118382312
Dispersal of molecular clouds by ionizing radiation
Feedback from massive stars is believed to be a key element in the evolution of molecular clouds. We use high-resolution 3D smoothed particle hydrodynamics simulations to explore the dynamical effects of a single O7 star-emitting ionizing photons at 1049 s−1 and located at the centre of a molecular cloud with mass 104 M⊙ and radius 6.4 pc; we also perform comparison simulations in which the ionizing star is removed. The initial internal structure of the cloud is characterized by its fractal dimension, which we vary between and 2.8, and the standard deviation of the approximately log-normal initial density PDF, which is σ10 = 0.38 for all clouds. (i) As regards star formation, in the short term ionizing feedback is positive, in the sense that star formation occurs much more quickly (than in the comparison simulations), in gas that is compressed by the high pressure of the ionized gas. However, in the long term ionizing feedback is negative, in the sense that most of the cloud is dispersed with an outflow rate of up to ∼10−2 M⊙yr−1, on a time-scale comparable with the sound-crossing time for the ionized gas (), and triggered star formation is therefore limited to a few per cent of the cloud's mass. We will describe in greater detail the statistics of the triggered star formation in a companion paper. (ii) As regards the morphology of the ionization fronts (IFs) bounding the H ii region and the systematics of outflowing gas, we distinguish two regimes. For low , the initial cloud is dominated by large-scale structures, so the neutral gas tends to be swept up into a few extended coherent shells, and the ionized gas blows out through a few large holes between these shells; we term these H ii regions shell dominated. Conversely, for high , the initial cloud is dominated by small-scale structures, and these are quickly overrun by the advancing IF, thereby producing neutral pillars protruding into the H ii region, whilst the ionized gas blows out through a large number of small holes between the pillars; we term these H ii regions pillar dominated. (iii) As regards the injection of bulk kinetic energy, by ∼1 Myr, the expansion of the H ii region has delivered a mass-weighted rms velocity of ∼6 km s−1; this represents less than 0.1 per cent of the total energy radiated by the O7 star.
119233345
The influence of the turbulent perturbation scale on pre-stellar core fragmentation and disc formation
The collapse of weakly turbulent pre-stellar cores is a critical stage in the process of star formation. Being highly non-linear and stochastic, the outcome of collapse can only be explored theoretically by performing large ensembles of numerical simulations. Standard practice is to quantify the initial turbulent velocity field in a core in terms of the amount of turbulent energy (or some equivalent) and the exponent in the power spectrum (n ≡−dlog Pk/dlog k). In this paper, we present a numerical study of the influence of the details of the turbulent velocity field on the collapse of an isolated, weakly turbulent, low-mass pre-stellar core. We show that, as long as n ≳ 3 (as is usually assumed), a more critical parameter than n is the maximum wavelength in the turbulent velocity field, λMAX. This is because λMAX carries most of the turbulent energy, and thereby influences both the amount and the spatial coherence of the angular momentum in the core. We show that the formation of dense filaments during collapse depends critically on λMAX, and we explain this finding using a force balance analysis. We also show that the core has only a high probability of fragmenting if λMAX > RCORE/2 (where RCORE is the core radius), the dominant mode of fragmentation involves the formation and break-up of filaments and although small protostellar discs (with radius RDISC≲ 20 au) form routinely, more extended discs are rare. In turbulent, low-mass cores of the type we simulate here, the formation of large, fragmenting protostellar discs is suppressed by early fragmentation in the filaments.
117249738
The hunt for the Higgs boson: the $WW \rightarrow \ell\nu\ell\nu$ final state at the ATLAS detector
118421464
DETAILED NUMERICAL SIMULATIONS ON THE FORMATION OF PILLARS AROUND H II REGIONS
We study the structural evolution of turbulent molecular clouds under the influence of ionizing radiation emitted from a nearby massive star by performing a high-resolution parameter study with the iVINE code. The temperature is taken to be 10 K or 100 K, the mean number density is either 100 cm{sup -3} or 300 cm{sup -3}. Furthermore, the turbulence is varied between Mach 1.5 and Mach 12.5, the main driving scale of the turbulence is varied between 1 pc and 8 pc. We vary the ionizing flux by an order of magnitude, corresponding to allowing between 0.5% and 5% of the mass in the domain to be ionized immediately. In our simulations, the ionizing radiation enhances the initial turbulent density distribution and thus leads to the formation of pillar-like structures observed adjacent to H II regions in a natural way. Gravitational collapse occurs regularly at the tips of the structures. We find a clear correlation between the initial state of the turbulent cold cloud and the final morphology and physical properties of the structures formed. The most favorable regime for the formation of pillars is Mach 4-10. Structures and therefore stars only form if the initial density contrast between the high-densitymore » unionized gas and the gas that is going to be ionized is lower than the temperature contrast between the hot and the cold gas. The density of the resulting pillars is determined by a pressure equilibrium between the hot and the cold gas. A thorough analysis of the simulations shows that the complex kinematical and geometrical structure of the formed elongated filaments reflects that of observed pillars to an impressive level of detail. In addition, we find that the observed line-of-sight velocities allow for a distinct determination of different formation mechanisms. Comparing the current simulations to previous results and recent observations, we conclude that, e.g., the pillars of creation in M16 formed by the mechanism proposed here and not by the radiation driven implosion of pre-existing clumps.« less
18698114
Protostellar discs formed from turbulent cores
We investigate the collapse and fragmentation of low-mass, trans-sonically turbulent pre-stellar cores, using smoothed particle hydrodynamics simulations. The initial conditions are slightly supercritical Bonnor–Ebert spheres, all with the same density profile, the same mass (MO= 6.1 M⊙) and the same radius (RO= 17 000 au), but having different initial turbulent velocity fields. 400 turbulent velocity fields have been generated, all scaled so that the mean Mach number is . Then, a subset of these (in total 11 setups), having a range of net angular momenta, j, has been evolved. The evolution of these turbulent cores is significantly different from the collapse of a rigidly rotating core. It is not strongly correlated with j. Instead, it is moderated by the formation of filamentary structures due to converging turbulent flows. A high fraction (9 out of 13, ∼69 per cent) of the individual protostars forming from turbulent cores are attended by resolved (R≥ 10 au) protostellar accretion discs, but only a very small fraction (1 out of 9, ∼11 per cent) of these discs is sufficiently cool and extended to develop non-linear gravitational instabilities and fragment. Protostars with discs show two distinct growth modes. They initially grow by direct gravitational collapse, followed by subsequent disc accretion.
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.
41233042
Flow‐driven cloud formation and fragmentation: results from Eulerian and Lagrangian simulations
The fragmentation of shocked flows in a thermally bistable medium provides a natural mechanism to form turbulent cold clouds as precursors to molecular clouds. Yet because of the large density and temperature differences and the range of dynamical scales involved, following this process with numerical simulations is challenging. We compare two-dimensional simulations of flow-driven cloud formation without self-gravity, using the Lagrangian smoothed particle hydrodynamics (SPH) code VINE and the Eulerian grid code PROTEUS. Results are qualitatively similar for both methods, yet the variable spatial resolution of the SPH method leads to smaller fragments and thinner filaments, rendering the overall morphologies different. Thermal and hydrodynamical instabilities lead to rapid cooling and fragmentation into cold clumps with temperatures below 300 K. For clumps more massive than 1 M ⊙ pc ―1 , the clump mass function has an average slope of -0.8. The internal velocity dispersion of the clumps is nearly an order of magnitude smaller than their relative motion, rendering it subsonic with respect to the internal sound speed of the clumps but supersonic as seen by an external observer. For the SPH simulations most of the cold gas resides at temperatures below 100 K, while the grid-based models show an additional, substantial component between 100 and 300 K. Independent of the numerical method, our models confirm that converging flows of warm neutral gas fragment rapidly and form high-density, low-temperature clumps as possible seeds for star formation.