101 research outputs found
An Alternative Accurate Tracer of Molecular Clouds: The "-Factor"
We explore the utility of CI as an alternative high-fidelity gas mass tracer
for Galactic molecular clouds. We evaluate the X-factor for the 609
m carbon line, the analog of the CO X-factor, which is the ratio of the
H column density to the integrated CO(1-0) line intensity. We use
3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming
clouds. We compare the emission of CI and CO for model clouds irradiated by 1
and 10 times the average background and demonstrate that CI is a comparable or
superior tracer of the molecular gas distribution for column densities up to cm. Our results hold for both reduced and full chemical
networks. For our fiducial Galactic cloud we derive an average of
cmKkms and of cmKkms.Comment: 5 pages, 4 figures, 1 table, accepted to MNRAS Letter
Smoothed Particle Hydrodynamics simulations of expanding HII regions. I. Numerical methods and tests
We describe a new algorithm for including the dynamical effects of ionizing
radiation in SPH simulations, and we present several examples of how the
algorithm can be applied to problems in star formation. We use the HEALPix
software to tessellate the sky and to solve the equation of ionization
equilibrium along a ray towards each of the resulting tesserae. We exploit the
hierarchical nature of HEALPix to make the algorithm adaptive, so that fine
angular resolution is invoked only where it is needed, and the computational
cost is kept low. We present simulations of (i) the spherically symmetric
expansion of an HII region inside a uniform-density, non--self-gravitating
cloud; (ii) the spherically symmetric expansion of an HII region inside a
uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre
HII region inside a uniform-density, non--self-gravitating cloud, resulting in
rocket acceleration and dispersal of the cloud; and (iv) radiatively driven
compression and ablation of a core overrun by an HII region. The new algorithm
provides the means to explore and evaluate the role of ionizing radiation in
regulating the efficiency and statistics of star formation.Comment: 12 pages, 16 figures, simulation movies available at
http://galaxy.ig.cas.cz/~richard/HIIregion
The influence of the turbulent perturbation scale on prestellar core fragmentation and disk formation
The collapse of weakly turbulent prestellar 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 \equiv -d log Pk /d log
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 prestellar 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, {\lambda}_MAX. This is because {\lambda}_MAX
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
{\lambda}_MAX, and we explain this finding using a force balance analysis. We
also show that the core only has a high probability of fragmenting if
{\lambda}_MAX > 0.5 R_CORE (where R_CORE is the core radius); that the dominant
mode of fragmentation involves the formation and break-up of filaments; and
that, although small protostellar disks (with radius R_DISK <= 20 AU) form
routinely, more extended disks are rare. In turbulent, low-mass cores of the
type we simulate here, the formation of large, fragmenting protostellar disks
is suppressed by early fragmentation in the filaments.Comment: 11 pages, 7 figures; accepted for publication by MNRA
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 (Hubber et al. 2010) and the
HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We
find that the incident ionizing flux is the critical parameter determining the
evolution: high fluxes simply disperse the cloud, whereas low fluxes trigger
star formation. We find a clear connection between the intensity of the
incident flux and the parameters of star formation.Comment: 4 pages, 2 figures, conference proceedings, IAU Symposium 270 (eds.
Alves, Elmegreen, Girart, Trimble
ALMA observations of atomic carbon in z~4 dusty star-forming galaxies
We present ALMA [CI]() (rest frequency 492 GHz) observations for a
sample of 13 strongly-lensed dusty star-forming galaxies originally discovered
at 1.4mm in a blank-field survey by the South Pole Telescope. We compare these
new data with available [CI] observations from the literature, allowing a study
of the ISM properties of extreme dusty star-forming galaxies spanning
a redshift range . Using the [CI] line as a tracer of the molecular
ISM, we find a mean molecular gas mass for SPT-DSFGs of
M. This is in tension with gas masses derived via low- CO
and dust masses; bringing the estimates into accordance requires either (a) an
elevated CO-to-H conversion factor for our sample of and a gas-to-dust ratio , or (b) an high carbon abundance . Using observations of a range of additional atomic
and molecular lines (including [CI], [CII], and multiple transitions of CO), we
use a modern Photodissociation Region code (3D-PDR) to assess the physical
conditions (including the density, UV radiation field strength, and gas
temperature) within the ISM of the DSFGs in our sample. We find that the ISM
within our DSFGs is characterised by dense gas permeated by strong UV fields.
We note that previous efforts to characterise PDR regions in DSFGs may have
significantly underestimated the density of the ISM. Combined, our analysis
suggests that the ISM of extreme dusty starbursts at high redshift consists of
dense, carbon-rich gas not directly comparable to the ISM of starbursts in the
local Universe.Comment: 21 pages, 12 figures. Accepted for publication in MNRA
Observing gas and dust in simulations of star formation with Monte Carlo radiation transport on Voronoi meshes
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.Ionizing feedback from massive stars dramatically affects the interstellar medium local to star-forming regions. Numerical simulations are now starting to include enough complexity to produce morphologies and gas properties that are not too dissimilar from observations. The comparison between the density fields produced by hydrodynamical simulations and observations at given wavelengths relies however on photoionization/chemistry and radiative transfer calculations. We present here an implementation of Monte Carlo radiation transport through a Voronoi tessellation in the photoionization and dust radiative transfer code MOCASSIN. We show for the first time a synthetic spectrum and synthetic emission line maps of a hydrodynamical simulation of a molecular cloud affected by massive stellar feedback. We show that the approach on which previous work is based, which remapped hydrodynamical density fields on to Cartesian grids before performing radiative transfer/photoionization calculations, results in significant errors in the temperature and ionization structure of the region. Furthermore, we describe the mathematical process of tracing photon energy packets through a Voronoi tessellation, including optimizations, treating problematic cases and boundary conditions. We perform various benchmarks using both the original version of MOCASSIN and the modified version using the Voronoi tessellation. We show that for uniform grids, or equivalently a cubic lattice of cell generating points, the new Voronoi version gives the same results as the original Cartesian grid version of MOCASSIN for all benchmarks. For non-uniform initial conditions, such as using snapshots from smoothed particle hydrodynamics simulations, we show that the Voronoi version performs better than the Cartesian grid version, resulting in much better resolution in dense regions.Peer reviewe
PDF_CHEM: fast simulations of the chemical ISM using probability distributions
Interstellar matter and star formatio
Ionisation-induced star formation III: Effects of external triggering on the IMF in clusters
We report on Smoothed Particle Hydrodynamics (SPH) simulations of the impact
on a turbulent M star--forming molecular cloud of
irradiation by an external source of ionizing photons. We find that the
ionizing radiation has a significant effect on the gas morphology, but a less
important role in triggering stars. The rate and morphology of star formation
are largely governed by the structure in the gas generated by the turbulent
velocity field, and feedback has no discernible effect on the stellar initial
mass function. Although many young stars are to be found in dense gas located
near an ionization front, most of these objects also form when feedback is
absent. Ionization has a stronger effect in diffuse regions of the cloud by
sweeping up low--density gas that would not otherwise form stars into
gravitationally--unstable clumps. However, even in these regions, dynamical
interactions between the stars rapidly erase the correlations between their
positions and velocities and that of the ionization front.Comment: 12 pages, 16 figures (some downgraded to fit on astro-ph), accepted
for publication in MNRA
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