45 research outputs found
Profiles of interstellar cloud filaments. Observational effects in synthetic sub-millimetre observations
Sub-millimetre observations suggest that the filaments of interstellar clouds
have rather uniform widths and can be described with the so-called Plummer
profiles. The shapes of the filament profiles are linked to their physical
state. Before drawing conclusions on the observed column density profiles, we
must evaluate the observational uncertainties. We want to estimate the bias
that could result from radiative transfer effects or from variations of submm
dust emissivity. We use cloud models obtained with magnetohydrodynamic
simulations and carry out radiative transfer calculations to produce maps of
sub-millimetre emission. Column densities are estimated based on the synthetic
observations. For selected filaments, the estimated profiles are compared to
those derived from the original column density. Possible effects from spatial
variations of dust properties are examined. With instrumental noise typical of
the Herschel observations, the parameters derived for nearby clouds are correct
to within a few percent. The radiative transfer effects have only a minor
effect on the results. If the signal-to-noise ratio is degraded by a factor of
four, the errors become significant and for half of the examined filaments the
values cannot be constrained. The errors increase in proportion to the cloud
distance. Assuming the resolution of Herschel instruments, the model filaments
are barely resolved at a distance of ~400 pc and the errors in the parameters
of the Plummer function are several tens of per cent. The Plummer parameters,
in particular the power-law exponent p, are sensitive to noise but can be
determined with good accuracy using Herschel data. One must be cautious about
possible line-of-sight confusion. In our models, a large fraction of the
filaments seen in the column density maps are not continuous structures in
three dimensions.Comment: 12 pages, 14 figures, accepted to A&
Aspect Ratio Dependence of the Free-Fall Time for Non-Spherical Symmetries
We investigate the collapse of non-spherical substructures, such as sheets
and filaments, which are ubiquitous in molecular clouds. Such non-spherical
substructures collapse homologously in their interiors but are influenced by an
edge effect that causes their edges to be preferentially accelerated. We
analytically compute the homologous collapse timescales of the interiors of
uniform-density, self-gravitating filaments and find that the homologous
collapse timescale scales linearly with the aspect ratio. The characteristic
timescale for an edge driven collapse mode in a filament, however, is shown to
have a square root dependence on the aspect ratio. For both filaments and
circular sheets, we find that selective edge acceleration becomes more
important with increasing aspect ratio. In general, we find that lower
dimensional objects and objects with larger aspect ratios have longer collapse
timescales. We show that estimates for star formation rates, based upon gas
densities, can be overestimated by an order of magnitude if the geometry of a
cloud is not taken into account.Comment: 10 pages, 2 figures, accepted by ApJ, minor grammatical errors fixe
Physical properties of interstellar filaments
We analyze the physical parameters of interstellar filaments that we describe
by an idealized model of isothermal self-gravitating infinite cylinder in
pressure equilibrium with the ambient medium. Their gravitational state is
characterized by the ratio f_cyl of their mass line density to the maximum
possible value for a cylinder in a vacuum. Equilibrium solutions exist only for
f_cyl < 1. This ratio is used in providing analytical expressions for the
central density, the radius, the profile of the column density, the column
density through the cloud centre, and the fwhm. The dependence of the physical
properties on external pressure and temperature is discussed and directly
compared to the case of pressure-confined isothermal self-gravitating spheres.
Comparison with recent observations of the fwhm and the central column
density N_H(0) show good agreement and suggest a filament temperature of ~10 K
and an external pressure p_ext/k in the range 1.5x10^4 K/cm^3 to 5x10^4 K/cm^3.
Stability considerations indicate that interstellar filaments become
increasingly gravitationally unstable with mass line ratio f_cyl approaching
unity. For intermediate f_cyl>0.5 the instabilities should promote core
formation through compression, with a separation of about five times the fwhm.
We discuss the nature of filaments with high mass line densities and their
relevance to gravitational fragmentation and star formation.Comment: 18 pages, 12 figures accepted for publication (13/4/2012
Gravitational collapse of polytropic, magnetized, filamentary clouds
When the gas of a magnetized filamentary cloud obeys a polytropic equation of
state, gravitational collapse of the cloud is studied using a simplified model.
We concentrate on the radial distribution and restrict ourselves to the purely
toroidal magnetic field. If the axial motions and poloidal magnetic fields are
sufficiently weak, we could reasonably expect our solutions to be a good
approximation. We show that while the filament experiences gravitational
condensation and the density at the center increases, the toroidal flux-to-mass
ratio remains constant. A series of spatial profiles of density, velocity and
magnetic field for several values of the toroidal flux-to-mass ratio and the
polytropic index, is obtained numerically and discussed.Comment: accepted by MNRA
LABOCA mapping of the infrared dark cloud MSXDC G304.74+01.32
Infrared dark clouds (IRDCs) likely represent very early stages of high-mass
star/star cluster formation. In this study, we aim to determine the physical
properties and spatial distribution of dense clumps in the IRDC MSXDC
G304.74+01.32 (G304.74), and bring these characteristics into relation to
theories concerning the origin of IRDCs and their fragmentation into clumps and
star-forming cores. G304.74 was mapped in the 870 m dust continuum with
the LABOCA bolometer on APEX. Archival MSX and IRAS infrared data were used to
study the nature and properties of the submillimetre clumps within the cloud.
There are 8 clumps within G304.74 which are not associated with mid-infrared
(MIR) emission. Some of them are candidates for being/harbouring high-mass
starless cores (HMSCs). We compared the clump masses and their spatial
distribution in G304.74 with those in several other recently studied IRDCs.
There is a high likelihood that the clump mass distributions in G304.74 and in
several other IRDCs represent the samples of the same parent distribution. In
most cases the spatial distributions of clumps in IRDCs do not deviate
significantly from random distributions. This is consistent with the idea that
the origin of IRDCs, and their further sub-fragmentation down to scales of
clumps is caused by supersonic turbulence in accordance with results from giant
molecular clouds.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Velocity field and star formation in the Horsehead nebula
Using large scale maps in C18O(2-1) and in the continuum at 1.2mm obtained at
the IRAM-30m antenna with the Heterodyne Receiver Array (HERA) and MAMBO2, we
investigated the morphology and the velocity field probed in the inner layers
of the Horsehead nebula. The data reveal a non--self-gravitating (m/mvir = 0.3)
filament of dust and gas (the "neck", diameter = 0.15-0.30 pc) connecting the
Horsehead western ridge, a Photon-Dominated Region illuminated by sigmaOri, to
its parental cloud L1630. Several dense cores are embedded in the ridge and the
neck. One of these cores appears particularly peaked in the 1.2 mm continuum
map and corresponds to a feature seen in absorption on ISO maps around 7 micr.
Its \cdo emission drops at the continuum peak, suggestive of molecular
depletion onto cold grains. The channel maps of the Horsehead exhibit an
overall north-east velocity gradient whose orientation swivels east-west,
showing a somewhat more complex structure than was recently reported by
\cite{pound03} using BIMA CO(1-0) mapping. In both the neck and the western
ridge, the material is rotating around an axis extending from the PDR to L1630
(angular velocity=1.5-4.0 km/s). Moreover, velocity gradients along the
filament appear to change sign regularly (3 km/s/pc, period=0.30 pc) at the
locations of embedded integrated intensity peaks. The nodes of this oscillation
are at the same velocity. Similar transverse cuts across the filament show a
sharp variation of the angular velocity in the area of the main dense core. The
data also suggest that differential rotation is occurring in parts of the
filament. We present a new scenario for the formation and evolution of the
nebula and discuss dense core formation inside the filament.Comment: 11 pages, 9 figures, A&
Star Clusters
This review concentrates almost entirely on globular star clusters. It
emphasises the increasing realisation that few of the traditional problems of
star cluster astronomy can be studied in isolation: the influence of the Galaxy
affects dynamical evolution deep in the core, and the spectrum of stellar
masses; in turn the evolution of the core determines the highest stellar
densities, and the rate of encounters. In this way external tidal effects
indirectly influence the formation and evolution of blue stragglers, binary
pulsars, X-ray sources, etc. More controversially, the stellar density appears
to influence the relative distribution of normal stars. In the opposite sense,
the evolution of individual stars governs much of the early dynamics of a
globular cluster, and the existence of large numbers of primordial binary stars
has changed important details of our picture of the dynamical evolution. New
computational tools which will become available in the next few years will help
dynamical theorists to address these questions.Comment: 10 pages, 3 figures, Te
An ammonia spectral map of the L1495-B218 filaments in the Taurus molecular cloud. I. Physical properties of filaments and dense cores
We present deep NH3 observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3° angular range using the K-band focal plane array on the 100 m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH3 (1, 1) and (2, 2) with a spectral resolution of 0.038 km s−1 and a spatial resolution of 31''. Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 and 500 μm. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8–15 K, velocity dispersions of 0.05–0.25 km s−1, and NH3 column densities of 5 × 1012 to 1 × 1014 cm−2. The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH3 structures, including 39 leaves and 16 branches. The masses of the NH3 sources range from 0.05 to 9.5 . The masses of NH3 leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests that these leaves are gravitationally unbound structures. Nine out of 39 NH3 leaves are gravitationally bound, and seven out of nine gravitationally bound NH3 leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar
Modes of Star Formation in Finite Molecular Clouds
We analytically investigate the modes of gravity-induced star formation
possible in idealized finite molecular clouds where global collapse competes
against both local Jeans instabilities and discontinuous edge instabilities. We
examine these timescales for collapse in spheres, discs, and cylinders, with
emphasis on the structure, size, and degree of internal perturbations required
in order for local collapse to occur before global collapse. We find that
internal, local collapse is more effective for the lower dimensional objects.
Spheres and discs, if unsupported against global collapse, must either contain
strong perturbations or must be unrealistically large in order for small
density perturbations to collapse significantly faster than the entire cloud.
We find, on the other hand, that filamentary geometry is the most favorable
situation for the smallest perturbations to grow before global collapse
overwhelms them and that filaments containing only a few Jeans masses and weak
density perturbations can readily fragment. These idealized solutions are
compared with simulations of star-forming regions in an attempt to delineate
the role of global, local, and edge instabilities in determining the
fragmentation properties of molecular clouds. The combined results are also
discussed in the context of recent observations of Galactic molecular clouds.Comment: 42 pages, 6 figures, 1 table, accepted by Ap
Dynamics of stellar black holes in young star clusters with different metallicities - I. Implications for X-ray binaries
We present N-body simulations of intermediate-mass (3000-4000 M-circle dot) young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z(circle dot)), including metal-dependent stellar evolution recipes and binary evolution. Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. In particular, massive metal-poor stars (Z = 0.3 Z(circle dot)) are enabled to form massive stellar black holes (MSBHs, with mass >= 25 M-circle dot) through direct collapse. We find that three-body encounters, and especially dynamical exchanges, dominate the evolution of the MSBHs formed in our simulations. In SCs with Z = 0.01 and 0.1 Z(circle dot), about 75 per cent of simulated MSBHs form from single stars and become members of binaries through dynamical exchanges in the first 100 Myr of the SC life. This is a factor of greater than or similar to 3 more efficient than in the case of low-mass (<25 M-circle dot) stellar black holes. A small but non-negligible fraction of MSBHs power wind-accreting (10-20 per cent) and Roche lobe overflow (RLO, 5-10 per cent) binary systems. The vast majority of MSBH binaries that undergo wind accretion and/or RLO were born from dynamical exchange. This result indicates that MSBHs can power X-ray binaries in low-metallicity young SCs, and is very promising to explain the association of many ultraluminous X-ray sources with low-metallicity and actively star-forming environments