We use magnetohydrodynamical simulations of converging flows to investigate the process of molecular cloud formation and evolution out of the magnetised ISM. Here, we study whether the observed subcritical HI clouds can become supercritical and hence allow the formation of stars within them. To do so, we vary the turbulent Mach number of the flows, as well as the initial magnetic field strength. We show that dense cores are able to build up under all conditions, but that star formation in these cores is either heavily delayed or completely suppressed if the initial field strength is B > 3 µG. To probe the effect of magnetic diffusion, we introduce a tilting angle ϕ between the flows and the uniform background magnetic field, which mimics nonideal MHD effects. Even with highly diffusive flows, the formed cores are devoid of star formation, because no magnetically supercritical regions are build up. Hence we conclude, that the problem of how supercritical cloud cores are generated still persists.
We use magnetohydrodynamical simulations of converging warm neutral medium flows to analyse the formation and global evolution of magnetised 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 timescales 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 timescales, 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.
Context. In cold and dense gas prior to the formation of young stellar objects, heavy molecular species (including CO) are accreted onto dust grains. Under these conditions H + 3 and its deuterated isotopologues become more abundant, enhancing the deuterium fraction of molecules such as N 2 H + that are formed via ion-neutral reactions. Because this process is extremely temperature sensitive, the abundance of these species is likely linked to the evolutionary stage of the source. Aims. We investigate how the abundances of o-H 2 D + and N 2 D + vary with evolution in high-mass clumps. Methods. We observed with APEX the ground-state transitions of o-H 2 D + near 372 GHz, and N 2 D + (3-2) near 231 GHz for three massive clumps in different evolutionary stages. The sources were selected within the G351.77-0.51 complex to minimise the variation of initial chemical conditions, and to remove distance effects. We modelled their dust continuum emission to estimate their physical properties, and also modelled their spectra under the assumption of local thermodynamic equilibrium to calculate beam-averaged abundances.Results. We find an anticorrelation between the abundance of o-H 2 D + and that of N 2 D + , with the former decreasing and the latter increasing with evolution. With the new observations we are also able to provide a qualitative upper limit to the age of the youngest clump of about 10 5 yr, comparable to its current free-fall time.Conclusions. We can explain the evolution of the two tracers with simple considerations on the chemical formation paths, depletion of heavy elements, and evaporation from the grains. We therefore propose that the joint observation and the relative abundance of o-H 2 D + and N 2 D + can act as an efficient tracer of the evolutionary stages of the star-formation process.
High-mass stars are expected to form from dense prestellar cores. Their precise formation conditions are widely discussed, including their virial condition, which results in slow collapse for super-virial cores with strong support by turbulence or magnetic fields, or fast collapse for sub-virial sources. To disentangle their formation processes, measurements of the deuterium fractions are frequently employed to approximately estimate the ages of these cores and to obtain constraints on their dynamical evolution. We here present 3D magneto-hydrodynamical simulations including for the first time an accurate non-equilibrium chemical network with 21 gas-phase species plus dust grains and 213 reactions. With this network we model the deuteration process in fully depleted prestellar cores in great detail and determine its response to variations in the initial conditions. We explore the dependence on the initial gas column density, the turbulent Mach number, the mass-to-magnetic flux ratio and the distribution of the magnetic field, as well as the initial ortho-to-para ratio of H 2 . We find excellent agreement with recent observations of deuterium fractions in quiescent sources. Our results show that deuteration is rather efficient, even when assuming a conservative ortho-to-para ratio of 3 and highly sub-virial initial conditions, leading to large deuterium fractions already within roughly a free-fall time. We discuss the implications of our results and give an outlook to relevant future investigations.
Both observational and theoretical research over the past decade has demonstrated that the probability distribution function (PDF) of the gas density in turbulent molecular clouds is a key ingredient for understanding star formation. It has recently been argued that the PDF of molecular clouds is a pure power-law distribution. It has been claimed that the log-normal part is ruled out when using only the part of the PDF up/down to which it is complete, that is where the column density contours are still closed. By using the results from high-resolution magnetohydrodynamical simulations of molecular cloud formation and evolution, we find that the column density PDF is indeed composed of a log-normal and, if including self-gravity, a power-law part. We show that insufficient sampling of a molecular cloud results in closed contours that cut off the log-normal part. In contrast, systematically increasing the field of view and sampling the entire cloud yields a completeness limit at the lower column densities, which also recovers the log-normal part. This demonstrates that the field of view must be sufficiently large for the PDF to be complete down to its log-normal part, which has important implications for predictions of star-formation activity based on the PDF.
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