In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with δ < −30• , with the SEST antenna at millimeter wavelengths. The sample contains 142 Low sources and 93 High, which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of ∼0.5 pc, a mass of ∼320 M for a dust temperature T d = 30 K, an H 2 density of 9.5 × 10 5 cm −3 , and a surface density of 0.4 g cm −2 . The median values are 0.4 pc, 102 M , 4 × 10 4 cm −3 , and 0.14 g cm −2 , respectively. The mean value of the luminosity-to-mass ratio, L/M 99 L /M , suggests that the sources are in a young, pre-ultracompact Hii phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or pointlike, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is ∼3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above M ∼ 100 M is best fitted with a power-law dN/dM ∝ M −α with α = 2.1, consistent with the Salpeter (1955) stellar IMF, with α = 2.35. On the other hand, the mass function of clumps with masses 10 M < ∼ M < ∼ 120 M is better fitted with a power law of slope α = 1.5, more consistent with the mass function of molecular clouds derived from gas observations.

Context. Theory predicts, and observations confirm, that the column density ratio of a molecule containing D to its counterpart containing H can be used as an evolutionary tracer in the low-mass star formation process. Aims. Since it remains unclear if the high-mass star formation process is a scaled-up version of the low-mass one, we investigated whether the relation between deuteration and evolution can be applied to the high-mass regime. Methods. With the IRAM-30 m telescope, we observed rotational transitions of N 2 D + and N 2 H + and derived the deuterated fraction in 27 cores within massive star-forming regions understood to represent different evolutionary stages of the massive-star formation process.Results. The abundance of N 2 D + is higher at the pre-stellar/cluster stage, then drops during the formation of the protostellar object(s) as in the low-mass regime, remaining relatively constant during the ultra-compact HII region phase. The objects with the highest fractional abundance of N 2 D + are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N 2 D + is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Conclusions. Our results indicate that the N 2 D + -to-N 2 H + column density ratio can be used as an evolutionary indicator in both lowand high-mass star formation, and that the physical conditions influencing the abundance of deuterated species likely evolve similarly during the processes that lead to the formation of both low-and high-mass stars.

Context. In the low-mass regime, molecular cores have spatially resolved temperature and density profiles allowing a detailed study of their chemical properties. It is found that the gas-phase abundances of C-bearing molecules in cold starless cores rapidly decrease with increasing density. Here the molecules tend to stick to the grains, forming ice mantles. Aims. We study CO depletion in a large sample of massive clumps, and investigate its correlation with evolutionary stage and with the physical parameters of the sources. Moreover, we study the gradients in Methods. From the ATLASGAL 870 µm survey we selected 102 clumps, which have masses in the range ∼10 2 −3 × 10 4 M , sampling different evolutionary stages. We use low-J emission lines of CO isotopologues and the dust continuum emission to infer the depletion factor f D . RATRAN one-dimensional models were also used to determine f D and to investigate the presence of depletion above a density threshold. The isotopic ratios and optical depth were derived with a Bayesian approach. Results. We find a significant number of clumps with a high degree of CO depletion, up to ∼20. Larger values are found for colder clumps, thus for earlier evolutionary phases. For massive clumps in the earliest stages of evolution we estimate the radius of the region where CO depletion is important to be a few tenths of a pc. The value of the [ 12 C]/[ 13 C] ratio is found to increase with distance from the Galactic centre, with a value of ∼66 ± 12 for the solar neighbourhood. The [ 18 O]/[ 17 O] ratio is approximately constant (∼4) across the inner Galaxy between 2 kpc and 8 kpc, albeit with a large range (∼2−6). Clumps are found with total masses derived from dust continuum emission up to ∼20 times higher than M vir , especially among the less evolved sources. These large values may in part be explained by the presence of depletion: if the CO emission comes mainly from the low-density outer layers, the molecules may be subthermally excited, leading to an overestimate of the dust masses. Conclusions. CO depletion in high-mass clumps seems to behave as in the low-mass regime, with less evolved clumps showing larger values for the depletion than their more evolved counterparts, and increasing for denser sources.

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

How do stars that are more massive than the Sun form, and thus how is the stellar initial mass function (IMF) established? Such intermediate-and high-mass stars may be born from relatively massive pre-stellar gas cores, which are more massive than the thermal Jeans mass. The Turbulent Core Accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Their internal pressure is provided by a combination of turbulence and magnetic fields. Alternatively, the Competitive Accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate-and high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (∼ 100 M ) clumps from mid-infrared extinction mapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration fractions of N 2 H + in these objects, which are consistent with them being starless. Here we present ALMA observations of these four clumps that probe the N 2 D + (3-2) line at 2.3 resolution. We find six N 2 D + cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of self-gravitating, magnetized (with Alfvén Mach number m A ∼ 1) and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ∼ 60 M , our results suggest that moderately enhanced magnetic fields (so that m A 0.3) may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF.

The enormous radiative and mechanical luminosities of massive stars impact a vast range of scales and processes, from the reionization of the universe, to the evolution of galaxies, to the regulation of the interstellar medium, to the formation of star clusters, and even to the formation of planets around stars in such clusters. Two main classes of massive star formation theory are under active study, Core Accretion and Competitive Accretion. In Core Accretion, the initial conditions are self-gravitating, centrally concentrated cores that condense with a range of masses from the surrounding, fragmenting clump environment. They then undergo relatively ordered collapse via a central disk to form a single star or a small-N multiple. In this case, the pre-stellar core mass function has a similar form to the stellar initial mass function. In Competitive Accretion, the material that forms a massive star is drawn more chaotically from a wider region of the clump without passing through a phase of being in a massive, coherent core. In this case, massive star formation must proceed hand in hand with star cluster formation. If stellar densities become very high near the cluster center, then collisions between stars may also help to form the most massive stars. We review recent theoretical and observational progress towards understanding massive star formation, considering physical and chemical processes, comparisons with low and intermediate-mass stars, and connections to star cluster formation.Comment: Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin

Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial conditions of high-mass star and star cluster formation. We present high-sensitivity and high-angular resolution IRAM PdBI observations of N 2 H + (1 − 0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly complex environment, consisting of several mildly supersonic filaments (σ NT /c s ∼ 1.5), separated in velocity by < 1 km s −1 . Where multiple spectral components are evident, moment analysis overestimates the non-thermal contribution to the linewidth by a factor ∼ 2. Large-scale velocity gradients evident in previous single-dish maps may be explained by the presence of substructure now evident in the interferometric maps. Whilst global velocity gradients are small (< 0.7 km s −1 pc −1 ), there is evidence for dynamic processes on local scales (∼ 1.5-2.5 km s −1 pc −1 ). Systematic trends in velocity gradient are observed towards several continuum peaks. This suggests that the kinematics are influenced by dense (and in some cases, starless) cores. These trends are interpreted as either infalling material, with accretion rates ∼ (7 ± 4)×10 −5 M yr −1 , or expanding shells with momentum ∼ 24 ±12 M km s −1 . These observations highlight the importance of high-sensitivity and high-spectral resolution data in disentangling the complex kinematic and physical structure of massive star forming regions.

In order to study the fragmentation of massive dense cores, which constitute the cluster cradles, we observed with the PdBI in the most extended configuration the continuum at 1.3 mm and the CO (2-1) emission of four massive cores. We detect dust condensations down to ∼ 0.3 M ⊙ and separate millimeter sources down to 0.4 ′′ or 1000 AU, comparable to the sensitivities and separations reached in optical/infrared studies of clusters. The CO (2-1) high angular resolution images reveal high-velocity knots usually aligned with previously known outflow directions. This, in combination with additional cores from the literature observed at similar mass sensitivity and spatial resolution, allowed us to build a sample of 18 protoclusters with luminosities spanning 3 orders of magnitude. Among the 18 regions, ∼ 30% show no signs of fragmentation, while 50% split up into 4 millimeter sources. We compiled a list of properties for the 18 massive dense cores, such as bolometric luminosity, total mass, and mean density, and found no correlation of any of these parameters with the fragmentation level. In order to investigate the combined effects of magnetic field, radiative feedback and turbulence in the fragmentation process, we compared our observations to radiation magneto-hydrodynamic simulations, and obtained that the low-fragmented regions are well reproduced in the magnetized core case, while the highly-fragmented regions are consistent with cores where turbulence dominates over the magnetic field. Overall, our study suggests that the fragmentation in massive dense cores could be determined by the initial magnetic field/turbulence balance in each particular core.