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.
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
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.
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.
17138642
Crystalline Silicates as a Probe of Disk Formation History
We present a new perspective on the crystallinity of dust in protoplanetary disks. The dominant crystallization by thermal annealing happens in the very early phases of disk formation and evolution. Both the disk properties and the level of crystallinity are thereby directly linked to the properties of the molecular cloud core from which the star+disk system was formed. We show that under the assumption of single-star formation, rapidly rotating clouds produce disks that after the main infall phase (i.e., in the optically revealed class II phase) are rather massive and have a high accretion rate but low crystallinity. Slowly rotating clouds, on the other hand, produce less massive disks with lower accretion rates but high levels of crystallinity. Cloud fragmentation and the formation of multiple stars complicates the problem and necessitates further study. The underlying physics of the model is insufficiently understood to provide the precise relationship between crystallinity, disk mass, and accretion rate. But the fact that with "standard" input physics the model produces disks that, in comparison to observations, appear to have either too high levels of crystallinity or too high disk masses demonstrates that the comparison of these models to observations can place strong constraints on the disk physics. The question to ask is not why some sources are so crystalline, but why some other sources have such a low level of crystallinity.
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.
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.