Search

In this contribution, we test our previously published one-dimensional PDR model for deriving total hydrogen volume densities from H i column density measurements in extragalactic regions by applying it to the Taurus molecular cloud, where its predictions can be compared to available data. Also, we make the first direct detailed comparison of our model to CO(1–0) and far-infrared emission.Using an incident ultraviolet flux G0 of 4.25 (χ = 5) throughout the main body of the cloud, we derive total hydrogen volume densities of ≈430 cm−3, consistent with the extensive literature available on Taurus. The distribution of the volume densities shows a log-normal shape with a hint of a power-law shape on the high density end. We convert our volume densities to H2 column densities assuming a cloud depth of 5 pc and compare these column densities to observed CO emission. We find a slope equivalent to a CO conversion factor relation that is on the low end of reported values for this factor in the literature [0.9 × 1020 cm−2 (K km s−1)−1], although this value is directly proportional to our assumed value of G0 as well as the cloud depth. We seem to underpredict the total hydrogen gas as compared to 100 μm dust emission, which as we speculate may be caused by a higher actual G0 incident on the Taurus cloud than is generally assumed. [ABSTRACT FROM AUTHOR]

We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non-ionizing ultraviolet radiation and X-rays from population synthesis models of young star clusters. For all our simulations we find that the H region expansion reduces the disordered component of the magnetic field, imposing a large-scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure-compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1-3 orders of magnitude lower. A high velocity dispersion ( km s) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large-scale H region expansion on the length and time-scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H region. However, the relative luminosity of ionizing and non-ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms. [ABSTRACT FROM AUTHOR]

We study the properties of clumps formed in three-dimensional weakly magnetized magnetohydrodynamic simulations of converging flows in the thermally bistable, warm neutral medium (WNM). We find the following. (1) Similarly to the situation in the classical two-phase medium, cold, dense clumps form through dynamically triggered thermal instability in the compressed layer between the convergent flows, and are often characterized by a sharp density jump at their boundaries though not always. (2) However, the clumps are bounded by phase-transition fronts rather than by contact discontinuities, and thus they grow in size and mass mainly by accretion of WNM material through their boundaries. (3) The clump boundaries generally consist of thin layers of thermally unstable gas, but these layers are often widened by the turbulence, and penetrate deep into the clumps. (4) The clumps are approximately in both ram and thermal pressure balance with their surroundings, a condition which causes their internal Mach numbers to be comparable to the bulk Mach number of the colliding WNM flows. (5) The clumps typically have mean temperatures , corresponding to the wide range of densities they contain under a nearly isothermal equation of state. (6) The turbulent ram pressure fluctuations of the WNM induce density fluctuations that then serve as seeds for local gravitational collapse within the clumps. (7) The velocity and magnetic fields tend to be aligned with each other within the clumps, although both are significantly fluctuating, suggesting that the velocity tends to stretch and align the magnetic field with it. (8) The typical mean field strength in the clumps is a few times larger than that in the WNM. (9) The magnetic field strength in the densest regions within the clumps has a mean value of but with a large scatter of nearly two orders of magnitude, implying that both sub- and supercritical cores are formed in the simulation. (10) In the final stages of the evolution, the clumps' growth drives them into gravitational instability, at which point star formation sets in, and the pressure in the clumps' centres increases even further. [ABSTRACT FROM AUTHOR]