14658394
Understanding the spectra of isolated Herbig stars in the frame of a passive disk model
We discuss spectral energy distributions of a sample of Herbig Ae/Be stars in the context of a passive irradiated disk model. The data have been presented earlier by Meeus et al. (2001), and preliminary interpretations ofthese data were given in that paper. While the spectra of Herbig Ae stars all show similarities, there is significant variation between the spectra, in particular in the shape of the mid-IR rise and in the presence or absence of a silicate feature. We explore the hypothesis that all these different spectra can be interpreted as pure disk spectra without additional components. Using the model of Dullemond et al. (2001) we deduce the disk parameters of a number of the sources, and find that for a large fraction of investigated sources, satisfactory fits can be obtained. The derived model parameters show that some group Ia sources can only be fit with radially increasing surface densities, indicating the presence of depleted inner disk regions. The steep-sloped SEDs of group IIa sources can be fit with very compact disks, probably representing disks with collapsed outer regions. The largest difficulties arise from sources that do not show significant silicate emission features. Our attempts to explain these objects with a pure geometric effect are only partially successful. It seems that these stars indeed require a strong depletion of small silicate grains.
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
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.
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.
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
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.
122655477
Flaws in the perfect bubble
Perfect bubbles like that surrounding the galactic HII region RCW 120 (Deharveng et al. 2009) have been interpreted as proof of concept for the collect and collapse (C & C) mechanism of triggered star formation. The cold, dusty clumps surrounding RCW 120 are aligned along an almost spherical shell. It has been inferred that these massive clumps, which sometimes harbour young stellar objects, have been formed via the fragmentation of the dense, swept-up shell. In order to better understand the triggering mechanisms at work in shells like RCW 120, we perform high-resolution, three dimensional SPH simulations of HII regions expanding into fractal molecular clouds. In a second step, we use RADMC-3D to compute the synthetic dust continuum emission from our simulations, in order to compare them with observations of RCW 120 made with APEX-LABOCA at 870 micron. We show that a distribution of clumps similar to the one seen in RCW 120 can readily be explained by a non-uniform underlying molecular cloud structure. Hence, a shell-like clump configuration around an HII region does not necessarily support the C & C scenario, but rather reflects the pre-existing, non-uniform density distribution of the molecular cloud into which the HII region expands.
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
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.