Massive star-forming regions with observed infall motions are good sites for studying the birth of massive stars. In this paper, 405 compact sources have been extracted from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) compact sources that also have been observed in the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey during Years 1 and 2. These observations are complemented with Spitzer GLIMPSE/MIPSGAL mid-IR survey data to help classify the elected starforming clumps into three evolutionary stages: pre-stellar, proto-stellar and UCHII regions. The results suggest that 0.05 g cm −2 is a reliable empirical lower bound for the clump surface densities required for massive-star formation to occur. The optically thick HCO + (1-0) and HNC(1-0) lines, as well as the optically thin N 2 H + (1-0) line were used to search for infall motions toward these sources. By analyzing the asymmetries of the optically thick HCO + (1-0) and HNC(1-0) lines and the mapping observations of HCO + (1-0), a total of 131 reliable infall candidates have been identified. The HCO + (1-0) line shows the highest occurrence of obvious asymmetric features, suggesting that it may be a better infall motion tracer than other lines such as HNC(1-0). The detection rates of infall candidates toward pre-stellar, proto-stellar and UCHII clumps are 0.3452, 0.3861 and 0.2152, respectively. The relatively high detection rate of infall candidates toward UCHII clumps indicates that many UCHII regions are still accreting matter. The peak column densities and masses of the infall candidates, in general, display a increasing trend with progressing evolutionary stages. However, the rough estimates of the mass infall rate show no obvious variation with evolutionary stage.

We present observations of the C-band 1 10 − 1 11 (4.8 GHz) and Ku-band 2 11 − 2 12 (14.5 GHz) K-doublet lines of H 2 CO and the C-band 1 10 − 1 11 (4.6 GHz) line of H 2 13 CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT). Our sample with 112 sources includes strong H 2 CO sources from the TMRT molecular line survey at C-band and other known H 2 CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C-and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H 2 12 CO/H 2 13 CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue's rotational mm-wave lines connecting our measured K-doublets, and to obtain 12 C/ 13 C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure (2013). Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12 C/ 13 C = (5.08±1.10)D GC + (11.86±6.60), with a correlation coefficient of 0.58. D GC refers to Galactocentric distances. Our 12 C/ 13 C ratios agree very well with the ones deduced from CN and C 18 O but are lower than those previously reported on the basis of H 2 CO, tending to suggest that the bulk of the H 2 CO in our sources was formed on dust grain mantles and not in the gas phase.

In this work, we aim to characterise high-mass clumps with infall motions. We selected 327 clumps from the Millimetre Astronomy Legacy Team 90-GHz (MALT90) survey, and identified 100 infall candidates. Combined with the results of He et al. (2015), we obtained a sample of 732 high-mass clumps, including 231 massive infall candidates and 501 clumps where infall is not detected. Objects in our sample were classified as pre-stellar, proto-stellar, HII or photo-dissociation region (PDR). The detection rates of the infall candidates in the pre-stellar, proto-stellar, HII and PDR stages are 41.2%, 36.6%, 30.6% and 12.7%, respectively. The infall candidates have a higher H 2 column density and volume density compared with the clumps where infall is not detected at every stage. For the infall candidates, the median values of the infall rates at the pre-stellar, proto-stellar, HII and PDR stages are 2.6×10 −3 , 7.0×10 −3 , 6.5×10 −3 and 5.5×10 −3 M yr −1 , respectively. These values indicate that infall candidates at later evolutionary stages are still accumulating material efficiently. It is interesting to find that both infall candidates and clumps where infall is not detected show a clear trend of increasing mass from the pre-stellar to proto-stellar, and to the HII stages. The power indices of the clump mass function (ClMF) are 2.04±0.16 and 2.17±0.31 for the infall candidates and clumps where infall is not detected, respectively, which agree well with the power index of the stellar initial mass function (2.35) and the cold Planck cores (2.0).

Aims. We seek to understand how the 4.8 GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H 2 CO, 12 CO, and 13 CO spectral lines. Methods. The four regions of MON R2 (60 × 90 ), S156 (50 × 70 ), DR17/L906 (40 × 60 ), and M17/M18 (70 × 80 ) were observed for H 2 CO (beam 10 ), H110α recombination (beam 10 ), 6 cm continuum (beam 10 ), 12 CO (beam 1 ), and 13 CO (beam 1 ). We compared the H 2 CO, 12 CO, 13 CO, and continuum distributions, and also the spectra line parameters of H 2 CO, 12 CO, and 13 CO. Column densities of H 2 CO, 13 CO, and H 2 were also estimated. Results. We found out that the H 2 CO distribution is similar to the 12 CO and the 13 CO distributions on a large scale. The correlation between the 13 CO and the H 2 CO distributions is better than between the 12 CO and H 2 CO distributions. The H 2 CO and the 13 CO tracers systematically provide consistent views of the dense regions. Their maps have similar shapes, sizes, peak positions, and molecular spectra and present similar central velocities and line widths. Such good agreement indicates that the H 2 CO and the 13 CO arise from similar regions.

Measurement of magnetic field strengths in a molecular cloud is essential for determining the criticality of magnetic support against gravitational collapse. In this paper, as part of the JCMT BISTRO survey, we suggest a new application of the Davis-Chandrasekhar-Fermi (DCF) method to estimate the distribution of magnetic field strengths in the OMC-1 region. We use observations of dust polarization emission at 450 and 850 μm, and C 18 O (3-2) spectral line data obtained with the JCMT. We estimate the volume density, the velocity dispersion, and the polarization angle dispersion in a box, 40″ × 40″ (5×5 pixels), which moves over the OMC-1 region. By substituting three quantities in each box with the DCF method, we get magnetic field strengths over the OMC-1 region. We note that there are very large uncertainties in the inferred field strengths, as discussed in detail in this paper. The field strengths vary from 0.8 to

Context. For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are strongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature. Aims. We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps. Methods. Three 218 GHz transitions (J K A K C = 3 03 -2 02 , 3 22 -2 21 , and 3 21 -2 20 ) of para-H 2 CO were observed with the 15m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H 2 CO 3 22 -2 21 /3 03 -2 02 and 3 21 -2 20 /3 03 -2 02 ratios. Results. The gas kinetic temperatures derived from the para-H 2 CO (3 21 -2 20 /3 03 -2 02 ) line ratios range from 30 to 61 K with an average of 46 ± 9 K. A comparison of kinetic temperature derived from para-H 2 CO, NH 3 , and the dust emission indicates that in many cases para-H 2 CO traces a similar kinetic temperature to the NH 3 (2,2)/(1,1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H 2 CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H 2 CO trace turbulence to a higher degree than NH 3 (2,2)/(1,1) in the massive clumps. The non-thermal velocity dispersions of para-H 2 CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.

We present 850 μm polarization observations of the L1689 molecular cloud, part of the nearby Ophiuchus molecular cloud complex, taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We observe three regions of L1689: the clump L1689N which houses the IRAS 16293-2433 protostellar system, the starless clump SMM-16, and the starless core L1689B. We use the Davis-Chandrasekhar-Fermi method to estimate plane-of-sky field strengths of 366±55 μG in L1689N, 284±34 μG in SMM-16, and 72±33 μG in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically transcritical with sub-Alfvénic turbulence. In all three regions, the inferred mean magnetic field direction is approximately perpendicular to the local filament direction identified in Herschel Space Telescope observations. The core-scale field morphologies for L1689N and L1689B are consistent with the cloud-scale field morphology measured by the Planck Space Observatory, suggesting that material can flow freely from large to small scales for these sources. Based on these magnetic field measurements, we posit that accretion from the cloud onto L1689N and L1689B may be magnetically regulated. However, in SMM-16, the clump-scale field is nearly perpendicular to the field seen on cloud scales by Planck, suggesting that it may be unable to efficiently accrete further material from its surroundings.

NGC 4945 is one of the nearest (D≈3.8 Mpc; 1 ′′ ≈ 19 pc) starburst galaxies. To investigate structure, dynamics, and composition of its dense nuclear gas, ALMA band 3 (λ≈3-4 mm) observations were carried out with ≈2 ′′ resolution. Measured were three HCN and two HCO + isotopologues, CS, C3H2, SiO, HCO, and CH3C2H. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10 ′′ ×2 ′′ reaching out to a galactocentric radius of r≈100 pc with position angle PA = 45 • ±2 • , inclination i = 75 • ±2 • and an unresolved bright central core of size < ∼ 2 ′′ . The continuum source, representing mostly free-free radiation from star forming regions, is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0 . ′′ 39 ± 0 . ′′ 14 northeast of the nuclear accretion disk defined by H2O maser emission. Near the systemic velocity but outside the nuclear disk, both HCN J=1→0 and CS J=2→1 delineate molecular arms of length > ∼ 15 ′′ ( > ∼ 285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ≈0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside (r < ∼ 100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojectd velocity of ≈50 km s −1 . All our molecular lines, with the notable exception of CH3C2H, show significant absorption near the systemic velocity (≈571 km s −1 ), within a range of ≈500 -660 km s −1 . Apparently, only molecular transitions with low critical H2-density (ncrit < ∼ 10 4 cm −3 ) do not show absorption. The velocity field of the nuclear disk, derived from CH3C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of Mtot = (2.1±0.2) × 10 8 M⊙ inside a galactocentric radius of 2 .′′ 45 (≈45 pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to ≈1 .′′ 5 (≈30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of 14 N/ 15 N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, with 15 N being less abundant than expected, the question for strong 15 N enrichment by massive star ejecta in starbursts still remains to be settled.