5,906 research outputs found
Our Knowledge of High-Mass Star Formation at the Dawn of Herschel
We review the theories and observations of high-mass star formation
emphasizing the differences with those of low-mass star formation. We hereafter
describe the progress expected to be achieved with Herschel, thanks notably to
Key Programmes dedicated to the earliest phases of high-mass star formation.Comment: 16 page
A multi-scale, multi-wavelength source extraction method: getsources
We present a multi-scale, multi-wavelength source extraction algorithm called
getsources. Although it has been designed primarily for use in the far-infrared
surveys of Galactic star-forming regions with Herschel, the method can be
applied to many other astronomical images. Instead of the traditional approach
of extracting sources in the observed images, the new method analyzes fine
spatial decompositions of original images across a wide range of scales and
across all wavebands. It cleans those single-scale images of noise and
background, and constructs wavelength-independent single-scale detection images
that preserve information in both spatial and wavelength dimensions. Sources
are detected in the combined detection images by following the evolution of
their segmentation masks across all spatial scales. Measurements of the source
properties are done in the original background-subtracted images at each
wavelength; the background is estimated by interpolation under the source
footprints and overlapping sources are deblended in an iterative procedure. In
addition to the main catalog of sources, various catalogs and images are
produced that aid scientific exploitation of the extraction results. We
illustrate the performance of getsources on Herschel images by extracting
sources in sub-fields of the Aquila and Rosette star-forming regions. The
source extraction code and validation images with a reference extraction
catalog are freely available.Comment: 31 pages, 27 figures, to be published in Astronomy & Astrophysic
The physics and modes of star cluster formation: simulations
We review progress in numerical simulations of star cluster formation. These
simulations involve the bottom-up assembly of clusters through hierarchical
mergers, which produces a fractal stellar distribution at young (~0.5 Myr)
ages. The resulting clusters are predicted to be mildly aspherical and highly
mass-segregated, except in the immediate aftermath of mergers. The upper
initial mass function within individual clusters is generally somewhat flatter
than for the aggregate population. Recent work has begun to clarify the factors
that control the mean stellar mass in a star-forming cloud and also the
efficiency of star formation. The former is sensitive to the thermal properties
of the gas while the latter depends both on the magnetic field and the initial
degree of gravitational boundedness of the natal cloud. Unmagnetized clouds
that are initially bound undergo rapid collapse, which is difficult to reverse
by ionization feedback or stellar winds.Comment: 21 pages, 10 figures. To appear as invited review article in a
special issue of the Phil. Trans. Royal Soc. A: Ch. 3 "Star clusters as
tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer
reviewed. LaTeX, requires rspublic.cls style fil
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Service user involvement in the evaluation of psycho-social intervention for self-harm: a systematic literature review
Background: The efficacy of interventions and treatments for self-harm is well researched. Previous reviews of the literature have highlighted the lack of definitively effective interventions for self-harm and have highlighted the need for future research. These recommendations are also reflected in clinical guidelines published by the National Institute for Health and Clinical Excellence (NICE, 2004) which also call for service user involvement in studies of treatment efficacy. Aims: A systematic review was undertaken to determine i) what contributions service users have made to the evaluation of psychosocial interventions ii) by what methods have service users been involved iii) in what ways could service user involvement supplement empirical evidence for interventions
Spiral arm triggering of star formation
We present numerical simulations of the passage of clumpy gas through a
galactic spiral shock, the subsequent formation of giant molecular clouds
(GMCs) and the triggering of star formation. The spiral shock forms dense
clouds while dissipating kinetic energy, producing regions that are locally
gravitationally bound and collapse to form stars. In addition to triggering the
star formation process, the clumpy gas passing through the shock naturally
generates the observed velocity dispersion size relation of molecular clouds.
In this scenario, the internal motions of GMCs need not be turbulent in nature.
The coupling of the clouds' internal kinematics to their externally triggered
formation removes the need for the clouds to be self-gravitating. Globally
unbound molecular clouds provides a simple explanation of the low efficiency of
star formation. While dense regions in the shock become bound and collapse to
form stars, the majority of the gas disperses as it leaves the spiral arm.Comment: 6 pages, 4 figures: IAU 237, Triggering of star formation in
turbulent molecular clouds, eds B. Elmegreen and J. Palou
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Making water privatisation illegal: new laws in Netherlands and Uruguay
Both Uruguay and Netherlands are legislating to make privatisation of water illegal. This report describes the processes and discusses the context of international laws
Gas dynamics in Massive Dense Cores in Cygnus-X
We study the kinematic properties of dense gas surrounding massive protostars
recognized by Bontemps et a. (2010) in a sample of five Massive Dense Cores in
Cygnus-X. We investigate whether turbulent support plays a major role in
stabilizing the core against fragmentation into Jeans-mass objects or
alternatively, the observed kinematics could indicate a high level of dynamics.
We present IRAM 30m single-dish (HCO+ and H13CO+) and IRAM PdBI high
angular-resolution observations of dense gas tracers (H13CO+ and H13CN) to
reveal the kinematics of molecular gas at scales from 0.03 to 0.1 pc. Radiative
transfer modeling shows that H13CO+ is depleted within the envelopes of massive
protostars and traces the bulk of material surrounding the protostars rather
than their inner envelopes. H13CN shows a better correspondence with the peak
of the continuum emission, possibly due to abundance anomalies and specific
chemistry in the close vicinity of massive protostars. Analyzing the
line-widths we show that the observed line-dispersion of H13CO+ at the scale of
MDCs is smaller than expected from the quasi-static, turbulent-core model. At
large-scales, global organized bulk motions are identified for 3 of the MDCs.
At small-scales, several spectral components are identified in all MDCs showing
filamentary structures and intrinsic velocity gradients towards the continuum
peaks. The dynamics of these flows show diversity among the sample and we link
this to the specific fragmentation properties of the MDCs. No clear evidence is
found for a turbulence regulated, equilibrium scenario within the sample of
MDCs. We propose a picture in which MDCs are not in equilibrium and their
dynamics is governed by small-scale converging flows, which may initiate
star-formation via their shears
A minimum hypothesis explanation for an IMF with a lognormal body and power law tail
We present a minimum hypothesis model for an IMF that resembles a lognormal
distribution at low masses but has a distinct power-law tail. Even if the
central limit theorem ensures a lognormal distribution of condensation masses
at birth, a power-law tail in the distribution arises due to accretion from the
ambient cloud, coupled with a non-uniform (exponential) distribution of
accretion times.Comment: 2 pages, 1 figure, to appear in IMF@50, eds. E. Corbelli, F. Palla,
and H. Zinnecker, Kluwer, Astrophysics and Space Science Librar
Sur la conjecture de Malle et le problème de Grunwald
We contribute to the Malle conjecture on the number N (K, G, y) of finite Galois extensions E of some number field K of finite group G and of discriminant of norm |N K/Q (d E)| ≤ y. We prove the lower bound part of the conjecture for every group G and every number field K containing a certain number field K 0 depending on G : N (K, G, y) ≥ y α(G) for y 1 and some specific exponent α(G) depending on G. To achieve this goal, we start from a regular Galois extension F/K(T) that we specialize. We prove a strong version of the Hilbert Irreducibility Theorem which counts the number of specialized extensions F t0 /K and not only the specialization points t 0 , and which provides some control of |N K/Q (d Ft 0)|. We can also prescribe the local behaviour of the specialized extensions at some primes. Consequently, we deduce new results on the local-global Grunwald problem, in particular for some non-solvable groups G
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