118425527
Radiation Driven Implosion and Triggered Star Formation
We present simulations of initially stable isothermal clouds exposed to ionizing radiation from a discrete external 
source, and identify the conditions that lead to radiatively driven implosion and star formation.We use the smoothed 
particle hydrodynamics code SEREN and a HEALPix-based photoionization algorithm to simulate the propagation 
of the ionizing radiation and the resulting dynamical evolution of the cloud. We find that the incident ionizing flux, 
ΦLyC, is the critical parameter determining the cloud evolution. At moderate fluxes, a large fraction of the cloud 
mass is converted into stars. As the flux is increased, the fraction of the cloud mass that is converted into stars 
and the mean masses of the individual stars both decrease. Very high fluxes simply disperse the cloud. Newly 
formed stars tend to be concentrated along the central axis of the cloud (i.e., the axis pointing in the direction of 
the incident flux). For given cloud parameters, the time, t�, at which star formation starts is proportional to Φ−1/3 
LyC . 
The pattern of star formation found in the simulations is similar to that observed in bright-rimmed clouds
14189523
iVINE – Ionization in the parallel tree/sph code VINE: first results on the observed age‐spread around O‐stars
We present a three-dimensional, fully parallelized, efficient implementation of ionizing ultraviolet (UV) radiation for smoothed particle hydrodynamics (SPH) including self-gravity. Our method is based on the SPH/TREE code VINE. We therefore call it iVINE (for Ionization + VINE). This approach allows detailed high-resolution studies of the effects of ionizing radiation from, for example, young massive stars on their turbulent parental molecular clouds. In this paper, we describe the concept and the numerical implementation of the radiative transfer for a plane-parallel geometry and we discuss several test cases demonstrating the efficiency and accuracy of the new method. As a first application, we study the radiatively driven implosion of marginally stable molecular clouds at various distances of a strong UV source and show that they are driven into gravitational collapse. The resulting cores are very compact and dense exactly as it is observed in clustered environments. Our simulations indicate that the time of triggered collapse depends on the distance of the core from the UV source. Clouds closer to the source collapse several 10 5 yr earlier than more distant clouds. This effect can explain the observed age spread in OB associations where stars closer to the source are found to be younger. We discuss possible uncertainties in the observational derivation of shock front velocities due to early stripping of protostellar envelopes by ionizing radiation.
118057756
Modelling Shear Flows with SPH and Grid Based Methods
Given the importance of shear flows for astrophysical gas dynamics, we study the evolution of the Kelvin-Helmholtz instability (KHI) analytically and numerically. We derive the dispersion relation for the two-dimensional KHI including viscous dissipation. The resulting expression for the growth rate is then used to estimate the intrinsic viscosity of four numerical schemes depending on code-specific as well as on physical parameters. Our set of numerical schemes includes the Tree-SPH code VINE, an alternative SPH formulation developed by Price (2008), and the finite-volume grid codes FLASH and PLUTO. In the first part, we explicitly demonstrate the effect of dissipation-inhibiting mechanisms such as the Balsara viscosity on the evolution of the KHI. With VINE, increasing density contrasts lead to a continuously increasing suppression of the KHI (with complete suppression from a contrast of 6:1 or higher). The alternative SPH formulation including an artificial thermal conductivity reproduces the analytically expected growth rates up to a density contrast of 10:1. The second part addresses the shear flow evolution with FLASH and PLUTO. Both codes result in a consistent non-viscous evolution (in the equal as well as in the different density case) in agreement with the analytical prediction. The viscous evolution studied with FLASH shows minor deviations from the analytical prediction.
118647466
THE TURBULENT FRAGMENTATION OF THE INTERSTELLAR MEDIUM: THE IMPACT OF METALLICITY ON GLOBAL STAR FORMATION
We study the influence of gas metallicity, turbulence, and non-equilibrium chemistry on the evolution of the two-phase interstellar medium (warm and cold atomic phases), and thereby constrain the initial conditions for star formation prevailing in turbulent gas. We perform high-resolution simulations in three dimensions, including a realistic non-equilibrium treatment of the ionization state of the gas, and examine both driven and decaying turbulence. This allows us to explore variations in the metallicity Z. In this paper, we study solar metallicity, Z = Z ☉, and low-metallicity, Z = 10–3 Z ☉, gas. For driven, large-scale turbulence, we find that the influence of the metallicity on the amount of mass in the cold gas component is small. However, in decaying turbulent conditions this picture is much changed. While cold regions survive in the case of solar metallicity, they are quickly heated and dispersed in low-metallicity gas. This result suggests that star formation can be suppressed in environments of low metallicity, unless a strong turbulent driver is acting on timescales shorter than a few turbulent crossing times. Inter alia this finding could explain the overall inefficient star formation as well as the burst-like mode of star formation found in metal-poor, gas-rich systems like dwarf galaxies.