High-mass Prestellar Cores Research Articles

Absence of High-mass Prestellar Cores in the Orion Giant Molecular Cloud

A fundamental difference between “core-fed” and “clump-fed” star-formation theories lies in the existence or absence of high-mass cores at the prestellar stage. However, only a handful of such cores have been observed. Here, different than previous search in distributed star-formation regions in the Galactic plane, we search for high-mass prestellar cores in the Orion GMC, by observing the seven most massive starless cores selected from previous deep continuum surveys. We present ALMA Atacama Compact Array Band 6 and Band 7 continuum and line observations toward the seven cores, in which we identify nine dense cores at both bands. The derived maximum core mass is less than 11 M ⊙, based on different dust temperatures. We find no high-mass prestellar cores in this sample, aligning with the results of previous surveys, thereby challenging the existence of such cores in Orion. Outside Orion, further detailed studies are needed for remaining high-mass prestellar core candidates to confirm their status as massive, starless cores.

Filamentary mass accretion towards the high-mass protobinary system G11.92–0.61 MM2

ABSTRACT We present deep, sub-arcsecond ($\sim$2000 au) resolution ALMA 0.82-mm observations of the former high-mass prestellar core candidate G11.92–0.61 MM2, recently shown to be an $\sim$500 au-separation protobinary. Our observations show that G11.92–0.61 MM2, located in the G11.92–0.61 protocluster, lies on a filamentary structure traced by 0.82-mm continuum and N$_2$H$^+$(4-3) emission. The N$_2$H$^+$(4-3) spectra are multipeaked, indicative of multiple velocity components along the line of sight. To analyse the gas kinematics, we performed pixel-by-pixel Gaussian decomposition of the N$_2$H$^+$ spectra using scousepy and hierarchical clustering of the extracted velocity components using acorns. Seventy velocity- and position-coherent clusters (called ‘trees’) are identified in the N$_2$H$^+$-emitting gas, with the eight largest trees accounting for $\gt $60 per cent of the fitted velocity components. The primary tree, with $\sim$20 per cent of the fitted velocity components, displays a roughly north–south velocity gradient along the filamentary structure traced by the 0.82-mm continuum. Analysing an $\sim$0.17 pc-long substructure, we interpret its velocity gradient of $\sim$10.5 km s$^{-1}$ pc$^{-1}$ as tracing filamentary accretion towards MM2 and estimate a mass inflow rate of $\sim 1.8\times 10^{-4}$ to 1.2$\times 10^{-3}$ M$_\odot$ yr$^{-1}$. Based on the recent detection of a bipolar molecular outflow associated with MM2, accretion on to the protobinary is ongoing, likely fed by the larger scale filamentary accretion flows. If 50 per cent of the filamentary inflow reaches the protostars, each member of the protobinary would attain a mass of 8 M$_\odot$ within $\sim 1.6\times 10^5$ yr, comparable to the combined time-scale of the 70-μm- and mid-infrared-weak phases derived for ATLASGAL-TOP100 massive clumps using chemical clocks.

The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). IX. Physical Properties and Spatial Distribution of Cores in IRDCs

The initial conditions found in infrared dark clouds (IRDCs) provide insights on how high-mass stars and stellar clusters form. We have conducted high-angular resolution and high-sensitivity observations toward thirty-nine massive IRDC clumps, which have been mosaicked using the 12 and 7 m arrays from the Atacama Large Millimeter/submillimeter Array. The targets are 70 μm dark massive (220–4900 M ⊙), dense (>104 cm−3), and cold (∼10–20 K) clumps located at distances between 2 and 6 kpc. We identify an unprecedented number of 839 cores, with masses between 0.05 and 81 M ⊙ using 1.3 mm dust continuum emission. About 55% of the cores are low-mass (<1 M ⊙), whereas ≲1% (7/839) are high-mass (≳27 M ⊙). We detect no high-mass prestellar cores. The most massive cores (MMC) identified within individual clumps lack sufficient mass to form high-mass stars without additional mass feeding. We find that the mass of the MMCs is correlated with the clump surface density, implying denser clumps produce more massive cores. There is no significant mass segregation except for a few tentative detections. In contrast, most clumps show segregation once the clump density is considered instead of mass. Although the dust continuum emission resolves clumps in a network of filaments, some of which consist of hub-filament systems, the majority of the MMCs are not found in the hubs. Our analysis shows that high-mass cores and MMCs have no preferred location with respect to low-mass cores at the earliest stages of high-mass star formation.

Fragmentation of the High-mass “Starless” Core G10.21-0.31: A Coherent Evolutionary Picture for Star Formation

G10.21-0.31 is a 70 μm dark high-mass starless core (M > 300 M ⊙ within r < 0.15 pc) identified in the Spitzer, Herschel, and APEX continuum surveys, and is believed to harbor the initial stages of high-mass star formation. We present Atacama Large Millimeter/submillimeter Array (ALMA) and Submillimeter Array observations to resolve the internal structure of this promising high-mass starless core. Sensitive high-resolution ALMA 1.3 mm dust continuum emission reveals three cores of mass ranging within 11–18 M ⊙, characterized by a turbulent fragmentation. Cores 1, 2, and 3 represent a coherent evolution of three different stages, characterized by outflows (CO and SiO), gas temperature (H2CO), and deuteration (N2D+/N2H+). We confirm the potential for formation of high-mass stars in G10.21 and explore the evolution path of high-mass star formation. Yet, no high-mass prestellar core is present in G10.21. This suggests a dynamical star formation where cores grow in mass over time.

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92–0.61 MM2

We present high-resolution (≲160 au) Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations of the high-mass prestellar core candidate G11.92−0.61 MM2, which reveal that this source is in fact a protobinary system with a projected separation of 505 au. The binary components, MM2E and MM2W, are compact (radii <140 au) sources within the partially optically thick dust emission with α 0.9 cm−1.3 mm = 2.47–2.94. The 1.3 mm brightness temperatures, T b = 68.4/64.6 K for MM2E/MM2W, imply internal heating and minimum luminosities L * > 24.7 L ⊙ for MM2E and L * > 12.6 L ⊙ for MM2W. The compact sources are connected by a “bridge” of lower-surface-brightness dust emission and lie within more extended emission that may correspond to a circumbinary disk. The circumprotostellar gas mass, estimated from ∼0.″2 resolution VLA 0.9 cm observations assuming optically thin emission, is 6.8 ± 0.9 M ⊙. No line emission is detected toward MM2E and MM2W in our high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J = 2–1, in absorption against the 1.3 mm continuum, which likely traces a layer of cooler molecular material surrounding the protostars. We also report the discovery of a highly asymmetric bipolar molecular outflow that appears to be driven by MM2E and/or MM2W in new deep, ∼0.″5 resolution (1685 au) ALMA 0.82 mm observations. This outflow, traced by low-excitation CH3OH emission, indicates ongoing accretion onto the protobinary system. Overall, the super-Alfvénic models of Mignon-Risse et al. agree well with the observed properties of the MM2E/MM2W protobinary, suggesting that this system may be forming in an environment with a weak magnetic field.

Linking high- and low-mass star formation

Context. Astronomers have yet to establish whether high-mass protostars form from high-mass prestellar cores, similar to their lower-mass counterparts, or from lower-mass fragments at the heart of a pre-protostellar cluster undergoing large-scale collapse. Part of the uncertainty is due to a shortage of envelope structure data on protostars of a few tens of solar masses, where we expect to see a transition from intermediate-mass star formation to the high-mass process. Aims. We sought to derive the masses, luminosities, and envelope density profiles for eight sources in Cygnus-X, whose mass estimates in the literature placed them in the sampling gap. Combining these sources with similarly evolved sources in the literature enabled us to perform a meta-analysis of protostellar envelope parameters over six decades in source luminosity. Methods. We performed spectral energy distribution fitting on archival broadband photometric continuum data from 1.2 to 850 μm to derive bolometric luminosities for our eight sources plus initial mass and radius estimates for modelling density and temperature profiles with the radiative-transfer package Transphere. Results. The envelope masses, densities at 1000 AU, outer envelope radii, and density power law indices as functions of bolometric luminosity all follow established trends in the literature spanning six decades in luminosity. Most of our sources occupy an intermediate to moderately high range of masses and luminosities, which helps to more firmly establish the continuity between low- and high-mass star formation mechanisms. Our density power law indices are consistent with observed values in the literature, which show no discernible trends with luminosity, and have a mean p = −1.4 ± 0.4. However, our sub-sample, with a mean power law index of −1.1 ± 0.3, is slightly flatter than would be expected for spherical envelopes in free fall (p = −1.5). Conclusions. We attribute flattened density profiles for our eight sources to one or more of the following: ongoing accretion from their natal filaments, convolution of sources with neighbours or the larger filament, spherical averaging of asymmetric features (for example fragments), or inflation of the envelope by a moderate far-ultraviolet field. Finally, we show that the trends in all of the envelope parameters for high-mass protostars are statistically indistinguishable from trends in the same variables for low-mass protostars.

Identification of pre-stellar cores in high-mass star forming clumps via H2D+ observations with ALMA

Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, which are the high-mass pre-stellar cores. However, no conclusive observation of such objects has been made to date. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark massive clumps. Our goal is to identify the core population that they harbour and to investigate their physical and chemical properties at high spatial resolution. Methods. We obtained Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 6 observations of continuum emission at 0.8 mm and of the ortho-H2D+ transition at 372 GHz towards the two clumps. We used the SCIMES algorithm to identify substructures (i.e. cores) in the position-position-velocity space, finding 16 cores. We modelled their observed spectra using a Bayesian fitting approach in the approximation of local thermodynamic equilibrium. We derived the centroid velocity, the line width, and the molecular column density maps. We also studied the correlation between the continuum and molecular data, which in general do not present the same structure. Results. We report, for the first time, the detection of ortho-H2D+ in high-mass star-forming regions performed with an interferometer. The molecular emission shows narrow and subsonic lines, suggesting that locally, the temperature of the gas is below 10 K. From the continuum emission, we estimated the cores’ total masses and compare them with the respective virial masses. We also computed the volume density values, which are found to be higher than 106 cm−3. Conclusions. Our data confirm that ortho-H2D+ is an ideal tracer of cold and dense gas. Interestingly, almost all the H2D+-identified cores are less massive than ≈13 M⊙, with the exception of one core in AG354, which could be as massive as 39 M⊙ under the assumption of low dust temperature (5 K). Furthermore, most of them are sub-virial and larger than their Jeans masses. These results are hence difficult to explain in the context of the turbulent accretion models, which predict massive and virialised pre-stellar cores. However, we cannot exclude that the cores are still in the process of accreting mass and that magnetic fields are providing enough support for the virialisation. ALMA could also be seeing only the innermost parts of the cores, and hence the cores’ total masses could be higher than inferred in this work. Furthermore, we note that the total masses of the investigated clumps are below the average for typical high-mass clumps, and thus studies of more massive sources are needed.

The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). I. Pilot Survey: Clump Fragmentation

Abstract The ALMA Survey of 70 μm dark High-mass clumps in Early Stages (ASHES) is designed to systematically characterize the earliest stages and constrain theories of high-mass star formation. Twelve massive (&gt;500 ), cold (≤15 K), 3.6–70 μm dark prestellar clump candidates, embedded in infrared dark clouds, were carefully selected in the pilot survey to be observed with the Atacama Large Millimeter/submillimeter Array (ALMA). We have mosaicked each clump (∼1 arcmin2) in continuum and line emission with the 12 m, 7 m, and Total Power (TP) arrays at 224 GHz (1.34 mm), resulting in ∼1.″2 resolution (∼4800 au, at the average source distance). As the first paper in the series, we concentrate on the continuum emission to reveal clump fragmentation. We detect 294 cores, from which 84 (29%) are categorized as protostellar based on outflow activity or “warm core” line emission. The remaining 210 (71%) are considered prestellar core candidates. The number of detected cores is independent of the mass sensitivity range of the observations and, on average, more massive clumps tend to form more cores. We find a large population of low-mass (&lt;1 ) cores and no high-mass (&gt;30 ) prestellar cores (maximum mass 11 ). From the prestellar core mass function, we derive a power-law index of 1.17 ± 0.10, which is slightly shallower than Salpeter. We used the minimum spanning tree (MST) technique to characterize the separation between cores and their spatial distribution, and to derive mass segregation ratios. While there is a range of core masses and separations detected in the sample, the mean separation and mass per clump are well explained by thermal Jeans fragmentation and are inconsistent with turbulent Jeans fragmentation. Core spatial distribution is well described by hierarchical subclustering rather than centrally peaked clustering. There is no conclusive evidence of mass segregation. We test several theoretical conditions and conclude that overall, competitive accretion and global hierarchical collapse scenarios are favored over the turbulent core accretion scenario.

On the ALMA observability of nascent massive multiple systems formed by gravitational instability

Massive young stellar object (MYSOs) form during the collapse of high-mass pre-stellar cores, where infalling molecular material is accreted through a centrifugally-balanced accretion disc that is subject to efficient gravitational instabilities. In the resulting fragmented accretion disc of the MYSO, gaseous clumps and low-mass stellar companions can form, which will influence the future evolution of massive protostars in the Hertzsprung-Russell diagram. We perform dust continuum radiative transfer calculations and compute synthetic images of disc structures modelled by the gravito-radiation-hydrodynamics simulation of a forming MYSO, in order to investigate the Atacama Large Millimeter/submillimeter Array (ALMA) observability of circumstellar gaseous clumps and forming multiple systems. Both spiral arms and gaseous clumps located at ~a few 100 au from the protostar can be resolved by interferometric ALMA Cycle 7 C43-8 and C43-10 observations at band 6 (1.2 mm), using a maximal 0.015" beam angular resolution and at least 10-30 min exposure time for sources at distances of 1-2 kpc. Our study shows that substructures are observable regardless of their viewing geometry or can be inferred in the case of an edge-viewed disc. The observation probability of the clumps increases with the gradually increasing efficiency of gravitational instability at work as the disc evolves. As a consequence, large discs around MYSOs close to the zero-age-main-sequence line exhibit more substructures than at the end of the gravitational collapse. Our results motivate further observational campaigns devoted to the close surroundings of the massive protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a probable signature of disc fragmentation and accretion variability.

Molecular analysis of a high-mass prestellar core candidate in W43-MM1

Context. 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. Aims. We aim to characterise the chemistry in this high-mass prestellar core candidate, referred to as W43-MM1 core #6, and its environment. Methods. 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. Results. 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 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 ~ 80 K. Conclusions. 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.

Lack of high-mass pre-stellar cores in the starless MDCs of NGC 6334

Context. The formation of high-mass stars remains unknown in many aspects. There are two competing families of models to explain the formation of high-mass stars. On the one hand, quasi-static models predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence. On the other hand, competitive accretion models predict that high-mass proto-stellar cores evolve from low or intermediate mass proto-stellar cores in dynamic environments. Aims. The aim of the present work is to bring observational constraints at the scale of high-mass cores (~0.03 pc). Methods. We targeted with ALMA and MOPRA a sample of nine starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study. Their mass and size (~110 M⊙ and r = 0.1 pc, respectively) are similar to the initial conditions used in the quasi-static family of models explaining for the formation of high-mass stars. We present ALMA 1.4 mm continuum observations that resolve the Jeans length (λJeans ~ 0.03 pc) and that are sensitive to the Jeans mass (MJeans ~ 0.65 M⊙) in the nine starless MDCs, together with ALMA-12CO(2–1) emission line observations. We also present HCO+(1–0), H13CO+(1–0) and N2H+(1–0) molecular lines from the MOPRA telescope for eight of the nine MDCs. Results. The nine starless MDCs have the mass reservoir to form high-mass stars according to the criteria by Baldeschi et al. (2017). Three of the starless MDCs are subvirialized with αvir ~ 0.35, and four MDCs show sign of collapse from their molecular emission lines. ALMA observations show very little fragmentation within the MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to &lt;3 M⊙ fragments. Therefore, the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer. Conclusions. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC 6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.

Detection of a high-mass prestellar core candidate in W43-MM1

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 13CS(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⊙ 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.

The earliest phases of high-mass star formation, as seen in NGC 6334 by Herschel-HOBYS

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel/PACS and SPIRE 70−500 μm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc × 1 pc ridge and two 0.8 pc × 0.8 pc hubs, with volume-averaged densities of ~105 cm-3. Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M⊙ for MDCs in NGC 6334. MDCs have temperatures of 9.5−40 K, masses of 75−1000 M⊙, and densities of 1 × 105−7 × 107 cm-3. Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 μm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1−7 × 104 yr and at most 3 × 105 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC 6334, favoring a scenario presented here, which simultaneously forms clouds, ridges, MDCs, and high-mass protostars.

Modified Bonnor-Ebert spheres with ambipolar diffusion heating

Magnetic fluctuations through the molecular cloud cores can produce ambipolar diffusion (AD) heating, which consequently can produce temperature gradients through the core. The aim of this paper is to investigate the effects of these produced temperature gradients on the radius and mass of the non-isothermal modified Bonnor-Ebert spheres (MBES). Here, we use the parameter $\kappa$ to represent the magnetic fluctuations through the molecular cloud cores. This parameter introduces the change of magnetic filed strength in the length-scale. The results show that increasing of $\kappa$ leads to an increase of the radius and mass of MBES. The most important result is existence of the gravitationally stable high-mass prestellar cores at the low-density molecular medium with great magnetic fluctuations.

High-Mass Star and Massive Cluster Formation in the Milky Way

This review examines the state-of-the-art knowledge of high-mass star and massive cluster formation, gained from ambitious observational surveys, which acknowledges the multiscale characteristics of these processes. After a brief overview of theoretical models and main open issues, we present observational searches for the evolutionary phases of high-mass star formation, first among high-luminosity sources and more recently among young massive protostars and the elusive high-mass prestellar cores. We then introduce the most likely evolutionary scenario for high-mass star formation, which emphasizes the link of high-mass star formation to massive cloud and cluster formation. Finally, we introduce the first attempts to search for variations of the star-formation activity and cluster formation in molecular cloud complexes in the most extreme star-forming sites and across the Milky Way. The combination of Galactic plane surveys and high–angular resolution images with submillimeter facilities such as Atacama Large Millimeter Array (ALMA) are prerequisites to make significant progress in the forthcoming decade.

HOBYS and W43-HERO: Two more steps toward a Galaxy-wide understanding of high-mass star formation

AbstractThe Herschel/HOBYS key program allows to statistically study the formation of 10−20 M⊙ stars. The IRAM/W43-HERO large program is itself dedicated to the much more extreme W43 molecular complex, which forms stars up to 50 M⊙. Both reveal high-density cloud filaments of several pc3, which are forming clusters of OB-type stars. Given their activity, these so-called mini-starburst cloud ridges could be seen as “miniature and instant models” of starburst galaxies. Both surveys also strongly suggest that high-mass prestellar cores do not exist, in agreement with the dynamical formation of cloud ridges. The HOBYS and W43 surveys are necessary steps towards Galaxy-wide studies of high-mass star formation.

HOBYS insights on high-mass star formation

The Herschel /HOBYS key program allows to statistically study the formation of 10 − 20 M ⊙ stars. It reveals high-density cloud filaments of several pc3 , which are forming clusters of OB-type stars. It also strongly suggests and higher-angular resolution images tend to confirm that high-mass prestellar cores do not exist.

The physical conditions in IRDC clumps fromHerschel/HIFI observations of H2O

Context. The earliest phases of high-mass star formation are poorly understood. Aims. Our goal is to determine the physical conditions and kinematic structure of massive star-forming cloud clumps. Methods. We analyze H$_2$O 557 GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps . By comparison with ground-based C$^{17}$O, N$_2$H$^+$, CH$_3$OH and NH$_3$ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H$_2$O and N$_2$H$^+$. Results. The observed water lines are complex with emission and absorption components. The absorption is red shifted and consistent with a cold envelope, while the emission is interpreted as resulting from protostellar outflows. The gas density in the clumps is $\sim$ 10$^7$ cm$^{-3}$. The o-H2O outflow column density is 0.3 to 3.0 10$^{14}$ cm$^{-2}$, the o-H2O absorption column density is between 1.5 10$^{14}$ and 2.6 10$^{15}$ cm$^-2$ with cold o-H2O abundances between 1.5 10$^{-9}$ and 3.1 10$^{-8}$. Conclusions. All clumps have high gas densities ($\sim$ 10$^7$ cm$^{-3}$) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H$_2$O N$_2$H$^+$, NH$_3$ and CH$_3$OH. We find that G28-MM is the most evolved, followed by G11-MM then G28-NH3. The least evolved clump is G11-NH3 which shows no sign-posts of star-formation. G11-NH3 is a high-mass pre-stellar core.

CO outflows from high-mass Class 0 protostars in Cygnus-X

As natural consequences of the accretion process, outflows are one of the few (indirect) tracers of accretion. We used CO(2-1) PdBI observations towards 6 MDCs in Cygnus-X, containing 9 high-mass cores, to investigate what the accretion process and origin of the material feeding the precursors of high-mass stars are. We compared our sample to low-mass objects from the literature and developed simple evolutionary models to reproduce the observables. We find that 8/9 high-mass cores drive clear individual outflows as true equivalents of Class 0 protostars in the high-mass regime. The remaining core has only a tentative outflow detection. It could be amongst the first examples of a true individual high-mass prestellar core. We find that the momentum flux of high-mass objects has a linear relation to the envelope mass, as a scale-up of the relations found for low-mass protostars. This suggests a fundamental proportionality between accretion rates and mass reservoir, suggesting identical collapse timescales for all masses. We conclude that if the pre-collapse evolution is quasi-static, the fragmentation scale (which is similar for all masses) would limit the size of the initial mass reservoirs, leading to shorter free-fall times for higher mass stars. However, as we find identical collapse timescales for all masses, a significant turbulent/magnetic support is needed to slow down the collapse of the more massive envelopes in a quasi-static view. With this support still to be discovered, and with indications of large dynamics in pre-collapse gas for high-mass star formation, we propose that such an identical collapse timescale implies that the initial densities, which should set the duration of the collapse, should be similar for all masses. This suggests that the mass that incorporates massive stars has to have been accreted in a dynamical way from larger scales than those of low-mass stars.

Initial highlights of the HOBYS key program, theHerschelimaging survey of OB young stellar objects

We present the initial highlights of the HOBYS key program, which are based on Herschel images of the Rosette molecular complex and maps of the RCW120 H ii region. Using both SPIRE at 250/350/500 μm and PACS at 70/160 μm or 100/160 μm, the HOBYS survey provides an unbiased and complete census of intermediate- to high-mass young stellar objects, some of which are not detected by Spitzer. Key core properties, such as bolometric luminosity and mass (as derived from spectral energy distributions), are used to constrain their evolutionary stages. We identify a handful of high-mass prestellar cores and show that their lifetimes could be shorter in the Rosette molecular complex than in nearby low-mass star-forming regions. We also quantify the impact of expanding H ii regions on the star formation process acting in both Rosette and RCW 120.