Understanding star formation in molecular clouds

Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentially available to form stars. Many of these clouds are likely to be pristine structures representing the initial conditions for star formation. The study presented here aims to construct and analyze accurate column density and dust temperature maps of IRDCs by using the first <i>Herschel<i/> data from the Hi-GAL galactic plane survey. These fundamental quantities, are essential for understanding processes such as fragmentation in the early stages of the formation of stars in molecular clouds. We have developed a simple pixel-by-pixel SED fitting method, which accounts for the background emission. By fitting a grey-body function at each position, we recover the spatial variations in both the dust column density and temperature within the IRDCs. This method is applied to a sample of 22 IRDCs exhibiting a range of angular sizes and peak column densities. Our analysis shows that the dust temperature decreases significantly within IRDCs, from background temperatures of 20–30 K to minimum temperatures of 8–15 K within the clouds, showing that dense molecular clouds are not isothermal. Temperature gradients have most likely an important impact on the fragmentation of IRDCs. Local temperature minima are strongly correlated with column density peaks, which in a few cases reach = 1×10<sup>23<sup/> cm<sup>-2<sup/>, identifying these clouds as candidate massive prestellar cores. Applying this technique to the full Hi-GAL data set will provide important constraints on the fragmentation and thermal properties of IRDCs, and help identify hundreds of massive prestellar core candidates.

We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground-based (CSO, JCMT, APEX, and IRAM-30 m) and space telescopes (Herschel and Planck). For the seven luminous ( L ⊙) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, and G10.3-0.1, we have performed single-component, modified blackbody fits to each pixel of the combined (sub)millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The ∼10″ resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3 kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed ∼10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (i.e., the enclosed mass profile, the column density probability distribution function (N-PDF), the two-point correlation function of column density, and the probability distribution function of clump/core separations). In particular, the massive molecular gas clumps located at the center of G10.6-0.4 and W49A, which contribute to a considerable fraction of their overall cloud masses, may be special OB cluster-forming environments as a direct consequence of global cloud collapse. These centralized massive molecular gas clumps also uniquely occupy much higher column densities than what is determined by the overall fit of power-law N-PDF. We have made efforts to archive the derived statistical quantities of individual target sources, to permit comparisons with theoretical frameworks, numerical simulations, and other observations in the future.

The total mass estimate of molecular clouds suffers from the uncertainty in the H2-CO conversion factor, the so-called XCO factor, which is used to convert the 12CO (1–0) integrated intensity to the H2 column density. We demonstrate the machine learning’s ability to predict the H2 column density from the 12CO, 13CO, and C18O (1–0) data set of four star-forming molecular clouds: Orion A, Orion B, Aquila, and M17. When the training is performed on a subset of each cloud, the overall distribution of the predicted column density is consistent with that of the Herschel column density. The total column density predicted and observed is consistent within 10 per cent, suggesting that the machine learning prediction provides a reasonable total mass estimate of each cloud. However, the distribution of the column density for values &amp;gt;∼2 × 1022 cm−2, which corresponds to the dense gas, could not be predicted well. This indicates that molecular line observations tracing the dense gas are required for the training. We also found a significant difference between the predicted and observed column density when we created the model after training the data on different clouds. This highlights the presence of different XCO factors between the clouds, and further training in various clouds is required to correct for these variations. We also demonstrated that this method could predict the column density towards the area not observed by Herschel if the molecular line and column density maps are available for the small portion, and the molecular line data are available for the larger areas.

We present continuum observations of the infrared dark cloud (IRDC) G48.66-0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to several molecular line observations. The Herschel maps are used to derive temperature and column density maps of G48 using a model based on a modified blackbody. We find that G48 has a relatively simple structure and is relatively isolated; thus this IRDC provides an excellent target to study the collapse and fragmentation of a filamentary structure in the absence of complicating factors such as strong external feedback. The derived temperature structure of G48 is clearly non-isothermal from cloud to core scale. The column density peaks are spatially coincident with the lowest temperatures (~ 17.5 K) in G48. A total cloud mass of ~390Msun is derived from the column density maps. By comparing the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS bands to evolutionary models, we find that two cores are likely to evolve into high-mass stars (M>8 Msun). The derived mean projected separation of point sources is smaller than in other IRDCs but in good agreement with theoretical predications for cylindrical collapse. We detect several molecular species such as CO, HCO+, HCN, HNC and N2H+. CO is depleted by a factor of ~3.5 compared to the expected interstellar abundance, from which we conclude that CO freezes out in the central region. Furthermore, the molecular clumps, associated with the sub-millimeter peaks in G48, appear to be gravitationally unbound or just pressure confined. The analysis of critical line masses in G48 show that the entire filament is collapsing, overcoming any internal support.

We present SOFIA-upGREAT observations of [C ii] emission of Infrared Dark Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus test models of the origins of such clouds. Several velocity components of [C ii] emission are detected, tracing structures that are at a wide range of distances in the Galactic plane. We find a main component that is likely associated with the IRDC and its immediate surroundings. This strongest emission component has a velocity similar to that of the 13CO(2–1) emission of the IRDC, but offset by ∼3 km s−1 and with a larger velocity width of ∼9 km s−1. The spatial distribution of the [C ii] emission of this component is also offset predominantly to one side of the dense filamentary structure of the IRDC. The C ii column density is estimated to be of the order of ∼1017–1018 cm−2. We compare these results to the [C ii] emission from numerical simulations of magnetized, dense gas filaments formed from giant molecular cloud (GMC) collisions, finding similar spatial and kinematic offsets. These observations and modellingof [C ii] add further to the evidence that IRDC G035.39-00.33 has been formed by a process of GMC–GMC collision, which may thus be an important mechanism for initiating star cluster formation.

We present star formation activity in the infrared dark cloud (IRDC) G53.2, a remarkable IRDC located at Galactic coordinates $(l,b)\sim(53^{\circ}.2,\,0^{\circ}.0)$ based on the census of young stellar object (YSO) candidates. IRDC G53.2 was previously identified as several IRDCs in mid-IR images, but it is in fact a long ($\gtrsim$45 pc) cloud, well consistent with a CO cloud at $v\sim23$ \kms\ (or at $d\sim$1.7 kpc). We present a point-source catalog of IRDC G53.2 that contains $\sim$370 sources from our photometry of the {\it Spitzer} MIPS 24 \um\ data and Galactic Legacy Infrared Mid-Plane Survey Extraordinaire Catalog. The classification of the identified sources based on their spectral index and control field analysis to remove field star contamination reveals that IRDC G53.2 is an active star-forming region with $\sim$300 YSO candidates. We compare the YSO classification based on spectral index, mid-IR colors, and the wavelength range used, which results in consistent classification, except for flat-spectrum objects, with some ambiguity between Class I and II. Comparison of the YSO population in IRDC G53.2 with those of other nearby star-forming clusters indicates that they are similar in age; on the other hand, stronger association with mid-IR stellar sources in IRDC G53.2 compared with other IRDCs indicates that IRDC G53.2 is at a later evolutionary stage among IRDCs. Spatial distribution of the YSO candidates in IRDC G53.2 shows a good correlation with $^{13}$CO column density and far-IR emission, and earlier-class objects tend to be more clustered in the regions with higher density.

Dense cores in massive, parsec-scale molecular clumps are sites that harbor protocluster formation. We present results from observations toward a hub–filament structure of a massive infrared (IR) dark cloud G14.225-0.506 using the Atacama Large Millimeter/submillimeter Array. The dense cores are revealed by the 1.3 mm dust continuum emission at an angular resolution of ∼1 . ″ 5 and are identified through the hierarchical dendrogram technique. Combining with the N2D+ 3–2 spectral line emission and gas temperatures derived from a previous NH3 study, we analyze the thermodynamic properties of the dense cores. The results show transonic and supersonic-dominated turbulent motions. There is an inverse correlation between the virial parameter and the column density, which implies that denser regions may undergo stronger gravitational collapse. Molecular outflows are identified in the CO 2–1 and SiO 5–4 emission, indicating active protostellar activities in some cores. Besides these star formation signatures revealed by molecular outflows in the dense cores, previous studies in the IR, X-ray, and radio wavelengths also found a rich and widespread population of young stellar objects, showing active star formation both inside and outside of the dense cloud.

Aims . Filamentary infrared dark clouds (IRDCs) are believed to represent the initial conditions for massive star and cluster formation. Methods . We investigated the IRDC G035.39-00.33 using SiO, H 13 CO + , CH 3 OH, and CS emission observed with ALMA at 3.5″ resolution (∼0.05 pc). The analysis of the SiO emission provides a record of shock activity within the cloud, offering insights into both the current level of star formation and the cloud’s formation mechanisms. Results . We identify several regions with broad SiO emission clearly associated with outflows, pinpointing the locations of ongoing star formation across the cloud. The ALMA images also reveal a series of spatially extended SiO emission spots with narrow line profiles aligned along an arc-like path that is also seen in CS and CH 3 OH emission. While the broad SiO emission is mainly associated with the main cloud filament, as seen in visual extinction, the narrow SiO arch is located at the edge of the cloud, far from the identified sites of star formation activity. The presence of these arc-like morphologies suggests that large-scale shocks may have compressed the gas in the surroundings of the G035.39-00.33 cloud, shaping its filamentary structure. By inspecting the large-scale radio continuum emission around G035.39-00.33, we find that this IRDC is part of a larger star-forming complex where the densest and coolest material appears at the interacting regions between a supernova remnant (SNR) and an expanding HII region. In particular, we hypothesise that this IRDC may be spatially coincident with the ionised expanding gas associated with the previously identified SNR G35.6-0.4. Conclusions . We suggest that collisions between giant molecular clouds and expanding gas flows from interacting SNRs and HII regions may be responsible for the observed arc-like structures. Such shock compressions could play an important role in the formation of IRDCs and in the potential triggering of star formation.

We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 um, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the `Centre-Ridge' and `South-Nest' sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.

The field of computer vision has greatly matured in the past decade, and many of the methods and techniques can be useful for astronomical applications. One example is in searching large imaging surveys for objects of interest, especially when it is difficult to specify the characteristics of the objects being searched for. We have developed a method using contour finding and convolution neural networks (CNNs) to search for Infrared Dark Clouds (IRDCs) in the Spitzer Galactic plane survey data. IRDCs can vary in size, shape, orientation, and optical depth, and are often located near regions with complex emission from molecular clouds and star formation, which can make the IRDCs difficult to reliably identify. False positives can occur in regions where emission is absent, rather than from a foreground IRDC. The contour finding algorithm we implemented found most closed figures in the mosaic and we developed rules to filter out some of the false positive before allowing the CNNs to analyze them. The method was applied to the Spitzer data in the Galactic plane surveys, and we have constructed a catalog of IRDCs which includes additional parts of the Galactic plane that were not included in earlier surveys.

The initial conditions of massive star and star cluster formation are expected to be cold, dense and high column density regions of the interstellar medium, which can reveal themselves via near, mid and even far-infrared absorption as Infrared Dark Clouds (IRDCs). Elucidating the dynamical state of IRDCs thus constrains theoretical models of these complex processes. In particular, it is important to assess whether IRDCs have reached virial equilibrium, where the internal pressure balances that due to the self-gravitating weight of the cloud plus the pressure of the external environmental. We study this question for the filamentary IRDC G035.39-00.33 by deriving mass from combined NIR & MIR extinction maps and velocity dispersion from C18O (1-0) & (2-1) line emission. In contrast to our previous moderately super-virial results based on 13CO emission and MIR-only extinction mapping, with improved mass measurements we now find that the filament is consistent with being in virial equilibrium, at least in its central parsec-wide region where ~1000 M_Sun snakes along several parsecs. This equilibrium state does not require large-scale net support or confinement by magnetic fields.

We have conducted a large-field simultaneous survey of 12CO, 13CO and C18O J = 1 – 0 emission toward the Orion A giant molecular cloud (GMC) with a sky coverage of ∼4.4 deg2 using the Purple Mountain Observatory (PMO)-13.7 m millimeter-wavelength telescope. We use the probability distribution function of the column density (N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC. The H2 column density, derived from the 13CO emission, of the GMC is dominated by a log-normal distribution in the range from ∼ 4 × 1021 to ∼ 1.5 × 1023 cm−2 with excesses both at the low-density and high-density ends. The excess of the low-density end is possibly caused by an extended and low-temperature (∼10 K) component with velocities in the range of 5 – 8 km s−1. Compared with the northern sub-regions, the southern sub-regions of the Orion A GMC contain less gas with column density in NH2 > 1.25 × 1022 cm−2. The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC. The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm, and it is found that the GMC is composed of two branches. All structures except one in the tree have virial parameters less than 2, indicating self-gravity is important on the spatial scales from ∼0.3 to ∼4 pc. Although power-laws and departures from lognormal distributions are found at the high-density end of N-PDFs for active star-forming regions, the N-PDFs of structures in the Orion A GMC are predominantly lognormal on scales from R∼0.4 to 4 pc.

We have performed an unbiased dense core survey toward the Orion A Giant Molecular Cloud in the C18O (J = 1–0) emission line taken with the Nobeyama Radio Observatory (NRO) 45 m telescope. The effective angular resolution of the map is 26″, which corresponds to ∼0.05 pc at a distance of 414 pc. By using the Herschel–Planck H2 column density map, we calculate the C18O fractional abundance and find that it is roughly constant over the column density range of ≲5 × 1022 cm−3, although a trend of C18O depletion is determined toward higher column density. Therefore, C18O intensity can follow the cloud structure reasonably well. The mean C18O abundance in Orion A is estimated to be 5.7 × 10−7, which is about three times larger than the fiducial value. We identified 746 C18O cores with astrodendro and classified 709 cores as starless cores. We compute the core masses by decomposing the Herschel–Planck dust column density using the relative proportions of the C18O integrated intensities of line-of-sight components. Applying this procedure, we attempt to remove the contribution of the background emission, i.e., the ambient gas outside the cores. Then, we derived mass function for starless cores and found that it resembles the stellar initial mass function (IMF). The CMF for starless cores, dN/dM, is fitted with a power-law relation of Mα with a power index of α = −2.25 ± 0.16 at the high-mass slope (≳0.44 M⊙). We also found that the ratio of each core mass to the total mass integrated along the line of sight is significantly large. Therefore, in the previous studies, the core masses derived from the dust image are likely to be overestimated by at least a factor of a few. Accordingly, such previous studies may underestimate the star formation efficiency of individual cores.

An estimate of the degree of CO-depletion (fD) provides information on the physical conditions occurring in the innermost and densest regions of molecular clouds. A key parameter in these studies is the size of the depletion radius, i.e. the radius within which the C-bearing species, and in particular CO, are largely frozen on to dust grains. A strong depletion state (i.e. fD &amp;gt; 10, as assumed in our models) is highly favoured in the innermost regions of dark clouds, where the temperature is &amp;lt;20 K and the number density of molecular hydrogen exceeds a few × 104 cm−3. In this work, we estimate the size of the depleted region by studying the Infrared Dark Cloud (IRDC) G351.77−0.51. Continuum observations performed with the Herschel Space Observatory and the LArge APEX BOlometer CAmera, together with APEX C18O and C17O J = 2→1 line observations, allowed us to recover the large-scale beam- and line-of-sight-averaged depletion map of the cloud. We built a simple model to investigate the depletion in the inner regions of the clumps in the filament and the filament itself. The model suggests that the depletion radius ranges from 0.02 to 0.15 pc, comparable with the typical filament width (i.e. ∼0.1 pc). At these radii, the number density of H2 reaches values between 0.2 and 5.5 × 105 cm−3. These results provide information on the approximate spatial scales on which different chemical processes operate in high-mass star-forming regions and also suggest caution when using CO for kinematical studies in IRDCs.

Context.Star formation takes place in giant molecular clouds, resulting in mass-segregated young stellar clusters composed of Sun-like stars, brown dwarfs, and massive O-type(50–100M⊙) stars.Aims.We aim to identify candidate hub-filament systems (HFSs) in the Milky Way and examine their role in the formation of the highest mass stars and star clusters.Methods.TheHerschelsurvey HiGAL has catalogued about 105clumps. Of these, approximately 35 000 targets are detected at the 3σlevel in a minimum of four bands. Using the DisPerSE algorithm we detect filamentary skeletons on 10′ × 10′ cut-outs of the SPIRE 250μm images (18′′ beam width) of the targets. Any filament with a total length of at least 55′′ (3 × 18′′) and at least 18′′ inside the clump was considered to form a junction at the clump. A hub is defined as a junction of three or more filaments. Column density maps were masked by the filament skeletons and averaged for HFS and non-HFS samples to compute the radial profile along the filaments into the clumps.Results.Approximately 3700 (11%) are candidate HFSs, of which about 2150 (60%) are pre-stellar and 1400 (40%) are proto-stellar. The filaments constituting the HFSs have a mean length of ~10–20 pc, a mass of ~5 × 104M⊙, and line masses (M∕L) of ~2 × 103M⊙pc−1. All clumps withL&gt; 104L⊙andL&gt; 105L⊙at distances within 2 and 5 kpc respectively are located in the hubs of HFSs. The column densities of hubs are found to be enhanced by a factor of approximately two (pre-stellar sources) up to about ten (proto-stellar sources).Conclusions.All high-mass stars preferentially form in the density-enhanced hubs of HFSs. This amplification can drive the observed longitudinal flows along filaments providing further mass accretion. Radiation pressure and feedback can escape into the inter-filamentary voids. We propose a “filaments to clusters” unified paradigm for star formation, with the following salient features: (a) low-intermediate-mass stars form slowly (106yr) in the filaments and massive stars form quickly (105yr) in the hub, (b) the initial mass function is the sum of stars continuously created in the HFS with all massive stars formed in the hub, (c) feedback dissipation and mass segregation arise naturally due to HFS properties, and explain the (d) age spreads within bound clusters and the formation of isolated OB associations.