ALBERT

Description: 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 $\mathcal {M}$ from 1to100) 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 lognormal 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 $p_{{\rm turb}}/{p_{{0}}}\ =23.07\, \left(\frac{n_{{0,\rm turb}}}{1\,{\rm cm}^{-3}}\right)^{-0.12} + 0.82 (\ln (1+b^{2}\mathcal {M}^{2}))^{1.49}\left(\frac{n_{{0,\rm turb}}}{1\,{\rm cm}^{-3}}\right)^{-1.6}$ . 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.

Description: We use magnetohydrodynamical simulations of converging warm neutral medium flows to analyse the formation and global evolution of magnetized and turbulent molecular clouds subject to supernova feedback from massive stars. We show that supernova feedback alone fails to disrupt entire, gravitationally bound, molecular clouds, but is able to disperse small-sized (~10 pc) regions on time-scales of less than 1 Myr. Efficient radiative cooling of the supernova remnant as well as strong compression of the surrounding gas result in non-persistent energy and momentum input from the supernovae. However, if the time between subsequent supernovae is short and they are clustered, large hot bubbles form that disperse larger regions of the parental cloud. On longer time-scales, supernova feedback increases the amount of gas with moderate temperatures ( T 300–3000 K). Despite its inability to disrupt molecular clouds, supernova feedback leaves a strong imprint on the star formation process. We find an overall reduction of the star formation efficiency by a factor of 2 and of the star formation rate by roughly factors of 2–4.

Description: The first supernova explosions are potentially relevant sources for the production of the first large-scale magnetic fields. For this reason, we present a set of high-resolution simulations studying the effect of supernova explosions on magnetized, primordial haloes. We focus on the evolution of an initially small-scale magnetic field formed during the collapse of the halo. We vary the degree of magnetization, the halo mass, and the amount of explosion energy in order to account for expected variations as well as to infer systematical dependences of the results on initial conditions. Our simulations suggest that core collapse supernovae with an explosion energy of 10 51 erg and more violent pair instability supernovae with 10 53 erg are able to disrupt haloes with masses up to about 10 6 and 10 7 M , respectively. The peak of the magnetic field spectra shows a continuous shift towards smaller k -values, i.e. larger length scales, over time reaching values as low as k = 4. On small scales, the magnetic energy decreases at the cost of the energy on large scales resulting in a well-ordered magnetic field with a strength up to ~10 –8 G depending on the initial conditions. The coherence length of the magnetic field inferred from the spectra reaches values up to 250 pc in agreement with those obtained from autocorrelation functions. We find the coherence length to be as large as 50 per cent of the radius of the supernova bubble. Extrapolating this relation to later stages, we suggest that significantly strong magnetic fields with coherence lengths as large as 1.5 kpc could be created. We discuss possible implications of our results on processes like recollapse of the halo, first galaxy formation, and the magnetization of the intergalactic medium.

Description: We analyse simulations of turbulent, magnetized molecular cloud cores focusing on the formation of Class 0 stage protostellar discs and the physical conditions in their surroundings. We show that for a wide range of initial conditions Keplerian discs are formed in the Class 0 stage already. In particular, we show that even subsonic turbulent motions reduce the magnetic braking efficiency sufficiently in order to allow rotationally supported discs to form. We therefore suggest that already during the Class 0 stage the fraction of Keplerian discs is significantly higher than 50 per cent, consistent with recent observational trends but significantly higher than predictions based on simulations with misaligned magnetic fields, demonstrating the importance of turbulent motions for the formation of Keplerian discs. We show that the accretion of mass and angular momentum in the surroundings of protostellar discs occurs in a highly anisotropic manner, by means of a few narrow accretion channels. The magnetic field structure in the vicinity of the discs is highly disordered, revealing field reversals up to distances of 1000 au. These findings demonstrate that as soon as even mild turbulent motions are included, the classical disc formation scenario of a coherently rotating environment and a well-ordered magnetic field breaks down. Hence, it is highly questionable to assess the magnetic braking efficiency based on non-turbulent collapse simulation. We strongly suggest that, in addition to the global magnetic field properties, the small-scale accretion flow and detailed magnetic field structure have to be considered in order to assess the likelihood of Keplerian discs to be present.

Description: Chemistry plays a key role in many astrophysical situations regulating the cooling and the thermal properties of the gas, which are relevant during gravitational collapse, the evolution of discs and the fragmentation process. In order to simplify the usage of chemical networks in large numerical simulations, we present the chemistry package krome , consisting of a python pre-processor which generates a subroutine for the solution of chemical networks which can be embedded in any numerical code. For the solution of the rate equations, we make use of the high-order solver DLSODES , which was shown to be both accurate and efficient for sparse networks, which are typical in astrophysical applications. krome also provides a large set of physical processes connected to chemistry, including photochemistry, cooling, heating, dust treatment and reverse kinetics. The package presented here already contains a network for primordial chemistry, a small metal network appropriate for the modelling of low metallicities environments, a detailed network for the modelling of molecular clouds, a network for planetary atmospheres, as well as a framework for the modelling of the dust grain population. In this paper, we present an extended test suite ranging from one-zone and 1D models to first applications including cosmological simulations with enzo and ramses and 3D collapse simulations with the flash code. The package presented here is publicly available at http://kromepackage.org/ and https://bitbucket.org/krome/krome_stable .

Description: We present synthetic ALMA observations of Keplerian, protostellar discs in the Class 0 stage studying the emission of molecular tracers like 13 CO, C 18 O, HCO + , H 13 CO + , N 2 H + , and H 2 CO. We model the emission of discs around low- and intermediate-mass protostars. We show that under optimal observing conditions ALMA is able to detect the discs already in the earliest stage of protostellar evolution, although the emission is often concentrated to the innermost 50 au. Therefore, a resolution of a few 0.1 arcsec might be too low to detect Keplerian discs around Class 0 objects. We also demonstrate that under optimal conditions for edge-on discs Keplerian rotation signatures are recognisable, from which protostellar masses can be inferred. For this we here introduce a new approach, which allows us to determine protostellar masses with higher fidelity than before. Furthermore, we show that it is possible to reveal Keplerian rotation even for strongly inclined discs and that ALMA should be able to detect possible signs of fragmentation in face-on discs. In order to give some guidance for future ALMA observations, we investigate the influence of varying observing conditions and source distances. We show that it is possible to probe Keplerian rotation in inclined discs with an observing time of 2 h and a resolution of 0.1 arcsec, even in the case of moderate weather conditions. Furthermore, we demonstrate that under optimal conditions, Keplerian discs around intermediate-mass protostars should be detectable up to kpc distances.

Description: We present simulations of star-forming filaments incorporating on of the largest chemical network used to date on-the-fly in a 3D-magnetohydrodynamic (MHD) simulation. The network contains 37 chemical species and about 300 selected reaction rates. For this, we use the newly developed package krome (Grassi et al.). We combine the krome package with an algorithm which allows us to calculate the column density and attenuation of the interstellar radiation field necessary to properly model heating and ionization rates. Our results demonstrate the feasibility of using such a complex chemical network in 3D-MHD simulations on modern supercomputers. We perform simulations with different strengths of the interstellar radiation field and the cosmic ray ionization rate. We find that, towards the centre of the filaments, there is gradual conversion of hydrogen from H to H 2 as well as of C + over C to CO. Moreover, we find a decrease of the dust temperature towards the centre of the filaments in agreement with recent HERSCHEL observations.

Description: We present collapse simulations of strongly magnetized, turbulent molecular cloud cores with masses ranging from 2.6 to 1000 M in order to study the influence of the initial conditions on the turbulence-induced disc formation mechanism proposed recently by Seifried et al. We find that Keplerian discs are formed in all cases independently of the core mass, the strength of turbulence or the presence of global rotation. The discs appear within a few kyr after the formation of the protostar, are 50–150 au in size, and have masses between 0.05 and a few 0.1 M . During the formation of the discs the mass-to-flux ratio stays well below the critical value of 10 for Keplerian disc formation. Hence, flux-loss alone cannot explain the formation of Keplerian discs. The formation of rotationally supported discs at such early phases is rather due to the disordered magnetic field structure and due to turbulent motions in the surroundings of the discs, two effects lowering the classical magnetic braking efficiency. Binary systems occurring in the discs are mainly formed via the disc capturing mechanism rather than via disc fragmentation, which is largely suppressed by the presence of magnetic fields.

Description: The formation of stars is usually accompanied by the launching of protostellar outflows. Observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) will soon revolutionize our understanding of the morphologies and kinematics of these objects. In this paper, we present synthetic ALMA observations of protostellar outflows based on numerical magnetohydrodynamic collapse simulations. We find significant velocity gradients in our outflow models and a very prominent helical structure within the outflows. We speculate that the disc wind found in the ALMA Science Verification Data of HD 163296 presents a first instance of such an observation.