Star Cluster Formation Research Articles

Failed supernova explosions increase the duration of star formation in globular clusters

The duration of SF in GC is an essential aspect for understanding their formation. Contrary to previous presumptions that all stars above 8 $M_ explode as CCSN recent evidence suggests a more complex scenario. We analyse iron spread observations from 55 GC to estimate the number of CCSN explosions before SF termination, thereby determining the SF duration. This work for the first time takes the possibility of failed CCSN into account, when estimating the SF duration. Two scenarios are considered: one where all stars explode as CCSN and another where only stars below 20 $M_ lead to CCSN as most CCSN models predict that no failed CCSN happen below 20 $M_ odot$. This establishes a lower ($ Myr$) and an upper ($ Myr$) limit for the duration of SF . Extending the findings of our previous paper, this study indicates a significant difference in SF duration based on CCSN outcomes, with failed CCSN extending SF by up to a factor of three. Additionally, a new code is introduced to compute the SF duration for a given CCSN model. The extended SF has important implications on GC formation, including enhanced pollution from stellar winds and increased binary star encounters. These results underscore the need for a refined understanding of CCSN in estimating SF durations and the formation of multiple stellar populations in GC

The formation and evolution of dark star clusters II: The impact of primordial mass segregation

The formation and evolution of dark star clusters II: The impact of primordial mass segregation

Dynamical accretion flows

Context. Investigating the flow of material along filamentary structures towards the central core can help provide insights into high-mass star formation and evolution. Aims. Our main motivation is to answer the question of what the properties of accretion flows are in star-forming clusters. We used data from the ALMA Evolutionary Study of High Mass Protocluster Formation in the Galaxy (ALMAGAL) survey to study 100 ALMAGAL regions at a ∼1″ resolution, located between ∼2 and 6 kpc. Methods. Making use of the ALMAGAL ∼1.3 mm line and continuum data, we estimated flow rates onto individual cores. We focus specifically on flow rates along filamentary structures associated with these cores. Our primary analysis is centered around position velocity cuts in H2CO (30, 3–20, 2), which allow us to measure the velocity fields surrounding these cores. Combining this work with column density estimates, we were able to derive the flow rates along the extended filamentary structures associated with cores in these regions. Results. We selected a sample of 100 ALMAGAL regions, covering four evolutionary stages from quiescent to protostellar, young stellar objects (YSOs), and HII regions (25 each). Using a dendrogram and line analysis, we identify a final sample of 182 cores in 87 regions. In this paper, we present 728 flow rates for our sample (4 per core), analysed in the context of evolutionary stage, distance from the core, and core mass. On average, for the whole sample, we derived flow rates on the order of ∼10−4 M⊙ yr−1 with estimated uncertainties of ±50%. We see increasing differences in the values among evolutionary stages, most notably between the less evolved (quiescent and protostellar) and more evolved (YSO and HII region) sources and we also see an increasing trend as we move further away from the centre of these cores. We also find a clear relationship between the calculated flow rates and core masses ∼M2/3, which is in line with the result expected from the tidal-lobe accretion mechanism. The significance of these relationships is tested with Kolmogorov–Smirnov and Mann-Whitney U tests. Conclusions. Overall, we see an increasing trend in the relationships between the flow rate and the three investigated parameters, namely: evolutionary stage, distance from the core, and core mass.

Filamentary Hierarchies and Superbubbles: Galactic Multiscale Magnetohydrodynamic Simulations of Giant Molecular Cloud to Star Cluster Formation

There is now abundant observational evidence that star formation is a highly dynamical process that connects filament hierarchies and supernova feedback from galaxy-scale kiloparsec filaments and superbubbles to giant molecular clouds (GMCs) on 100 pc scales and star clusters (1 pc). Here we present galactic multiscale MHD simulations that track the formation of structure from galactic down to subparsec scales in a magnetized, Milky Way–like galaxy undergoing supernova-driven feedback processes. We do this by adopting a novel zoom-in technique that follows the evolution of typical 3 kpc subregions without cutting out the surrounding galactic environment, allowing us to reach 0.28 pc resolution in the individual zoom-in regions. We find a wide range of morphologies and hierarchical structures, including superbubbles, turbulence, and kiloparsec atomic gas filaments hosting multiple GMC condensations that are often associated with superbubble compression, down to smaller-scale filamentary GMCs and star cluster regions within them. Gas accretion and compression ultimately drive filaments over a critical, scale-dependent line mass leading to gravitational instabilities that produce GMCs and clusters. In quieter regions, galactic shear can produce filamentary GMCs within flattened, rotating disklike structures on 100 pc scales. Strikingly, our simulations demonstrate the formation of helical magnetic fields associated with the formation of these disklike structures.

Massive star cluster formation

The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCH framework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCH by implementing the N-body code PETAR, thereby enabling TORCH to handle massive clusters forming from 106 M⊙ clouds with ≥105 individual stars. We present results from TORCH simulations of star clusters forming from 104, 105, and 106 M⊙ turbulent spherical gas clouds (named M4, M5, M6) of radius R = 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 101,2,3 M⊙ pc−2, after a free-fall time of tff = 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 are ϵ⋆ = M⋆/Mcloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies of ϵinst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 102 M⊙ pc−2) and massive (Mcloud ≥ 105 M⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars.

Massive star cluster formation

Two main mechanisms have classically been proposed for the formation of runaway stars. In the binary supernova scenario (BSS), a massive star in a binary explodes as a supernova, ejecting its companion. In the dynamical ejection scenario, a star is ejected during a strong dynamical encounter between multiple stars. We propose a third mechanism for the formation of runaway stars: the subcluster ejection scenario (SCES), where a subset of stars from an infalling subcluster is ejected out of the cluster via a tidal interaction with the contracting gravitational potential of the assembling cluster. We demonstrate the SCES in a star-by-star simulation of the formation of a young massive cluster from a 106 M⊙ gas cloud using the TORCH framework. This star cluster forms hierarchically through a sequence of subcluster mergers determined by the initial turbulent, spherical conditions of the gas. We find that these mergers drive the formation of runaway stars in our model. Late-forming subclusters fall into the central potential, where they are tidally disrupted, forming tidal tails of runaway stars that are distributed highly anisotropically. Runaways formed in the same SCES have similar ages, velocities, and ejection directions. Surveying observations, we identify several SCES candidate groups with anisotropic ejection directions. The SCES is capable of producing runaway binaries: two wide dynamical binaries in infalling subclusters were tightened through ejection. This allows for another velocity kick via subsequent via a subsequent BSS ejection. An SCES-BSS ejection is a possible avenue for the creation of hypervelocity stars unbound to the Galaxy. The SCES occurs when subcluster formation is resolved. We expect nonspherical initial gas distributions to increase the number of calculated runaway stars, bringing it closer to observed values. The observation of groups of runaway stars formed via the SCES can thus reveal the assembly history of their natal clusters.

Mapping Multiphase Metals in Star-forming Galaxies: A Spatially Resolved UV+Optical Study of NGC 5253

We present a pioneering, spatially resolved, multiphase gas abundance study on the blue compact dwarf galaxy NGC 5253, targeting 10 star-forming (SF) clusters inside six far-UV (FUV) Hubble Space Telescope/Cosmic Origins Spectrograph (COS) pointings with cospatial optical Very Large Telescope/MUSE observations throughout the galaxy. The SF regions span a wide range of ages (1–15 Myr) and are distributed at different radii (50–230 pc). We performed a robust absorption-line profile fitting on the COS spectra, covering 1065–1430 Å in the FUV, allowing an accurate computation of neutral-gas abundances for 13 different ions sampling eight elements. These values were then compared with the ionized-gas abundances, measured using the direct method on MUSE integrated spectra inside analog COS apertures. Our multiphase, spatially resolved comparisons find abundances, which are lower in the neutral gas than the ionized gas by 0.22, 0.80, and 0.58 dex for log(O/H), log(N/H), and log(N/O), respectively. We modeled the chemical abundance distributions and evaluated correlations as a function of the radius and age. It was found that, while N, O, and N/O abundances decrease as a function of age in the ionized gas, they increase with age in the neutral gas. No strong correlations for N, O, or N/O were observed as a function of the radius. The N/O and N/H offsets between the phases were found to decrease with age, providing evidence that chemical enrichment happens differentially, first in the ionized-gas phase around 2–5 Myrs (due to N-rich Wolf–Rayet stars) and then mixing out into the cold neutral gas on longer timescales of 10–15 Myr.

Impacts of stellar wind and supernovae on star cluster formation: Origins of extremely high N/O ratios and multiple stellar populations

Abstract We study metal enrichment originating from stellar wind and supernovae in low-metallicity clouds by performing three-dimensional radiation hydrodynamics simulations. We find that metals ejected from stellar wind are accumulated, leading to subsequent star formation in the nitrogen-enriched gas. During this early phase, the ${\rm N/O}$ ratios are similar to observed nitrogen-enriched galaxies (${\rm [N/O]}\gtrsim 0.5$). Then, once supernovae occur, the ${\rm N/O}$ ratios decrease significantly. If the duration of star formation is comparable to the time-scale of supernovae, the mass fraction of nitrogen-enriched stars reaches half the mass of star clusters. We suggest that the mass of the star cluster needs to exceed $\sim \!10^6$ M$_{\odot }$ to have multiple populations due to stellar wind, considering the condition for massive star cluster formation and the timescales of stellar evolution.

Suppressed Cosmic-Ray Energy Densities in Molecular Clouds from Streaming Instability-regulated Transport

Cosmic rays (CRs) are the primary driver of ionization in star-forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic-ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a 2000M ⊙ MC and the subsequent star formation including CRT using the STAR FORmation in Gaseous Environments framework implemented in the GIZMO code. We show that when CRT is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way–like environment the median CR ionization rate in the cloud is low (ζ ≲ 2 × 10−19 s−1) during the main star-forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated (ζ ≲ 6 × 10−18), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency by 50% to ∼13%. The initial mass function is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments.

Overview Results of JWST Observations of Star-forming Clusters in the Extreme Outer Galaxy

The extreme outer Galaxy (EOG), which we define as the region of the Milky Way with a galactocentric radius of more than 18 kpc, provides an excellent opportunity to study star formation in an environment significantly different from that in the solar neighborhood because of its lower metallicity and lower gas density. We carried out near- and mid-infrared (NIR and MIR) imaging observations toward two star-forming clusters located in the EOG using JWST NIRCam and MIRI with nine filters: F115W, F150W, F200W, F350W, F405N, F444W, F770W, F1280W, and F2100W. In this paper, we present an overview of the observations, data reduction, and initial results. The NIR sensitivity is approximately 10–80 times better than our previous observation with the Subaru 8.2 m telescope. Accordingly, the mass detection limit reaches to about 0.01–0.05 M ⊙, which is about 10 times better than the previous observations. At MIR wavelengths, the high sensitivity and resolution data enable us to resolve individual young stellar objects in such a distant region for the first time. The mass detection limit at MIR F770W filter reaches about 0.1–0.3 M ⊙. With these new observations, we have identified components of the clusters that previous surveys did not detect, including class 0 candidates, outflow/jet components, and distinctive nebular structures. These data will enable us to investigate the properties of star formation in the EOG at the same depth of detail as previous observations of star formation in the solar neighborhood.

Wide-binary eccentricity distribution in young star clusters: dependence on the binary separation and mass

ABSTRACT We study the wide-binary eccentricity (e) distribution in young star clusters and the role of turbulence in setting the form of the e distribution using magnetohydrodynamical simulations of star cluster formation. The simulations incorporate gravity, turbulence, magnetic fields, protostellar heating, and jets/outflows. We find that (1) simulations that employ purely compressive turbulence driving produce binaries with a superthermal e distribution [$\alpha \gt 1$ in $p(e) \propto e^\alpha$], while simulations with purely solenoidal driving or natural mixture of driving modes produce subthermal/thermal distributions ($\alpha \le$ 1), (2) the e distribution over the full range of binary separations in our simulations is set at the early stages of the star cluster formation process, (3) while binaries (separation of $r_{\mathrm{pair}} \le 1000\, \mathrm{AU}$) have subthermal to thermal e distributions ($\alpha \sim 0.8$), wide binaries ($r_{\mathrm{pair}} \gt 1000\, \mathrm{AU}$) have a superthermal distribution ($\alpha \sim 1.8$), and (4) low-mass binary systems (system masses of $M_{\mathrm{sys}} \le 0.8\, \mathrm{M_\odot }$) have a highly superthermal distribution ($\alpha \sim 2.4$), whereas high-mass systems ($M_{\mathrm{sys}} \gt 0.8\, \mathrm{M_\odot }$) exhibit a subthermal/thermal distribution ($\alpha \sim 0.8$). The binary eccentricity distribution is often modelled as a thermal distribution. However, our results suggest that the e distribution depends on the range of separation of the sampled binaries, which agrees with the findings from recent Gaia observations. We conclude that the dependence of the e distribution on the binary separation and mass is linked to the binary formation mechanism governed by the turbulent properties of the parent cloud.

MeerKAT observations of starburst galaxies and AGNs within the core of XMMXCS J2215.9−1738 at z = 1.46

ABSTRACT We present the first detailed radio study of the galaxy cluster XMMXCS J2215.9−1738 at z = 1.46 using MeerKAT L-band (1.3 GHz) observations. We combine our radio observation with archival optical and infrared data to investigate the star formation and active galactic nucleus (AGN) population within $R_{200}$ ($R =$ 0.8 Mpc) of the cluster centre. Using three selection criteria; the radio luminosity, the far-infrared radio ratio ($q_{\rm {IR}}$), and the mid-infrared colour, we distinguish galaxies with radio emission predominantly powered by star formation from that powered by AGNs. We selected 24 cluster members within $R_{\rm {200}}$ in the MeerKAT image based on either their photometric or spectroscopic redshift. We classified 12/24 (50 per cent) as galaxies whose radio emission is dominated by star-formation activity, 6/24 (25 per cent) as intermediate star-forming galaxies, and 6/24 (25 per cent) as AGN-dominated galaxies. Using the radio continuum luminosities of the star-forming cluster galaxies, we estimated an integrated star formation rate (SFR) value of 1700 $\pm$ 330 M$_{\odot }$ yr$^{-1}$ within $R_{200}$. We derived a mass-normalized integrated SFR value of $(570 \pm 110) \times 10^{-14}$ yr$^{-1}$. This supports previous observational and theoretical studies that indicated a rapid increase in star formation activity within the core of high-redshift clusters. We also show that the high-AGN fraction within the cluster core is consistent with previous cluster observations at $z \gt $ 1.5.

Violent Starbursts and Quiescence Induced by Far-ultraviolet Radiation Feedback in Metal-poor Galaxies at High Redshift

JWST observations of galaxies at z ≳ 8 suggest that they are more luminous and clumpier than predicted by most models, prompting several proposals on the physics of star formation and feedback in the first galaxies. In this paper, we focus on the role of ultraviolet (UV) radiation in regulating star formation by performing a set of cosmological radiation hydrodynamics simulations of one galaxy at subparsec resolution with different radiative feedback models. We find that the suppression of cooling by far-UV (FUV) radiation (i.e., H2 dissociating radiation) from Population II stars is the main physical process triggering the formation of compact and massive star clusters and is responsible for the bursty star formation observed in metal-poor galaxies at z ≳ 10. Indeed, artificially suppressing FUV radiation leads to a less intense continuous mode of star formation distributed into numerous but low-mass open star clusters. Due to the intense FUV field, low-metallicity clouds remain warm (∼104 K) until they reach a relatively high density (≳103 cm−3), before becoming self-shielded and transitioning to a colder (∼100 K), partially molecular phase. As a result, star formation is delayed until the clouds accumulate enough mass to become gravitationally unstable. At this point, the clouds undergo rapid star formation, converting gas into stars with high efficiency. We therefore observe exceptionally bright galaxies (10 times brighter than for continuous star formation) and subsequent quenched “dead” galaxies that did not form stars for tens of Myr.

Probing the physics of star formation (ProPStar)

Context. The detections of narrow channels of accretion toward protostellar disks, known as streamers, have increased in number in the last few years. However, it is unclear whether streamers are a common feature around protostars that were previously missed, or if they are a rare phenomenon. Aims. Our goals are to obtain the incidence of streamers toward a region of clustered star formation and to trace the origins of their gas to determine whether they originate within the filamentary structure of molecular clouds or from beyond. Methods. We used combined observations of the nearby NGC 1333 star-forming region, carried out with the NOEMA interferometer and the IRAM 30m single dish. Our observations cover the area between the systems IRAS 4 and SVS 13. We traced the chemically fresh gas within NGC 1333 with HC3N molecular gas emission and the structure of the fibers in this region with N2H+ emission. We fit multiple velocity components in both maps and used clustering algorithms to recover velocity-coherent structures. Results. We find streamer candidates toward 7 out of 16 young stellar objects within our field of view. This represents an incidence of approximately 40% of young stellar objects with streamer candidates in a clustered star-forming region. The incidence increases to about 60% when we only considered embedded protostars. All streamers are found in HC3N emission. Conclusions. Given the different velocities between HC3N and N2H+ emission, and because by construction, N2H+ traces the fiber structure, we suggest that the gas that forms the streamers comes from outside the fibers. This implies that streamers can connect cloud material that falls onto the filaments with protostellar disk scales.

Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift z ≈ 10.2 (ref. 1). However, it is an intrinsically ultraviolet faint galaxy, in the range of those now thought to drive the reionization of the Universe2–4. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs (pc)5,6. Here we report JWST observations of the Cosmic Gems. The light of the galaxy is resolved into five star clusters located in a region smaller than 70 pc. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of about 106M⊙. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 105M⊙ pc−2, three orders of magnitude higher than typical young star clusters in the local Universe7. Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, that is, proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shaping the properties of galaxies during the epoch of reionization.

Most nearby young star clusters formed in three massive complexes.

Efforts to unveil the structure of the local interstellar medium and its recent star-formation history have spanned the past 70 years (refs. 1-6). Recent studies using precise data from space astrometry missions have revealed nearby, newly formed star clusters with connected origins7-12. Nonetheless, mapping young clusters across the entire sky back to their natal regions has been hindered by a lack of clusters with precise radial-velocity data. Here we show that 155 out of 272 (57%) high-quality young clusters13,14 within 1 kiloparsec of the Sun arise from three distinct spatial volumes. This conclusion is based on the analysis of data from the third Gaia release15 and other large-scale spectroscopic surveys. At present, dispersed throughout the solar neighbourhood, their past positions more than 30 million yearsago reveal that these families of clusters each formed in one of three compact, massive star-forming complexes. One of these families includes all of the young clusters near the Sun-the Taurus and Scorpius-Centaurus star-forming complexes16,17. We estimate that more than 200 supernovae were produced from these families and argue that these clustered supernovae produced both the Local Bubble18 and the largest nearby supershell GSH 238+00+09 (ref. 19), both of which are clearly visible in modern three-dimensional dust maps20-22.

Great Balls of FIRE

Context. Despite the nearly hundred gravitational-wave detections reported by the LIGO-Virgo-KAGRA Collaboration, the question of the cosmological origin of merging binary black holes (BBHs) remains open. The two main formation channels generally considered are from isolated field binaries or via dynamical assembly in dense star clusters. Aims. Here we focus on understanding the dynamical formation of merging BBHs within massive clusters in galaxies of different masses. Methods. To this end, we applied a new framework to consistently model the formation and evolution of massive star clusters in zoom-in cosmological simulations of galaxies. Each simulation, taken from the FIRE project, provides a realistic star formation environment, with a unique star formation history, that hosts realistic giant molecular clouds that constitute the birthplace of star clusters. Combined with the code for star cluster evolution CMC, we are able to produce populations of dynamically formed merging BBHs across cosmic time in different environments. Results. As the most massive star clusters preferentially form in dense massive clouds of gas, we find that, despite their low metallicities favouring the creation of black holes, low-mass galaxies contain few massive clusters and therefore make a limited contribution to the global production of dynamically formed merging BBHs. Furthermore, we find that massive clusters can host hierarchical BBH mergers with clear, identifiable physical properties. Looking at the evolution of the BBH merger rate in different galaxies, we find strong correlations between BBH mergers and the most extreme episodes of star formation. Finally, we discuss the implications for future LIGO-Virgo-KAGRA gravitational wave observations.

Probing the physics of star formation (ProPStar)

Context. Electron fraction and cosmic-ray ionization rates in star-forming regions are important quantities in astrochemical modeling and are critical to the degree of coupling between neutrals, ions, and electrons, which regulates the dynamics of the magnetic field. However, these are difficult quantities to estimate. Aims. We aim to derive the electron fraction and cosmic-ray ionization rate maps of an active star-forming region. Methods. We combined observations of the nearby NGC 1333 star-forming region carried out with the NOEMA interferometer and IRAM 30 m single dish to generate high spatial dynamic range maps of different molecular transitions. We used the DCO+ and H13CO+ ratio (in addition to complementary data) to estimate the electron fraction and produce cosmic-ray ionization rate maps. Results. We derived the first large-area electron fraction and cosmic-ray ionization rate resolved maps in a star-forming region, with typical values of 10−65 and 10−16.5 s−1, respectively. The maps present clear evidence of enhanced values around embedded young stellar objects (YSOs). This provides strong evidence for locally accelerated cosmic rays. We also found a strong enhancement toward the northwest region in the map that might be related either to an interaction with a bubble or to locally generated cosmic rays by YSOs. We used the typical electron fraction and derived a magnetohydrodynamic (MHD) turbulence dissipation scale of 0.054 pc, which could be tested with future observations. Conclusions. We found a higher cosmic-ray ionization rate compared to the canonical value for N(H2) = 1021−1023 cm−2 of 10−17 s−1 in the region, and it is likely generated by the accreting YSOs. The high value of the electron fraction suggests that new disks will form from gas in the ideal-MHD limit. This indicates that local enhancements of ζ(H2), due to YSOs, should be taken into account in the analysis of clustered star formation.

Polarized Light from Massive Protoclusters (POLIMAP). I. Dissecting the Role of Magnetic Fields in the Massive Infrared Dark Cloud G28.37+0.07

Magnetic fields may play a crucial role in setting the initial conditions of massive star and star cluster formation. To investigate this, we report SOFIA-HAWC+ 214 μm observations of polarized thermal dust emission and high-resolution GBT-Argus C18O(1-0) observations toward the massive Infrared Dark Cloud (IRDC) G28.37+0.07. Considering the local dispersion of B-field orientations, we produce a map of the B-field strength of the IRDC, which exhibits values between ∼0.03 and 1 mG based on a refined Davis–Chandrasekhar–Fermi method proposed by Skalidis & Tassis. Comparing to a map of inferred density, the IRDC exhibits a B–n relation with a power-law index of 0.51 ± 0.02, which is consistent with a scenario of magnetically regulated anisotropic collapse. Consideration of the mass-to-flux ratio map indicates that magnetic fields are dynamically important in most regions of the IRDC. A virial analysis of a sample of massive, dense cores in the IRDC, including evaluation of magnetic and kinetic internal and surface terms, indicates consistency with virial equilibrium, sub-Alfvénic conditions, and a dominant role for B-fields in regulating collapse. A clear alignment of magnetic field morphology with the direction of the steepest column density gradient is also detected. However, there is no preferred orientation of protostellar outflow directions with the B-field. Overall, these results indicate that magnetic fields play a crucial role in regulating massive star and star cluster formation, and therefore they need to be accounted for in theoretical models of these processes.

Kinematics and Star Formation in the Hub–Filament System G6.55-0.1

Hub–filament systems (HFSs) being the potential sites of formation of star clusters and high-mass stars, provide a testbed for the current theories that attempt to explain star formation globally. It is thus important to study a large number of HFSs using both intensity and velocity information to constrain these objects better observationally. Here, we present a study of the HFS associated with G6.55-0.1 using newly obtained observations of the radio continuum and the J = 2–1 transition of CO, 13CO, and C18O. The radio continuum maps show multiple peaks that coincide with far-infrared dust continuum peaks, indicating the presence of more than one young massive star in the hub of the HFS. We used the velocity information from the C18O(2–1) map to (a) show that the source G6.55-0.1 is not physically associated with the supernova remnant W28 and (b) disentangle and identify the velocity components genuinely associated with G6.55-0.1. Among the velocity-coherent structures identified in the region, we conclude that only the two filaments at 13.8 and 17.3 km s−1 contribute a total mass accretion rate of 3000 M ⊙ Myr−1 to the hub. Both the filaments also show a V-shaped structure, characteristic of gravitational collapse, in their velocity profile at the location of the hub. The estimated mass per unit length of the segments of the filaments is smaller than the critical line masses derived from virial equilibrium considerations. This suggests that the filaments are not gravitationally collapsing as a whole, although their inner parts clearly show evidence of collapse in the form of young star-forming cores. We further conclude that the observed velocity gradients are consistent with the gravitational collapse of the main source in the region as estimated from its mass and size.