Molecular Clumps Research Articles

CHIMPS: physical properties of molecular clumps across the inner Galaxy

The latest generation of high-angular-resolution unbiased Galactic plane surveys in molecular-gas tracers are enabling the interiors of molecular clouds to be studied across a range of environments. The CO Heterodyne Inner Milky Way Plane Survey (CHIMPS) simultaneously mapped a sector of the inner Galactic plane, within 27.8° ≲ ℓ ≲ 46.2° and |b|≤ 0°.5, in 13CO (3–2) and C18O (3–2) at an angular resolution of 15 arcsec. The combination of the CHIMPS data with 12CO (3–2) data from the CO High Resolution Survey (COHRS) has enabled us to perform a voxel-by-voxel local-thermodynamic-equilibrium (LTE) analysis, determining the excitation temperature, optical depth, and column density of 13CO at each ℓ, b, v position. Distances to discrete sources identified by FELLWALKER in the 13CO (3–2) emission maps were determined, allowing the calculation of numerous physical properties of the sources, and we present the first source catalogues in this paper. We find that, in terms of size and density, the CHIMPS sources represent an intermediate population between large-scale molecular clouds identified by CO and dense clumps seen in thermal dust continuum emission, and therefore represent the bulk transition from the diffuse to the dense phase of molecular gas. We do not find any significant systematic variations in the masses, column densities, virial parameters, mean excitation temperature, or the turbulent pressure over the range of Galactocentric distance probed, but we do find a shallow increase in the mean volume density with increasing Galactocentric distance. We find that inter-arm clumps have significantly narrower linewidths, and lower virial parameters and excitation temperatures than clumps located in spiral arms. When considering the most reliable distance-limited subsamples, the largest variations occur on the clump-to-clump scale, echoing similar recent studies that suggest that the star-forming process is largely insensitive to the Galactic-scale environment, at least within the inner disc.

A Supernova Remnant Counterpart for HESS J1832−085

Abstract We examine the new Galactic supernova remnant (SNR) candidate, G23.11+0.18, as seen by the Murchison Widefield Array radio telescope. We describe the morphology of the candidate and find a spectral index of −0.63 ± 0.05 in the 70–170 MHz domain. Coincident TeV gamma-ray detection in High Energy Stereoscopic System (HESS) data supports the SNR nature of G23.11+0.18 and suggests that G23.11+0.18 is accelerating particles beyond TeV energies, thus making this object a promising new cosmic-ray hadron source candidate. The remnant cannot be seen in current optical, infrared and X-ray data sets. We do find, however, a dip in CO-traced molecular gas at a line-of-sight velocity of ∼85 km s−1, suggesting the existence of a G23.11+0.18 progenitor wind-blown bubble. Furthermore, the discovery of molecular gas clumps at a neighboring velocity toward HESS J1832−085 adheres to the notion that a hadronic gamma-ray production mechanism is plausible toward the north of the remnant. Based on these morphological arguments, we propose an interstellar medium association for G23.11+0.18 at a kinematic distance of 4.6 ± 0.8 kpc.

Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario

AbstractWe present a unified description of the scenario of global hierarchical collapse (GHC). GHC constitutes a flow regime of (non-homologous) collapses within collapses, in which all scales accrete from their parent structures, and small, dense regions begin to contract at later times, but on shorter time-scales than large, diffuse ones. The different time-scales allow for most of the clouds’ mass to be dispersed by the feedback from the first massive stars, maintaining the cloud-scale star formation rate low. Molecular clouds (MCs), clumps, and cores are not in equilibrium, but rather are either undergoing contraction or dispersal. The main features of GHC are as follows: (1) The gravitational contraction is initially very slow, and begins when the cloud still consists of mostly atomic gas. (2) Star-forming MCs are in an essentially pressureless regime, causing filamentary accretion flows from the cloud to the core scale to arise spontaneously. (3) Accreting objects have longer lifetimes than their own free-fall time, due to the continuous replenishment of material. (4) The clouds’ total mass and its molecular and dense mass fractions increase over time. (5) The clouds’ masses stop growing when feedback becomes important. (6) The first stars appear several megayears after global contraction began, and are of low mass; massive stars appear a few megayears later, in massive hubs. (7) The minimum fragment mass may well extend into the brown-dwarf regime. (8) Bondi–Hoyle–Lyttleton-like accretion occurs at both the protostellar and the core scales, accounting for an IMF with slope dN/dM ∝ M−2. (9) The extreme anisotropy of the filamentary network explains the difficulty in detecting large-scale infall signatures. (10) The balance between inertial and gravitationally driven motions in clumps evolves during the contraction, explaining the approach to apparent virial equilibrium, from supervirial states in low-column density clumps and from subvirial states in dense cores. (11) Prestellar cores adopt Bonnor–Ebert-like profiles, but are contracting ever since when they may appear to be unbound. (12) Stellar clusters develop radial age and mass segregation gradients. We also discuss the incompatibility between supersonic turbulence and the observed scalings in the molecular hierarchy. Since gravitationally formed filaments do not develop shocks at their axes, we suggest that a diagnostic for the GHC scenario should be the absence of strong shocks in them. Finally, we critically discuss some recent objections to the GHC mechanism.

Nobeyama 45 m Cygnus-X CO Survey. II. Physical Properties of C18O Clumps

Abstract We report the statistical physical properties of the C18O(J = 1–0) clumps present in a prominent cluster-forming region, Cygnus X, using the data set obtained by the Nobeyama 45 m radio telescope. This survey covers 9 deg2 of the northern and southern regions of Cygnus X, and, in total, 174 C18O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2–1 pc, velocity dispersions of <2.2 km s−1, gas masses of 30–3000 M ⊙, and H2 densities of (0.2–5.5) × 104 cm−3. We confirm that the C18O clumps in the northern region have a higher H2 density than those in the southern region, supporting the existence of a difference in the evolutionary stages, consistent with the star-formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H2 density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C18O clump mass function shows two spectral index components, α = −1.4 in 55–140 M ⊙ and α = −2.1 in >140 M ⊙, which are consistent with the low- and intermediate-mass parts of the Kroupa’s initial mass function. The spectral index of the star-forming clumps >140 M ⊙ is consistent with that of the starless clumps ranging from 55–140 M ⊙, suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star-formation efficiency of molecular clumps (10%), about 10 C18O clumps having a gas mass of >103 M ⊙ will evolve into open clusters containing one or more OB stars.

Photoevaporation of Molecular Gas Clumps Illuminated by External Massive Stars: Clump Lifetimes and Metallicity Dependence

Abstract We perform a suite of 3D radiation hydrodynamics simulations of photoevaporation of molecular gas clumps illuminated by external massive stars. We study the fate of solar-mass clumps and derive their lifetimes by varying the gas metallicity over a range of . Our simulations incorporate radiation transfer of far- and extreme-ultraviolet photons and follow atomic/molecular line cooling and dust–gas collisional cooling. Nonequilibrium chemistry is coupled with the radiative transfer and hydrodynamics in a self-consistent manner. We show that radiation-driven shocks compress gas clumps to have a volume that is set by the pressure equilibrium with the hot ambient gas. Radiative cooling enables metal-rich clumps to condense and have small surface areas where photoevaporative flows are launched. For our fiducial setup with an O-type star at a distance of 0.1 pc, the resulting photoevaporation rate is as small as for metal-rich clumps, but it is larger for metal-poor clumps that have larger surface areas. The clumps are continuously accelerated away from the radiation source by the so-called rocket effect and can travel over ∼1 pc within the lifetime. We also study the photoevaporation of clumps in a photodissociation region. Photoelectric heating is inefficient for metal-poor clumps that contain a smaller amount of grains, and thus they survive for over 105 yr. We conclude that the gas metallicity strongly affects the clump lifetime and thus determines the strength of feedback from massive stars in star-forming regions.

MILLIMETER AND FAR-IR STUDY OF THE IRDC SDC 341.232-0.268

We analyze the molecular gas and dust associated with the infrared dark cloud SDC 341.232-0.268 in order to investigate the characteristics and parameters of the gas, determine the evolutionary status of four embedded EGO candidates, and establish possible infall or outflow gas motions. We base our study on 12 CO(2-1), 13 CO(2-1), and C18 O(2-1) data obtained with the APEX telescope, molecular data of high density tracers from the MALT90 survey and IR images from Spitzer, Herschel and ATLASGAL. The study reveals two clumps at −44 km/s towards the IRDC, with densities of > 104 cm^{−3}, typical of IRDCs, while high density tracers show H2 densities > 105 . FIR images reveals the presence of cold dust linked to the molecular clumps and EGOs. A comparison of the spectra of the optically thin and optically thick molecular lines towards the EGOs suggests the existence of infall and outflow motions.

The role of molecular filaments in the origin of the prestellar core mass function and stellar initial mass function

Context. The origin of the stellar initial mass function (IMF) is one of the most debated issues in astrophysics. Aims. Here we explore the possible link between the quasi-universal filamentary structure of star-forming molecular clouds and the origin of the IMF. Methods. Based on our recent comprehensive study of filament properties from Herschel Gould Belt survey observations, we derive, for the first time, a good estimate of the filament mass function (FMF) and filament line mass function (FLMF) in nearby molecular clouds. We use the observed FLMF to propose a simple toy model for the origin of the prestellar core mass function (CMF), relying on gravitational fragmentation of thermally supercritical but virialized filaments. Results. We find that the FMF and the FLMF have very similar shapes and are both consistent with a Salpeter-like power-law function (dN/dlog Mline ∝ Mline−1.5±0.1) in the regime of thermally supercritical filaments (Mline > 16 M⊙ pc−1). This is a remarkable result since, in contrast, the mass distribution of molecular clouds and clumps is known to be significantly shallower than the Salpeter power-law IMF, with dN/dlog Mcl ∝ Mcl−0.7. Conclusions. Since the vast majority of prestellar cores appear to form in thermally transcritical or supercritical filaments, we suggest that the prestellar CMF and by extension the stellar IMF are at least partly inherited from the FLMF through gravitational fragmentation of individual filaments.

Thermal instability through the outer half of quasi-static spherically symmetric molecular clumps and cores

Thermal instability (TI) is a trigger mechanism, which can explain formation of condensations through some regions of the interstellar clouds. Our goal here is to investigate some conditions for occurrence of TI and formation of pre-condensations through the outer half a quasi-static spherical molecular clump or core. The inner half is nearly singular and ambiguous so out of scope of this research. We consider a spherically symmetric molecular cloud in quasi-static and thermally equilibrium state, and we use the linear perturbation method to investigate occurrence of TI through its outer half. The origin of perturbations are assumed to be as Inside-Rush-Perturbation (IRP) with outward perturbed velocity at inner region of the cloud, and Outside-Rush-Perturbation (ORP) with inward perturbed velocity originated at the outer parts of the cloud. The local thermal balance at the outer half of the molecular cloud leads to a local loosely constrained power-law relation between the pressure and density as $p \propto \rho^{1+\chi}$, where $-0.4\lesssim \chi\lesssim 0.05$ depends on the functional form of the net cooling function. Physically, the value of $\chi$ depends on the power of dependence of magnetic field to the density, $\eta$, and also on the value of magnetic field gradient, $\zeta$. For strong magnetic field (smaller $\eta$) and/or large field gradient (greater $\zeta$), the value of $\chi$ decreases, and vice versa. The results show that increasing of the value of $\chi$ leads to form a flatter density profiles at the thermally equilibrium outer half of the molecular clump or core, and to occur more thermally unstable IRP and ORP with smaller growth time-scales, and vice versa.

12CO and 13CO J = 3–2 observations toward N11 in the Large Magellanic Cloud

Aims. After 30 Doradus, N11 is the second largest and brightest nebula in the Large Magellanic Cloud (LMC). This large nebula has several OB associations with bright nebulae at its surroundings. N11 was previously mapped at the lowest rotational transitions of 12CO (J = 1–0 and 2–1), and in some particular regions, pointings of the 13CO J = 1–0 and 2–1 lines were also performed. Observations of higher CO rotational transitions are needed to map gas with higher critical densities, which are useful to study the physical conditions of the gas component and its relation with the UV radiation more accurately. Methods. Using the Atacama Submillimeter Telescope Experiment, we mapped the whole extension of the N11 nebula in the 12CO J = 3–2 line and three subregions in the 13CO J = 3–2 line. The regions mapped in the 13CO J = 3–2 were selected with the criterion that they were to be exposed to the radiation in different ways: a region lying across the nebula, which is related to the OB association LH10 (N11B), another region that it is associated with the southern part of the nebula, which is related to the OB association LH13 (N11D), and finally an area farther away in the southwest without any embedded OB association (N11I). Results. We found that the morphology of the molecular clouds lying in each region shows some signatures that could be explained by the expansion of the nebulae and the action of the radiation. Fragmentation generated in a molecular shell due to the expansion of the N11 nebula is suggested. The integrated line ratios 12CO/13CO show evidence of selective photodissociation of the 13CO, and probably other mechanisms such as chemical fractionation. The values found for the integrated line ratios 12CO J = 3–2/1–0 are in agreement with values that were assumed in previous works, and the CO contribution to the continuum at 870 μm was derived directly. The distribution of the integrated line ratios 12CO J = 3–2/2–1 show indications of stellar feedback in N11B and N11D. The ratio between the virial and local thermal equilibrium (LTE) mass (Mvir/MLTE) is higher than unity in all analyzed molecular clumps, which suggests that the clumps are not gravitationally bounded and may be supported by external pressure. A non-LTE analysis suggests that we map gas with densities of about a few 103 cm−3. The molecular clump at N11B, the unique molecular feature with direct evidence of ongoing star formation, is the densest of the clumps we analyzed.

IRAS 23385+6053: an embedded massive cluster in the making

Context. This study is part of the CORE project, an IRAM/NOEMA large program consisting of observations of the millimeter continuum and molecular line emission towards 20 selected high-mass star-forming regions. The goal of the program is to search for circumstellar accretion disks, study the fragmentation process of molecular clumps, and investigate the chemical composition of the gas in these regions. Aims. We focus on IRAS 23385+6053, which is believed to be the least-evolved source of the CORE sample. This object is characterized by a compact molecular clump that is IR-dark shortward of 24 μm and is surrounded by a stellar cluster detected in the near-IR. Our aim is to study the structure and velocity field of the clump. Methods. Observations were performed at ~1.4 mm and employed three configurations of NOEMA and additional single-dish maps, merged with the interferometric data to recover the extended emission. Our correlator setup covered a number of lines from well-known hot core tracers and a few outflow tracers. The angular (~0′′.45–0′′.9) and spectral (0.5 km s−1) resolutions were sufficient to resolve the clump in IRAS 23385+6053 and investigate the existence of large-scale motions due to rotation, infall, or expansion. Results. We find that the clump splits into six distinct cores when observed at sub-arcsecond resolution. These are identified through their 1.4 mm continuum and molecular line emission. We produce maps of the velocity, line width, and rotational temperature from the methanol and methyl cyanide lines, which allow us to investigate the cores and reveal a velocity and temperature gradient in the most massive core. We also find evidence of a bipolar outflow, possibly powered by a low-mass star. Conclusions. We present the tentative detection of a circumstellar self-gravitating disk lying in the most massive core and powering a large-scale outflow previously known in the literature. In our scenario, the star powering the flow is responsible for most of the luminosity of IRAS 23385+6053 (~3000 L⊙). The other cores, albeit with masses below the corresponding virial masses, appear to be accreting material from their molecular surroundings and are possibly collapsing or on the verge of collapse. We conclude that we are observing a sample of star-forming cores that is bound to turn into a cluster of massive stars.

A Large-field J = 1 – 0 Survey of CO and Its Isotopologues toward the Cassiopeia A Supernova Remnant

Abstract We have conducted a large-field simultaneous survey of 12CO, 13CO, and C18O J = 1 − 0 emission toward the Cassiopeia A (Cas A) supernova remnant (SNR), which covers a sky area of 3.°5 × 3.°1. The Cas giant molecular cloud (GMC) mainly consists of three individual clouds with masses on the order of 104−105 M ☉. The total mass derived from the 13CO emission of the GMC is 2.1 × 105 M ☉ and is 9.5 × 105 M ☉ from the 12CO emission. Two regions with broadened (6–7 km s−1) or asymmetric 12CO line profiles are found in the vicinity (within a 10′ × 10′ region) of the Cas A SNR, indicating possible interactions between the SNR and the GMC. Using the GAUSSCLUMPS algorithm, 547 13CO clumps are identified in the GMC, 54% of which are supercritical (i.e., α vir < 2). The mass spectrum of the molecular clumps follows a power-law distribution with an exponent of −2.20. The pixel-by-pixel column density of the GMC can be fitted with a log-normal probability distribution function (N-PDF). The median column density of molecular hydrogen in the GMC is 1.6 × 1021 cm−2 and half the mass of the GMC is contained in regions with H2 column density lower than 3 × 1021 cm−2, which is well below the threshold of star formation. The distribution of the YSO candidates in the region shows no agglomeration.

Molecular clumps towards compact H ii regions

Molecular clumps towards compact H ii regions

Photoevaporation of Jeans-unstable molecular clumps

We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV (6 eV $< {\rm h}\nu$ < 13.6 eV) radiation, through 3D radiative transfer hydrodynamical simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H$_2$. We run a set of simulations considering different clump masses ($M=10-200$ M$_\odot$) and impinging fluxes ($G_0=2\times 10^3-8\times 10^4$ in Habing units). In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H$_2$ mass by a factor 40-90%. Then, a weak ($\mathcal{M}\simeq 2$) shock develops and travels towards the centre of the clump, which collapses while loosing mass from its surface. All considered clumps remain gravitationally unstable even if radiation rips off most of the clump mass, showing that external FUV radiation is not able to stop clump collapse. However, the FUV intensity regulates the final H$_2$ mass available for star formation: for example, for $G_0 < 10^4$ more than 10% of the initial clump mass survives. Finally, for massive clumps ($\sim 100$ M$_\odot$) the H$_2$ mass increases by 25-50% during the collapse, mostly because of the rapid density growth that implies a more efficient H$_2$ self-shielding.

Star Clusters in the Galactic tidal field, from birth to dissolution

AbstractWe study the evolution of star clusters in the Galactic tidal field starting from their birth in molecular clumps. Our model clusters form according to the local-density-driven cluster formation model in which the stellar density profile is steeper than that of gas. As a result, clusters resist the gas expulsion better than predicted by earlier models.We vary the impact of the Galactic tidal field λ, considering different Galactocentric distances (3-18 kpc), as well as different cluster sizes. Our model clusters survive the gas expulsion independent of λ.We investigated the relation between the cluster mass at the onset of secular evolution and their dissolution time. The model clusters formed with a high star-formation efficiency (SFE) follow a tight mass-dependent dissolution relation, in agreement with previous theoretical studies. However, the low-SFE models present a shallower mass-dependent relation than high-SFE clusters, and most dissolve before reaching 1 Gyr (cluster teenage mortality).

M 31 circum-nuclear region: A molecular survey with the IRAM interferometer

We analysed molecular observations performed at IRAM interferometer in CO(1-0) of the circum-nuclear region (within 250 pc) of Andromeda with 2.9″ = 11 pc resolution. We detected 12 molecular clumps in this region, corresponding to a total molecular mass of (8.4 ± 0.4)×104 M⊙. These clumps follow Larson’s mass-size relation, but lie well above the velocity-size relation. We discuss the possibility that these clumps are probably not virialised, but are transient agglomerations of smaller entities that might be virialised. Three of these clumps have been detected in CO(2-1) in a previous work, and we find a temperature line ratio below 0.5 in this work. With a radiative transfer analysis, we show that this gas is in non-local thermal equilibrium with a low excitation temperature (Tex = 5 − 9 K). We find a surface beam filling factor of order 5% and a gas density in the range 60 − 650 cm−3, which is well below the critical density. With a gas-to-stellar mass fraction of 4 × 10−4 and dust-to-gas ratio of 0.01, this quiescent region has exhausted its gas budget. Its spectral energy distribution is compatible with passive templates assembled from elliptical galaxies. While weak dust emission is present in the region, we show that no star formation is present and support the previous results that the dust is heated by the old and intermediate stellar population. We study the possibility that this region lies formally in the low-density part of the Kennicutt-Schmidt law in a regime where the star formation rate estimators are not completely reliable. We confirm the quiescence of the inner part of this galaxy known to lie on the green valley.

Nobeyama 45 m mapping observations toward Orion A. III. Multi-line observations toward an outflow-shocked region, Orion Molecular Cloud 2 FIR 4

Abstract We present the results of mapping observations toward an outflow-shocked region, OMC-2 FIR 4, using the Nobeyama 45 m telescope. We observed the area in 13CO (J = 1–0), C18O (J = 1–0), N2H+ (J = 1–0), CCS (JN = 87–76), HCO+ (J = 1–0), H13CO+ (J = 1–0), HN13C (J = 1–0), H13CN (J = 1–0), DNC (J = 1–0), N2D+ (J = 1–0), and DC3N (J = 9–8). We detected a dense molecular clump that contains FIR 4/5. We also detected, in the 13CO line, blueshifted and redshifted components driven presumably by protostellar outflows in this region. The axes of the FIR 3 and VLA 13 outflows, projected on the plane of the sky, appear to point to the FIR 4 clump, suggesting that it may be compressed by protostellar outflows from Class I sources, FIR 3 and VLA 13. Applying a hyperfine fit of N2H+ lines, we estimated the excitation temperature to be ∼20 K. The high excitation temperature is consistent with the fact that the clump contains protostars. CCS emission was detected in this region for the first time. Its abundance is estimated to be a few × 10−12, indicating that the region is chemically evolved at ∼105 yr, which is comparable to the typical lifetime of Class I protostars. This timescale is consistent with the scenario that star formation in FIR 4 is triggered by dynamical compression of the protostellar outflows. The [HNC]/[HCN] ratio was evaluated to be ∼0.5 in the dense clump and the outflow lobes, whereas it is somewhat larger in the envelope of the dense clump. The small [HNC]/[HCN] ratio indicates that HNC formation was prevented due to high temperatures. Such high temperatures seem to be consistent with the scenario that either protostellar radiation, or outflow compression, or both affected the thermal properties of this region.

A high-resolution study of carbon radio recombination lines towards Cassiopeia A

Carbon Radio Recombination Lines trace the interface between molecular and atomic gas. We present GMRT observations of Carbon Radio Recombination Lines (CRRL), C$\alpha$244-C$\alpha$250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical depth maps of these CRRLs. The emission shows a wide range of spatial and velocity structures, some of which are unresolved within our synthesis beam of 0.29 pc and velocity channel of 0.55 km/s. These variations in the emission optical depth and line width are indicative of inhomogeneity and fragmentation in the intervening Perseus Arm gas. We compare the distribution of the CRRL emission with that of diffuse and dense molecular gas using existing CO and H$_2$CO studies. We find that the CO emission in the -47 km/s Perseus Arm component is primarily concentrated along an elongated structure detected in our CRRL maps, to the south of which lies the high-density molecular clumps traced by H$_2$CO. This spatial distribution of CRRL and molecular tracers is similar to what one would observe for a Photo Dissociation Region (PDR). In the other Perseus Arm component centered at -37 km/s, there is evidence for high column density (N$_{H_2}$ ~ $10^{22}$cm$^{-2}$) molecular clumps embedded in diffuse CO as well as CRRL emission towards the center of Cassiopeia A. We propose that the CRRL emissions coincident with molecular tracers originates from the line of sight integrated component of the C$^{+}$ envelope of the molecular gas.

ATLASGAL-selected massive clumps in the inner Galaxy

Context. High-mass stars are formed within massive molecular clumps, where a large number of stars form close together. The evolution of the clumps with different masses and luminosities is mainly regulated by their high-mass stellar content and the formation of such objects is still not well understood. Aims. In this work, we characterise the mid-J CO emission in a statistical sample of 99 clumps (TOP100) selected from the ATLASGAL survey that are representative of the Galactic proto-cluster population. Methods. High-spatial resolution APEX-CHAMP+ maps of the CO (6–5) and CO (7–6) transitions were obtained and combined with additional single-pointing APEX-FLASH+ spectra of the CO (4–3) line. The data were convolved to a common angular resolution of 13.′′4. We analysed the line profiles by fitting the spectra with up to three Gaussian components, classified as narrow or broad, and computed CO line luminosities for each transition. Additionally, we defined a distance-limited sample of 72 sources within 5 kpc to check the robustness of our analysis against beam dilution effects. We have studied the correlations of the line luminosities and profiles for the three CO transitions with the clump properties and investigate if and how they change as a function of the evolution. Results. All sources were detected above 3-σ in all three CO transitions and most of the sources exhibit broad CO emission likely associated with molecular outflows. We find that the extension of the mid-J CO emission is correlated with the size of the dust emission traced by the Herschel-PACS 70 μm maps. The CO line luminosity (LCO) is correlated with the luminosity and mass of the clumps. However, it does not correlate with the luminosity-to-mass ratio. Conclusions. The dependency of the CO luminosity with the properties of the clumps is steeper for higher-J transitions. Our data seem to exclude that this trend is biased by self-absorption features in the CO emission, but rather suggest that different J transitions arise from different regions of the inner envelope. Moreover, high-mass clumps show similar trends in CO luminosity as lower mass clumps, but are systematically offset towards larger values, suggesting that higher column density and (or) temperature (of unresolved) CO emitters are found inside high-mass clumps.

Stellar mass spectrum within massive collapsing clumps

Context. The stellar mass spectrum is an important property of the stellar cluster and a fundamental quantity to understand our Universe. The fragmentation of diffuse molecular cloud into stars is subject to physical processes such as gravity, turbulence, thermal pressure, and magnetic field. Aims. The final mass of a star is believed to be a combined outcome of a virially unstable reservoir and subsequent accretion. We aim to clarify the roles of different supporting energies, notably the thermal pressure and magnetic field, in determining the stellar mass. Methods. Following our previous studies, we performed a series of numerical experiments of stellar cluster formation inside an isolated molecular clump. We investigated whether any characteristic mass is introduced into the fragmentation processes by changing the effective equation of state (EOS) of the diffuse gas, that is to say gas whose density is below the critical density at which dust becomes opaque to its radiation, and the strength of the magnetic field. Results. The EOS of the diffuse gas, including the bulk temperature and polytropic index, does not significantly affect the shape of the stellar mass spectrum. The presence of magnetic field slightly modifies the shape of the mass spectrum only when extreme values are applied. Conclusions. This study confirms that the peak of the initial mass function is primarily determined by the adiabatic high-density end of the EOS that mimics the radiation inside the high-density gas. Furthermore, the shape of the mass spectrum is mostly sensitive to the density PDF and the magnetic field likely only a secondary role. In particular, we stress that the Jeans mass at the mean cloud density and at the critical density are not responsible for setting the peak.

Investigating Fragmentation of Gas Structures in OB Cluster-forming Molecular Clump G33.92+0.11 with 1000 au Resolution Observations of ALMA

Abstract We report new ∼1000 au spatial resolution observations of 225 GHz dust continuum emission toward the OB cluster-forming molecular clump G33.92+0.11. On parsec scales, this molecular clump presents a morphology with several arm-like dense gas structures surrounding the two central massive (≳100 M ⊙) cores. From the new higher resolution observations, we identified 28 localized, spatially compact dust continuum emission sources, which may be candidates of young stellar objects (YSOs). Only one of them is not embedded within known arm-like (or elongated) dense gas structures. The spatial separations of these compact sources can be very well explained by Jeans lengths. We found that G33.92+0.11 may be consistently described by a marginally centrifugally supported, Toomre unstable accretion flow that is approximately in a face-on projection. The arm-like overdensities are a natural consequence of the Toomre instability, which can fragment to form YSOs in shorter timescales than the timescale of the global clump contraction. On our resolved spatial scales, there is no evidence yet that the fragmentation is halted by turbulence, the magnetic field, or stellar feedback.