107 research outputs found
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
Search for starless clumps in the ATLASGAL survey
In this study, we present an unbiased sample of the earliest stages of
massive star formation across 20 square-degree of the sky. Within the region
10deg < l < 20deg and |b| < 1deg, we search the ATLASGAL survey at 870 micron
for dense gas condensations. These clumps are carefully examined for
indications of ongoing star formation using YSOs from the GLIMPSE source
catalog as well as sources in the 24 micron MIPSGAL images, to search for
starless clumps. We calculate the column densities as well as the kinematic
distances and masses for sources where the v_lsr is known from spectroscopic
observations. Within the given region, we identify 210 starless clumps with
peak column densities > 1 x 10e23 cm^(-2). In particular, we identify potential
starless clumps on the other side of the Galaxy. The sizes of the clumps range
between 0.1 pc and 3 pc with masses between a few tens of solar masses up to
several ten thousands of solar masses. Most of them may form massive stars, but
in the 20 square-degree we only find 14 regions massive enough to form stars
more massive than 20 solar masses and 3 regions with the potential to form
stars more massive than 40 40 solar masses. The slope of the high-mass tail of
the clump mass function for clumps on the near side of the Galaxy is 2.2 and,
therefore, Salpeter-like. We estimate the lifetime of the most massive starless
clumps to be 60000 yr. The sample offers a uniform selection of starless
clumps. In the large area surveyed, we only find a few potential precursors of
stars in the excess of 40 solar masses. It appears that the lifetime of these
clumps is somewhat shorter than their free-fall times, although both values
agree within the errors. In addition, these are ideal objects for detailed
studies and follow-up observations.Comment: 15 pages plus appendix, in total 44 pages, accepted for publication
in Astronomy & Astrophysics, full tables will be added soo
ATLASGAL - towards a complete sample of massive star forming clumps
By matching infrared-selected, massive young stellar objects (MYSOs) and compact HII 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{ring operator} <lPeer reviewedFinal Accepted Versio
Survey of ortho-H2D+in high-mass star-forming regions
Context. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H3+ isotopologues (e.g. ortho-H2D+) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive. Aims. We present a large sample of o-H2D+ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. Methods. APEX observations of the ground-state transition of o-H2D+ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-Averaged abundances of o-H2D+ with respect to H2, X(o-H2D+), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. Results. We detect 16 sources in o-H2D+ and find clear correlations between X(o-H2D+) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-To-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H3+ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H2D+) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H13CO+, DCO+, and C17O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Conclusions. Our study presents the largest sample of o-H2D+ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H2D+ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence
ATLASGAL - towards a complete sample of massive star forming clumps
By matching infrared-selected, massive young stellar objects (MYSOs) and compact HII 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 {ring operator} < l < 350 {ring operator} and 10 {ring operator} < l < 60 {ring operator} with | b | < 1 {ring operator} . 5. Combined with an existing sample of radio-selected methanol masers and compact HII 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 HII-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 HII-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. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
LEGO-II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc
The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the
physical, kinematic, and chemical properties of extragalactic systems, yet exploiting these observations requires a detailed
understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (âŒ3 pc) resolution
observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (âŒ100 pc Ă 100 pc
at 11 kpc), which were taken as part of the âLEGOâ IRAM-30m large project. We find that the spatial extent or brightness of
the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must
be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e.
line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust
temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently
emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density
(1022 cmâ2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cmâ2;
<20 K). We determine XCO(1â0) ⌠0.3 Ă 1020 cmâ2 (K km sâ1)
â1, and αHCN(1â0) ⌠30 M (K km sâ1 pc2)
â1, which both differ
significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular
line emission
OGHReS: Large-scale filaments in the outer Galaxy
Filaments are a ubiquitous morphological feature of the molecular interstellar medium and are identified as sites of star formation. In recent years, more than 100 large-scale filaments (with a length > 10 pc) have been observed in the inner Milky Way. As they appear linked to Galactic dynamics, studying those structures represents an opportunity to link kiloparsec-scale phenomena to the physics of star formation, which operates on much smaller scales. In this letter, we use newly acquired Outer Galaxy High Resolution Survey (OGHReS) 12CO(2-1) data to demonstrate that a significant number of large-scale filaments are present in the outer Galaxy as well. The 37 filaments identified appear tightly associated with inter-arm regions. In addition, their masses and linear masses are, on average, one order of magnitude lower than similar-sized molecular filaments located in the inner Galaxy, showing that Milky Way dynamics is able to create very elongated features in spite of the lower gas supply in the Galactic outskirts
ATLASGAL - Molecular fingerprints of a sample of massive star-forming clumps
We have conducted a 3-mm molecular-line survey towards 570 high-mass star-forming clumps, using the Mopra telescope. The sample is selected from the 10 000 clumps identified by the ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) survey and includes all of the most important embedded evolutionary stages associated with massive star formation, classified into five distinct categories (quiescent, protostellar, young stellar objects, H ii regions, and photon-dominated regions). The observations were performed in broad-band mode with frequency coverage of 85.2-93.4 GHz and a velocity resolution of 1/40.9 km s '1, detecting emission from 26 different transitions. We find significant evolutionary trends in the detection rates, integrated line intensities, and abundances of many of the transitions and also identify a couple of molecules that appear to be invariant to changes in the dust temperature and evolutionary stage [N 2 H + (1-0) and HN 13 C (1-0)]. We use the K-ladders for CH 3 C 2 H (5-4) and CH 3 CH (5-4) to calculate the rotation temperatures and find around one-third of the quiescent clumps have rotation temperatures that suggest the presence of an internal heating source. These sources may constitute a population of very young protostellar objects that are still dark at 70 mum and suggest that the fraction of truly quiescent clumps may only be a few per cent. We also identify a number of line ratios that show a strong correlation with the evolutionary stage of the embedded objects and discuss their utility as diagnostic probes of evolution
SEDIGISM-ATLASGAL: Dense Gas Fraction and Star Formation Efficiency Across the Galactic Disk
By combining two surveys covering a large fraction of the molecular material in the Galactic disk we investigate the role the spiral arms play in the star formation process. We have matched clumps identified by ATLASGAL with their parental GMCs as identified by SEDIGISM, and use these giant molecular cloud (GMC) masses, the bolometric luminosities, and integrated clump masses obtained in a concurrent paper to estimate the dense gas fractions () and the instantaneous star forming efficiencies (i.e., ). We find that the molecular material associated with ATLASGAL clumps is concentrated in the spiral arms (âŒ60 per cent found within ±10 km s of an arm). We have searched for variations in the values of these physical parameters with respect to their proximity to the spiral arms, but find no evidence for any enhancement that might be attributable to the spiral arms. The combined results from a number of similar studies based on different surveys indicate that, while spiral-arm location plays a role in cloud formation and HI to H conversion, the subsequent star formation processes appear to depend more on local environment effects. This leads us to conclude that the enhanced star formation activity seen towards the spiral arms is the result of source crowding rather than the consequence of a any physical process
SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium
The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing. Aims. We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. © 2017 ESO
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