Search

Context. The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive-star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1–0) and CO(2–1). The recent detection of 23 clouds in the higher excitation transition CO(3–2) within the XUV disk of M83 thus requires an explanation. Aims. We test the hypothesis introduced to explain the non-detections and recent detection simultaneously: The clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having dense star-forming clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3–2) emission and are surrounded by less dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there. Methods. We utilize new high-resolution ALMA CO(3–2) observations of a subset (11) of the 23 clouds in the XUV disk of M83. Results. We confirm the compactness of the CO(3–2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6–9 pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, a local star-forming cloud with massive-star formation. Conclusions. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud's internal physics rather than by external triggers. This simple model explains the current observations of clouds with ongoing massive-star formation, although it may require some adjustment, for example including the effect of cloud evolution, to describe star formation in molecular clouds more generally. [ABSTRACT FROM AUTHOR]

Context. The impact of stellar feedback on the Kennicutt–Schmidt (KS) law, which relates the star formation rate (SFR) to the surface gas density, is a topic of ongoing debate. The interpretation of high-resolution observations of individual clouds is challenging due to the various processes at play simultaneously and inherent biases. Therefore, a numerical investigation is necessary to understand the role of stellar feedback and identify observable signatures. Aims. In this study we investigate the impact of stellar feedback on the KS law, aiming to identify distinct signatures that can be observed and analysed. By employing magnetohydrodynamic simulations of an isolated cloud, we specifically isolate the effects of high-mass star radiation feedback and protostellar jets. High-resolution numerical simulations are a valuable tool for isolating the impact of stellar feedback on the star formation process, while also allowing us to assess how observational biases may affect the derived relation. Methods. We used high-resolution (<0.01 pc) magnetohydrodynamic numerical simulations of a 104M⊙ cloud and followed its evolution under different feedback prescriptions. The set of simulations contained four types of feedback: one with only protostellar jets, one with ionising radiation from massive stars (>8 M⊙), one with the combination of the two, and one without any stellar feedback. In order to compare these simulations with the existing observational results, we analysed their evolution by adopting the same techniques applied in the observational studies. Then, we simulated how the same analyses would change if the data were affected by typical observational biases: counting young stellar objects (YSO) to estimate the SFR, the limited resolution for the column density maps, and a sensitivity threshold for detecting faint embedded YSOs. Results. Our analysis reveals that the presence of stellar feedback strongly influences the shape of the KS relation and the star formation efficiency per free-fall time (ϵff). The impact of feedback on the relation is primarily governed by its influence on the cloud's structure. Additionally, the evolution of ϵff throughout the star formation event suggests that variations in this quantity can mask the impact of feedback in observational studies that do not account for the evolutionary stage of the clouds. Although the ϵff measured in our clouds is higher than what is usually observed in real clouds, upon applying prescriptions to mimic observational biases we recover a good agreement with the expected values. From that, we can infer that observations tend to underestimate the total SFR. Moreover, this likely indicates that the physics included in our simulations is sufficient to reproduce the basic mechanisms that contribute to setting ϵff. Conclusions. We demonstrate the interest of employing numerical simulations to address the impact of early feedback on star formation laws and to correctly interpret observational data. This study will be extended to other types of molecular clouds and ionising stars, sampling different feedback strengths, to fully characterise the impact of H II regions on star formation. [ABSTRACT FROM AUTHOR]

Aims. Our goal is to better understand the origin and the star-formation history of regions NGC 6334 and NGC 6357. We focus our study on the kinematics of young stars (young stellar objects and OB stars) in both regions mainly on the basis of the Gaia DR2 data. Methods. For both regions, we compiled catalogs of OB stars and young stellar objects from the literature and complemented them using VPHAS+ DR2 and Spitzer IRAC/GLIMPSE photometry catalogues. We applied a cross-match with the Gaia DR2 catalog to obtain information on the parallax and transverse motion. Results. We confirm that NGC 6334 and NGC 6357 are in the far side of the Saggitarius-Carina arm at a distance of 1.76 kpc. For NGC 6357, OB stars show strong clustering and ordered star motion with Vlon ∼–10.7 km s−1 and Vlat ∼3.7 km s−1, whereas for NGC 6334, no significant systemic motion was observed. The OB stars motions and distribution in NGC 6334 suggest that it should be classified as an association. Ten runaway candidates may be related to NGC 6357 and two to NGC 6334, respectively. The spatial distributions of the runaway candidates in and around NGC 6357 favor a dynamical (and early) ejection during the cluster(s) formation. Because such stars are likely to be ejected during a cluster's formation, the fact that not as many such stars are observed towards NGC 6334 suggests different formation conditions than have been assumed for NGC 6357. [ABSTRACT FROM AUTHOR]

Aims. We aim to investigate the impact of the ionized radiation from the M 16 H II region on the surrounding molecular cloud and on its hosted star formation. Methods. To present comprehensive multi-wavelength observations towards the M 16 H II region, we used new CO data and existing infrared, optical, and submillimeter data. The 12CO J = 1−0, 13CO J = 1−0, and C18O J = 1−0 data were obtained with the Purple Mountain Observatory (PMO) 13.7 m radio telescope. To trace massive clumps and extract young stellar objects (YSOs) associated with the M 16 H II region, we used the ATLASGAL and GLIMPSE I catalogs, respectively. Results. From CO data, we discern a large-scale filament with three velocity components. Because these three components overlap with each other in both velocity and space, the filament may be made of three layers. The M 16 ionized gas interacts with the large-scale filament and has reshaped its structure. In the large-scale filament, we find 51 compact cores from the ATLASGAL catalog, 20 of them being quiescent. The mean excitation temperature of these cores is 22.5 K, while this is 22.2 K for the quiescent cores. This high temperature observed for the quiescent cores suggests that the cores may be heated by M 16 and do not experience internal heating from sources in the cores. Through the relationship between the mass and radius of these cores, we obtain that 45% of all the cores are massive enough to potentially form massive stars. Compared with the thermal motion, the turbulence created by the nonthermal motion is responsible for the core formation. For the pillars observed towards M 16, the H II region may give rise to the strong turbulence. [ABSTRACT FROM AUTHOR]

Aims. To constrain models of high-mass star formation it is important to identify the massive dense cores (MDCs) that are able to form high-mass star(s). This is one of the purposes of the Herschel/HOBYS key programme. Here, we carry out the census and characterise of the properties of the MDCs population of the NGC 6357 H II region. Methods. Our study is based on the Herschel/PACS and SPIRE 70−500 μm images of NGC 6357 complemented with (sub-)millimetre and mid-infrared data. We followed the procedure established by the Herschel/HOBYS consortium to extract ~0.1 pc massive dense cores using the getsources software. We estimated their physical parameters (temperatures, masses, luminosities) from spectral energy distribution (SED) fitting. Results. We obtain a complete census of 23 massive dense cores, amongst which one is found to be IR-quiet and twelve are starless, representing very early stages of the star-formation process. Focussing on the starless MDCs, we have considered their evolutionary status, and suggest that only five of them are likely to form a high-mass star. Conclusions. We find that, contrarily to the case in NGC 6334, the NGC 6357 region does not exhibit any ridge or hub features that are believed to be crucial to the massive star formation process. This study adds support for an empirical model in which massive dense cores and protostars simultaneously accrete mass from the surrounding filaments. In addition, the massive star formation in NGC 6357 seems to have stopped and the hottest stars in Pismis 24 have disrupted the filaments. [ABSTRACT FROM AUTHOR]

Context. The star formation process requires the dust and gas present in the Milky Way to self-assemble into dense reservoirs of neutral material where the new generation of stars will emerge. Star-forming regions are usually studied in the context of Galactic surveys, but dedicated observations are sometimes needed when the study reaches beyond the survey area. Aims. A better understanding of the star formation process in the Galaxy can be obtained by studying several regions. This allows increasing the sample of objects (clumps, cores, and stars) for further statistical works and deeper follow-up studies. Here, we studied the G345.5+1.5 region, which is located slightly above the Galactic plane, to understand its star formation properties. Methods. We combined Large Apex BOlometer CAmera (LABOCA) and 12CO(4−3) transition line (NANTEN2) observations complemented with the Hi-GAL and Spitzer-GLIMPSE surveys to study the star formation toward this region. We used the Clumpfind algorithm to extract the clumps from the 870 μm and 12CO(4−3) data. Radio emission at 36 cm was used to estimate the number of H II regions and to remove the contamination from the free–free emission at 870 μm. We employed color–color diagrams and spectral energy distribution (SED) slopes to distinguish between prestellar and protostellar clumps. We studied the boundedness of the clumps through the virial parameter. Finally, we estimated the star formation efficiency (SFE) and star formation rate (SFR) of the region and used the Schmidt–Kennicutt diagram to compare its ability to form stars with other regions of the Galactic plane. Results. Of the 13 radio sources that we found using the MGPS-2 catalog, 7 are found to be associated with H II regions corresponding to late-B or early-O stars. We found 45 870 μm clumps with diameters between 0.4 and 1.2 pc and masses between 43 M⊙ and 3923 M⊙, and 107 12CO clumps with diameters between 0.4 and 1.3 pc and masses between 28 M⊙ and 9433 M⊙. More than 50% of the clumps are protostellar and bounded and are able to host (massive) star formation. High SFR and SFR density (ΣSFR) values are associated with the region, with an SFE of a few percent. Conclusions. With submillimeter, CO transition, and short-wavelength infrared observations, our study reveals a population of massive stars, protostellar and bound starless clumps, toward G345.5+1.5. This region is therefore actively forming stars, and its location in the starburst quadrant of the Schmidt–Kennicutt diagram is comparable to other star-forming regions found within the Galactic plane. [ABSTRACT FROM AUTHOR]

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 <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. [ABSTRACT FROM AUTHOR]

Aims. We aim to identify bipolar Galactic H II regions and to understand their parental cloud structures, morphologies, evolution, and impact on the formation of new generations of stars. Methods. We use the Spitzer-GLIMPSE, Spitzer-MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar H II regions and to examine their morphologies. We search for their exciting star(s) using NIR data from the 2MASS, UKIDSS, and VISTA surveys. Massive molecular clumps are detected near these bipolar nebulae, and we estimate their temperatures, column densities, masses, and densities. We locate Class 0/I young stellar objects (YSOs) in their vicinities using the Spitzer and Herschel-PACS emission. Results. Numerical simulations suggest bipolar H II regions form and evolve in a two-dimensional flat- or sheet-like molecular cloud. We identified 16 bipolar nebulae in a zone of the Galactic plane between ℓ ± 60° and |b| < 1°. This small number, when compared with the 1377 bubble H II regions in the same area, suggests that most H II regions form and evolve in a three-dimensional medium. We present the catalogue of the 16 bipolar nebulae and a detailed investigation for six of these. Our results suggest that these regions formed in dense and flat structures that contain filaments. We find that bipolar H II regions have massive clumps in their surroundings. The most compact and massive clumps are always located at the waist of the bipolar nebula, adjacent to the ionised gas. These massive clumps are dense, with a mean density in the range of 105 cm−3 to several 106 cm−3 in their centres. Luminous Class 0/I sources of several thousand solar luminosities, many of which have associated maser emission, are embedded inside these clumps. We suggest that most, if not all, massive 0/I YSO formation has probably been triggered by the expansion of the central bipolar nebula, but the processes involved are still unknown. Modelling of such nebula is needed to understand the star formation processes at play. [ABSTRACT FROM AUTHOR]

Aims. The formation of high-mass stars is still debated. For this reason, several projects such as Herschel-HOBYS are focussed on the study of the earliest phases of massive star formation. As a result, massive star-forming complexes such as NGC 6334 and NGC 6357 have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to better characterise the environments of these cores we need to understand the previous massive star formation history. To better characterise the environment of these massive dense cores we study the previous high-mass star formation and how these stars act on their environments. Methods. This study is based on the spectral classification of the OB stars identified towards NGC 6334 and NGC 6357 with spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of 109 stars across these regions we were able to evaluate the following: distance, age, mass, global star-forming efficiency (SFE), and star formation rate (SFR) of the regions. The physical conditions of the ionised gas for both complexes was also derived. Results.We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1 kpc to 2.2 kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ~10 Myr are broadly spread across these complexes, while younger stars are mainly located in the H II regions and stellar clusters. Our data also suggests that some of the young stars can be considered runaway stars. We evaluate a SFE of 0.019+0:008 -0:007 and 0.021+0:004 -0:003 and a SFR of 1:1 X 103 ± 300 M☉Myr-1 and 1:7 X 103 ± 400 M☉ Myr-1 for NGC 6334 and NGC 6357, respectively. We note that 29 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 are more impacted by X-ray flux and stellar winds than in NGC 6334. Finally, from the analysis of nebular lines (H , [NII], and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated. [ABSTRACT FROM AUTHOR]

We present a comprehensive statistical analysis of star-forming objects located in the vicinities of 1 360 bubble structures throughout the Galactic Plane and their local environments. The compilation of ~70 000 star-forming sources, found in the proximity of the ionized (Hii) regions and detected in both Hi-GAL and GLIMPSE surveys, provided a broad overview of the different evolutionary stages of star-formation in bubbles, from prestellar objects to more evolved young stellar objects (YSOs). Surface density maps of star-forming objects clearly reveal an evolutionary trend where more evolved star-forming objects are found spatially located near the center, while younger star-forming objects are found at the edge of the bubbles. We derived dynamic ages for a subsample of 182 Hii regions for which kinematic distances and radio continuum flux measurements were available. We detect ~80% more star-forming sources per unit area in the direction of bubbles than in the surrounding fields. We estimate ~10% clump formation efficiency (CFE) of Hi-GAL clumps in bubbles, twice the CFE in fields not affected by feedback. We find higher CFE of protostellar clumps in younger bubbles, whose density of the bubble shells is higher. We argue that the formation rate from prestellar to protostellar phase is probably higher during the early stages of the bubble expansion. Evaluation of the fragmentation time inside the shell of bubbles advocates the preexistence of clumps in the medium before the bubble, as supported by numerical simulations. Approximately 23% of the Hi-GAL clumps are found located in the direction of a bubble, with 15% for prestellar clumps and 41% for protostellar clumps. We argue that the high fraction of protostellar clumps may be due to the acceleration of the star-formation process cause by the feedback of the (Hii) bubbles. [ABSTRACT FROM AUTHOR]

Context. Triggered star formation around Hii regions could be an important process. The Galactic Hii region RCW79 is a prototypical object for triggered high-mass star formation. Aims. We aim to obtain a census of the young stellar population observed at the edges of the Hii region and to determine the properties of the young sources in order to characterize the star formation processes that take place at the edges of this ionized region. Methods. We take advantage of Herschel data from the surveys HOBYS, "Evolution of Interstellar Dust", and Hi-Gal to extract compact sources. We use the algorithm getsources. We complement the Herschel data with archival 2MASS, Spitzer, and WISE data to determine the physical parameters of the sources (e.g., envelope mass, dust temperature, and luminosity) by fitting the spectral energy distribution. Results. We created the dust temperature and column density maps along with the column density probability distribution function (PDF) for the entire RCW79 region. We obtained a sample of 50 compact sources in this region, 96% of which are situated in the ionization-compressed layer of cold and dense gas that is characterized by the column density PDF with a double-peaked lognormal distribution. The 50 sources have sizes of ~0.1-0.4 pc with a typical value of ~0.2 pc, temperatures of ~11-26 K, envelope masses of ~6-760 M⊙, densities of ~0.1-44 × 105 cm-3, and luminosities of ~19-12 712 L⊙. The sources are classified into 16 class 0, 19 intermediate, and 15 class I objects. Their distribution follows the evolutionary tracks in the diagram of bolometric luminosity versus envelope mass (Lbol - Menv) well. A mass threshold of 140 M⊙, determined from the Lbol - Menv diagram, yields 12 candidate massive dense cores that may form high-mass stars. The core formation efficiency (CFE) for the 8 massive condensations shows an increasing trend of the CFE with density. This suggests that the denser the condensation, the higher the fraction of its mass transformation into dense cores, as previously observed in other high-mass star-forming regions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Herschel survey of the Galactic Plane (Hi-GAL) is a continuum Galactic plane survey in five wavebands at 70, 160, 250, 350 and 500 µm. From such images, about 150 000 sources have been extracted for which the distance determination is a challenge. In this context the velocity of these sources has been determined thanks to a large number of molecular data cubes. But to convert the velocity to kinematic distance, one needs to adopt a rotation curve for our Galaxy. For three different samples of tracers, we test different analytical forms. We find that the power-law expression, θ(R)/θ0 = 1.022 (R/R0)0.0803 with R0, θ0 = 8.34 kpc, 240 kms-1 is a good and easily manipulated expression for the distance determination process. [ABSTRACT FROM AUTHOR]

Context. The expansion of Hii regions can trigger the formation of stars. An overdensity of young stellar objects is observed at the edges of Hii regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between Hii -region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. Aims. We aim to characterize the star formation observed at the edges of Hii regions by studying the properties of young stars that form there. We aim to detect young sources, derive their properties and their evolution stage in order to discuss the possible causal link between the first-generation massive stars that form the Hii region and the young sources observed at their edges. Methods. We have observed the Galactic Hii region RCW 120 with Herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500 µm. We produced temperature and H2 column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at Herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. Results. The overall temperatures of the region (without background subtraction) range from 15 K to 24 K. The warmest regions are observed towards the ionized gas. The coldest regions are observed outside the ionized gas and follow the emission of the cold material previously detected at 870 mand 1.3 mm. The H2 column density map reveals the distribution of the cold medium to be organized in filaments and highly structured. Column densities range from 7 × 1021 cm-2 up to 9 × 1023 cm-2 without background subtraction. The cold regions observed outside the ionized gas are the densest and host star formation when the column density exceeds 2 × 1022 cm-2. The most reliable 35 compact sources are discussed. Using existing CO data and morphological arguments we show that these sources are likely to be associated with the RCW 120 region. These sources' volume densities range from 2 × 105 cm-3 to 108 cm-3. Five sources have envelope masses larger than 50 M∡ and are all observed in high column density regions (>7 × 1022 cm-2). We find that the evolutionary stage of the sources primarily depends on the density of their hosting condensation and is not correlated with the distance to the ionizing star. Conclusions. The Herschel data, with their unique sampling of the far infrared domain, have allowed us to characterize the properties of compact sources observed towards RCW 120 for the first time. We have also been able to determine the envelope temperature, envelope mass and evolutionary stage of these sources. Using these properties we have shown that the density of the condensations that host star formation is a key parameter of the star-formation history, irrespective of their projected distance to the ionizing stars. [ABSTRACT FROM AUTHOR]

Context. The surroundings of H II regions can have a profound influence on their development, morphology, and evolution. This paper explores the effect of the environment on H II regions in the MonR2 molecular cloud. Aims. We aim to investigate the density structure of envelopes surrounding H II regions and to determine their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an ideal target since it hosts an H II region association, which has been imaged by the Herschel PACS and SPIRE cameras as part of the HOBYS key programme. Methods. Column density and temperature images derived from Herschel data were used together to model the structure of H II bubbles and their surrounding envelopes. The resulting observational constraints were used to follow the development of the Mon R2 ionised regions with analytical calculations and numerical simulations. Results. The four hot bubbles associated with H II regions are surrounded by dense, cold, and neutral gas envelopes, which are partly embedded in filaments. The envelope's radial density profiles are reminiscent of those of low-mass protostellar envelopes. The inner parts of envelopes of all four H II regions could be free-falling because they display shallow density profiles: ρ(r) ∝ r-q with q ≤ 1:5. As for their outer parts, the two compact H II regions show a ρ(r) ∝ r-2 profile, which is typical of the equilibrium structure of a singular isothermal sphere. In contrast, the central UCH II region shows a steeper outer profile, ρ(r) ∝ r-2.5, that could be interpreted as material being forced to collapse, where an external agent overwhelms the internal pressure support. Conclusions. The size of the heated bubbles, the spectral type of the irradiating stars, and the mean initial neutral gas density are used to estimate the ionisation expansion time, texp ∼ 0:1 Myr, for the dense UCH II and compact H II regions and ∼0:35 Myr for the extended H II region. Numerical simulations with and without gravity show that the so-called lifetime problem of H II regions is an artefact of theories that do not take their surrounding neutral envelopes with slowly decreasing density profiles into account. The envelope transition radii between the shallow and steeper density profiles are used to estimate the time elapsed since the formation of the first protostellar embryo, tinf ∼ 1 Myr, for the ultra-compact, 1:5-3 Myr for the compact, and greater than ∼6 Myr for the extended H II regions. These results suggest that the time needed to form a OB-star embryo and to start ionising the cloud, plus the quenching time due to the large gravitational potential amplified by further in-falling material, dominates the ionisation expansion time by a large factor. Accurate determination of the quenching time of H II regions would require additional small-scale observationnal constraints and numerical simulations including 3D geometry effects. [ABSTRACT FROM AUTHOR]

Context. The expansion of Galactic H II regions can trigger the formation of a new generation of stars. However, little is know about the physical conditions that prevail in these regions. Aims. We study the physical conditions that prevail in specific zones towards expanding H II regions that trace representative media such as the photodissociation region, the ionized region, and condensations with and without ongoing star formation. Methods. We use the SPIRE Fourier Transform Spectrometer (FTS) on board Herschel to observe the H II region RCW 120. Continuum and lines are observed in the 190-670 μm range. Line intensities and line ratios are obtained and used as physical diagnostics of the gas. We used the Meudon PDR code and the RADEX code to derive the gas density and the radiation field at nine distinct positions including the PDR surface and regions with and without star-formation activity. Results. For the different regions we detect the atomic lines [NII] at 205 μm and [CI] at 370 and 609 μm, the 12CO ladder between the J = 4 and J = 13 levels and the 13CO ladder between the J = 5 and J = 14 levels, as well as CH+ in absorption. We find gas temperatures in the range 45-250 K for densities of 104-106 cm-3, and a high column density on the order of NH ~ 1022 cm-2 that is in agreement with dust analysis. The ubiquitousness of the atomic and CH+ emission suggests the presence of a low-density PDR throughout RCW 120. High-excitation lines of CO indicate the presence of irradiated dense structures or small dense clumps containing young stellar objects, while we also find a less dense medium (NH ~ 1020 cm-2) with high temperatures (80-200 K). [ABSTRACT FROM AUTHOR]

Context. The determination of the physical conditions in molecular clouds is a key step towards our understanding of their formation and evolution of associated star formation. We investigate the density, temperature, and column density of both dust and gas in the photodissociation regions (PDRs) located at the interface between the atomic and cold molecular gas of the NGC 7023 reflection nebula. We study how young stars affect the gas and dust in their environment. Aims. Several Herschel Space Telescope programs provide a wealth of spatial and spectral information of dust and gas in the heart of PDRs. We focus our study on Spectral and Photometric Image Receiver (SPIRE) Fourier-Transform Spectrometer (FTS) fully sampled maps that allow us for the first time to study the bulk of cool/warm dust and warm molecular gas (CO) together. In particular, we investigate if these populations spatially coincide, if and how the medium is structured, and if strong density and temperature gradients occur, within the limits of the spatial resolution obtained with Herschel. Methods. The SPIRE FTS fully sampled maps at different wavelengths are analysed towards the northwest (NW) and the east (E) PDRs in NGC 7023. We study the spatial and spectral energy distribution of a wealth of intermediate rotational 12CO 4 ⩽ Ju ⩽ 13 and 13CO 5 ⩽ Ju ⩽ 10 lines. A radiative transfer code is used to assess the gas kinetic temperature, density, and column density at different positions in the cloud. The dust continuum emission including Spitzer, the Photoconductor Array Camera and Spectrometer (PACS), and SPIRE photometric and the Institute for Radio Astronomy in the Millimeter Range (IRAM) telescope data is also analysed. Using a single modified black body and a radiative transfer model, we derive the dust temperature, density, and column density. Results. The cloud is highly inhomogeneous, containing several irradiated dense structures. Excited 12CO and 13CO lines and warm dust grains localised at the edge of the dense structures reveal high column densities of warm/cool dense matter. Both tracers give a good agreement in the local density, column density, and physical extent, leading to the conclusion that they trace the same regions. The derived density profiles show a steep gradient at the cloud edge reaching a maximum gas density of 105-106 cm-3 in the PDR NGC 7023 NW and 104-105 cm-3 in the PDR NGC 7023 E and a subsequent decrease inside the cloud. Close to the PDR edges, the dust temperature (30 K and 20 K for the NW and E PDRs, respectively) is lower than the gas temperature derived from CO lines (65-130 K and 45-55 K, respectively). Further inside the cloud, the dust and gas temperatures are similar. The derived thermal pressure is about 10 times higher in NGC 7023 NW than in NGC 7023 E. Comparing the physical conditions to the positions of known young stellar object candidates in NGC 7023 NW, we find that protostars seem to be spatially correlated with the dense structures. Conclusions. Our approach combining both dust and gas delivers strong constraints on the physical conditions of the PDRs. We find dense and warm molecular gas of high column density in the PDRs. [ABSTRACT FROM AUTHOR]

Aims. This work aims to improve the current understanding of the interaction between Hii regions and turbulent molecular clouds.We propose a new method to determine the age of a large sample of OB associations by investigating the development of their associated Hii regions in the surrounding turbulent medium. Methods. Using analytical solutions, one-dimensional (1D), and three-dimensional (3D) simulations, we constrained the expansion of the ionized bubble depending on the turbulence level of the parent molecular cloud. A grid of 1D simulations was then computed in order to build isochrone curves for Hii regions in a pressure–size diagram. This grid of models allowed us to date a large sample of OB associations that we obtained from the Hii Region Discovery Survey (HRDS). Results. Analytical solutions and numerical simulations showed that the expansion of Hii regions is slowed down by the turbulence up to the point where the pressure of the ionized gas is in a quasi-equilibrium with the turbulent ram pressure. Based on this result, we built a grid of 1D models of the expansion of Hii regions in a profile based on Larson’s laws. We take the 3D turbulence into account with an effective 1D temperature profile. The ages estimated by the isochrones of this grid agree well with literature values of well known regions such as Rosette, RCW 36, RCW 79, and M16. We thus propose that this method can be used to find ages of young OB associations through the Galaxy and also in nearby extra-galactic sources. [ABSTRACT FROM AUTHOR]

Aims. We investigate the star formation activity in the molecular complex associated with the Galactic H ii region Sh2-90. Methods. We obtain the distribution of the ionized and cold neutral gas using radio-continuum and Herschel observations. We use near-infrared and Spitzer data to investigate the stellar content of the complex. We discuss the evolutionary status of embedded massive young stellar objects (MYSOs) using their spectral energy distribution. Results. The Sh2-90 region presents a bubble morphology in the mid-infrared. Radio observations suggest it is an evolved H ii region with an electron density ~144 cm-3, emission measure ~ 6.7 × 104 cm-6 pc and an ionized mass ~55 M⊙. From Herschel and CO (J = 3 − 2) observations we found that the H ii region is part of an elongated extended molecular cloud (H2 column density ≥ 3 × 1021 cm-2 and dust temperature 18−27 K) of total mass ≥ 1 × 104 M⊙. We identify the ionizing cluster of Sh2-90, the main exciting star being an O8–O9 V star. Five cold dust clumps, four mid-IR blobs around B stars, and a compact H ii region are found at the edge of the bubble. The velocity information derived from CO data cubes suggest that most of them are associated with the Sh2-90 region. One hundred and twenty-nine low mass (≤3 M⊙) YSOs have been identified, and they are found to be distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found. We suggest that multi-generation star formation is present in the complex. From evidence of interaction, time scales involved, and evolutionary status of stellar/protostellar sources, we argue that the star formation at the edges of Sh2-90 might have been triggered. However, several young sources in this complex are probably formed by some other processes. [ABSTRACT FROM AUTHOR]

Context. The formation processes and the evolutionary stages of high-mass stars are poorly understood compared to low-mass stars. Large-scale surveys are needed to provide an unbiased census of high column density sites that can potentially host precursors to high-mass stars. Aims. The ATLASGAL survey covers 420 sq. degree of the Galactic plane, between −80◦ < ℓ < +60◦ at 870 μm. Here we identify the population of embedded sources throughout the inner Galaxy.With this catalog we first investigate the general statistical properties of dust condensations in terms of their observed parameters, such as flux density and angular size. Then using mid-infrared surveys we aim to investigate their star formation activity and the Galactic distribution of star-forming and quiescent clumps. Our ultimate goal is to determine the statistical properties of quiescent and star-forming clumps within the Galaxy and to constrain the star formation processes. Methods. We optimized the source extraction method, referred to as MRE-GCL, for the ATLASGAL maps in order to generate a catalog of compact sources. This technique is based on multiscale filtering to remove extended emission from clouds to better determine the parameters corresponding to the embedded compact sources. In a second step we extracted the sources by fitting 2D Gaussians with the Gaussclumps algorithm. Results. We have identified in total 10861 compact submillimeter sources with fluxes above 5σ. Completeness tests show that this catalog is 97% complete above 5σ and >99% complete above 7σ. Correlating this sample of clumps with mid-infrared point source catalogs (MSX at 21.3 μm andWISE at 22 μm), we have determined a lower limit of 33% that is associated with embedded protostellar objects.We note that the proportion of clumps associated with mid-infrared sources increases with increasing flux density, achieving a rather constant fraction of ∼75% of all clumps with fluxes over 5 Jy/beam being associated with star formation. Examining the source counts as a function of Galactic longitude, we are able to identify the most prominent star-forming regions in the Galaxy. Conclusions. We present here the compact source catalog of the full ATLASGALsurvey and investigate their characteristic properties. From the fraction of the likely massive quiescent clumps (∼25%), we estimate a formation time scale of ∼7.5 ± 2.5 × 104 yr for the deeply embedded phase before the emergence of luminous young stellar objects. Such a short duration for the formation of high-mass stars in massive clumps clearly proves that the earliest phases have to be dynamic with supersonic motions. [ABSTRACT FROM AUTHOR]

Aims. Ionization feedback should impact the probability distribution function (PDF) of the column density of cold dust around the ionized gas. We aim to quantify this effect and discuss its potential link to the core and initial mass function (CMF/IMF). Methods. We used Herschel column density maps of several regions observed within the HOBYS key program in a systematic way: M 16, the Rosette and Vela C molecular clouds, and the RCW 120 Hii region. We computed the PDFs in concentric disks around the main ionizing sources, determined their properties, and discuss the effect of ionization pressure on the distribution of the column density. Results. We fitted the column density PDFs of all clouds with two lognormal distributions, since they present a "double-peak" or an enlarged shape in the PDF. Our interpretation is that the lowest part of the column density distribution describes the turbulent molecular gas, while the second peak corresponds to a compression zone induced by the expansion of the ionized gas into the turbulent molecular cloud. Such a double peak is not visible for all clouds associated with ionization fronts, but it depends on the relative importance of ionization pressure and turbulent ram pressure. A power-law tail is present for higher column densities, which are generally ascribed to the effect of gravity. The condensations at the edge of the ionized gas have a steep compressed radial profile, sometimes recognizable in the flattening of the power-law tail. This could lead to an unambiguous criterion that is able to disentangle triggered star formation from pre-existing star formation. Conclusions. In the context of the gravoturbulent scenario for the origin of the CMF/IMF, the double-peaked or enlarged shape of the PDF may affect the formation of objects at both the low-mass and the high-mass ends of the CMF/IMF. In particular, a broader PDF is required by the gravo-turbulent scenario to fit the IMF properly with a reasonable initial Mach number for the molecular cloud. Since other physical processes (e.g., the equation of state and the variations among the core properties) have already been said to broaden the PDF, the relative importance of the different effects remains an open question. [ABSTRACT FROM AUTHOR]

Context. Herschel far-infrared imaging observations have revealed the density structure of the interface between H II regions and molecular clouds in great detail. In particular, pillars and globules are present in many high-mass star-forming regions, such as the Eagle nebula (M16) and the Rosette molecular cloud, and understanding their origin will help characterize triggered star formation. Aims. The formation mechanisms of these structures are still being debated. The initial morphology of the molecular cloud and its turbulent state are key parameters since they generate deformations and curvatures of the shell during the expansion of the H II region. Recent numerical simulations have shown how pillars can arise from the collapse of the shell in on itself and how globules can be formed from the interplay of the turbulent molecular cloud and the ionization from massive stars. The goal here is to test this scenario through recent observations of two massive star-forming regions, M16 and the Rosette molecular cloud. Methods. First, the column density structure of the interface between molecular clouds and associated H II regions was characterized using column density maps obtained from far-infrared imaging of the Herschel HOBYS key programme. Then, the DisPerSe algorithm was used on these maps to detect the compressed layers around the ionized gas and pillars in different evolutionary states. Column density profiles were constructed. Finally, their velocity structure was investigated using CO data, and all observational signatures were tested against some distinct diagnostics established from simulations. Results. The column density profiles have revealed the importance of compression at the edge of the ionized gas. The velocity properties of the structures, i.e. pillars and globules, are very close to what we predict from the numerical simulations. We have identified a good candidate of a nascent pillar in the Rosette molecular cloud that presents the velocity pattern of the shell collapsing on itself, induced by a high local curvature. Globules have a bulk velocity dispersion that indicates the importance of the initial turbulence in their formation, as proposed from numerical simulations. Altogether, this study re-enforces the picture of pillar formation by shell collapse and globule formation by the ionization of highly turbulent clouds. [ABSTRACT FROM AUTHOR]

Aims. We observed the L1506 filament, which is located in the Taurus molecular complex, with the Herschel PACS and SPIRE instruments. Our aim is to prove the variation in grain properties along the entire length of the filament. In particular, we want to determine above which gas density this variation arises and what changes in the grain optical properties/size distribution are required. Methods. We use the 3D radiative transfer code CRT, coupled to the dust emission and extinction code DustEM, to model the emission and extinction of the dense filament. We test a range of optical properties and size distributions for the grains: dust of the diffuse interstellar medium (interstellar PAHs and amorphous carbons and silicates) and both compact and fluffy aggregates. Results. We find that the grain opacity has to increase across the filament to fit simultaneously the near-IR extinction and Herschel emission profiles of L1506. We interpret this change to be a consequence of the coagulation of dust grains to form fluffy aggregates. Grains similar to those in the diffuse medium have to be present in the outer layers of the cloud, whereas aggregates must prevail above gas densities of a few 103 H/cm3. This corresponds to line-of-sights with visual extinction in the V band of the order of 2 to 3. The dust opacity at 250 μm is raised by a factor of 1.8 to 2.2, while the grain average size is increased by a factor of 5. These exact numbers depend naturally on the dust model chosen to fit the data. Our findings agree with the constraints given by the study of the gas molecular lines. Using a simple approach, we show that the aggregates may have time to form inside the filament within the cloud lifetime. Our model also characterises the density structure of the filament, showing that the filament width is not constant along L1506 but instead varies by a factor of the order of 4. Conclusions. We confirm the need for an increase in the far-IR dust opacity to explain the emission and extinction in L1506C, which we interpret as being due to dust growth. We also show that this opacity variation is valid along the entire length of the L1506 dense filament. [ABSTRACT FROM AUTHOR]

In order to characterise the cloud structures responsible for the formation of high-mass stars, we present Herschel observations of the DR21 environment. Maps of the column density and dust temperature unveil the structure of the DR21 ridge and several connected filaments. The ridge has column densities higher than 1023 cm-2 over a region of 2.3 pc2. It shows substructured column density profiles and branches into two major filaments in the north. The masses in the filaments range between 130 and 1400 M☉, whereas the mass in the ridge is 15 000 M☉. The accretion of these filaments onto the DR21 ridge, suggested by a previous molecular line study, could provide a continuous mass inflow to the ridge. In contrast to the striations seen in, e.g., the Taurus region, these filaments are gravitationally unstable and form cores and protostars. These cores formed in the filaments potentially fall into the ridge. Both inflow and collisions of cores could be important to drive the observed high-mass star formation. The evolutionary gradient of star formation running from DR21 in the south to the northern branching is traced by decreasing dust temperature. This evolution and the ridge structure can be explained by two main filamentary components of the ridge that merged first in the south. [ABSTRACT FROM AUTHOR]

Context. Because of their relatively simple morphology, "bubble" Hit regions have been instrumental to our understanding of star formation triggered by H n regions. With the far-infrared (FIR) spectral coverage of the Herschel satellite, we can access the wavelengths where these regions emit the majority of their energy through their dust emission. Aims. We wish to learn about the duet temperature distribution in and surrounding bubble H II regions and to calculate the mass and column density of regions of interest, in order to better understand ongoing star formation. Additionally, we wish to determine whether and how the spectral index of the dust opacity, β, varies with dust temperature. Any such relationship would imply that dust properties vary with environment. Methods. Using aperture photometry and fits to the spectral energy distribution, we determine the average temperature, β-value, and mass for regions of interest within eight bubble Hit regions. Additionally, we compute maps of the dust temperature and column density. Results. At Herschel wavelengths (70 pm to 500 pm), the emission associated with Hit regions is dominated by the cool dust in their photodissociation regions (PDRs). We find average dust temperatures of 26 K along the PDRs, with little variation between the H II regions in the sample, while local filaments and infrared dark clouds average 19 K and 15 K respectively. Higher temperatures lead to higher values of the Jeans mass, which may affect future star formation. The mass of the material in the PDR, collected through the expansion of the H II region, in between ∼ 300 Mʘ and ∼ 10000 Mʘ for the H II regions studied here. These masses are in rough agreement with the expected masses swept up during the expansion of the H II regions~ Approximately 20% of the total FIR emission is from the direction of the bubble central regions. This suggests that we are detecting emission from the "near-side" and "far-side" PDRs along the line of sight and that bubbles are three-dimensional structures. We find only weak support for a relationship between dust temperature and β, of a form similar to that caused by noise and calibration uncertainties alone. [ABSTRACT FROM AUTHOR]

Context. In the project galactic cold cores we are carrying out Herschel photometric observations of cold regions of the interstellar clouds as previously identified with the Planck satellite. The aim of the project is to derive the physical properties of the population of cold clumps and to study its connection to ongoing and future star formation. Aims. We examine the cloud structure around the Planck detections in 71 fields observed with the Herschel SPIRE instrument by the summer of 2011. We wish to determine the general physical characteristics of the fields and to examine the morphology of the clouds where the cold high column density clumps are found. Methods. Using the Herschel SPIRE data, we derive colour temperature and column density maps of the fields. Together with ancillary data, we examine the infrared spectral energy distributions of the main clumps. The clouds are categorised according to their large scale morphology. With the help of recently released WISE satellite data, we look for signs of enhanced mid-infrared scattering ("coreshine"), an indication of growth of the dust grains, and have a first look at the star formation activity associated with the cold clumps. Results. The mapped clouds have distances ranging from ∼100 pc to several kiloparsecs and cover a range of sizes and masses from cores of less than 10 M☉ to clouds with masses in excess of 10 000 M☉. Most fields contain some filamentary structures and in about half of the cases a filament or a few filaments dominate the morphology. In one case out of ten, the clouds show a cometary shape or have sharp boundaries indicative of compression by an external force. The width of the filaments is typically ∼0.2-0.3 pc. However, there is significant variation from 0.1 pc to 1 pc and the estimates are sensitive to the methods used and the very definition of a filament. Enhanced mid-infrared scattering, coreshine, was detected in four clouds with six additional tentative detections. The cloud LDN183 is included in our sample and remains the best example of this phenomenon. About half of the fields are associated with active star formation as indicated by the presence of mid-infrared point sources. The mid-infrared sources often coincide with structures whose sub-millimetre spectra are still dominated by the cold dust. [ABSTRACT FROM AUTHOR]

Context. Star-forming complexes are large structures exhibiting massive star-formation at different stages of evolution, from dense cores to well-developed Hii regions. They are very interesting for the study of the formation and evolution of stars. NGC 6334 and NGC 6357 are two active and relatively nearby star-forming complexes. From the extinction map and the sub-mm cold dust emission, and because they have similar velocities, these regions are most likely connected. However, located in the direction of the Galactic center their radial velocity is not representative of their distance. An alternative is then to determine the distance of NGC 6334 and NGC 6357 from their stellar content. Aims. Our aim is to perform a census of O-B3 ionising stars in NGC 6334 and NGC 6357, to determine the extinction coefficient, and the distance of both regions. A census of O-B3 stars is an essential basis for estimating the statistical lifetime of the earliest massive star-forming phases. Methods. We performed a U, B, V, and R photometric survey of a large area covering NGC 6334 and NGC 6357 with the VIMOS (ESO-VLT) and the MOSAIC (CTIO) instruments. This allows us to have a complete census of O to B3 stars up to V = 22.6 mag. The OB stars are selected based on their U -- B and B -- V colors. The most robust extinction coefficient is determined from color--color plots before computing the distance of the OB stars. Results. We find a higher value than typical of the diffuse interstellar medium for RV of 3.53±0.08 and 3.56±0.15 for NGC 6357 and NGC 6334, respectively. Adopting these RV values, the distances of NGC 6357 and NGC 6334 are 1.9 ± 0.4 kpc and 1.7 ± 0.3 kpc. We conclude that, within the error bars, both regions are thus at the same distance of 1.75 kpc (weighted mean). We confirm that the value of RV is linked to the large dust grain content. In particular, we found that there are more very small grains in NGC 6357 than in NGC 6334, suggesting that NGC 6357 could be more evolved than NGC 6334. Placed in the Galactic context, the NGC 6334- NGC 6357 complex appears to be located at the inner edge of the Sagittarius-Carina arm. Our census of O to B3 stars leads to a count of ∼230, which allows us to determine the statistical lifetime of the earliest phases of the massive stars. The starless and the protostellar phases have a mean statistical lifetime of ∼1.5 × 104 yr and ∼2.2 × 105 yr, respectively. [ABSTRACT FROM AUTHOR]

Context. H II regions and planetary nebulae (PNe) both emit at radio and infrared (IR) wavelengths, and angularly small H II regions can be mistaken for PNe. This problem of classification is most severe for H II regions in an early evolutionary stage, those that are extremely distant, or those that are both young and distant. Previous work has shown that H II regions and PNe can be separated based on their infrared colors. Aims. Using data from the Herschel Hi-GAL survey, as well as WISE and the Spitzer MIPSGAL and GLIMPSE surveys, we wish to establish characteristic IR colors that can be used to distinguish between H II regions and PNe. Methods. We perform aperture photometry measurements for a sample of 126 H II regions and 43 PNe at wavelengths from 8.0 μm to 500 μm. Results. We find that H II regions and PNe have distinct IR colors. The most robust discriminating color criteria are [F12/F8] < 0.3, [F160/F12] > 1.3, and [F160/F24] > 0.8 (or alternately [F160/F22] > 0.8), where the brackets indicate the log of the flux ratio. All three of these criteria are individually satisfied by over 98% of our sample of H II regions and by ∼10% of our sample of PNe. Combinations of these colors are more robust in separating the two populations; for example all H II regions and no PNe satisfy [F12/F8] < 0.4 and [F160/F22] > 0.8.When applied to objects of unknown classification, these criteria prove useful in separating the two populations. The dispersion in color is relatively small for H II regions; this suggests that any evolution in these colors with time for H II regions must be relatively modest. The spectral energy distributions (SEDs) of H II regions can be separated into "warm" and "cold" components. The "cold" component is well-fit by a grey-body of temperature 25 K. The SEDs of nearly two-thirds of our sample of H II regions peak at 160 μm and one third peak at 70 μm. For PNe, 67% of the SEDs peak at 70 μm, 23% peak at either 22 μm or 24μm, and 9% (two sources) peak at 160 μm. [ABSTRACT FROM AUTHOR]