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.
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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.
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With the GREAT receiver at the Stratospheric Observatory for Infrared Astronomy (SOFIA), nine massive molecular clumps have been observed in the ammonia 32+−22− line at 1.8 THz in a search for ...signatures of infall. The sources were selected from the ATLASGAL submillimeter dust continuum survey of our Galaxy. Clumps with high masses covering a range of evolutionary stages based on their infrared properties were chosen. The ammonia line was detected in all sources, leading to five new detections and one confirmation of a previous detection of redshifted absorption in front of their strong THz continuum as a probe of infall in the clumps. These detections include two clumps embedded in infrared dark clouds. The measured velocity shifts of the absorptions compared to optically thin C17O (3–2) emission are 0.3–2.8 km s-1, corresponding to fractions of 3% to 30% of the free-fall velocities of the clumps. The ammonia infall signature is compared with complementary data of different transitions of HCN, HNC, CS, and HCO+, which are often used to probe infall because of their blue-skewed line profiles. The best agreement with the ammonia results is found for the HCO+ (4–3) transitions, but the latter is still strongly blended with emission from associated outflows. This outflow signature is far less prominent in the THz ammonia lines, which confirms it as a powerful probe of infall in molecular clumps. Infall rates in the range from 0.3 to 16 × 10-3 M⊙/yr were derived with a tentative correlation with the virial parameters of the clumps. The new observations show that infall on clump scales is ubiquitous through a wide range of evolutionary stages, from L/M covering about ten to several hundreds.
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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.
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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. Newborn stars form within the localized, high density regions of molecular clouds. The sequence and rate at which stars form in dense clumps and the dependence on local and global ...environments are key factors in developing descriptions of stellar production in galaxies. Aims. We seek to observationally constrain the rate and latency of star formation in dense massive clumps that are distributed throughout the Galaxy and to compare these results to proposed prescriptions for stellar production. Methods. A sample of 24 μm-based Class I protostars are linked to dust clumps that are embedded within molecular clouds selected from the APEX Telescope Large Area Survey of the Galaxy. We determine the fraction of star-forming clumps, f∗, that imposes a constraint on the latency of star formation in units of a clump’s lifetime. Protostellar masses are estimated from models of circumstellar environments of young stellar objects from which star formation rates are derived. Physical properties of the clumps are calculated from 870 μm dust continuum emission and NH3 line emission. Results. Linear correlations are identified between the star formation rate surface density, ΣSFR, and the quantities ΣH2/τff and ΣH2/τcross, suggesting that star formation is regulated at the local scales of molecular clouds. The measured fraction of star forming clumps is 23%. Accounting for star formation within clumps that are excluded from our sample due to 24 μm saturation, this fraction can be as high as 31%, which is similar to previous results. Dense, massive clumps form primarily low mass (<1–2 M⊙) stars with emergent 24 μm fluxes below our sensitivity limit or are incapable of forming any stars for the initial 70% of their lifetimes. The low fraction of star forming clumps in the Galactic center relative to those located in the disk of the Milky Way is verified.
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ABSTRACT
ATLASGAL is an 870-µm dust survey of 420 deg2 the inner Galactic plane and has been used to identify ∼10 000 dense molecular clumps. Dedicated follow-up observations and complementary ...surveys are used to characterize the physical properties of these clumps, map their Galactic distribution, and investigate the evolutionary sequence for high-mass star formation. The analysis of the ATLASGAL data is ongoing: We present an up-to-date version of the catalogue. We have classified 5007 clumps into four evolutionary stages (quiescent, protostellar, young stellar objects and H ii regions) and find similar numbers of clumps in each stage, suggesting a similar lifetime. The luminosity-to-mass (Lbol/Mfwhm) ratio curve shows a smooth distribution with no significant kinks or discontinuities when compared to the mean values for evolutionary stages indicating that the star formation process is continuous and that the observational stages do not represent fundamentally different stages or changes in the physical mechanisms involved. We compare the evolutionary sample with other star formation tracers (methanol and water masers, extended green objects and molecular outflows) and find that the association rates with these increases as a function of evolutionary stage, confirming that our classification is reliable. This also reveals a high association rate between quiescent sources and molecular outflows, revealing that outflows are the earliest indication that star formation has begun and that star formation is already ongoing in many of the clumps that are dark even at 70 µm.
Context.
The Serpens filament, a prominent elongated structure in a relatively nearby molecular cloud, is believed to be at an early evolutionary stage, so studying its physical and chemical ...properties can shed light on filament formation and early evolution.
Aims.
The main goal is to address the physical and chemical properties as well as the dynamical state of the Serpens filament at a spatial resolution of ~0.07 pc and a spectral resolution of ≲0.1 km s
−1
.
Methods.
We performed
13
CO (1–0), C
18
O (1–0), C
17
O (1–0),
13
CO (2–1), C
18
O (2–1), and C
17
O (2–1) imaging observations toward the Serpens filament with the Institut de Radioastronomie Millimétrique 30-m and Atacama Pathfinder EXperiment telescopes.
Results.
Widespread narrow
13
CO (2–1) self-absorption is observed in this filament, causing the
13
CO morphology to be different from the filamentary structure traced by C
18
O and C
17
O. Our excitation analysis suggests that the opacities of C
18
O transitions become higher than unity in most regions, and this analysis confirms the presence of widespread CO depletion. Further we show that the local velocity gradients have a tendency to be perpendicular to the filament’s long axis in the outskirts and parallel to the large-scale magnetic field direction. The magnitudes of the local velocity gradients decrease toward the filament’s crest. The observed velocity structure can be a result of gravity-driven accretion flows. The isochronic evolutionary track of the C
18
O freeze-out process indicates the filament is young with an age of ≲2 Myr.
Conclusions.
We propose that the Serpens filament is a newly-formed slightly-supercritical structure which appears to be actively accreting material from its ambient gas.
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Context.
Understanding the details of the formation process of massive (i.e.
M
≳ 8–10
M
⊙
) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their ...entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging.
Aims.
We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process.
Methods.
We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH
3
CCH), acetonitrile (CH
3
CN), formaldehyde (H
2
CO), and methanol (CH
3
OH).
Results.
We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 10
3
to 10
5
AU during the warm-up phase. We succeed in reproducing the observed column densities of CH
3
CCH and CH
3
CN, but H
2
CO and CH
3
OH agree less with the observed values. The total (massive) star formation time is found to be ~5.2 × 10
5
yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: ~5 × 10
4
yr for 70-μm weak, ~1.2 × 10
5
yr for mid-IR weak, ~2.4 × 10
5
yr for mid-IR bright, and ~1.1 × 10
5
yr for HII-region phases.
Conclusions.
With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales.
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Aims. The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative ...analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. Methods. We combine our recent detections of C super(0) in these disks with observations of other carbon reservoirs (CO, C super(+), C sub(2) H) and gas-mass and warm-gas tracers (HD, O super(0)), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Results. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of < or =0.1 M sub(?). The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is, at most, moderately depleted compared to the interstellar medium, with C/H sub(gas)= (0.1?1.5) x 10 super(-4). HD 100546 itself is a lambda Bootis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with C/H sub(gas)= (0.2?5.0) x 10 super(-6) and C / O > 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed C/H sub(gas) in the outer disk of TW Hya and allows to make predictions for other systems.
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