Massive Star Formation Research Articles

Pattern finding in millimetre-wave spectra of massive young stellar objects

Massive stars (M* > 8 M⊙) play a pivotal role in shaping their galactic surroundings due to their high luminosity and intense ionizing radiation. However, the precise mechanisms governing the formation of massive stars remain elusive. Complex organic molecules (COMs) offer an avenue for studying star formation across the low- to high-mass spectrum because COMs are found in every young stellar object (YSO) phase and offer insight into the structure and temperature. We aim to unveil patterns in the evolution of COM chemistry in 41 massive young stellar objects (MYSOs) sourced from diverse catalogues, using Atacama Large Millimeter/Submillimeter Array Band 6 spectra. Previous line analysis of these sources revealed the presence of methanol, methyl acetylene, and methyl cyanide with diverse excitation temperatures (a few tens to hundreds of Kelvin) and column densities (spanning two to four orders of magnitude in range), indicating a possible evolutionary path across sources. However, such analyses usually involve manual line extraction and rotational diagram fitting. We improved upon this process by directly retrieving the physicochemical state of MYSOs from their dimensionally reduced spectra. We used a locally linear embedding to find a lower-dimensional projection for the physicochemical parameters obtained from individual line analysis. We identified clusters of similar MYSOs in the embedded space using a Gaussian mixture model, revealing three groups of MYSOs corresponding to distinct physicochemical conditions: (i) cold, COM-poor sources, (ii) warm, medium-COM-abundance sources, and (iii) hot, COM-rich sources. Principal component analysis (PCA) of the source spectra further supported an evolutionary path across MYSO groups. Finally, by training a simple random forest model on the first few PCA components, we found that the physicochemical state of MYSOs in our sample can be derived directly from the spectra. Our results highlight the effectiveness of dimensionality reduction in obtaining clear physical insights directly from MYSO spectra.

ALMA detections of circumstellar disks in the giant H ii region M17. Probing the intermediate- to high-mass pre-main-sequence population

Our current understanding is that intermediate- to high-mass stars form in a way similar to low-mass stars, through disk accretion. The expected shorter formation timescales, higher accretion rates, and increasingly strong radiation fields compared to their lower-mass counterparts may lead to significantly different physical conditions that play a role in disk formation, evolution, and the possibility of (sub)stellar companion formation therein. We searched for the mm counterparts of four intermediate- to high-mass ($4-10$ young stellar objects (YSOs) in the giant H ii region M17 at a distance of 1.7 kpc. These objects expose their photospheric spectrum such that their location on the pre-main-sequence (PMS) is well established. They have a circumstellar disk that is likely remnant of the formation process. With ALMA we detected, for the first time, these four YSOs in M17, in Band 6 and 7, as well as four other serendipitous objects. In addition to the flux measurements, the source size and spectral index provide important constraints on the physical mechanism(s) producing the observed emission. We applied different models to estimate the dust and gas mass contained in the disks. All our detections are spatially unresolved, constraining the source size to $<120$,au, and have a spectral index in the range $0.5-2.7$. The derived (upper limits on) the disk dust masses are on the order of a few M_⊕, and estimations of the upper limits on the gas mass vary between 10^-5 and 10^-3 Our modeling suggests that the inner disks of the target YSOs are dust depleted. In two objects (B331 and B268) free-free emission indicates the presence of ionized material around the star. The four serendipitous detections are likely low-mass YSOs. We compared the derived disk masses of our M17 targets to those obtained for YSOs in low-mass star-forming regions (SFRs) and Herbig stars, as a function of stellar mass, age, luminosity, and outer disk radius. The M17 sample, though small, is both the most massive and the youngest sample, yet has the lowest mean disk mass. The studied intermediate- to high-mass PMS stars are surrounded by low-mass compact disks that likely no longer offer a significant contribution to either the final stellar mass or the formation of a planetary system. Along with the four serendipitous discoveries, our findings show the capability of ALMA to probe disks in relatively distant high-mass SFRs, and offer tentative evidence of the influence of the massive star formation environment on disk formation, lifetime, and evolution.

Protostellar Outflows at the EarliesT Stages (POETS) VI. Evidence of disk-wind in G11.92-0.61 MM1

Magnetohydrodynamic disk-winds are thought to play a key role in the formation of massive stars by providing the fine-tuning between accretion and ejection, where excess angular momentum is redirected away from the disk, allowing further mass growth of a young protostar. However, only a limited number of disk-wind sources have been detected to date. To better constrain the exact mechanism of this phenomenon, expanding the sample is critical. We performed a detailed analysis of the disk-wind candidate G11.92-0.61 MM1 by estimating the physical parameters of the massive protostellar system and constraining the wind-launching mechanism. Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of G11.92-0.61 MM1 were conducted in September 2021 with ALMA's longest baselines, which provided a synthesised beam of ∼30 mas. We obtained high-resolution images of the CH_3CN (varv_8=1 and varv=0), CH_3OH, SO_2, and SO molecular lines, as well as the 1.3 mm continuum. Our high-resolution molecular data allowed us to refine the parameters of the disk-outflow system in MM1. The rotating disk is resolved into two regions with distinct kinematics: the inner region ($<$300 au) is traced by high-velocity emission of high-excitation CH_3CN lines and shows a Keplerian rotation; the outer region ($>$300 au), traced by mid-velocity CH_3CN emission, rotates in a sub-Keplerian regime. The central source is estimated to be ∼20 M_⊙, which is about half the mass estimated in previous lower-resolution studies. A strong collimated outflow is traced by SO and SO_2 emission up to ∼3400 au around MM1a. The SO and SO_2 emissions show a rotation-dominated velocity pattern, a constant specific angular momentum, and a Keplerian profile that suggests a magneto-centrifugal disk-wind origin with launching radii of ∼50-100 au. G11.92-0.61 MM1 appears to be one of the clearest cases of molecular line-traced disk-winds detected around massive protostars.

Massive Clusters and OB Associations as Output of Massive Star Formation in Gaia Era

Over the past two decades, our understanding of star formation has undergone a major shift, driven by a wealth of data from infrared, submillimeter and radio surveys. The emerging view depicts star formation as a hierarchical process, which predominantly occurs along filamentary structures in the interstellar medium. These structures span a wide range of spatial scales, ultimately leading to the birth of young stars, which distribute in small groups, clusters and OB associations. Given the inherently complex and dynamic nature of star formation, a comprehensive understanding of these processes can only be achieved by examining their end products—namely, the distribution and properties of young stellar populations. In the Gaia era, the nearby OB associations are now characterised with unprecedented detail, allowing for a robust understanding of their formation histories. Nevertheless, to fully grasp the mechanisms of star formation and its typical scale, it is essential to study the much larger associations, which constitute the backbones of spiral arms. The large catalogues of young open clusters that have emerged from Gaia DR3 offer a valuable resource for investigating star formation on larger spatial scales. While the cluster parameters listed in these catalogues are still subject to many uncertainties and systematic errors, ongoing improvements in data analysis and upcoming Gaia releases promise to enhance the accuracy and reliability of these measurements. This review aims to provide a comprehensive summary of recent advancements and a critical assessment of the datasets available.

Dynamic massive star formation: Radio flux variability in UCHII regions

Theoretical models of early accretion during the formation process of massive stars have predicted that regions exhibit radio variability on timescales of decades. However, large-scale searches for such temporal variations with sufficient sensitivity have not yet been carried out. Our aim is to identify regions with variable radio wavelength fluxes and to investigate the properties of the identified objects, especially those with the highest level of variability. We compared the peak flux densities of 86 ultracompact regions measured by the GLOSTAR and CORNISH surveys and identified variables that show flux variations higher than 30% over the ∼8 yr timespan between these surveys. We found a sample of 38 variable regions, which is the largest sample identified to date. The overall occurrence of variability is 44±5%, suggesting that variation in regions is significantly more common than prediction. The variable regions are found to be younger than nonvariable regions, all of them meeting the size criterion of hypercompact (HC) regions. We studied the seven regions that show the highest variability (the ``Top7'') with variations > 100%. The Top7 variable regions are optically thick at 4--8 GHz and compact, suggesting they are in a very early evolutionary stage of or regions. There is a significant correlation between variability and the spectral index of the radio emission. No dependence is observed between the variations and the properties of the sources' natal clumps traced by submillimeter continuum emission from dust, although variable regions are found in clumps at an earlier evolutionary stage.

VLA 22 GHz Imaging of Massive Star Formation in Local Wolf–Rayet Galaxies

We present 22 GHz imaging of regions of massive star formation within the Local Wolf–Rayet Galaxy Sample, a NSF Karl G. Jansky Very Large Array survey of 30 local galaxies showing spectral features of Wolf–Rayet (WR) stars. These spectral features are present in galaxies with young super star clusters (SSCs), and are an indicator of large concentrations of massive stars. We present a catalog of 92 individually identified regions of likely free–free emission associated with potential young SSCs located in these WR galaxies. The free–free fluxes from these maps allow extinction-free estimates of the Lyman continuum rates, masses, and luminosities of the emission regions. Thirty-nine of these regions meet the minimum Lyman continuum rate to contain at least once SSC, and 29 of these regions could contain individual SSCs massive enough to test specific theories on star formation and feedback inhibition in SSCs, requiring follow-up observations at higher spatial resolution. The resulting catalog provides sources for future molecular line and IR studies into the properties of SSC formation.

High Mass Star Formation - a (short) review

Massive star formation is one of the most enigmatic topics in astrophysi- cal research. It involves complex mechanisms such as accretion, competitive accretion, and stellar collisions. A comprehensive understanding requires multiwavelength studies, as different phases of star formation emit distinct radiative signatures. Massive stars significantly influence their surroundings by forming HII regions and injecting energy into the interstellar medium (ISM) via stellar winds and ultraviolet radiation. This review explore the theoretical and observational perspectives on massive star formation, highlighting the challenges, processes, and their profound impact on galactic evo- lution.

The SOFIA Massive (SOMA) Star Formation Q-band follow-up

Context. Evidence that the chemical characteristics around low- and high-mass protostars are similar has been found: notably, a variety of carbon-chain species and complex organic molecules (COMs) form around both types. On the other hand, the chemical compositions around intermediate-mass (IM) protostars (2 M⊙ < m* < 8 M⊙) have not been studied with large samples. In particular, it is unclear the extent to which carbon-chain species form around them. Aims. We aim to obtain the chemical compositions of a sample of IM protostars, focusing particularly on carbon-chain species. We also aim to derive the rotational temperatures of HC5N to confirm whether carbon-chain species are formed in the warm gas around these stars. Methods. We conducted Q-band (31.5–50 GHz) line survey observations toward 11 mainly IM protostars with the Yebes 40 m radio telescope. The target protostars were selected from a subsample of the source list of the SOFIA Massive Star Formation project. Assuming local thermodynamic equilibrium, we derived the column densities of the detected molecules and the rotational temperatures of HC5N and CH3 OH. Results. Nine carbon-chain species (HC3N, HC5N, C3H, C4H linear-H2CCC, cyclic-C3H2, CCS, C3S, and CH3CCH), three COMs (CH3OH, CH3CHO, and CH3CN), H2CCO, HNCO, and four simple sulfur-bearing species (13CS, C34S, HCS+, and H2CS) are detected. The rotational temperatures of HC5N are derived to be ~20–30 K in three IM protostars (Cepheus E, HH288, and IRAS 20293+3952). The rotational temperatures of CH3OH are derived in five IM sources and found to be similar to those of HC5N. Conclusions. The rotational temperatures of HC5N around the three IM protostars are very similar to those around low- and high-mass protostars. These results indicate that carbon-chain molecules are formed in lukewarm gas (~20–30 K) around IM protostars via the warm carbon-chain chemistry process. Thus, carbon-chain formation occurs ubiquitously in the warm gas around protostars across a wide range of stellar masses. Carbon-chain molecules and COMs coexist around most of the target IM protostars, which is similar to the situation for low- and high-mass protostars. In summary, the chemical characteristics around protostars are the same in the low-, intermediate- and high-mass regimes.

Exploring Lenticular Galaxy Formation in Field Environments Using NewHorizon: Evidence for Counterrotating Gas Accretion as a Formation Channel

The formation pathways of lenticular galaxies (S0s) in field environments remain a matter of debate. We utilize the cosmological hydrodynamic simulation, NewHorizon, to investigate the issue. We select two massive star formation quenched S0s as our main sample. By closely tracing their physical and morphological evolution, we identify two primary formation channels: mergers and counterrotating gas accretion. The former induces central gas inflow due to gravitational and hydrodynamic torques, triggering active central star formation, which quickly depletes the gas of the galaxy. Counterrotating gas accretion overall has a similar outcome but more exclusively through hydrodynamic collisions between the preexisting and newly accreted gas. Both channels lead to S0 morphology, with gas angular momentum cancellation being a crucial mechanism. These formation pathways quench star formation on a short timescale (<Gyr) compared to the timescales of environmental effects. We also discuss how counterrotating gas accretion may explain the origin of S0s with ongoing star formation and the frequently observed gas–star misaligned kinematics in S0s.

The role of thermal instability in accretion outbursts in high-mass stars

High-mass young stellar objects (HMYSOs) can exhibit episodic bursts of accretion, accompanied by intense outflows and luminosity variations. Understanding the underlying mechanisms driving these phenomena is crucial for elucidating the early evolution of massive stars and their feedback on star formation processes. Thermal instability (TI) due to hydrogen ionisation is among the most promising mechanisms of episodic accretion in low-mass ($M_* protostars. Its role in HMYSOs has not yet been determined. Here we investigate the properties of TI outbursts in young massive ($M_* 5$ msun ) stars, and compare them to those that have been observed to date. We employed a 1D numerical model to simulate TI outbursts in HMYSO accretion discs. We varied the key model parameters, such as stellar mass, mass accretion rate onto the disc, and disc viscosity, to assess the TI outburst properties. Our simulations show that modelled TI bursts can replicate the durations and peak accretion rates of long outbursts (a few years to decades) observed in HMYSOs with similar mass characteristics. However, they struggle with short-duration bursts (less than a year) with short rise times (a few weeks or months), suggesting the need for alternative mechanisms. Moreover, while our models match the durations of longer bursts, they fail to reproduce the multiple outbursts seen in some HMYSOs, regardless of model parameters. We also emphasise the significance of not just evaluating model accretion rates and durations, but also performing photometric analysis to thoroughly evaluate the consistency between model predictions and observational data. Our findings suggest that some other plausible mechanisms, such as gravitational instabilities and disc fragmentation, can be responsible for generating the observed outburst phenomena in HMYSOs, and we underscore the need for further investigation into alternative mechanisms driving short outbursts. However, the physics of TI is crucial in sculpting the inner disc physics in the early bright epoch of massive star formation, and comprehensive parameter space exploration; the use of 2D modelling is essential to obtaining a more detailed understanding of the underlying physical processes. By bridging theoretical predictions with observational constraints, this study contributes to advancing our knowledge of HMYSO accretion physics and the early evolution of massive stars.

The physical properties of cluster chains

We explore the kinematics and star formation history of the Scorpius Centaurus (Sco-Cen) OB association following the initial identification of sequential, linearly aligned chains of clusters. Building upon our characterization of the Corona Australis (CrA) chain, we now analyze two additional major cluster chains that exhibit similar characteristics: the Lower Centaurus Crux (LCC) and Upper Scorpius (Upper Sco) chains. All three cluster chains display distinct sequential patterns in 1) the 3D spatial distribution, 2) age, 3) velocity, and 4) mass. The Upper-Sco chain is the most massive and complex cluster chain, possibly consisting of two or more overlapping subchains. We discuss the possible formation of cluster chains and argue for a scenario where feedback from the most massive star formation episode 15 Myr ago initiated the formation of these spatio-temporal cluster sequences. Our results identify cluster chains as a distinct type of stellar structure with well-defined physical properties, formed in environments capable of sustaining stellar feedback over timescales of 5--10 Myr. We find that around 40&lt;!PCT!&gt; of the stellar population in Sco-Cen formed due to triggered star formation, with 35&lt;!PCT!&gt; forming along the three cluster chains. We conclude that cluster chains could be common structures in OB associations, particularly in regions that have similar natal environments as Sco-Cen. Beyond their significance for star formation and stellar feedback, they appear to be promising laboratories for chemical enrichment and the transport of elements from one generation to the next in the same star-forming region.

Compact and high excitation molecular clumps in the extended ultraviolet disk of M83

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.

Observational studies of S-bearing molecules in massive star-forming regions

S-bearing molecules are powerful tools for determining the physical conditions inside a massive star-forming region. The abundances of S-bearing molecules, including H$_ $S, H$_ $CS, and HCS$^ $, are highly dependent on physical and chemical changes, which means that they are good tracers of the evolutionary stage of massive star formation. We present observational results of H$_ $S 1$_ $, H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, SiO 4-3, HC$_ $N 19-18, and C18O 1-0 toward a sample of 51 late-stage massive star-forming regions, and study the relationships between H$_ $S, H$_ $CS, HCS$^ $, and SiO in hot cores. We discuss the chemical connections of these S-bearing molecules based on the relations between the relative abundances in our sources. H$_ $34S 1$_ $, as the isotopic line of H$_ $S 1$_ $, was used to correct the optical depths of H$_ $S 1$_ $. Beam-averaged column densities of all molecules were calculated, as were the abundances of H$_ $S, H$_ $CS, and HCS$^ $ relative to H$_ $, which were derived from C18O. Results from a chemical model that included gas, dust grain surface, and icy mantle phases, were compared with the observed abundances of H$_2$S, H$_2$CS, and HCS$^+$ molecules. H$_ $S 1$_ $, H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, and HC$_ $N 19-18 were detected in 50 of the 51 sources, SiO 4-3 was detected in 46 sources, and C18O 1-0 was detected in all sources. The Pearson correlation coefficients between H$_ $CS and HCS$^+$ normalized by H$_ $ and H$_ $S are 0.94 and 0.87, respectively, and a tight linear relationship with a slope of 1.00 and 1.09 is found; this relationship is 0.77 and 0.98 between H$_ $S and H$_ $CS and 0.76 and 0.97 between H$_ $S and HCS$^ $. The full widths at half maxima of H$_ $34S 1$_ $, H$_ $CS 5$_ $, HCS$^ $ 4-3, and HC$_ $N 19-18 in each source are similar to each other, which indicates that they may trace similar regions. By comparing the observed abundance with model results, we see that there is one possible time (2-3times 105 yr) a which each source in the model matches the measured abundances of H$_ $S, H$_ $CS, and HCS$^ $. The abundances of HCS$^ $, H$_ $CS, and H$_ $S increase with the SiO abundance in these sources, which implies that shock chemistry may be playing a large role. The close abundance relation of H$_2$S, H$_2$CS, and HCS$^+$ and the similar line widths in observational results indicate that these three molecules could be chemically linked, with HCS$^+$ and H$_2$CS the most correlated. The comparison of the observational results with chemical models shows that the abundances can be reproduced for almost all the sources at a specific time. The observational results, including the abundances in these sources need to be considered in further modeling of H$_ $S, H$_ $CS, and HCS$^ $ in hot cores with shock chemistry.

Exploring massive star early evolution: the case of the Herschel 36 A triple system

ABSTRACT Theoretical models show that some massive stars have not yet arrived at the zero-age main sequence (ZAMS) at the end of the accretion phase. At that time, they have lost their thick envelopes and thus could be optically visible. Although some candidates to optically observable ZAMS stars have been reported, the evolutionary status of none of them has been confirmed yet. The O-type triple system Herschel 36 A (H36A) is one of these candidates. We present the quantitative spectral analysis of the individual stellar components of H36A and investigate the evolutionary status of the system by contrasting main-sequence and pre-main-sequence models. Overall, the derived parameters suggest that the components of H36A could be pre-main-sequence stars going through the very last contraction to the ZAMS. However, the possibility of them already being on the main sequence is not yet ruled out. This study highlights the importance of considering multiple evolutionary models and shows that H36A represents a key object for understanding massive star formation and early evolution.

ATOMS: ALMA three-millimetre observations of massive star-forming regions–XVII. High-mass star-formation through a large-scale collapse in IRAS 15394-5358

ABSTRACT Hub-filament systems are considered as natural sites for high-mass star formation. Kinematic analysis of the surroundings of hub-filaments is essential to better understand high-mass star formation within such systems. In this work, we present a detailed study of the massive Galactic protocluster IRAS 15394$-$5358, using continuum and molecular line data from the ALMA three-millimetre observations of massive star-forming regions (ATOMS) survey. The 3 mm dust continuum map reveals the fragmentation of the massive ($\rm M=843~{\rm M}_{\odot }$) clump into six cores. The core C-1A is the largest (radius = 0.04 pc), the most massive ($\rm M=157~{\rm M}_{\odot }$), and lies within the dense central region, along with two smaller cores ($\rm M=7~and~3~{\rm M}_{\odot }$). The fragmentation process is consistent with the thermal Jeans fragmentation mechanism and virial analysis shows that all the cores have small virial parameter values ($\rm \alpha _{vir}\lt \lt 2$), suggesting that the cores are gravitationally bound. The mass versus radius relation indicates that three cores can potentially form at least a single massive star. The integrated intensity map of $\rm H^{13}CO^{+}$ shows that the massive clump is associated with a hub-filament system, where the central hub is linked with four filaments. A sharp velocity gradient is observed towards the hub, suggesting a global collapse where the filaments are actively feeding the hub. We discuss the role of global collapse and the possible driving mechanisms for the massive star formation activity in the protocluster.

Massive Star Formation Starts in Subvirial Dense Clumps Unless Resisted by Strong Magnetic Fields

Knowledge of the initial conditions of high-mass star formation is critical for theoretical models, but are not well observed. Built on our previous characterization of a Galaxy-wide sample of 463 candidate high-mass starless clumps (HMSCs), here we investigate the dynamical state of a representative subsample of 44 HMSCs (radii 0.13–1.12 pc) using Green Bank Telescope NH3 (1,1) and (2,2) data from the Radio Ammonia Mid-Plane Survey pilot data release. By fitting the two NH3 lines simultaneously, we obtain velocity dispersion, gas kinetic temperature, NH3 column density and abundance, Mach number, and virial parameter. Thermodynamic analysis reveals that most HMSCs have Mach number <5, inconsistent with what have been considered in theoretical models. All but one (43 out of 44) of the HMSCs are gravitationally bound with virial parameter α vir < 2. Either these massive clumps are collapsing or magnetic field strengths of 0.10–2.65 mG (average 0.51 mG) would be needed to prevent them from collapsing. The estimated B-field strength correlates tightly with density, Best/mG=0.269(nH2/104cm−3)0.61 , with a similar power-law index as found in observations but a factor of 4.6 higher in strength. For the first time, the initial dynamical state of high-mass formation regions has been statistically constrained to be subvirial, in contradiction to theoretical models in virial equilibrium and in agreement with the lack of observed massive starless cores. The findings urge future observations to quantify the magnetic field support in the prestellar stage of massive clumps, which has rarely been explored so far, toward a full understanding of the physical conditions that initiate massive star formation.

JCMT 850 μm Continuum Observations of Density Structures in the G35 Molecular Complex

Filaments are believed to play a key role in high-mass star formation. We present a systematic study of the filaments and their hosting clumps in the G35 molecular complex using James Clerk Maxwell Telescope SCUBA-2 850 μm continuum data. We identified five clouds in the complex and 91 filaments within them, some of which form 10 hub–filament systems (HFSs), each with at least three hub-composing filaments. We also compiled a catalog of 350 dense clumps, 183 of which are associated with the filaments. We investigated the physical properties of the filaments and clumps, such as mass, density, and size, and their relation to star formation. We find that the global mass–length trend of the filaments is consistent with a turbulent origin, while the hub-composing filaments of high line masses (m l > 230 M ⊙ pc−1) in HFSs deviate from this relation, possibly due to feedback from massive star formation. We also find that the most massive and densest clumps (R > 0.2 pc, M > 35 M ⊙, Σ > 0.05 g cm−2) are located in the filaments and in the hubs of HFSs, with the latter bearing a higher probability of the occurrence of high-mass star-forming signatures, highlighting the preferential sites of HFSs for high-mass star formation. We do not find significant variation in the clump mass surface density across different evolutionary environments of the clouds, which may reflect the balance between mass accretion and stellar feedback.

Signatures of Mass Segregation from Competitive Accretion and Monolithic Collapse

The two main competing theories proposed to explain the formation of massive (>10 M ⊙) stars—competitive accretion and monolithic core collapse—make different observable predictions for the environment of the massive stars during, and immediately after, their formation. Proponents of competitive accretion have long predicted that the most massive stars should have a different spatial distribution to lower-mass stars, through the stars being either mass segregated or being in areas of higher relative densities or sitting deeper in gravitational potential wells. We test these predictions by analyzing a suite of smoothed-particle hydrodynamics simulations where star clusters form massive stars via competitive accretion with and without feedback. We find that the most massive stars have higher relative densities, and sit in deeper potential wells, only in simulations in which feedback is not present. When feedback is included, only half of the simulations have the massive stars residing in deeper potential wells, and there are no other distinguishing signals in their spatial distributions. Intriguingly, in our simple models for monolithic core collapse, the massive stars may also end up in deeper potential wells because if massive cores fragment then the stars that form are also massive, and dominate their local environs. We find no robust diagnostic test in the spatial distributions of massive stars that can distinguish their formation mechanisms, and so other predictions for distinguishing between competitive accretion and monolithic collapse are required.

Evidence of a Forming Nucleus in the Fourcade–Figueroa Galaxy

We analyze data from the IRAS, Wide-field Infrared Survey Explorer, and Planck satellites, revealing an unresolved dust condensation at the center of the Fourcade–Figueroa galaxy (ESO270-G017), which may correspond to a forming nucleus. We model the condensation’s continuum spectrum in the spectral range from 3 to 1300 μm using the DUSTY code. The best-fit model, based on the chi-square test, indicates that the condensation is a shell with an outer temperature of T out ≈ 12 K and an inner boundary temperature of T i ≈ 500 K. The shell’s outer radius is r o = 86.2 pc, and the inner cavity radius is r i = 0.082 pc. The condensation produces an extinction A V = 50 mag, and its luminosity is L c = 1.08 × 1034 W, which would correspond to a burst of massive star formation approximately similar to the central 5 pc of R 136 in the LMC and NGC 3603, the ionizing cluster of a giant Carina arm H ii region. The comparison with normal, luminous, and ultraluminous IR galaxies leads us to consider this obscured nucleus as the nearest and weakest object of this category.

Gravitational collapse at low to moderate Mach numbers: The relationship between star formation efficiency and the fraction of mass in the massive object

The formation of massive objects via gravitational collapse is relevant both for explaining the origin of the first supermassive black holes and in the context of massive star formation. Here, we analyze simulations of the formation of massive objects pursued by different groups and in various environments, concerning the formation of supermassive black holes, primordial stars, as well as present-day massive stars. We focus here particularly on the regime of small virial parameters, that is, low ratios of the initial kinetic to gravitational energy, low to moderate Mach numbers, and the phase before feedback is very efficient. We compare the outcomes of collapse under different conditions using dimensionless parameters, particularly the star formation efficiency є*, the fraction ƒ* of mass in the most massive object relative to the total stellar mass, and the fraction ƒtot of mass of the most massive object as a function of the total mass. We find that in all simulations analyzed here, ƒtot increases as a function of є*, although the steepness of the increase depends on the environment. The relation between ƒ* and є* is found to be more complex and also strongly depends on the number of protostars present at the beginning of the simulations. We show that a collision parameter, estimated as the ratio of the system size divided by the typical collision length, allows us to approximately characterize whether collisions are important. A high collision parameter implies a steeper increase in the relation between ƒtot and є*. We analyze the statistical correlation between the dimensionless quantities using the Spearman coefficient and further confirm via a machine learning analysis that good predictions of ƒ* can be obtained from є* together with a rough estimate of the collision parameter. This suggests that a good estimate of the mass of the most massive object can be obtained once the maximum efficiency for a given environment is known and an estimate for the collision parameter has been determined.