ABSTRACT
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 ∼0.9 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 N2H+ (1–0) and HN13C (1–0). We use the K-ladders for CH3C2H (5–4) and CH3CH (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 $\mu$m 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.
Radio continuum observations using the Australia telescope compact array at 5.5, 9.0, 17.0 and 22.8 GHz have detected free–free emission associated with 45 of 49 massive young stellar objects and ...H ii regions. Of these, 26 sources are classified as ionized jets (12 of which are candidates), 2 as ambiguous jets or disc winds, 1 as a disc-wind, 14 as H ii regions and 2 were unable to be categorized. Classification as ionized jets is based upon morphology, radio flux and spectral index, in conjunction with previous observational results at other wavelengths. Radio luminosity and momentum are found to scale with bolometric luminosity in the same way as low-mass jets, indicating a common mechanism for jet production across all masses. In 13 of the jets, we see associated non-thermal/optically thin lobes resulting from shocks either internal to the jet and/or at working surfaces. 10 jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of α = −0.55 consistent with Fermi acceleration in shocks. This shows that magnetic fields are present, in agreement with models of jet formation incorporating magnetic fields. Since the production of collimated radio jets is associated with accretion processes, the results presented in this paper support the picture of disc-mediated accretion for the formation of massive stars with an upper limit on the jet phase lasting approximately 6.5 × 104 yr. Typical mass-loss rates in the jet are found to be 1.4 × 10−5 M⊙ yr−1 with associated momentum rates of the order of (1–2) × 10−2 M⊙ km s−1 yr−1.
We have observed 99 mid-infrared-bright, massive young stellar objects and compact H ii regions drawn from the Red MSX source survey in the J = 3−2 transition of 12CO and 13CO, using the James Clerk ...Maxwell Telescope. 89 targets are within 6 kpc of the Sun, covering a representative range of luminosities and core masses. These constitute a relatively unbiased sample of bipolar molecular outflows associated with massive star formation. Of these, 59, 17 and 13 sources (66, 19 and 15 per cent) are found to have outflows, show some evidence of outflow, and have no evidence of outflow, respectively. The time-dependent parameters of the high-velocity molecular flows are calculated using a spatially variable dynamic time-scale. The canonical correlations between the outflow parameters and source luminosity are recovered and shown to scale with those of low-mass sources. For coeval star formation, we find the scaling is consistent with all the protostars in an embedded cluster providing the outflow force, with massive stars up to ∼30 M⊙ generating outflows. Taken at face value, the results support the model of a scaled-up version of the accretion-related outflow-generation mechanism associated with discs and jets in low-mass objects with time-averaged accretion rates of ∼10−3 M⊙ yr−1 on to the cores. However, we also suggest an alternative model, in which the molecular outflow dynamics are dominated by the entrained mass and are unrelated to the details of the acceleration mechanism. We find no evidence that outflows contribute significantly to the turbulent kinetic energy of the surrounding dense cores.
The Red MSX Source (RMS) survey has identified a sample of ∼1200 massive young stellar objects (MYSOs), compact and ultra-compact H ii regions from a sample of ∼2000 MSX and Two Micron All Sky Survey ...(2MASS) colour-selected sources. We have used the 100-m Green Bank Telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards ∼600 RMS sources located within the northern Galactic plane. We have identified 308 H2O masers which corresponds to an overall detection rate of ∼50 per cent. We find no significant difference in the detection rate for H ii regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity, we find the H2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity.
We detect ammonia emission towards 479 of these massive young stars, which corresponds to ∼80 per cent. Ammonia is an excellent probe of high-density gas allowing us to measure key parameters such as gas temperatures, opacities and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, full width at half-maximum linewidth and total NH3 column density for the sample are approximately 22 K, 2 km s−1 and 2 × 1015 cm−2, respectively. We find that the NH3 (1,1) linewidth and kinetic temperature are correlated with luminosity, and finding no underlying dependence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the linewidth-clump mass relationship.
The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.
Context.
The Galactic plane has been observed extensively by a large number of Galactic plane surveys from infrared to radio wavelengths at an angular resolution below 40′′. However, a 21 cm line and ...continuum survey with comparable spatial resolution is lacking.
Aims.
The first half of THOR data (
l
= 14.0°−37.9°, and
l
= 47.1°−51.2°, |
b
|≤ 1.25°) has been published in our data release 1 paper. With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (10′′–40′′), including a new H
I
dataset for the whole THOR survey region (
l
= 14.0−67.4° and |
b
|≤ 1.25°). As we published the results of OH lines and continuum emission elsewhere, we concentrate on the H
I
analysis in this paper.
Methods.
With the
Karl G. Jansky
Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤40′′. At
L
Band, the WIDAR correlator at the VLA was set to cover the 21 cm H
I
line, four OH transitions, a series of H
nα
radio recombination lines (RRLs;
n
= 151 to 186), and eight 128 MHz-wide continuum spectral windows, simultaneously.
Results.
We publish all OH and RRL data from the C-configuration observations, and a new H
I
dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H
I
emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out, likely due to higher spatial and velocity crowding of structures at the positive velocities. Compared to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well, but with significant material in the inter-arm regions. With the C-configuration-only H
I
+continuum data, we produce an H
I
optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this
τ
map, we corrected the H
I
emission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H
I
mass with optical depth correction in the survey region is 4.7 × 10
8
M
⊙
, 31% more than the mass derived assuming the emission is optically thin. If we applied this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5 × 10
9
M
⊙
. Comparing the H
I
with existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions.
Conclusions.
The high-sensitivity and resolution THOR H
I
dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction issignificant for H
I
column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H
I
data provide the basis for high-angular-resolution studies of the interstellar medium in different phases.
ABSTRACT
We have used catalogues from several Galactic plane surveys and dedicated observations to investigate the relationship between various maser species and Galactic star-forming clumps, as ...identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. The maser transitions of interest are the 6.7 and 12.2-GHz methanol masers, 22.2-GHz water masers, and the masers emitting in the four ground-state hyperfine structure transitions of hydroxyl. We find clump association rates for the water, hydroxyl and methanol masers to be 56, 39, and 82 per cent, respectively, within the Galactic longitude range of 60○ > ℓ > −60○. We investigate the differences in physical parameters between maser associated clumps and the full ATLASGAL sample, and find that clumps coincident with maser emission are more compact with increased densities and luminosities. However, we find the physical conditions within the clumps are similar for the different maser species. A volume density threshold of n(H2) > 104.1 cm−3 for the 6.7-GHz methanol maser found in our previous study is shown to be consistent across for all maser species investigated. We find limits that are required for the production of maser emission to be 500 L⊙ and 6 M⊙, respectively. The evolutionary phase of maser associated clumps is investigated using the L/M ratio of clumps coincident with maser emission, and these have similar L/M ranges (∼100.2−102.7 L⊙/M⊙) regardless of the associated transitions. This implies that the conditions required for the production of maser emission only occur during a relatively narrow period during a star’s evolution. Lower limits of the statistical lifetimes for each maser species are derived, ranging from ∼0.4−2 × 104 yr and are in good agreement with the ‘straw man’ evolutionary model previously presented.
We present a detailed statistical study of massive star formation in the environment of 322 Spitzer mid-infrared bubbles by using the Red MSX Source (RMS) survey for massive young stellar objects ...(YSOs). Using a combination of simple surface density plots and a more sophisticated angular cross-correlation function analysis, we show that there is a statistically significant overdensity of RMS YSOs towards the bubbles. There is a clear peak in the surface density and angular cross-correlation function of YSOs projected against the rim of the bubbles. By investigating the autocorrelation function of the RMS YSOs, we show that this is not due to intrinsic clustering of the RMS YSO sample. RMS YSOs and Spitzer bubbles are essentially uncorrelated with each other beyond a normalized angular distance of two bubble radii. The bubbles associated with RMS YSOs tend to be both smaller and thinner than those that are not associated with YSOs. We interpret this tendency to be due to an age effect, with YSOs being preferentially found around smaller and younger bubbles. We find no evidence to suggest that the YSOs associated with the bubbles are any more luminous than the rest of the RMS YSO population, which suggests that the triggering process does not produce a top-heavy luminosity function or initial mass function. We suggest that it is likely that the YSOs were triggered by the expansion of the bubbles and estimate that the fraction of massive stars in the Milky Way formed by this process could be between 14 and 30 per cent.
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.
We present first results of the H2O Southern Galactic Plane Survey (HOPS), using the Mopra Radio Telescope with a broad-band backend and a beam size of about 2 arcmin. We have observed 100 deg2 of ...the southern Galactic plane at 12 mm (19.5-27.5 GHz), including spectral line emission from H2O masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and H2O maser emission. We find 540 H2O masers, of which 334 are new detections. The strongest maser is 3933 Jy and the weakest is 0.7 Jy, with 62 masers over 100 Jy. In 14 maser sites, the spread in the velocity of the H2O maser emission exceeds 100 km s−1. In one region, the H2O maser velocities are separated by 351.3 km s−1. The rms noise levels are typically between 1 and 2 Jy, with 95 per cent of the survey under 2 Jy. We estimate completeness limits of 98 per cent at around 8.4 Jy and 50 per cent at around 5.5 Jy. We estimate that there are between 800 and 1500 H2O masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH3 (11,9) and (8,6) emission towards G19.61−0.23 and in the NH3 (3,3) line towards G23.33−0.30.
The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the ...northern GLIMPSE region (10degrees < l < 65degrees), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1".5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam super(-1). Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14" and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7sigma detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.
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