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

Jeux mortels

Le formalisme ludique de la Disparition , de Perec, roman de trois cents pages composé sans la lettre E , cache un jeu mortel. Car le livre encrypte un discours d'urgence, dont les termes principaux sont la privation, le manque et la mort. Au terme d'une telle lecture, le littéraire en vient à figurer iconiquement le littéral : ce que transcrit le roman, finalement, c'est l'itinéraire écrit d'une survie.Behind and beyond the formalist games of Georges Perec's la Disparition, a three hundred-page novel written without the letter E, a far more deadly game becomes apparent. For la Disparition encodes an urgent discourse on privation, loss, and death. In the course of such a reading, the literary comes iconically to subsume the literal: what the novel transcribes, in the final analysis, is a written itinerary of survival

Jeux mortels

Le formalisme ludique de la Disparition , de Perec, roman de trois cents pages composé sans la lettre E , cache un jeu mortel. Car le livre encrypte un discours d'urgence, dont les termes principaux sont la privation, le manque et la mort. Au terme d'une telle lecture, le littéraire en vient à figurer iconiquement le littéral : ce que transcrit le roman, finalement, c'est l'itinéraire écrit d'une survie.Behind and beyond the formalist games of Georges Perec's la Disparition, a three hundred-page novel written without the letter E, a far more deadly game becomes apparent. For la Disparition encodes an urgent discourse on privation, loss, and death. In the course of such a reading, the literary comes iconically to subsume the literal: what the novel transcribes, in the final analysis, is a written itinerary of survival

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

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

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

The circumstellar environment of low-mass protostars

We present a complete 1.3 mm continuum mapping survey of the embedded young stellar objects (YSOs) in the Taurus molecular cloud. We have also imaged several isolated Bok globules, as well as protostellar objects in the Perseus cluster. Our maps, taken with the IRAM 30 m telescope and the MPIfR bolometer arrays, are sensitive to the column density structure of the sources on spatial scales ranging from 1 500-5 000 AU to > 15 000-50 000 AU. For the protostellar envelopes mapped in Taurus, the results are roughly consistent with the predictions of the self-similar inside-out collapse model of Shu and collaborators. The envelopes observed in Bok globules are also qualitatively consistent with these predictions, providing the effects of magnetic pressure are included in the model. By contrast, the envelopes of Class 0 protostars in Perseus have finite radii ≾ 10 000 AU and are a factor of 3 to 12 denser than is predicted by the standard model. In cluster-forming regions, individual protostellar collapse thus appears to be induced in compact condensations resembling more finite-sized Bonnor-Ebert condensations than singular isothermal spheres. Accordingly, the beginning of protostellar evolution is suggested to be more violent, with larger accretion rates, in protoclusters compared to regions of distributed star formation like Taurus. Follow-up line observations of the envelopes' velocity fields are required to confirm this suggestion. We also find that roughly half of the Class I infrared sources of Taurus are either at the very end of the main accretion phase or already in the pre-main sequence phase. These sources are surrounded by only remnant, finite-sized envelopes (Menv4200 AU ≾ 0.01 M⊙ and Rout ≾ 1 500 AU). Lastly, our 1.3 mm continuum images reveal the presence of new candidate pre-stellar condensations and/or Class 0 protostars in the close environment of 8 Taurus Class I YSOs, 2 Bok globules, and 3 Perseus protostars

Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey

The APEX Telescope Large Area Survey: The Galaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 \mu m. It covers +/- 60 deg in Galactic longitude and aims to find all massive clumps at various stages of high-mass star formation in the inner Galaxy, particularly the earliest evolutionary phases. We aim to determine properties such as the gas kinetic temperature and dynamics of new massive cold clumps found by ATLASGAL. Most importantly, we derived their kinematical distances from the measured line velocities. We observed the ammonia (J,K) = (1,1) to (3,3) inversion transitions toward 862 clumps of a flux-limited sample of submm clumps detected by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey (GRS). We determined distances for a subsample located at the tangential points (71 sources) and for 277 clumps whose near/far distance ambiguity is resolved. Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K. They have a wide range of NH3 linewidths, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities with an NH3 abundance in the range of 5 to 30 * 10^{-8}. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The high detection rate (87%) confirms ammonia as an excellent probe of the molecular content of the massive, cold clumps revealed by ATLASGAL. A clear trend of increasing rotational temperatures and linewidths with evolutionary stage is seen for source samples ranging from 24 \mu m dark clumps to clumps with embedded HII regions. The survey provides the largest ammonia sample of high-mass star forming clumps and thus presents an important repository for the characterization of statistical properties of the clumps and the selection of subsamples for detailed, high-resolution follow-up studies

Cluster Formation in Protostellar Outflow-Driven Turbulence

Most, perhaps all, stars go through a phase of vigorous outflow during formation. We examine, through 3D MHD simulation, the effects of protostellar outflows on cluster formation. We find that the initial turbulence in the cluster-forming region is quickly replaced by motions generated by outflows. The protostellar outflow-driven turbulence (``protostellar turbulence'' for short) can keep the region close to a virial equilibrium long after the initial turbulence has decayed away. We argue that there exist two types of turbulence in star-forming clouds: a primordial (or ``interstellar'') turbulence and a protostellar turbulence, with the former transformed into the latter mostly in embedded clusters such as NGC 1333. Since the majority of stars are thought to form in clusters, an implication is that the stellar initial mass function is determined to a large extent by the stars themselves, through outflows which individually limit the mass accretion onto forming stars and collectively shape the environments (density structure and velocity field) in which most cluster members form. We speculate that massive cluster-forming clumps supported by protostellar turbulence gradually evolve towards a highly centrally condensed ``pivotal'' state, culminating in rapid formation of massive stars in the densest part through accretion.Comment: 11 pages (aastex format), 2 figures submitted to ApJ

Environmental regulation of cloud and star formation in galactic bars

The strong time-dependence of the dynamics of galactic bars yields a complex and rapidly evolving distribution of dense gas and star forming regions. Although bars mainly host regions void of any star formation activity, their extremities can gather the physical conditions for the formation of molecular complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical simulation of a Milky Way-like galaxy, we probe these conditions to explore how and where bar (hydro-)dynamics favours the formation or destruction of molecular clouds and stars. The interplay between the kpc-scale dynamics (gas flows, shear) and the parsec-scale (turbulence) is key to this problem. We find a strong dichotomy between the leading and trailing sides of the bar, in term of cloud fragmentation and in the age distribution of the young stars. After orbiting along the bar edge, these young structures slow down at the extremities of the bar, where orbital crowding increases the probability of cloud-cloud collision. We find that such events increase the Mach number of the cloud, leading to an enhanced star formation efficiency and finally the formation of massive stellar associations, in a fashion similar to galaxy-galaxy interactions. We highlight the role of bar dynamics in decoupling young stars from the clouds in which they form, and discuss the implications on the injection of feedback into the interstellar medium, in particular in the context of galaxy formation.Comment: MNRAS accepte