Context. We model the dust and free-free continuum emission in the high-mass star-forming region Sagittarius B2. Aims. We want to reconstruct the 3D density and dust temperature distribution, as a crucial input to follow-up studies of the gas velocity field and molecular abundances. Methods. We employ the 3D radiative transfer program RADMC-3D to calculate the dust temperature self-consistently, providing a given initial density distribution. This density distribution of the entire cloud complex is then recursively reconstructed, based on available continuum maps, including both single-dish and high-resolution interferometric maps that cover a wide frequency range (ν = 40 GHz−4 THz). The model covers spatial scales from 45 pc down to 100 au, i.e., a spatial dynamic range of 10 5 . Results. We find that the density distribution of Sagittarius B2 can be reasonably well fitted by applying a superposition of spherical cores with Plummer-like density profiles. To reproduce the spectral energy distribution, we position Sgr B2(N) along the line of sight behind the plane containing Sgr B2(M). We find that the entire cloud complex comprises a total gas mass of 8.0 × 10 6 M within a diameter of 45 pc. This corresponds to an averaged gas density of 170 M pc −3 . We estimate stellar masses of 2400 M and 20 700 M and luminosities of 1.8 × 10 6 L and 1.2 × 10 7 L for Sgr B2(N) and Sgr B2(M), respectively. We report H 2 column densities of 2.9 × 10 24 cm −2 for Sgr B2(N) and 2.5 × 10 24 cm −2 for Sgr B2(M) in a 40 beam. For Sgr B2(S), we derive a stellar mass of 1100 M , a luminosity of 6.6 × 10 5 L , and an H 2 column density of 2.2 × 10 24 cm −2 in a 40 beam. We calculate a star formation efficiency of 5% for Sgr B2(N) and 50% for Sgr B2(M). This indicates that most of the gas content in Sgr B2(M) has already been converted to stars or dispersed.

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.

Context. High-mass stars form in clusters, but neither the early fragmentation processes nor the detailed physical processes leading to the most massive stars are well understood. Aims. We aim to understand the fragmentation, as well as the disk formation, outflow generation, and chemical processes during high-mass star formation on spatial scales of individual cores. Methods. Using the IRAM Northern Extended Millimeter Array (NOEMA) in combination with the 30 m telescope, we have observed in the IRAM large program CORE the 1.37 mm continuum and spectral line emission at high angular resolution (~0.4″) for a sample of 20 well-known high-mass star-forming regions with distances below 5.5 kpc and luminosities larger than 104 L⊙. Results. We present the overall survey scope, the selected sample, the observational setup, and the main goals of CORE. Scientifically, we concentrated on the mm continuum emission on scales on the order of 1000 AU. We detect strong mm continuum emission from all regions, mostly due to the emission from cold dust. The fragmentation properties of the sample are diverse. We see extremes where some regions are dominated by a single high-mass core whereas others fragment into as many as 20 cores. A minimum-spanning-tree analysis finds fragmentation at scales on the order of the thermal Jeans length or smaller suggesting that turbulent fragmentation is less important than thermal gravitational fragmentation. The diversity of highly fragmented vs. singular regions can be explained by varying initial density structures and/or different initial magnetic field strengths. Conclusions. A large sample of high-mass star-forming regions at high spatial resolution allows us to study the fragmentation properties of young cluster-forming regions. The smallest observed separations between cores are found around the angular resolution limit which indicates that further fragmentation likely takes place on even smaller spatial scales. The CORE project with its numerous spectral line detections will address a diverse set of important physical and chemical questions in the field of high-mass star formation.

We present the results of combined NH 3 (1,1) and (2,2) line emission observed with the Very Large Array and the Effelsberg 100 m telescope of the Infrared Dark Cloud G14.225-0.506. The NH 3 emission reveals a network of filaments constituting two hub-filament systems. Hubs are associated with gas of rotational temperature T rot ∼15 K, non-thermal velocity dispersion σ NT ∼1 km s −1 , and exhibit signs of star formation, while filaments appear to be more quiescent (T rot ∼11 K, σ NT ∼0.6 km s −1 ). Filaments are parallel in projection and distributed mainly along two directions, at PA∼10 • and 60 • , and appear to be coherent in velocity. The averaged projected separation between adjacent filaments is between 0.5 pc and 1 pc, and the mean width of filaments is 0.12 pc. Cores within filaments are separated by ∼0.33±0.09 pc, which is consistent with the predicted fragmentation of an isothermal gas cylinder due to the 'sausage'-type instability. The network of parallel filaments observed in G14.225-0.506 is consistent with the gravitational instability of a thin gas layer threaded by magnetic fields. Overall, our data suggest that magnetic fields might play an important role in the alignment of filaments, and polarization measurements in the entire cloud would lend further support to this scenario. Subject headings: stars: formation -ISM: clouds -ISM: individual objects (G14.225-0.506)

Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH + and discuss its relevance in the context of hydride absorption in diffuse gas with a low H 2 fraction. The confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with different H 2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. , and HF column densities promises to be a faithful tracer of the distribution of the H 2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD) simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH + molecule is a better tracer of almost purely atomic hydrogen gas than H itself, since H can also be present in gas with a significant molecular content, but ArH + singles out gas that is >99.9% atomic.

In order to shed light on the main physical processes controlling fragmentation of massive dense cores, we present a uniform study of the density structure of 19 massive dense cores, selected to be at similar evolutionary stages, for which their relative fragmentation level was assessed in a previous work. We inferred the density structure of the 19 cores through a simultaneous fit of the radial intensity profiles at 450 and 850 µm (or 1.2 mm in two cases) and the Spectral Energy Distribution, assuming spherical symmetry and that the density and temperature of the cores decrease with radius following power-laws. Even though the estimated fragmentation level is strictly speaking a lower limit, its relative value is significant and several trends could be explored with our data. We find a weak (inverse) trend of fragmentation level and density power-law index, with steeper density profiles tending to show lower fragmentation, and vice versa. In addition, we find a trend of fragmentation increasing with density within a given radius, which arises from a combination of flat density profile and high central density and is consistent with Jeans fragmentation. We considered the effects of rotational-to-gravitational energy ratio, non-thermal velocity dispersion, and turbulence mode on the density structure of the cores, and found that compressive turbulence seems to yield higher central densities. Finally, a possible explanation for the origin of cores with concentrated density profiles, which are the cores showing no fragmentation, could be related with a strong magnetic field, consistent with the outcome of radiation magnetohydrodynamic simulations.

Context. In high-mass (≥7 M ) star formation (SF) studies, high-angular resolution is crucial for resolving individual protostellar outflows (and possibly accretion disks) from the complex contribution of nearby (high-and low-mass) young stellar objects (YSO). Previous interferometric studies have focused mainly on single objects. Aims. A sensitive survey at high angular resolution is required to investigate outflow processes in a statistically significant sample of high-mass YSOs and on spatial scales relevant to testing theories. Methods. We selected a sample of 40 high-mass YSOs from water masers observed within the BeSSeL Survey. We investigated the 3D velocity and spatial structures of the molecular component of massive outflows at milli-arcsecond angular resolution using multi-epoch Very Long Baseline Array (VLBA) observations of 22 GHz water masers. We also characterize the ionized component of the flows using deep images of the radio continuum emission with resolutions of ∼0. 2, at 6, 13, and 22 GHz with the Jansky Very Large Array (JVLA). Results. We report the first results obtained for a subset of 11 objects from the sample. The water maser measurements provide us with a very accurate description of the molecular gas kinematics. This in turn enables us to estimate the momentum rate of individual outflows, varying in the range 10 −3 -10 0 M yr −1 km s −1 , among the highest values reported in the literature. In all the observed objects, the continuum emission at 13 and 22 GHz has a compact structure, with its position coincident with that of the water masers. The 6 GHz continuum consists of either compact components (mostly well aligned with the 13 and/or 22 GHz sources) or extended emission (either highly elongated or approximately spherical), which can be offset by up to a few arcseconds from the water masers. The unresolved continuum emission associated with the water masers likely points to the YSO location. The comparison of the radio continuum morphology to the maser spatial and 3D velocity distribution shows that five out of eleven high-mass YSOs emit a collimated outflow, with a flow semi-opening angle in the range 10• -30• . The remaining six sources present a more complicated relationship between the geometry of the radio continuum and water maser velocity pattern; therefore, no firm conclusions can be drawn regarding their outflow structure. In two sources, the 6 GHz continuum emission shows a highly elongated structure with a negative spectral index down to −1.2. We interpret this finding in terms of synchrotron emission from relativistic electrons accelerated in strong shocks, which indicates that non-thermal continuum emission could be common in high-mass protostellar jets. The Lyman continua derived from bolometric luminosities always exceed those obtained from the radio luminosities. Conclusions. These first results suggest that collimated outflows or jets can be common in high-mass YSOs and, in a couple of cases, provide hints that magnetic fields could be important in driving ...

Context. An ever growing number of observational and theoretical evidence suggests that the deuterated fraction (column density ratio between a species containing D and its hydrogenated counterpart, D frac ) is an evolutionary indicator both in the low-and the highmass star formation process. However, the role of surface chemistry in these studies has not been quantified from an observational point of view. Aims. Because many abundant species, such as NH 3 , H 2 CO, and CH 3 OH, are actively produced on ice mantles of dust grains during the early cold phases, their D frac is expected to evolve differently from species formed only (or predominantly) in the gas, such as N 2 H + , HNC, HCN, and their deuterated isotopologues. The differences are expected to be relevant especially after the protostellar birth, in which the temperature rises, causing the evaporation of ice mantles. Methods. To compare how the deuterated fractions of species formed only in the gas and partially or uniquely on grain surfaces evolve with time, we observed rotational transitions of CH 3 OH, 13 CH 3 OH, CH 2 DOH, and CH 3 OD at 3 mm and 1.3 mm, of NH 2 D at 3 mm with the IRAM-30 m telescope, and the inversion transitions (1, 1) and (2, 2) of NH 3 with the GBT, towards most of the cores already observed in N 2 H + , N 2 D + , HNC, and DNC. Results. NH 2 D is detected in all but two cores, regardless of the evolutionary stage. D frac (NH 3 ) is on average above 0.1 and does not change significantly from the earliest to the most evolved phases, although the highest average value is found in the protostellar phase (∼0.3). Few lines of CH 2 DOH and CH 3 OD are clearly detected, and then only towards protostellar cores or externally heated starless cores. In quiescent starless cores, we have only one doubtful detection of CH 2 DOH. Conclusions. This work clearly confirms an expected different evolutionary trend of the species formed exclusively in the gas (N 2 D + and N 2 H + ) and those formed partially (NH 2 D and NH 3 ) or totally (CH 2 DOH and CH 3 OH) on grain mantles. It also reinforces the idea that D frac (N 2 H + ) is the best tracer of massive starless cores, while high values of D frac (CH 3 OH) seem fairly good tracers of the early protostellar phases, where the evaporation or sputtering of the grain mantles is most efficient.