N. Schneider

From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt Survey

We summarize the first results from the Gould Belt survey, obtained toward the Aquila Rift and Polaris Flare regions during the science demonstration phase of Herschel. Our 70-500 micron images taken in parallel mode with the SPIRE and PACS cameras reveal a wealth of filamentary structure, as well as numerous dense cores embedded in the filaments. Between ~ 350 and 500 prestellar cores and ~ 45-60 Class 0 protostars can be identified in the Aquila field, while ~ unbound starless cores and no protostars are observed in the Polaris field. The prestellar core mass function (CMF) derived for the Aquila region bears a strong resemblance to the stellar initial mass function (IMF), already confirming the close connection between the CMF and the IMF with much better statistics than earlier studies. Comparing and contrasting our Herschel results in Aquila and Polaris, we propose an observationally-driven scenario for core formation according to which complex networks of long, thin filaments form first within molecular clouds, and then the densest filaments fragment into a number of prestellar cores via gravitational instability.

ATLASGAL - The APEX telescope large area survey of the galaxy at 870 μm

Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explore s the southern sky at submillimeter wavelengths, both in continuum and in spectral line emission. Studying continuum emission from interstellar dust is essential to locate the highest densit y regions in the interstellar medium, and to derive their masses, column densities, density structures, and larger scale morpholog ies. In particular, the early stages of (massive) star forma tion are still quite mysterious: only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characteriz ed sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submillimeter wavelengths also represents a pioneering work in preparation for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 µm, with a beam of 19. ′′ 2. Taking advantage of its large field of view (11. ′ 4) and excellent sensitivity, we have started an unbiased survey of the whole Galactic Plane accessible to APEX, with a typical noise level of 50‐70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we have covered ∼95 deg 2 of the Galactic Plane. These data reveal∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the c ompact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact Hii regions or young embedded clusters, thus tracing more evolved stages after star formation has occurred. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M⊙. In this first introductory paper, we show preliminary resul ts from these ongoing observations, and discuss the mid- and long-term perspectives of the survey.

The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X

Aims. Our current knowledge of high-mass star formation is mainly based on follow-up studies of bright sources found by IRAS, and is thus biased against its earliest phases, inconspicuous at infrared wavelengths. We therefore started searching, in an unbiased way and in the closest high-mass star-forming complexes, for the high-mass analogs of low-mass pre-stellar cores and class 0 protostars. nMethods.We have made an extensive 1.2 mm continuum mosaicing study of the Cygnus X molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. The ~3°^(σ2) imaged areas cover all the high-column density (A_V ≥ 15 mag) clouds of this nearby (~1.7 kpc) cloud complex actively forming OB stars. We then compared our millimeter maps with mid-infrared images, and have made SiO(2-1) follow-up observations of the best candidate progenitors of high-mass stars. nResults. Our complete study of Cygnus X with ~0.09 pc resolution provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores (FWHM size ~0.1 pc, M_(1.2 mm) = 4-950 M_☉, volume-averaged density ~10^5 cm^(-3), among which ~42 are probable precursors of high-mass stars. A large fraction of the Cygnus X dense cores (2/3 of the sample) remain undetected by the MSX satellite, regardless of the mass range considered. Among the most massive (≥40 M_☉) cores, infrared-quiet objects are driving powerful outflows traced by SiO emission. Our study qualifies 17 cores as good candidates for hosting massive infrared-quiet protostars, while up to 25 cores potentially host high-luminosity infrared protostars. We fail to discover the high-mass analogs of pre-stellar dense cores (~0.1 pc, > 10^4 cm^-3)) in Cygnus X, but find several massive starless clumps (~0.8 pc, 7 × 10^3 cm^(-3)) that might be gravitationally bound. nConclusions. Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the infrared-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity infrared phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (~3 × 10^4 yr and < 10^3 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.

The Aquila prestellar core population revealed by Herschel

The origin and possible universality of the stellar initial mass function (IMF) is a major issue in astrophysics. One of the main objectives of the Herschel Gould Belt Survey is to clarify the link between the prestellar core mass function (CMF) and the IMF. We present and discuss the core mass function derived from Herschel data for the large population of prestellar cores discovered with SPIRE and PACS in the Aquila Rift cloud complex at d ~ 260 pc. We detect a total of 541 starless cores in the entire ~11 deg^2 area of the field imaged at 70-500 micron with SPIRE/PACS. Most of these cores appear to be gravitationally bound, and thus prestellar in nature. Our Herschel results confirm that the shape of the prestellar CMF resembles the stellar IMF, with much higher quality statistics than earlier submillimeter continuum ground-based surveys.

Cluster-formation in the Rosette molecular cloud at the junctions of filaments (Corrigendum)

For many years feedback processes generated by OB-stars in molecular clouds, including expanding ionization fronts, stellar winds, or UV-radiation, have been proposed to trigger subsequent star formation. However, hydrodynamic models including radiation and gravity show that UV-illumination has little or no impact on the global dynamical evolution of the cloud. The Rosette molecular cloud, irradiated by the NGC2244 cluster, is a template region for triggered star-formation, and we investigated its spatial and density structure by applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and probability density functions (PDFs) on Herschel column density maps, obtained within the HOBYS key program. The analysis reveals not only the filamentary structure of the cloud but also that all known infrared clusters except one lie at junctions of filaments, as predicted by turbulence simulations. The PDFs of sub-regions in the cloud show systematic differences. The two UV-exposed regions have a double-peaked PDF we interprete as caused by shock compression. The deviations of the PDF from the log-normal shape typically associated with low- and high-mass star-forming regions at Av~3-4m and 8-10m, respectively, are found here within the very same cloud. This shows that there is no fundamental difference in the density structure of low- and high-mass star-forming regions. We conclude that star-formation in Rosette - and probably in high-mass star-forming clouds in general - is not globally triggered by the impact of UV-radiation. Moreover, star formation takes place in filaments that arose from the primordial turbulent structure built up during the formation of the cloud. Clusters form at filament mergers, but star formation can be locally induced in the direct interaction zone between an expanding HII--region and the molecular cloud.

Dynamic star formation in the massive DR21 filament

The formation of massive stars is a highly complex process in which it is not clear whether the star-forming gas is in global gravitational collapse or in an equilibrium state, supported by turbulence. By studying one of the most massive and dense star-forming regions in the Galaxy at a distance of less than 3 kpc, the filament containing the well-known sources DR21 and DR21(OH), we expect to find observational signatures that allow to discriminate between the two views. We use molecular line data from our 13CO 1-0, CS 2-1, and N2H+ 1-0 survey of the Cygnus X region obtained with the FCRAO and high-angular resolution observations of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM 30m telescope. We observe a complex velocity field and velocity dispersion in the DR21 filament in which regions of highest column-density, i.e. dense cores, have a lower velocity dispersion than the surrounding gas and velocity gradients that are not (only) due to rotation. Infall signatures in optically thick line profiles of HCO+ and 12CO are observed along and across the whole DR21 filament. From modelling the observed spectra, we obtain a typical infall speed of 0.6 km/s and mass accretion rates of the order of a few 10^-3 Msun/yr for the two main clumps constituting the filament. These massive (4900 and 3300 Msun) clumps are both gravitationally contracting. All observed kinematic features in the DR21 filament can be explained if it is formed by the convergence of flows at large scales and is now in a state of global gravitational collapse. Whether this convergence of flows originated from self-gravity at larger scales or from other processes can not be settled with the present study. The observed velocity field and velocity dispersion are consistent with results from (magneto)-hydrodynamic simulations where the cores lie at the stagnation points of convergent turbulent flows.

Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites

We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 um, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the Centre-Ridge and South-Nest sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.

Filamentary structures and compact objects in the Aquila and Polaris clouds observed by Herschel

Our PACS and SPIRE images of the Aquila Rift and part of the Polaris Flare regions, taken during the science demonstration phase of Herschel discovered fascinating, omnipresent filamentary structures that appear to be physically related to compact cores. We briefly describe a new multi-scale, multi-wavelength source extraction method used to detect objects and measure their parameters in our Herschel images. All of the extracted starless cores (541 in Aquila and 302 in Polaris) appear to form in the long and very narrow filaments. With its combination of the far-IR resolution and sensitivity, Herschel directly reveals the filaments in which the dense cores are embedded; the filaments are resolved and have deconvolved widths of 35 arcsec in Aquila and 59 arcsec in Polaris (9000 AU in both regions). Our first results of observations with Herschel enable us to suggest that in general dense cores may originate in a process of fragmentation of complex networks of long, thin filaments, likely formed as a result of an interplay between gravity, interstellar turbulence, and magnetic fields. To unravel the roles of the processes, one has to obtain additional kinematic and polarization information; these follow-up observations are planned.

Initial highlights of the HOBYS key program, the Herschel imaging survey of OB young stellar objects

We present the initial highlights of the HOBYS key program, which are based on Herschel images of the Rosette molecular complex and maps of the RCW120 H ii region. Using both SPIRE at 250/350/500 μm and PACS at 70/160 μm or 100/160 μm, the HOBYS survey provides an unbiased and complete census of intermediate- to high-mass young stellar objects, some of which are not detected by Spitzer. Key core properties, such as bolometric luminosity and mass (as derived from spectral energy distributions), are used to constrain their evolutionary stages. We identify a handful of high-mass prestellar cores and show that their lifetimes could be shorter in the Rosette molecular complex than in nearby low-mass star-forming regions. We also quantify the impact of expanding H ii regions on the star formation process acting in both Rosette and RCW 120.

A new view of the Cygnus X region - KOSMA

Context. The Cygnusxa0X region is one of the richest star formation sites in the Galaxy. There is a long-standing discussion about whether the region is a chance superposition of several complexes along the line of sight or a single coherent complex at a distance of 1.5 to 2xa0kpc. Aims. Combining a 13 COxa02