuclei, the heating efficiency is ∼0.15, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions.

Using 3D hydrodynamic calculations we simulate formation of molecular clouds in the Galaxy. The simulations take into account molecular hydrogen chemical kinetics, cooling and heating processes. Comprehensive gravitational potential accounts for contributions from the stellar bulge, two- and four-armed spiral structure, stellar disc, dark halo and takes into account self-gravitation of the gaseous component. Gas clouds in our model form in the spiral arms due to shear and wiggle instabilities and turn into molecular clouds after t ≳ 100 Myr. At the times t ∼ 100–300 Myr the clouds form hierarchical structures and agglomerations with the sizes of 100 pc and greater. We analyse physical properties of the simulated clouds and find that synthetic statistical distributions like mass spectrum, ‘mass–size’ relation and velocity dispersion are close to those observed in the Galaxy. The synthetic l–v (galactic longitude–radial velocity) diagram of the simulated molecular gas distribution resembles observed one and displays a structure with appearance similar to molecular ring of the Galaxy. Existence of this structure in our modelling can be explained by superposition of emission from the galactic bar and the spiral arms at ∼3–4 kpc.

We study the merging and evolution of isolated supernova (SN) remnants in a stellar cluster into a collective superbubble, with the help of 3D hydrodynamic simulations. We particularly focus on the transition stage when the isolated SN remnants gradually combine to form a superbubble. We find that when the SN rate is high (ν_sn ∼ 10⁻⁹ pc⁻³ yr⁻¹), the merging phase lasts for ∼10⁴ yr, for n = 1–10 cm⁻³, and the merging phase lasts for a longer time (∼0.1 Myr or more) for lower SN rates (ν_sn ≤ 10⁻¹⁰ pc⁻³ yr⁻¹). During this transition phase, the growing superbubble is filled with dense and cool fragments of shells, and most of the energy is radiated away during this merging process. After passing through the intermediate phase, the superbubble eventually settles on to a new power-law wind asymptote that is smaller than estimated in a continuous wind model. This results in a significant (more than several times) underestimation of the mechanical luminosity needed to feed the bubble. We determine the X-ray and H α surface brightnesses as functions of time for such merging SNe in a stellar cluster and find that clusters with high SN rate shine predominantly in soft X-rays and H α. In particular, a low value of the volume-averaged H α-to-H β ratio and its large spread can be a good indicator of the transition phase of merging SNe.

HD molecules can be an important cooling agent of the primordial gas behind the shock waves originated through merging of the dark matter haloes at e