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.
117947175
Algorithmic comparisons of decaying, isothermal, compressible turbulence I. Low-resolution simulations with fixed grids
A chlorinated ethylene-alpha olefin-nonconjugated diolefin terpolymer having a chlorine content of from about 0.5 to 14 percent by weight characterized by superior viscosity stability on storage or exposure to elevated temperatures is obtained by chlorinating an ethylene-alpha olefin-nonconjugated diolefin terpolymer wherein said nonconjugated diolefin is an acyclic nonconjugated diolefin characterized in that one olefinic bond is monosubstituted and terminal and the other olefinic bond is trisubstituted. A vulcanizable rubber composition comprising said chlorinated ethylene-alpha olefin-nonconjugated diolefin terpolymer and cure active agents is also provided.
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
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.
2674301
Constraints on the progenitor systems of type Ia supernovae
Bolometric light curves provide a valuable insight into the nature of type Ia supernovae. We present an analysis of sixteen well-observed type Ia supernovae. Constraints are placed on several global parameters concerning the progenitor system, explosion mechanism, and subsequent radiation transport. By fitting a radioactive decay energy deposition function to the quasi-exponential phase (50 to 100 days after maximum light), it is found that the ejected mass varies by at least a factor of two. This result suggests that a sub Chandrasekhar mass white dwarf could be the progenitor system of some type Ia supernovae. We find that the range in the amount of synthesized $\rm ^{56}Ni$ indicates a significant variation in the burning mechanism. In order to explain a factor of ten range in the observed bolometric luminosity, more detailed modeling of the explosion mechanism is required.
5645723
DRIVING TURBULENCE AND TRIGGERING STAR FORMATION BY IONIZING RADIATION
We present high-resolution simulations on the impact of ionizing radiation of massive O stars on the surrounding turbulent interstellar medium (ISM). The simulations are performed with the newly developed software iVINE which combines ionization with smoothed particle hydrodynamics (SPH) and gravitational forces. We show that radiation from hot stars penetrates the ISM, efficiently heats cold low-density gas and amplifies overdensities seeded by the initial turbulence. The formation of observed pillar-like structures in star-forming regions (e.g. in M16) can be explained by this scenario. At the tip of the pillars gravitational collapse can be induced, eventually leading to the formation of low-mass stars. Detailed analysis of the evolution of the turbulence spectra shows that UV radiation of O stars indeed provides an excellent mechanism to sustain and even drive turbulence in the parental molecular cloud.
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