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The stellar initial mass function (IMF) is critical to our understanding of star formation and the effects of young stars on their environment. On large scales, it enables us to use tracers such as UV or Halpha emission to estimate the star formation rate of a system and interpret unresolved star clusters across the universe. So far, there is little firm evidence of large-scale variations of the IMF, which is thus generally considered universal. Stars form from cores and it is now possible to estimate core masses and compare the core mass function (CMF) with the IMF, which it presumably produces. The goal of the ALMA-IMF large program is to measure the core mass function at high linear resolution (2700 au) in 15 typical Milky Way protoclusters spanning a mass range of 2500 to 32700 Msun. In this work, we used two different core extraction algorithms to extract about 680 gravitationally bound cores from these 15 protoclusters. We adopt per core temperature using the temperature estimate from the PPMAP Bayesian method. A power-law fit to the CMF of the sub-sample of cores above the 1.64 Msun completeness limit, 330 cores, through the maximum likelihood estimate technique yields a slope of 1.97 +/- 0.06, significantly flatter than the 2.35 Salpeter slope. Assuming a self-similar mapping between the CMF and the IMF, this result implies that these 15 high-mass protoclusters will generate atypical IMFs. This sample is the largest to date produced and analysed self-consistently, derived at matched physical resolution, with per-core temperature estimates and cores as massive as 150 Msun. We provide the raw source extraction catalogues and the source derived size, temperature, mass, and spectral indices in the 15 protoclusters., Comment: Accepted by A&A

A crucial aspect in addressing the challenge of measuring the core mass function, that is pivotal for comprehending the origin of the initial mass function, lies in constraining the temperatures of the cores. We aim to measure the luminosity, mass, column density and dust temperature of star-forming regions imaged by the ALMA-IMF large program. High angular resolution mapping is required to capture the properties of protostellar and pre-stellar cores and to effectively separate them from larger features, such as dusty filaments. We employed the point process mapping (PPMAP) technique, enabling us to perform spectral energy distribution fitting of far-infrared and submillimeter observations across the 15 ALMA-IMF fields, at an unmatched 2.5" angular resolution. By combining the modified blackbody model with near-infrared data, we derived bolometric luminosity maps. We estimated the errors impacting values of each pixel in the temperature, column density, and luminosity maps. Subsequently, we employed the extraction algorithm getsf on the luminosity maps in order to detect luminosity peaks and measure their associated masses. We obtained high-resolution constraints on the luminosity, dust temperature, and mass of protoclusters, that are in agreement with previously reported measurements made at a coarser angular resolution. We find that the luminosity-to-mass ratio correlates with the evolutionary stage of the studied regions, albeit with intra-region variability. We compiled a PPMAP source catalog of 313 luminosity peaks using getsf on the derived bolometric luminosity maps. The PPMAP source catalog provides constraints on the mass and luminosity of protostars and cores, although one source may encompass several objects. Finally, we compare the estimated luminosity-to-mass ratio of PPMAP sources with evolutionary tracks and discuss the limitations imposed by the 2.5" beam., Comment: 35 pages, 19 figures, 5 tables Accepted by A&A

We present significant improvements to our previous work on noise reduction in {\sl Herschel} observation maps by defining sparse filtering tools capable of handling, in a unified formalism, a significantly improved noise reduction as well as a deconvolution in order to reduce effects introduced by the limited instrumental response (beam). We implement greater flexibility by allowing a wider choice of parsimonious priors in the noise-reduction process. More precisely, we introduce a sparse filtering and deconvolution approach approach of type $l^2$-$l^p$, with $p > 0$ variable and apply it to a larger set of molecular clouds using {\sl Herschel} 250 $\mu $m data in order to demonstrate their wide range of application. In the {\sl Herschel} data, we are able to use this approach to highlight extremely fine filamentary structures and obtain singularity spectra that tend to show a significantly less $\log$-normal behavior and a filamentary nature in the less dense regions. We also use high-resolution adaptive magneto-hydrodynamic simulation data to assess the quality of deconvolution in such a simulated beaming framework., Comment: 24 pages, 32 figures, official acceptance date on 16/05/2024 (Astronomy & Astrophysics)

The ALMA-IMF Large Program provides multi-tracer observations of 15 Galactic massive protoclusters at matched sensitivity and spatial resolution. We focus on the dense gas kinematics of the G353.41 protocluster traced by N$_2$H$^+$ (1$-$0), with an spatial resolution $\sim$0.02 pc. G353.41, at a distance of $\sim$2 kpc, has a mass of $\sim$2500 M$_{\odot}$ within $1.3\times1.3$ pc$^2$. We extract the N$_2$H$^+$ isolated line component and we decompose it by fitting up to 3 Gaussian velocity components. This allows us to identify velocity structures that are impossible to identify in the traditional position-velocity diagram. We identify multiple velocity gradients on large ($\sim$1 pc) and small scales ($\sim$0.2 pc). We find good agreement between the N$_2$H$^+$ velocities and the previously reported DCN core velocities, suggesting that cores are kinematically coupled to the dense gas in which they form. We measure 9 converging ``V-shaped'' velocity gradients ($\sim20$ km/s/pc), located in filaments, which are sometimes associated with cores near their point of convergence. The average inflow timescale is $\sim67$ kyr, or about twice the free-fall time of cores in the same area ($\sim33$ kyr) but substantially shorter than protostar lifetime estimates ($\sim$0.5 Myr). We derive mass accretion rates in the range of $(0.35-8.77)\,\times\,10^{-4}$ M$_{\odot}$/yr. This feeding might lead to further filament collapse and formation of new cores. We suggest that the protocluster is collapsing on large scales, but the velocity signature of collapse is slow compared to pure free-fall. These data are consistent with a comparatively slow global protocluster contraction under gravity, and faster core formation within, suggesting the formation of multiple generations of stars over the protocluster lifetime., Comment: Accepted in A&A, 22 pages, 21 figures, 1 interactive figure, 4 tables

During the process of star formation, the dense gas undergoes significant chemical evolution leading to the emergence of a rich variety of molecules associated with hot cores and hot corinos. However, the physical and chemical conditions involved in this evolution are poorly constrained. We provide here a full inventory of the emission from complex organic molecules (COMs) to investigate the physical structure and chemical composition of six high-mass protostellar envelopes. We aim to investigate the conditions for the emergence of COMs in hot cores. We performed an unbiased spectral survey towards six infrared-quiet massive clumps between 159 GHz and 374 GHz with the APEX 12 m telescope. We detect up to 11 COMs, of which at least five COMs are detected towards all sources. Towards all the objects, most of the COM emission is found to be cold, with respect to the typical temperatures at which COMs are found, with a temperature of 30 K and extended with a size of ~0.3 pc. Although for our sample of young massive clumps the bulk of the gas has a cold temperature, we also detect emission from COMs originating from the immediate vicinity of the protostar revealing a compact and hot component of the envelope. Only three out of the six sources exhibit a hot gas component. We find a gradual emergence of the warm component in terms of size and temperature, together with an increasing molecular complexity, allowing us to establish an evolutionary sequence for our sample based on COMs. Our findings confirm that our sample of infrared-quiet massive clumps are in an early evolutionary stage during which the bulk of the gas is cold. The presence of COMs is found to be characteristic of these early evolutionary stages. We suggest that the emergence of hot cores is preceded by a phase in which mostly O-bearing COMs appear first with similar abundances to hot corinos albeit with larger source sizes., Comment: 36 pages, 18 figures, accepted for publication in A&A

Sites associated with high-mass star and cluster formation exhibit a so-called hot core phase, characterized by high temperatures and column densities of complex organic molecules. We built a comprehensive census of hot core candidates towards the ALMA-IMF protoclusters based on the detection of two CH3OCHO emission lines at 216.1 GHz. We used the source extraction algorithm GExt2D to identify peaks of methyl formate (CH3OCHO) emission that is a complex species commonly observed towards sites of star formation. We built up a catalog of 76 hot core candidates with masses ranging from about 0.2 to 80 Msun , of which 56 are new detections. A large majority of these objects are compact, rather circular, with deconvolved FWHM sizes of about 2300 au on average. About 30% of our sample of methyl formate sources have core masses above 8 Msun within sizes ranging from about 1000 au to 13400 au, which well correspond to archetypical hot cores. The origin of the CH3OCHO emission toward the lower-mass cores can be explained by a mixture of contribution from shocks, or may correspond to objects in a more evolved state, i.e. beyond the hot core stage. We find that the fraction of hot core candidates increases with the core mass. The large fraction of hot core candidates towards the most massive cores suggests that they rapidly enter the hot core phase and feedback effects from the forming protostar(s) impact their environment on short time-scales.

We aimed to measure the CMF in the evolved W33-Main star-forming protocluster to compare it with CMF recently obtained in other Galactic star-forming regions, including the ones included in the ALMA-IMF program. We used observations from the ALMA-IMF large program: 2'x2' maps of emission from the continuum and selected lines at 1.3mm and 3mm observed by the ALMA 12m only antennas. Our angular resolution was typically 1'', that is 2400au at a distance of 2.4kpc. The lines we analysed are CO(2-1), SiO(5-4), N2H+(1-0), H41alpha as well as He41alpha blended with C41alpha. We built a census of dense cores in the region, and we measured the associated CMF based on a core-dependent temperature value. We confirmed the 'evolved' status of W33-Main by identifiying three HII regions within the field, and to a lesser extent based on the number and extension of N2H+ filaments. We produced a filtered core catalog of 94 candidates, that we refined to take into account the contamination of the continuum by free-free and line emission, obtaining 80 cores with masses that range from 0.03 to 13.2Msun. We fitted the resulting high-mass end of the CMF with a single power law of the form N(log(M)) ~ M^alpha, obtaining alpha = -1.44(+0.16)(-0.22), slightly steeper but consistent with the Salpeter index. We categorized our cores in pre- and protostellar, mostly based on outlow activity and hot core nature. We found the prestellar CMF to be steeper than a Salpeter-like distribution, and the protostellar CMF to be slightly top heavy. We found a higher proportion of cores within the HII regions and their surroundings than in the rest of the field. We also found that the cores' masses were rather low (maximum mass of 13Msun).

We present a catalog of 315 protostellar outflow candidates detected in SiO J=5-4 in the ALMA-IMF Large Program, observed with ~2000 au spatial resolution, 0.339 km/s velocity resolution, and 2-12 mJy/beam (0.18-0.8 K) sensitivity. We find median outflow masses, momenta, and kinetic energies of ~0.3 M$_{\odot}$, 4 M$_{\odot}$ km/s, and 10$^{45}$ erg, respectively. Median outflow lifetimes are 6,000 years, yielding median mass, momentum, and energy rates of $\dot{M}$ = 10$^{-4.4}$ M$_{\odot}$ yr$^{-1}$, $\dot{P}$ = 10$^{-3.2}$ M$_{\odot}$ km/s yr$^{-1}$, and $\dot{E}$ = 1 L$_{\odot}$. We analyze these outflow properties in the aggregate in each field. We find correlations between field-aggregated SiO outflow properties and total mass in cores (~3$-$5$\sigma$), and no correlations above 3$\sigma$ with clump mass, clump luminosity, or clump luminosity-to-mass ratio. We perform a linear regression analysis and find that the correlation between field-aggregated outflow mass and total clump mass - which has been previously described in the literature - may actually be mediated by the relationship between outflow mass and total mass in cores. We also find that the most massive SiO outflow in each field is typically responsible for only 15-30% of the total outflow mass (60% upper limit). Our data agree well with the established mechanical force-bolometric luminosity relationship in the literature, and our data extend this relationship up to L $\geq$ 10$^6$ L$_{\odot}$ and $\dot{P}$ $\geq$ 1 M$_{\odot}$ km/s yr$^{-1}$. Our lack of correlation with clump L/M is inconsistent with models of protocluster formation in which all protostars start forming at the same time., Comment: 46 pages, 14 figures, 10 tables. This publication has an associated Zenodo entry, which can be found here: https://zenodo.org/records/8350595

ALMA-IMF is an Atacama Large Millimeter/submillimeter Array (ALMA) Large Program designed to measure the core mass function (CMF) of 15 protoclusters chosen to span their early evolutionary stages. It further aims to understand their kinematics, chemistry, and the impact of gas inflow, accretion, and dynamics on the CMF. We present here the first release of the ALMA-IMF line data cubes (DR1), produced from the combination of two ALMA 12m-array configurations. The data include 12 spectral windows, with eight at 1.3mm and four at 3mm. The broad spectral coverage of ALMA-IMF (~6.7 GHz bandwidth coverage per field) hosts a wealth of simple atomic, molecular, ionised, and complex organic molecular lines. We describe the line cube calibration done by ALMA and the subsequent calibration and imaging we performed. We discuss our choice of calibration parameters and optimisation of the cleaning parameters, and we demonstrate the utility and necessity of additional processing compared to the ALMA archive pipeline. As a demonstration of the scientific potential of these data, we present a first analysis of the DCN (3-2) line. We find that DCN traces a diversity of morphologies and complex velocity structures, which tend to be more filamentary and widespread in evolved regions and are more compact in the young and intermediate-stage protoclusters. Furthermore, we used the DCN (3-2) emission as a tracer of the gas associated with 595 continuum cores across the 15 protoclusters, providing the first estimates of the core systemic velocities and linewidths within the sample. We find that DCN (3-2) is detected towards a higher percentage of cores in evolved regions than the young and intermediate-stage protoclusters and is likely a more complete tracer of the core population in more evolved protoclusters. The full ALMA 12m-array cubes for the ALMA-IMF Large Program are provided with this DR1 release., Comment: 75 pages (21 main body; 54 appendix), 37 figures. The ALMA-IMF DR1 line release is hosted at https://dataverse.harvard.edu/dataverse/alma-imf

We present new $^{13}$CO(1-0), C$^{18}$O(1-0), HCO$^{+}$(1-0) and H$^{13}$CO$^{+}$(1-0) maps from the IRAM 30m telescope, and a spectrally-resolved [CII] 158 $\mu$m map observed with the SOFIA telescope towards the massive DR21 cloud. This traces the kinematics from low- to high-density gas in the cloud which allows to constrain the formation scenario of the high-mass star forming DR21 ridge. The molecular line data reveals that the sub-filaments are systematically redshifted relative to the dense ridge. We demonstrate that [CII] unveils the surrounding CO-poor gas of the dense filaments in the DR21 cloud. We also show that this surrounding gas is organized in a flattened cloud with curved redshifted dynamics perpendicular to the ridge. The sub-filaments thus form in this curved and flattened mass reservoir. A virial analysis of the different lines indicates that self-gravity should drive the evolution of the ridge and surrounding cloud. Combining all results we propose that bending of the magnetic field, due to the interaction with a mostly atomic colliding cloud, explains the velocity field and resulting mass accretion on the ridge. This is remarkably similar to what was found for at least two nearby low-mass filaments. We tentatively propose that this scenario might be a widespread mechanism to initiate star formation in the Milky Way. However, in contrast to low-mass clouds, gravitational collapse plays a role on the pc scale of the DR21 ridge because of the higher density. This allows more effective mass collection at the centers of collapse and should facilitate massive cluster formation., Comment: 32 pages, 28 figures, accepted in ApJ

The origin of the stellar initial mass function (IMF) and its relation with the core mass function (CMF) are actively debated issues with important implications in astrophysics. Recent observations in the W43 molecular complex of top-heavy CMFs, with an excess of high-mass cores compared to the canonical mass distribution, raise questions about our understanding of the star formation processes and their evolution in space and time. We aim to compare populations of protostellar and prestellar cores in three regions imaged in the ALMA-IMF Large Program. We created an homogeneous core catalogue in W43, combining a new core extraction in W43-MM1 with the catalogue of W43-MM2&MM3 presented in a previous work. Our detailed search for protostellar outflows enabled us to identify between 23 and 30 protostellar cores out of 127 cores in W43-MM1 and between 42 and 51 protostellar cores out of 205 cores in W43-MM2&MM3. Cores with neither outflows nor hot core emission are classified as prestellar candidates. We found a similar fraction of cores which are protostellar in the two regions, about 35%. This fraction strongly varies in mass, from 15-20% at low mass, between 0.8 and 3$M_{\odot} $ up to about 80% above 16$M_{\odot}$. Protostellar cores are found to be, on average, more massive and smaller in size than prestellar cores. Our analysis also revealed that the high-mass slope of the prestellar CMF in W43, $\alpha=-1.46_{-0.19}^{+0.12}$, is consistent with the Salpeter slope, and thus the top-heavy form measured for the global CMF, $\alpha=-0.96$, is due to the protostellar core population. Our results could be explained by clump-fed models in which cores grow in mass, especially during the protostellar phase, through inflow from their environment. The difference between the slopes of the prestellar and protostellar CMFs moreover implies that high-mass cores grow more in mass than low-mass cores., Comment: 28 pages, 18 figures. Accepted for publication in A&A. Title and appendices updated

Among the most central open questions regarding the initial mass function (IMF) of stars is the impact of environment on the shape of the core mass function (CMF) and thus potentially on the IMF. The ALMA-IMF Large Program aims to investigate the variations in the core distributions with cloud characteristics, as diagnostic observables of the formation process and evolution of clouds. The present study focuses on the W43-MM2&MM3 mini-starburst, whose CMF has recently been found to be top-heavy with respect to the Salpeter slope. W43-MM2&MM3 harbors a rich cluster that contains a statistically significant number of cores, which was previously characterized in Paper III. We applied a multi-scale decomposition technique to the ALMA 1.3 mm and 3 mm continuum images to define six subregions. For each subregion we characterized the high column density probability distribution function, n-PDF, and the shape of the cloud gas using the 1.3 mm image. Using the core catalog, we investigate correlations between the CMF and cloud and core properties. We classify the subregions into different stages of evolution, from quiescent to burst to post-burst, based on the surface number density of cores, number of outflows, and UCHii presence. The high-mass end of the subregion CMFs varies from being close to the Salpeter slope (quiescent) to top-heavy (burst and post-burst). Moreover, the second tail of the n-PDF varies from steep, to flat like observed for the high mass star-forming clouds. We found that subregions with flat second n-PDF tails display top-heavy CMFs. The CMF may evolve from Salpeter to top-heavy throughout the star formation process from the quiescent to the burst phase. This scenario raises the question of if the CMF might revert again to Salpeter as the cloud approaches the end of its star formation stage, a hypothesis that remains to be tested., Comment: Accepted for publication in A&A (december, 5th 2022) after language editing; 25 pages, 14 figures

We present N-PDFs of 29 Galactic regions obtained from Herschel imaging at high angular resolution, covering diffuse and quiescent clouds, and those showing low-, intermediate-, and high-mass star formation (SF), and characterize the cloud structure using the Delta-variance tool. The N-PDFs are double-log-normal at low column densities, and display one or two power law tails (PLTs) at higher column densities. For diffuse, quiescent, and low-mass SF clouds, we propose that the two log-normals arise from the atomic and molecular phase, respectively. For massive clouds, we suggest that the first log-normal is built up by turbulently mixed H2 and the second one by compressed (via stellar feedback) molecular gas. Nearly all clouds have two PLTs with slopes consistent with self-gravity, where the second one can be flatter or steeper than the first one. A flatter PLT could be caused by stellar feedback or other physical processes that slow down collapse and reduce the flow of mass toward higher densities. The steeper slope could arise if the magnetic field is oriented perpendicular to the LOS column density distribution. The first deviation point (DP), where the N-PDF turns from log-normal into a PLT, shows a clustering around values of a visual extinction of AV (DP1) around 2-5. The second DP, which defines the break between the two PLTs, varies strongly. Using the Delta-variance, we observe that the AV value, where the slope changes between the first and second PLT, increases with the characteristic size scale in the variance spectrum. We conclude that at low column densities, atomic and molecular gas is turbulently mixed, while at high column densities, the gas is fully molecular and dominated by self-gravity. The best fitting model N-PDFs of molecular clouds is thus one with log-normal low column density distributions, followed by one or two PLTs., Comment: Astronomy and Astrophysics in presss

W43-MM1 is a young region, very rich in terms of high-mass star formation. We aim to systematically identify the massive cores which contain a hot core and compare their molecular composition. We used ALMA high-spatial resolution (2500 au) data of W43-MM1 to identify line-rich protostellar cores and make a comparative study of their temperature and molecular composition. The identification of hot cores is based on both the spatial distribution of the complex organic molecules and the contribution of molecular lines relative to the continuum intensity. We rely on the analysis of CH3CN and CH3CCH to estimate the temperatures of the selected cores. Finally, we rescale the spectra of the different hot cores based on their CH3OCHO line intensities to directly compare the detections and line intensities of the other species. W43-MM1 turns out to be a region rich in massive hot cores with at least 1 less massive and 7 massive hot cores. The excitation temperature of CH3CN is of the same order for all of them (120-160 K). There is a factor of up to 30 difference in the intensity of the complex organic molecules (COMs) lines. However the molecular emission of the hot cores appears to be the same within a factor 2-3. This points towards both a similar chemical composition and excitation of most of the COMs over these massive cores, which span about an order of magnitude in core mass. In contrast, CH3CCH emission is found to preferentially trace more the envelope, with a temperature ranging from 50 K to 90 K. Core 1, the most massive hot core of W43-MM1, shows a richer line spectrum than the other cores. In core 2, the emission of O-bearing molecules does not peak at the dust continuum core center; the blue and red shifted emission correspond to the outflow lobes, suggesting a formation via the sublimation of the ice mantles through shocks or UV irradiation on the walls of the cavity., Comment: 19 pages, 17 figures. Accepted for publication in A&A

We present APEX, infrared and radio continuum observations of the G345.88-1.10 hub filament system which is a newly discovered star-forming cloud that hosts an unusually bright bipolar infrared nebulosity at its centre. At a distance of 2.26$^{+0.30}_{-0.21}$ kpc, G345.88-1.10 exhibits a network of parsec-long converging filaments. At the junction of these filaments lie four infrared-quiet fragments. The densest fragment (with M=210 M$_{\odot}$, R$_{\rm{eff}}=0.14$ pc) sits at the centre of a wide (opening angle of $\sim$ 90$\pm$15$^{o}$) bipolar nebulosity. $^{12}$CO(2-1) observations show that these infrared-bright nebulosities are spatially associated with a powerful molecular outflow from the central fragment. Negligible radio continuum and no H30$\alpha$ emission is detected towards the cavities, seemingly excluding that ionising radiation drives the evolution of the cavities. Furthermore, radiative transfer simulations are unable to reproduce the observed combination of a low-luminosity ($\lesssim$ 500 L$_{\odot}$) central source and a surrounding high-luminosity ($\sim 4000$ L$_{\odot}$) mid-infrared-bright bipolar cavity. This suggests that radiative heating from a central protostar cannot be responsible for the illumination of the outflow cavities. To our knowledge, this is the first reported object of this type. The rarity of objects like G345.88-1.10 is likely related to a very short phase in the massive star and/or cluster formation process that was so far unidentified. We discuss whether mechanical energy deposition by one episode or successive episodes of powerful mass accretion in a collapsing hub might explain the observations. While promising in some aspects, a fully coherent scenario that explains the presence of a luminous bipolar cavity centred on an infrared-dark fragment remains elusive at this point., Comment: 23 pages, 22 figures, accepted in A&A

The ALMA-IMF Large Program observed the W43-MM2-MM3 ridge, whose 1.3mm and 3mm ALMA 12m array continuum images reach a 2500au spatial resolution. We used both the best-sensitivity and the line-free ALMA-IMF images, reduced the noise with the multi-resolution segmentation technique MnGSeg, and derived the most complete and most robust core catalog possible. Using two different extraction software packages, getsf and GExt2D, we identified 200 compact sources, whose 100 common sources have on average fluxes consistent to within 30%. We filtered sources with non-negligible free-free contamination and corrected fluxes from line contamination, resulting in a W43-MM2-MM3 catalog of 205 getsf cores. With a median deconvolved FWHM size of 3400au, core masses range from 0.1Msun to 70Msun and the getsf catalog is 90% complete down to 0.8Msun. The high-mass end of the core mass function (CMF) of W43-MM2-MM3 is top-heavy compared to the canonical IMF. Fitting the cumulative CMF with a single power law of the form N(>logM)\propto M^a, we measured a=-0.95\pm0.04, compared to the canonical a=-1.35 Salpeter IMF slope. The slope of the CMF is robust with respect to map processing, extraction software package, and reasonable variations in the assumptions taken to estimate core masses. We explore several assumptions on how cores transfer their mass to stars and sub-fragment to predict the IMF resulting from the W43-MM2-MM3 CMF. In stark contrast to the commonly accepted paradigm, our result argues against the universality of the CMF shape. More robust functions of the star-formation efficiency and core sub-fragmentation are required to better predict the resulting IMF, here suggested to remain top-heavy at the end of the star-formation phase. If confirmed, the IMFs emerging from starburst events could inherit their top-heavy shape from their parental CMFs, challenging the IMF universality., Comment: 28 pages, 16 figures, 6 tables, A&A accepted on 02/08/22, revised following the language referee report

During star formation, the dense gas undergoes significant chemical evolution leading to the emergence of a rich variety of molecules associated with hot cores and hot corinos. The physical and chemical conditions are poorly constrained; the early phases of emerging hot cores in particular represent an unexplored territory. We provide here a full molecular inventory of a massive protostellar core that is proposed to be a precursor of a hot core. We performed an unbiased spectral survey towards the hot core precursor associated with clump G328.2551-0.5321 between 159GHz and 374GHz. To identify the spectral lines, we used rotational diagrams and radiative transfer modelling assuming LTE. We detected 39 species and 26 isotopologues, and were able to distinguish a warm and compact inner region, a colder more extended envelope, and the kinematic signatures of the accretion shocks that have previously been observed with ALMA. We associate most of the emission of the small molecules with the cold gas, while the molecular emission of the warm gas is enriched by complex organic molecules (COMs). We find a high abundance of S-bearing molecules in the cold gas phase suggesting a low sulphur depletion, with a factor of > 1%. We identify nine COMs in the warm gas, four in the cold gas, and four towards the accretion shocks. The high abundances of S-bearing species originating from the undisturbed gas may suggest a contribution from shocked gas at the outflow cavity walls. The molecular composition of the warm gas is similar to that of both hot cores and hot corinos, but the molecular abundances are closer to the values found towards hot corinos than to values found towards hot cores. Considering the compactness of the warm region and its moderate temperature, we suggest that thermal desorption has not been completed towards this object yet, representing an early phase of the emergence of hot cores., Comment: 68 pages, 59 figures. Accepted by A&A after refereeing and language corrections. Abstract abbreviated

We present the first data release of the ALMA-IMF Large Program, which covers the 12m-array continuum calibration and imaging. The ALMA-IMF Large Program is a survey of fifteen dense molecular cloud regions spanning a range of evolutionary stages that aims to measure the core mass function (CMF). We describe the data acquisition and calibration done by the Atacama Large Millimeter/submillimeter Array (ALMA) observatory and the subsequent calibration and imaging we performed. The image products are combinations of multiple 12m array configurations created from a selection of the observed bandwidth using multi-term, multi-frequency synthesis imaging and deconvolution. The data products are self-calibrated and exhibit substantial noise improvements over the images produced from the delivered data. We compare different choices of continuum selection, calibration parameters, and image weighting parameters, demonstrating the utility and necessity of our additional processing work. Two variants of continuum selection are used and will be distributed: the "best-sensitivity" data, which include the full bandwidth, including bright emission lines that contaminate the continuum, and "cleanest", which select portions of the spectrum that are unaffected by line emission. We present a preliminary analysis of the spectral indices of the continuum data, showing that the ALMA products are able to clearly distinguish free-free emission from dust emission, and that in some cases we are able to identify optically thick emission sources. The data products are made public with this release., Comment: Data released on Zenodo at https://zenodo.org/record/5702966. Accepted for publication in A&A

The ALMA-IMF Large Program imaged a total noncontiguous area of 53pc2, covering 15 extreme, nearby protoclusters of the Milky Way. They were selected to span relevant early protocluster evolutionary stages. Our 1.3mm and 3mm observations provide continuum images that are homogeneously sensitive to point-like cores with masses of 0.2 and 0.6Msun, respectively, with a matched spatial resolution of 2000au. We also detect lines that probe the protocluster structure, kinematics, chemistry, and feedback over scales from clouds to filaments to cores. We classify ALMA-IMF protoclusters as Young, Intermediate, or Evolved based on the amount of dense gas in the cloud that has potentially been impacted by HII regions. The ALMA-IMF catalog contains 700 cores that span a mass range of 0.15-250Msun at a typical size of 2100au. We show that this core sample has no significant distance bias and can be used to build core mass functions at similar physical scales. Significant gas motions, which we highlight here in the G353.41 region, are traced down to core scales and can be used to look for inflowing gas streamers and to quantify the impact of the possible associated core mass growth on the shape of the CMF with time. Our first analysis does not reveal any significant evolution of the matter concentration from clouds to cores or from the youngest to more evolved protoclusters, indicating that cloud dynamical evolution and stellar feedback have for the moment only had a slight effect on the structure of high-density gas in our sample. Furthermore, the first-look analysis of the line richness toward bright cores indicates that the survey encompasses several tens of hot cores, of which we highlight the most massive in the G351.77 cloud. Their homogeneous characterization can be used to constrain the emerging molecular complexity in protostars of high to intermediate masses., Comment: 27 pages, 9 figures, accepted for publication by A&A

IRAS 20319+3958 in Cygnus X South is a rare example of a free-floating globule (mass ~240 Msun, length ~1.5 pc) with an internal HII region created by the stellar feedback of embedded intermediate-mass stars, in particular, one Herbig Be star. Here, we present a Herschel/HIFI CII 158 mu map of the whole globule and a large set of other FIR lines (mid-to high-J CO lines observed with Herschel/PACS and SPIRE, the OI 63 mu line and the CO 16-15 line observed with upGREAT on SOFIA), covering the globule head and partly a position in the tail. The CII map revealed that the whole globule is probably rotating. Highly collimated, high-velocity CII emission is detected close to the Herbig Be star. We performed a PDR analysis using the KOSMA-tau PDR code for one position in the head and one in the tail. The observed FIR lines in the head can be reproduced with a two-component model: an extended, non-clumpy outer PDR shell and a clumpy, dense, and thin inner PDR layer, representing the interface between the HII region cavity and the external PDR. The modelled internal UV field of ~2500 Go is similar to what we obtained from the Herschel FIR fluxes, but lower than what we estimated from the census of the embedded stars. External illumination from the ~30 pc distant Cyg OB2 cluster, producing an UV field of ~150-600 G0 as an upper limit, is responsible for most of the CII emission. For the tail, we modelled the emission with a non-clumpy component, exposed to a UV-field of around 140 Go., Comment: accepted by Astronomy & Astrophysics

The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescope to map 84 deg^2 of the Galactic plane between l = -60 deg and l = +31 deg in several molecular transitions, including 13CO(2-1) and C18O(2-1), thus probing the moderately dense (~10^3 cm^-3) component of the interstellar medium. With an angular resolution of 30'' and a typical 1-sigma sensitivity of 0.8-1.0 K at 0.25 km/s velocity resolution, it gives access to a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the first and fourth Galactic quadrants, allowing us to constrain the large scale distribution of cold molecular gas in the inner Galaxy. In this paper we provide an updated overview of the full survey and the data reduction procedures used. We also assess the quality of these data and describe the data products that are being made publicly available as part of this first data release (DR1). We present integrated maps and position-velocity maps of the molecular gas and use these to investigate the correlation between the molecular gas and the large scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic centre. We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensity peaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond to the Galactic centre and well known star forming complexes, revealing that the 13CO emission is concentrated in a small number of complexes rather than evenly distributed along spiral arms., Comment: MNRAS, in press

We use the 13CO(2-1) emission from the SEDIGISM high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the SCIMES algorithm. This work compiles a cloud catalogue with a total of 10663 molecular clouds, 10300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases, and thus require further follow up work in order to be confirmed., Comment: 25 pages (+ appendices, 15 pages), 26 figures, MNRAS

The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question on the origin and evolution of mass segregation is still open. Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation. We separated the Herschel column density map of NGC 2264 in three subregions and compared their cloud power spectra using a multiscale segmentation technique. We identified in the whole NGC 2264 cloud a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 Msun to 53 Msun. Although clumps have been detected all over the cloud, the central subregion of NGC 2264 concentrates most of the massive, bound clumps. The local surface density and the mass segregation ratio indeed indicate a strong degree of mass segregation for the 15 most massive clumps, with a median $\Sigma_6$ three time that of the whole clumps population and $\Lambda_{MSR}$ about 8. We showed that this cluster of massive clumps is forming within a high-density cloud ridge, itself formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center and will soon start in the southern subregion. Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation., Comment: 26 pages, 19 figures. Accepted for publication in A&A, abstract abridged for arXiv

Observations of the interstellar medium (ISM) show a complex density and velocity structure which is in part attributed to turbulence. We here present a self-contained introduction to the multifractal formalism in a microcanonical version which allows us for the first time to compute precise turbulence characteristic parameters from a single observational map without the need for averages in a grand ensemble of statistical observables. We focus on studying the 250 mu-m Herschel map of the Musca filament and make use of MHD simulations. We find a clear signature of a multiplicative cascade in Musca with an inertial range from 0.05 to 0.65 pc. We show that the proposed microcanonical approach provides singularity spectra which are truly scale invariant as required to validate any method to analyze multifractality. The obtained, for the first time precise enough, singularity spectrum of Musca is clearly not as symmetric as usually observed in log-normal behavior. We claim that the ISM towards Musca features more a log-Poisson shape of its singularity spectrum. Since log-Poisson behavior is claimed to exist when dissipation is stronger for rare events in turbulent flows in contrast to more homogeneous (in volume and time) dissipation events, we suggest that this deviation from log-normality could trace enhanced dissipation in rare events at small scales, which may explain or is at least consistent with the dominant filamentary structure in Musca. Moreover we find that sub-regions in Musca tends to show different multifractal properties. It strongly suggests that different types of dynamics exist inside the Musca cloud. These differences between sub-regions appear only after eliminating noise features which have the tendency to "log-normalize" an observational map. Our study sets up fundamental tools which will be applied to other galactic clouds and simulations in forthcoming studies., Comment: 25 pages. 27 figures. Submitted to Astronomy & Astrophysics, November 2020

The Herschel Gould Belt survey mapped the nearby (d < 500 pc) star-forming regions to understand better how the prestellar phase influences the star formation process. Here we report a complete census of dense cores in a 15 deg2 area of the Serpens star-forming region located between d=420 pc and 484 pc. The PACS and SPIRE cameras imaged this cloud from 70micron to 500micron. With the multi-wavelength source extraction algorithm getsources, we extract 833 sources, of which 709 are starless cores and 124 are candidate proto-stellar cores. We obtain temperatures and masses for all the sample, classifying the starless cores in 604 prestellar cores and 105 unbound cores. Our census of sources is 80% complete for masses larger than 0.8 Msun overall. We produce the core mass function (CMF) and compare it with the initial mass function (IMF). The prestellar CMF is consistent with log-normal trend up to 2 Msun, after which it follows a power-law with slope of -2.05+/-0.34. The tail of its CMF is steeper but still compatible with the IMF for the region we studied in this work. We also extract the filaments network of the Serpens region, finding that 81% of prestellar cores lie on filamentary structures. The spatial association between cores and filamentary structure supports the paradigm, suggested by otherHerschelobservations, that prestellar cores mostly form on filaments. Serpens is confirmed to be a young, low-mass and active star-forming region., Comment: 22 pages, 17 figures

Context. Dense molecular filaments are ubiquituous in the interstellar medium, yet their internal physical conditions and formation mechanism remain debated. Aims. We study the kinematics and physical conditions in the Musca filament and the Chamaeleon-Musca complex to constrain the physics of filament formation. Methods. We produced CO(2-1) isotopologue maps with the APEX telescope that cut through the Musca filament. We further study a NANTEN2 $^{12}$CO(1-0) map of the Musca cloud and the HI emission of the Chamaeleon-Musca complex. Results. The Musca cloud contains multiple velocity components. Radiative transfer modelling of the CO emission indicates that the Musca filament consists of a cold ($\sim$10 K), dense (n$_{H_2}\sim$10$^4$ cm$^{-3}$) crest, which is best described with a cylindrical geometry. Connected to the crest, a separate gas component at T$\sim$15 K and n$_{H_2}\sim$10$^3$ cm$^{-3}$ is found, the so-called strands. The filament crest has a transverse velocity gradient that is linked to the kinematics of the nearby ambient cloud. Studying the large scale kinematics, we find coherence of the asymmetric kinematics from the 50 pc HI cloud down to the Musca filament. We also report a strong [C$^{18}$O]/[$^{13}$CO] abundance drop by an order of magnitude from the filament crest to the strands over a distance $<$ 0.2 pc in a weak far-ultraviolet (FUV) field. Conclusions. The dense Musca filament crest is a long-lived (several crossing times), dynamic structure that can form stars in the near future because of continuous mass accretion. This mass accretion appears to be triggered by a HI cloud-cloud collision, which bends the magnetic field around dense filaments. This bending of the magnetic field is then responsible for the observed asymmetric accretion scenario of the Musca filament, which is, for instance, seen as a V-shape in the position-velocity (PV) diagram., Comment: 24 pages, 26 figures

(Abridged) In this paper, we present analyses of images taken with the Herschel ESA satellite from 70mu to 500mu. We first constructed column density and dust temperature maps. Next, we identified compact cores in the maps, and characterize the cores using modified blackbody fits to their SEDs: we identified 684 starless cores, of which 199 are bound and potential prestellar cores, and 132 protostars. We also matched the Herschel-identified young stars with GAIA sources to model distance variations across the Perseus cloud. We measure a linear gradient function with right ascension and declination for the entire cloud. From the SED fits, mass and temperature of cores were derived. The core mass function can be modelled with a log-normal distribution that peaks at 0.82~$M_\sun$ suggesting a star formation efficiency of 0.30. The high-mass tail can be modelled with a power law of slope $\sim-2.32$, close to the Salpeter's value. We also identify the filamentary structure of Perseus, confirming that stars form preferentially in filaments. We find that the majority of filaments where star formation is ongoing are transcritical against their own internal gravity because their linear masses are below the critical limit of 16~$M_\sun$pc$^{-1}$ above which we expect filaments to collapse. We find a possible explanation for this result, showing that a filament with a linear mass as low as 8~$M_\sun$pc$^{-1}$ can be already unstable. We confirm a linear relation between star formation efficiency and slope of dust probability density function and a similar relation is also seen with the core formation efficiency. We derive a lifetime for the prestellar core phase of $1.69\pm0.52$~Myr for Perseus but different regions have a wide range in prestellar core fractions, hint that star-formation has started only recently in some clumps. We also derive a free-fall time for prestellar cores of 0.16~Myr., Comment: Comparison with Zari et al's (2016) work improved once Eleonora Zari told us that, contrarily to what we wrote, their data are publicly available (this was not written in the first version of their paper put on arxiv, then our misunderstaning). The modification is small, a paragraph on page 11, and does not have any impact on the content of the paper. Modification allowed by A&A editor

FEEDBACK is a SOFIA legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star forming regions in the 158 $\mu$m (1.9 THz) line of CII and the 63 $\mu$m (4.7 THz) line of OI. We employ the 14 pixel LFA and 7 pixel HFA upGREAT instrument to spectrally resolve (0.24 MHz) these FIR structure lines. With an observing time of 96h, we will cover $\sim$6700 arcmin$^2$ at 14.1$''$ angular resolution for the CII line and 6.3$''$ for the OI line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. The CII line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The OI line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of HII region bubbles, namely RCW49, RCW79, and RCW120. These CII maps, together with the less explored OI 63 $\mu$m line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.

Observations with the Herschel Space Telescope have established that most of the star forming gas is organised in interstellar filaments, a finding that is supported by numerical simulations of the supersonic interstellar medium (ISM) where dense filamentary structures are ubiquitous. We aim to understand the formation of these dense structures by performing observations covering the $^{12}$CO(4-3), $^{12}$CO(3-2), and various CO(2-1) isotopologue lines of the Musca filament, using the APEX telescope. The observed CO intensities and line ratios cannot be explained by PDR (photodissociation region) emission because of the low ambient far-UV field that is strongly constrained by the non-detections of the [C II] line at 158 $\mu$m and the [O I] line at 63 $\mu$m, observed with the upGREAT receiver on SOFIA, as well as a weak [C I] 609 $\mu$m line detected with APEX. We propose that the observations are consistent with a scenario in which shock excitation gives rise to warm and dense gas close to the highest column density regions in the Musca filament. Using shock models, we find that the CO observations can be consistent with excitation by J-type low-velocity shocks. A qualitative comparison of the observed CO spectra with synthetic observations of dynamic filament formation simulations shows a good agreement with the signature of a filament accretion shock that forms a cold and dense filament from a converging flow. The Musca filament is thus found to be dense molecular post-shock gas. Filament accretion shocks that dissipate the supersonic kinetic energy of converging flows in the ISM may thus play a prominent role in the evolution of cold and dense filamentary structures., Comment: 16 pages, 17 figures

The mass growth of protostars is a central element to the determination of fundamental stellar population properties such as the initial mass function. Constraining the accretion history of individual protostars is therefore an important aspect of star formation research. The goal of the study presented here is to determine whether high-mass (proto)stars gain their mass from a compact (<0.1pc) fixed-mass reservoir of gas, often referred to as dense cores, in which they are embedded, or whether the mass growth of high-mass stars is governed by the dynamical evolution of the parsec-scale clump that typically surrounds them. To achieve this goal, we performed a 350micron continuum mapping of 11 infrared dark clouds, along side some of their neighbouring clumps, with the ArT\'eMiS camera on APEX. By identifying about 200 compact ArT\'eMiS sources, and matching them with Herschel Hi-GAL 70micron sources, we have been able to produce mass vs. temperature diagrams. We compare the nature (i.e. starless or protostellar) and location of the ArT\'eMiS sources in these diagrams with modelled evolutionary tracks of both core-fed and clump-fed accretion scenarios. We argue that the latter provide a better agreement with the observed distribution of high-mass star-forming cores. However, a robust and definitive conclusion on the question of the accretion history of high-mass stars requires larger number statistics., Comment: 23 pages, 44 figures, accepted for publication in MNRAS

The accretion history of protostars remains widely mysterious even though it represents one of the best ways to understand the protostellar collapse that leads to the formation of stars. Molecular outflows are here used to characterize the protostellar accretion phase in W43-MM1. The W43-MM1 protocluster host a sufficient number of protostars to statistically investigate molecular outflows in a single, homogeneous region. We used the CO(2-1) and SiO(5-4) line datacubes, taken as part of an ALMA mosaic with a 2000 AU resolution, to search for protostellar outflows, evaluate the influence that the environment has on these outflows' characteristics and put constraints on outflow variability in W43-MM1. We discovered a rich cluster of 46 outflow lobes, driven by 27 protostars with masses of 1-100 Msun. The complex environment inside which these outflow lobes develop has a definite influence on their length, limiting the validity of using outflows' dynamical timescales as a proxy of the ejection timescale in clouds with high dynamics and varying conditions. We performed a detailed study of Position-Velocity (PV) diagrams of outflows that revealed clear events of episodic ejection. The time variability of W43-MM1 outflows is a general trend and is more generally observed than in nearby, low- to intermediate-mass star-forming regions. The typical timescale found between two ejecta, about 500 yr, is consistent with that found in nearby protostars. If ejection episodicity reflects variability in the accretion process, either protostellar accretion is more variable or episodicity is easier to detect in high-mass star-forming regions than in nearby clouds. The timescale found between accretion events could be resulting from disk instabilities, associated with bursts of inflowing gas arising from the dynamical environment of high-mass star-forming cores., Comment: Accepted for publication in Astronomy & Astrophysics (23 pages, 15 figures)

Complex organic molecules (COMs) have been detected in a few Class 0 protostars but their origin is not well understood. Going beyond studies of individual objects, we want to investigate the origin of COMs in young protostars on a statistical basis. We use the CALYPSO survey performed with the IRAM PdBI to search for COMs at high angular resolution in a sample of 26 solar-type protostars, including 22 Class 0 and four Class I objects. Methanol is detected in 12 sources and tentatively in one source, which represents half of the sample. Eight sources (30%) have detections of at least three COMs. We find a strong chemical differentiation in multiple systems with five systems having one component with at least three COMs detected but the other component devoid of COM emission. The internal luminosity is found to be the source parameter impacting the most the COM chemical composition of the sources, while there is no obvious correlation between the detection of COM emission and that of a disk-like structure. A canonical hot-corino origin may explain the COM emission in four sources, an accretion-shock origin in two or possibly three sources, and an outflow origin in three sources. The CALYPSO sources with COM detections can be classified into three groups on the basis of the abundances of oxygen-bearing molecules, cyanides, and CHO-bearing molecules. These chemical groups correlate neither with the COM origin scenarii, nor with the evolutionary status of the sources if we take the ratio of envelope mass to internal luminosity as an evolutionary tracer. We find strong correlations between molecules that are a priori not related chemically (for instance methanol and methyl cyanide), implying that the existence of a correlation does not imply a chemical link. [abridged], Comment: Accepted for publication in Astronomy and Astrophysics

Classical hot cores are rich in molecular emission, and they show a high abundance of complex organic molecules (COMs). The emergence of molecular complexity is poorly constrained in the early evolution of hot cores. Using the Atacama Large Millimeter Array we put observational constraints on the physical location of COMs in a high-mass protostellar envelope associated with the G328.2551-0.5321 clump. The protostar is single down to ~400au scales and we resolve the emission region of COMs. Using thermodynamic equilibrium modelling of the available 7.5 GHz bandwidth around ~345 GHz, we detect emission from 10 COMs, and identify a line of deuterated water (HDO). The most extended emission originates from methanol, methyl formate and formamide. Together with HDO, these molecules are found to be associated with both the accretion shocks and the inner envelope, for which we estimate a moderate temperature of $T_{\rm kin}\sim$110 K. Our findings reveal a significant difference in the distribution of COMs. O-bearing COMs, such as ethanol, acetone, and ethylene glycol are almost exclusively found and show a higher abundance towards the accretion shocks with $T_{\rm kin}\sim$180 K. Whereas N-bearing COMs with a CN group, such as vinyl and ethyl cyanide peak on the central position, thus the protostar and the accretion disk. This is the first observational evidence for a large column density of COMs seen towards accretion shocks at the centrifugal barrier at the inner envelope. Since the molecular composition is dominated by that of the accretion shocks and the radiatively heated hot inner region is very compact, we propose this source to be a precursor to a classical, radiatively heated hot core., Comment: Accepted for publication in A&A (17 pages, 9 figures). The abstract is abbreviated

We present a detailed study of the Orion B clouds (d~400 pc), imaged with the PACS/SPIRE cameras at 70-500 $\mu$m by the Herschel Gould Belt survey (HGBS). We release new high-res. maps of column density and dust temperature. In the filamentary sub-regions NGC2023/2024, NGC2068/2071, and L1622, 1768 starless dense cores were identified, ~28-45% of which are self-gravitating prestellar cores. A total of 76 protostellar dense cores were also found. The typical lifetime of the prestellar cores was found to be $t_{\rm pre}=1.7_{-0.6}^{+0.8}$ Myr. The prestellar core mass function (CMF) peaks at ~0.5 $M_\odot$ and is consistent with a power law with log slope -1.27$\pm$0.24 at the high-mass end, compared to the Salpeter slope of -1.35. In this region, we confirm the existence of a transition in prestellar core formation efficiency (CFE) around a fiducial value A_V_bg~7 mag in background visual extinction, similar to the trend observed with Herschel in other clouds. This is not a sharp threshold, but a smooth transition between a regime with very low prestellar CFE at A_V_bg<5 and a regime with higher, roughly constant CFE at A_V_bg$\gtrsim$10. The total mass in the form of prestellar cores represents only ~20% of the dense molecular cloud gas at A_V_bg$\gtrsim$7 mag. About 60-80% of the prestellar cores are closely associated with filaments, and this fraction increases up to >90% when a more complete sample of filamentary structures is considered. Interestingly, the median separation between nearest core neighbors corresponds to the typical inner filament width of ~0.1 pc commonly observed in nearby molecular clouds. Analysis of the CMF observed as a function of background cloud column density shows that the most massive prestellar cores are spatially segregated in the highest column density areas, and suggests that both higher- and lower-mass prestellar cores may form in denser filaments., Comment: In press in Astronomy and Astrophysics. arXiv admin note: text overlap with arXiv:1507.05926

High-mass analogues of low-mass prestellar cores are searched for to constrain the models of high-mass star formation. Several high-mass cores, at various evolutionary stages, have been recently identified towards the massive star-forming region W43-MM1 and amongst them a high-mass prestellar core candidate. We aim to characterise the chemistry in this high-mass prestellar core candidate, referred to as W43-MM1 core #6, and its environment. Using ALMA high-spatial resolution data of W43-MM1, we have studied the molecular content of core #6 and a neighbouring high-mass protostellar core, referred to as #3, which is similar in size and mass to core #6. We first subtracted the continuum emission using a method based on the density distribution of the intensities on each pixel. Then, from the distribution of detected molecules, we identified the molecules centred on the prestellar core candidate (core #6) and those associated to shocks related to outflows and filament formation. Then we constrained the column densities and temperatures of the molecules detected towards the two cores. While core #3 appears to contain a hot core with a temperature of about 190 K, core #6 seems to have a lower temperature in the range from 20 K to 90 K from a rotational diagram analysis. We have considered different source sizes for core #6 and the comparison of the abundances of the detected molecules towards the core with various interstellar sources shows that it is compatible with a core of size 1000 au with $T = 20-90$ K or a core of size 500 au with $T \sim 80$ K. Core #6 of W43-MM1 remains one of the best high-mass prestellar core candidates even if we cannot exclude that it is at the very beginning of the protostellar phase of high-mass star formation., Comment: 18 pages, 8 figures, Accepted for publication in Astronomy & Astrophysics

We present the Multiscale non-Gaussian Segmentation (MnGSeg) analysis technique. This wavelet based method combines the analysis of the probability distribution function (PDF) of map fluctuations as a function of spatial scales and the power spectrum analysis of a map. This technique allows us to extract the non-Gaussianities identified in the multiscaled PDFs usually associated with turbulence intermittency and to spatially reconstruct the Gaussian and the non-Gaussian component of the map. This new technique can be applied on any data set. Here, it is applied on a Herschel column density map of the Polaris flare cloud. The first component has by construction a self-similar fractal geometry as the one produced by fractional Brownian motion simulations. The second component is called the coherent component, by opposition to fractal, and includes a network of filamentary structures which demonstrates a spatial hierarchical scaling, i.e. filaments inside filaments. The power spectrum analysis of both components proves that the Fourier power spectrum of the initial map is dominated by the power of the coherent filamentary structures across almost all spatial scales. The coherent structures contribute progressively, more and more from large to smaller scales, without producing any break in the inertial range. We suggest that this behaviour is induced, at least partly, by inertial-range intermittency, a well known phenomenon for turbulent flows. We also demonstrate that the MnGSeg technique is a very sensitive signal analysis technique, which allows the extraction of the cosmic infrared background (CIB) signal present in the Polaris flare submillimeter observations and the detection of a characteristic scale for 0.1 < l < 0.3 pc whose origin could partly be the transition of regimes dominated by incompressible turbulence versus compressible modes and other physical processes, such as gravity., Comment: 17 pages, 16 figures. Submitted to A&A

SPICA, the cryogenic infrared space telescope recently pre-selected for a `Phase A' concept study as one of the three remaining candidates for ESA's fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager (SPICA-POL, now called B-BOP), which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetized Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100-350 micron images of linearly polarized dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 micron images will also have a factor ~30 higher resolution than Planck polarization data. This will make B-BOP a unique tool for characterizing the statistical properties of the magnetized interstellar medium and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars., Comment: Accepted for publication in PASA; 37 pages, 16 figures

Understanding the formation mechanisms of protoplanetary disks and multiple systems, and their pristine properties, is a key question for modern astrophysics. The properties of the youngest disks, embedded in rotating infalling protostellar envelopes, have largely remained unconstrained up to now. In the framework of the IRAM-PdBI CALYPSO survey, we have obtained sub-arcsecond observations of the dust continuum emission at 231 GHz and 94 GHz, for a sample of 16 solar-type Class 0 protostars. In an attempt to identify disk-like structures embedded at small scales in the protostellar envelopes, we model the dust continuum emission visibility profiles using both Plummer-like envelope models and envelope models including additional Gaussian disk-like components. Our analysis shows that in the CALYPSO sample, 11 of the 16 Class 0 protostars are better reproduced by models including a disk-like dust continuum component contributing to the flux at small scales, but less than 25% of these candidate protostellar disks are resolved at radii > 60 au. Including all available literature constraints on Class 0 disks at subarcsecond scales, we show that our results are representative: most (> 72% in a sample of 26 protostars) Class 0 protostellar disks are small and emerge only at radii < 60 au. Our multiplicity fraction at scales 100-5000 au is in global agreement with the multiplicity properties of Class I protostars at similar scales. We confront our observational constraints on the disk size distribution in Class 0 protostars to the typical disk properties from protostellar formation models. Because they reduce the centrifugal radius, and produce a disk size distribution peaking at radii <100 au during the main accretion phase, magnetized models of rotating protostellar collapse are favored by our observations., Comment: Accepted for publication in Astronomy \& Astrophysics. 24 pages of main text, 22 pages of additional content (under the form of three appendices) - revised version including language and typos corrections

Aims. To constrain the physical processes that lead to the birth of high-mass stars it is mandatory to study the very first stages of their formation. We search for high-mass analogs of low-mass prestellar cores in W43-MM1. Methods. We conducted a 1.3 mm ALMA mosaic of the complete W43-MM1 cloud, which has revealed numerous cores with ~ 2000 au FWHM sizes. We investigated the nature of cores located at the tip of the main filament, where the clustering is minimum. We used the continuum emission to measure the core masses and the $^{13}$CS(5-4) line emission to estimate their turbulence level. We also investigated the prestellar or protostellar nature of these cores by searching for outflow signatures traced by CO(2-1) and SiO(5-4) line emission, and for molecular complexity typical of embedded hot cores. Results. Two high-mass cores of ~ 1300 au diameter and ~ $60~M_\odot$ mass are observed to be turbulent but gravitationally bound. One drives outflows and is associated with a hot core. The other core, W43-MM1\#6, does not yet reveal any star formation activity and thus is an excellent high-mass prestellar core candidate., Comment: 7 pages, 6 figures. Accepted for publication in A&A Letter

The fragmentation mode of high-mass molecular clumps and the properties of the central rotating structures surrounding the most luminous objects have yet to be comprehensively characterised. Using the IRAM NOrthern Extended Millimeter Array (NOEMA) and the IRAM 30-m telescope, the CORE survey has obtained high-resolution observations of 20 well-known highly luminous star-forming regions in the 1.37 mm wavelength regime in both line and dust continuum emission. We present the spectral line setup of the CORE survey and a case study for W3(H2O). At ~0.35" (700 AU at 2 kpc) resolution, the W3(H2O) clump fragments into two cores (West and East), separated by ~2300 AU. Velocity shifts of a few km/s are observed in the dense-gas tracer, CH3CN, across both cores, consistent with rotation and perpendicular to the directions of two bipolar outflows, one emanating from each core. The kinematics of the rotating structure about W3(H2O) W shows signs of differential rotation of material, possibly in a disk-like object. The observed rotational signature around W3(H2O) E may be due to a disk-like object, an unresolved binary (or multiple) system, or a combination of both. We fit the emission of CH3CN (12-11) K = 4-6 and derive a gas temperature map with a median temperature of ~165 K across W3(H2O). We create a Toomre Q map to study the stability of the rotating structures against gravitational instability. The rotating structures appear to be Toomre unstable close to their outer boundaries, with a possibility of further fragmentation in the differentially-rotating core W3(H2O) W. Rapid cooling in the Toomre-unstable regions supports the fragmentation scenario. Combining millimeter dust continuum and spectral line data toward the famous high-mass star-forming region W3(H2O), we identify core fragmentation on large scales, and indications for possible disk fragmentation on smaller spatial scales., Comment: 23 pages, 26 figures, accepted for publication in Astronomy and Astrophysics

The central area (40"x40") of the bipolar nebula S106 was mapped in the OI line at 63.2 micron with high angular (6") and spectral resolution, using GREAT on board SOFIA. The OI emission distribution is compared to the CO 16-15, CII 158 micron, and CO 11-10 lines, mm-molecular lines, and continuum. It is composed of several velocity components in the range from -30 km/s to 25 km/s. The high-velocity blue- and redshifted emission can be explained as arising from accelerated photodissociated (PDR) gas associated with a dark lane close to the massive binary system S106 IR, and from shocks caused by the stellar wind and/or a disk--envelope interaction. At velocities from -9 to -4 km/s and 0.5 to 8 km/s line wings are observed that we attribute to cooling in PDRs created by the ionizing radiation impinging on the cavity walls. The bulk velocity range is dominated by PDR emission from the clumpy molecular cloud. Modelling the emission in the different velocity ranges with the KOSMA-tau code constrains a radiation field chi of a few times 10^4 and densities n of a few times 10^4 cm^-3. Considering self-absorption of the OI line results in higher densities (up to 10^6 cm^-3) only for the gas component seen at high blue- and red velocities. The dark lane has a mass of 275 Msun and shows a velocity difference of 1.4 km/s along its projected length of 1 pc, determined from H13CO+ 1-0 mapping. It can be interpreted as a massive accretion flow, or the remains of it, linked to S106 IR/FIR. The most likely explanation is that the binary system is at a stage of its evolution where gas accretion is counteracted by the stellar winds and radiation, leading to the very complex observed spatial and kinematic emission distribution of the various tracers., Comment: Final version as it will appear in Asronomy&Astrophysics, accepted 22.5.2018

The formation of the most massive O-type stars is poorly understood. We present a case study of a young massive clump from the ATLASGAL survey, G328.2551-0.5321. It exhibits a bolometric luminosity of 1.3$\times$10$^4$ L$_{\odot}$ corresponding to a current protostellar mass of $\sim$11 and 16 M$_{\odot}$. We analyze high angular-resolution observations with ALMA at $\sim$0.17" corresponding a physical scale of $\sim$400 au in dust continuum and molecular lines. The dust continuum emission reveals a single high-mass protostellar envelope and shows evidence for a marginally resolved continuum source. We detect a rotational line of CH$_3$OH within its $v_{\rm t}$=1 torsionally excited state revealing two bright peaks of emission spatially offset from the dust continuum peak, and exhibiting a distinct velocity component $\pm$4.5 km s$^{-1}$ offset compared to the source $v_{\rm lsr}$. Local thermodynamic equilibrium analysis suggests N(CH$_3$OH)=1.2$-$2$\times$10$^{19}$ cm$^{-2}$, and kinetic temperatures of 160$-$200 K at the position of these peaks. Their velocity shifts correspond well to the expected Keplerian velocity around a central object with 15M$_{\odot}$ consistent with the mass estimate based on the source's bolometric luminosity. We propose a picture where the CH$_3$OH emission peaks trace the accretion shocks around the centrifugal barrier, pinpointing the interaction region between the collapsing envelope and an accretion disk. Because the HC$_3$N $v_{\rm 7}$=1e ($J$=38-37) line shows compact emission, and a velocity pattern consistent with models of Keplerian rotation, we suggest that this could be a new tracer for compact accretion disks around high-mass protostars. The estimated physical properties of the accretion disk suggest a specific angular momentum several times larger than typically observed towards low-mass protostars., Comment: Manuscript submitted to A&A, and revised after first referee report (abstract abbreviated)

Understanding the processes that determine the stellar Initial Mass Function (IMF) is a critical unsolved problem, with profound implications for many areas of astrophysics. In molecular clouds, stars are formed in cores, gas condensations which are sufficiently dense that gravitational collapse converts a large fraction of their mass into a star or small clutch of stars. In nearby star-formation regions, the core mass function (CMF) is strikingly similar to the IMF, suggesting that the shape of the IMF may simply be inherited from the CMF. Here we present 1.3 mm observations, obtained with ALMA, the world's largest interferometer, of the active star-formation region W43-MM1, which may be more representative of the Galactic-disk regions where most stars form. The unprecedented resolution of these observations reveals, for the first time, a statistically robust CMF at high masses, with a slope that is markedly shallower than the IMF. This seriously challenges our understanding of the origin of the IMF., Comment: 20 pages, 6 Figures, 2 Tables Nature Astronomy, 2018

We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 {\mu}m, 160 {\mu}m, 250 {\mu}m, 350 {\mu}m, and 500 {\mu}m by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C18O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments., Comment: 26 pages, 21 figures, full catalogue available from A&A

The SPICA mid and far-infrared telescope will address fundamental issues in our understanding of star formation and ISM physics in galaxies. A particular hallmark of SPICA is the outstanding sensitivity enabled by the cold telescope, optimized detectors, and wide instantaneous bandwidth throughout the mid- and far-infrared. The spectroscopic, imaging and polarimetric observations that SPICA will be able to collect will help in clarifying the complex physical mechanisms which underlie the baryon cycle of galaxies. In particular: (i) The access to a large suite of atomic and ionic fine-structure lines for large samples of galaxies will shed light on the origin of the observed spread in star formation rates within and between galaxies. (ii) Observations of HD rotational lines (out to $\sim$10 Mpc) and fine structure lines such as [CII] 158 $\mu$m (out to $\sim$100 Mpc) will clarify the main reservoirs of interstellar matter in galaxies, including phases where CO does not emit. (iii) Far-infrared spectroscopy of dust and ice features will address uncertainties in the mass and composition of dust in galaxies, and the contributions of supernovae to the interstellar dust budget will be quantified by photometry and monitoring of supernova remnants in nearby galaxies. (iv) Observations of far-infrared cooling lines such as [OI] 63 $\mu$m from star-forming molecular clouds in our Galaxy will evaluate the importance of shocks to dissipate turbulent energy. The paper concludes with requirements for the telescope and instruments, and recommendations for the observing strategy., Comment: Accepted for publication in PASA; 19 pages, 8 figures, 1 table

The formation of massive stars is still not well understood. Accumulating a large amount of mass infalling within a single entity in spite of radiation pressure is possible if, among several other conditions, enough thermal energy is released. Despite numerous water line observations, with the Herschel Space Observatory, in most of the sources observations were not able to trace the emission from the hot core around the newly forming protostellar object. We want to probe the physical conditions and water abundance in the inner layers of the host protostellar object NGC7538-IRS1 using a highly excited H2O line. Water maser models predict that several THz water masers should be detectable in these objects. We present SOFIA observations of the o-H2O 8(2,7)-7(3,4) line at 1296.41106 GHz and a 6(1,6)-5(2,3) 22 GHz e-MERLIN map of the region (first-ever 22 GHz images made after the e-MERLIN upgrade). In order to be able to constrain the nature of the emission - thermal or maser - we use near-simultaneous observations of the 22 GHz water maser performed with the Effelsberg radiotelescope and e-MERLIN. A thermal water model using the RATRAN radiative transfer code is presented based on HIFI pointed observations. Molecular water abundances are derived for the hot core. The H2O 8(2,7)- 7(3,4) line is detected toward NGC7538-IRS1 with one feature at the source velocity (-57.7 km/s) and another one at -48.4 km/s. We propose that the emission at the source velocity is consistent with thermal excitation and is excited in the innermost part of the IRS1a massive protostellar object's closest circumstellar environment. The other emission is very likely the first detection of a water THz maser line, pumped by shocks due to IRS1b outflow, in a star-forming region. Assuming thermal excitation of the THz line, the water abundance in NGC7538-IRS1's hot core is estimated to be 5.2x10^{-5} with respect to H2., Comment: accepted to A&A, 15 pages

Recent studies have suggested that star formation (SF) in dense gas may be governed by essentially the same law in Galactic clouds and external galaxies. This conclusion remains controversial, however, because different tracers have been used to probe the dense gas mass in Galactic and extragalactic studies. We conducted observations of Aquila, Oph., and Orion B in HCN(1-0), HCO$^+$(1-0), and their isotopomers to calibrate the HCN and HCO$^+$ lines used as dense gas tracers in extragalactic studies and to test the possible universality of the star formation efficiency in dense gas, SFE$_{dense}$. H$^{13}$CO$^+$ and H$^{13}$CN were observed to be good tracers of the filaments detected with Herschel. Comparing the luminosities $L_{HCN}$ and $L_{HCO^+}$ with the reference masses $M_{Herschel}^{Av>8}$, the empirical conversion factors $\alpha_{Herschel-HCN}$ and $\alpha_{Herschel-HCO^+}$ were found to be anti-correlated with the local FUV strength. In agreement with Pety et al. (2017), HCN and HCO$^+$ were also found to trace gas down to Av>2. As a result, published extragalactic HCN studies must be tracing all of the moderate density gas. Estimating the contribution of this moderate density gas from the typical column density PDFs in nearby clouds, we obtained the following $G_0$-dependent HCN conversion factor for external galaxies:$\alpha_{Herschel-HCN}^{fit'}=64\times G_0^{-0.34}$. Re-estimating the dense gas masses in external galaxies with $\alpha_{Herschel-HCN}^{fit'}$, we found that SFE$_{dense}$ is remarkably constant over 8 orders of magnitude in dense gas mass. Our results confirm that SFE$_{dense}$ of galaxies is quasi-universal on a wide range of scales from ~1-10pc to >10kpc. Based on the tight link between SF and filamentary structure found in Herschel studies of nearby clouds, we argue that SFE$_{dense}$ is primarily set by the microphysics of SF in filaments., Comment: A&A accepted for publication