Context. The formation processes and the evolutionary stages of high-mass stars are poorly understood compared to low-mass stars. Large-scale surveys are needed to provide an unbiased census of high ...column density sites that can potentially host precursors to high-mass stars. Aims. The ATLASGAL survey covers 420 sq. degree of the Galactic plane, between −80° < ℓ < +60° at 870 μm. Here we identify the population of embedded sources throughout the inner Galaxy. With this catalog we first investigate the general statistical properties of dust condensations in terms of their observed parameters, such as flux density and angular size. Then using mid-infrared surveys we aim to investigate their star formation activity and the Galactic distribution of star-forming and quiescent clumps. Our ultimate goal is to determine the statistical properties of quiescent and star-forming clumps within the Galaxy and to constrain the star formation processes. Methods. We optimized the source extraction method, referred to as MRE-GCL, for the ATLASGAL maps in order to generate a catalog of compact sources. This technique is based on multiscale filtering to remove extended emission from clouds to better determine the parameters corresponding to the embedded compact sources. In a second step we extracted the sources by fitting 2D Gaussians with the Gaussclumps algorithm. Results. We have identified in total 10861 compact submillimeter sources with fluxes above 5σ. Completeness tests show that this catalog is 97% complete above 5σ and >99% complete above 7σ. Correlating this sample of clumps with mid-infrared point source catalogs (MSX at 21.3 μm and WISE at 22 μm), we have determined a lower limit of 33% that is associated with embedded protostellar objects. We note that the proportion of clumps associated with mid-infrared sources increases with increasing flux density, achieving a rather constant fraction of ~75% of all clumps with fluxes over 5 Jy/beam being associated with star formation. Examining the source counts as a function of Galactic longitude, we are able to identify the most prominent star-forming regions in the Galaxy. Conclusions. We present here the compact source catalog of the full ATLASGAL survey and investigate their characteristic properties. From the fraction of the likely massive quiescent clumps (~25%), we estimate a formation time scale of ~ 7.5 ± 2.5 × 104 yr for the deeply embedded phase before the emergence of luminous young stellar objects. Such a short duration for the formation of high-mass stars in massive clumps clearly proves that the earliest phases have to be dynamic with supersonic motions.
By matching infrared-selected, massive young stellar objects (MYSOs) and compact H ii regions in the Red MSX Source survey to massive clumps found in the submillimetre ATLASGAL (APEX Telescope Large ...Area Survey of the Galaxy) survey, we have identified ∼1000 embedded young massive stars between 280° < ℓ < 350° and 10° < ℓ < 60° with | b | < 1
$_{.}^{\circ}$
5. Combined with an existing sample of radio-selected methanol masers and compact H ii regions, the result is a catalogue of ∼1700 massive stars embedded within ∼1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and H ii-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and H ii-region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main-sequence lifetimes in the most luminous sources.
Context. The formation of high mass stars and clusters occurs in giant molecular clouds. Objects in evolved stages of massive star formation such as protostars, hot molecular cores, and ultracompact ...HII regions have been studied in more detail than earlier, colder objects. Further progress thus requires the analysis of the time before massive protostellar objects can be probed by their infrared emission. With this in mind, the APEX Telescope Large Area Survey of the whole inner Galactic plane at 870 μm (ATLASGAL) has been carried out to provide a global view of cold dust and star formation at submillimetre wavelengths. Aims. We derive kinematic distances to a large sample of massive cold dust clumps from their measured line velocities. We estimate masses and sizes of ATLASGAL sources, for which the kinematic distance ambiguity is resolved. Methods. The ATLASGAL sample is divided into groups of sources, which are located close together, mostly within a radius of 2 pc, and have velocities in a similar range with a median velocity dispersion of ~1 km s-1. We use NH3, N2H+, and CS velocities to calculate near and far kinematic distances to those groups. Results. We obtain 296 groups of ATLASGAL sources in the first quadrant and 393 groups in the fourth quadrant, which are coherent in space and velocity. We analyse HI self-absorption and HI absorption to resolve the kinematic distance ambiguity to 689 complexes of submm clumps. They are associated with 12CO emission probing large-scale structure and 13CO (1–0) line as well as the 870 μm dust continuum on a smaller scale. We obtain a scale height of ~28 ± 2 pc and displacement below the Galactic midplane of ~−7 ± 1 pc. Within distances from 2 to 18 kpc ATLASGAL clumps have a broad range of gas masses with a median of 1050 M⊙ as well as a wide distribution of radii with a median of 0.4 pc. Their distribution in galactocentric radii is correlated with spiral arms. Conclusions. Using a statistically significant ATLASGAL sample we derive a power-law exponent of −2.2 ± 0.1 of the clump mass function. This is consistent with the slope derived for clusters and with that of the stellar initial mass function. Examining the power-law index for different galactocentric distances and various source samples shows that it is independent of environment and evolutionary phase. Fitting the mass-size relationship by a power law gives a slope of 1.76 ± 0.01 for cold sources such as IRDCs and warm clumps associated with HII regions.
Context. The APEX Telescope Large Area Survey: the GALaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 μm. It covers ±60° 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. Aims. 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. Methods. 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. Results. Most ATLASGAL clumps are cold with rotational temperatures from 10−30 K with a median of 17 K. They have a wide range of NH3 linewidths (1−7 km s-1) with 1.9 km s-1 as median, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities from 1014 to 1016 cm-2 (median of 2 × 1015 cm-2) with an NH3 abundance in the range of 5 to 30 × 10-8. ATLASGAL sources are massive, ≳100 M⊙, and a fraction of clumps with a broad linewidth is in virial equilibrium. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The comparison of the NH3 lines as high-density probes with the GRS 13CO emission as low-density envelope tracer yields broader linewidths for 13CO than for NH3. The small differences in derived clump velocities between NH3 (representing dense core material) and 13CO (representing more diffuse molecular cloud gas) suggests that the cores are essentially at rest relative to the surrounding giant molecular cloud. Conclusions. 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 μm 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.
Observations of interstellar dust are often used as a proxy for total gas column density NH. By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and ...2MASS, with accurate (opacity-corrected) H i column densities and newly published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ353, reddening E(B − V), and the total proton column density NH in the range (1-30) × 1020 cm−2, along sightlines with no molecular gas detections in emission. We derive an NH/E(B − V) ratio of (9.4 1.6) × 1021 cm−2 mag−1 for purely atomic sightlines at b > 5 ° , which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity 353 = τ353/NH, when moving from the low column density (NH < 5 × 1020 cm−2) to the moderate column density (NH > 5 × 1020 cm−2) regime, and suggest that this rise is due to the evolution of dust grains in the atomic interstellar medium. Failure to account for H i opacity can cause an additional apparent rise in 353 of the order of a further ∼20%. We estimate molecular hydrogen column densities N H 2 from our derived linear relations, and hence derive the OH/H2 abundance ratio of XOH ∼ 1 × 10−7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H 2 in the range N H 2 ∼ (0.1−10) × 1021 cm−2. This suggests that OH may be used as a reliable proxy for H2 in this range, which includes sightlines with both CO-dark and CO-bright gas.
The Columbia UniversityemdashUniversidad de Chile CO Survey of the southern Milky Way is used to separate the CO(1-0) emission of the fourth Galactic quadrant within the solar circle into its ...dominant components, giant molecular clouds (GMCs). After the subtraction of an axisymmetric model of the CO background emission in the inner southern Galaxy, 92 GMCs are identified, and for 87 of them the twofold distance ambiguity is solved. Their total molecular mass is M(H sub(2)) = 1.14 + or - 0.05 x 10 super(8) M sub(middot in circle), accounting for around 40% of the molecular mass estimated from an axisymmetric analysis of the H sub(2) volume density in the Galactic disk, M(H sub(2)) sub(disk) = 3.03 x 10 super(8) M sub(middot in circle). The large-scale spiral structure in the southern Galaxy, within the solar circle, is traced by the GMCs in our catalog; three spiral arm segments, the Centaurus, Norma, and 3 kpc expanding arm, are analyzed. After fitting a logarithmic spiral arm model to the arms, tangent directions at 310degrees, 330degrees, and 338degrees, respectively, are found, consistent with previous values from the literature. A complete CS(2-1) survey toward IRAS point-like sources with far-IR colors characteristic of ultracompact H II regions is used to estimate the massive star formation rate per unit H sub(2) mass (MSFR) and the massive star formation efficiency ( epsilon ) for GMCs. The average MSFR for GMCs is 0.41 + or - 0.06 L sub(middot in circle)/M sub(middot in circle), and for the most massive clouds in the Norma arm it is 0.58 + or - 0.09 L sub(middot in circle)/M sub(middot in circle). Massive star formation efficiencies of GMCs are, on average, 3% of their available molecular mass.
Context.
The edges of ionized (H
II
) regions are important sites for the formation of (high-mass) stars. Indeed, at least 30% of the Galactic high-mass-star formation is observed there. The ...radiative and compressive impact of the H
II
region could induce star formation at the border following different mechanisms such as the collect and collapse or the radiation-driven implosion (RDI) models and change their properties.
Aims.
We aim to study the properties of two zones located in the photo dissociation region (PDR) of the Galactic H
II
region RCW 120 and discuss them as a function of the physical conditions and young star contents found in both clumps.
Methods.
Using the APEX telescope, we mapped two regions of size 1.5′ × 1.5′ toward the most massive clump of RCW 120 hosting young massive sources and toward a clump showing a protrusion inside the H
II
region and hosting more evolved low-mass sources. The
12
CO (
J
= 3−2),
13
CO (
J
= 3−2) and C
18
O (
J
= 3−2) lines observed, together with
Herschel
data, are used to derive the properties and dynamics of these clumps. We discuss their relation with the hosted star formation.
Results.
Assuming local thermodynamic equilibrium, the increase of velocity dispersion and
T
ex
are found toward the center of the maps, where star-formation is observed with
Herschel
. Furthermore, both regions show supersonic Mach numbers (7 and 17 in average). No substantial information has been gathered about the impact of far ultraviolet radiation on C
18
O photodissociation at the edges of RCW 120. The fragmentation time needed for CC to be at work is equivalent to the dynamical age of RCW 120 and the properties of region B are in agreement with bright-rimmed clouds.
Conclusions.
Although conclusions from this fragmentation model should be taken with caution, it strengthens the fact that, together with evidence of compression, CC might be at work at the edges of RCW 120. Additionally, the clump located at the eastern part of the PDR is a good candidate pre-existing clump where star-formation may be induced by the RDI mechanism.
Context. The formation of high-mass stars remains unknown in many aspects. There are two competing families of models to explain the formation of high-mass stars. On the one hand, quasi-static models ...predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence. On the other hand, competitive accretion models predict that high-mass proto-stellar cores evolve from low or intermediate mass proto-stellar cores in dynamic environments. Aims. The aim of the present work is to bring observational constraints at the scale of high-mass cores (~0.03 pc). Methods. We targeted with ALMA and MOPRA a sample of nine starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study. Their mass and size (~110 M⊙ and r = 0.1 pc, respectively) are similar to the initial conditions used in the quasi-static family of models explaining for the formation of high-mass stars. We present ALMA 1.4 mm continuum observations that resolve the Jeans length (λJeans ~ 0.03 pc) and that are sensitive to the Jeans mass (MJeans ~ 0.65 M⊙) in the nine starless MDCs, together with ALMA-12CO(2–1) emission line observations. We also present HCO+(1–0), H13CO+(1–0) and N2H+(1–0) molecular lines from the MOPRA telescope for eight of the nine MDCs. Results. The nine starless MDCs have the mass reservoir to form high-mass stars according to the criteria by Baldeschi et al. (2017). Three of the starless MDCs are subvirialized with αvir ~ 0.35, and four MDCs show sign of collapse from their molecular emission lines. ALMA observations show very little fragmentation within the MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to <3 M⊙ fragments. Therefore, the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer. Conclusions. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC 6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.
Analyzing the kinematics of filamentary molecular clouds is a crucial step toward understanding their role in the star formation process. Therefore, we study the kinematics of 283 filament candidates ...in the inner Galaxy, that were previously identified in the ATLASGAL dust continuum data. The 13CO(2 – 1) and C18O(2 – 1) data of the SEDIGISM survey (Structure, Excitation, and Dynamics of the Inner Galactic Inter Stellar Medium) allows us to analyze the kinematics of these targets and to determine their physical properties at a resolution of 30′′ and 0.25 km s−1. To do so, we developed an automated algorithm to identify all velocity components along the line-of-sight correlated with the ATLASGAL dust emission, and derive size, mass, and kinematic properties for all velocity components. We find two-third of the filament candidates are coherent structures in position-position-velocity space. The remaining candidates appear to be the result of a superposition of two or three filamentary structures along the line-of-sight. At the resolution of the data, on average the filaments are in agreement with Plummer-like radial density profiles with a power-law exponent of p ≈ 1.5 ± 0.5, indicating that they are typically embedded in a molecular cloud and do not have a well-defined outer radius. Also, we find a correlation between the observed mass per unit length and the velocity dispersion of the filament of m ∝ σv2 $m \propto \sigma_{\rm{v}}^2$ m∝ σ v 2 . We show that this relation can be explained by a virial balance between self-gravity and pressure. Another possible explanation could be radial collapse of the filament, where we can exclude infall motions close to the free-fall velocity.
Context.
The formation processes of massive stars are still unclear, but a picture is emerging involving accretion disks and molecular outflows in what appears to be a scaled-up version of low-mass ...star formation. A census of outflow activity toward high-mass star-forming clumps in various evolutionary stages has the potential to shed light on high-mass star formation.
Aims.
We conducted an outflow survey toward ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) clumps using SEDIGISM (structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) data and aimed to obtain a large sample of clumps exhibiting outflow activity in different evolutionary stages.
Methods.
We identify the high-velocity wings of the
13
CO lines, which indicate outflow activity, toward ATLASGAL clumps by (1) extracting the simultaneously observed
13
CO (2–1) and C
18
O (2–1) spectra from SEDIGISM, and (2) subtracting Gaussian fits to the scaled C
18
O (core emission) from the
13
CO line after considering opacity broadening.
Results.
We detected high-velocity gas toward 1192 clumps out of a total sample of 2052, corresponding to an overall detection rate of 58%. Outflow activity has been detected in the earliest (apparently) quiescent clumps (i.e., 70 μm weak) to the most evolved H
II
region stages (i.e., 8 μm bright with tracers of massive star formation). The detection rate increases as a function of evolution (quiescent = 51%, protostellar = 47%, YSO = 57%, UC H
II
regions = 76%).
Conclusions.
Our sample is the largest outflow sample identified so far. The high detection rate from this large sample is consistent with the results of similar studies reported in the literature and supports the scenario that outflows are a ubiquitous feature of high-mass star formation. The lower detection rate in early evolutionary stages may be due to the fact that outflows in the early stages are weak and difficult to detect. We obtain a statistically significant sample of outflow clumps for every evolutionary stage, especially for outflow clumps in the earliest stage (i.e., 70 μm dark). The detections of outflows in the 70 μm dark clumps suggest that the absence of 70 μm emission is not a robust indicator of starless and/or pre-stellar cores.
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