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
119113777
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.
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.
16069010
Supernova 2006aj and the associated X-Ray Flash 060218
Aims. We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova – dubbed SN 2006aj. Methods. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Results. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z = 0.0331 ± 0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.
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.
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
117249738
The hunt for the Higgs boson: the $WW \rightarrow \ell\nu\ell\nu$ final state at the ATLAS detector
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.