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ABSTRACT In their early, formative stages star clusters can undergo rapid dynamical evolution leading to strong gravitational interactions and ejection of “runaway” stars at high velocities. While O/B runaway stars have been well studied, lower-mass runaways are so far very poorly characterized, even though they are expected to be much more common. We carried out spectroscopic observations with MAG2-MIKE to follow-up 27 high priority candidate runaways consistent with having been ejected from the Orion Nebula Cluster (ONC) $$\gt 2.5$$ Myr ago, based on Gaia astrometry. We derive spectroscopic youth indicators (Li and H $$\alpha$$) and radial velocities, enabling detection of bona fide runaway stars via signatures of youth and 3D traceback. We successfully confirmed 11 of the candidates as low-mass Young Stellar Objects (YSOs) on the basis of our spectroscopic criteria and derived radial velocities (RVs) with which we performed 3D traceback analysis. Three of these confirmed YSOs have kinematic ejection ages $$\gt 4\:$$ Myr, with the oldest being 4.7 Myr. Assuming that these stars indeed formed in the ONC and were then ejected, this yields an estimate for the overall formation time of the ONC to be at least $$\sim 5\:$$ Myr, i.e. about 10 free-fall times, and with a mean star formation efficiency per free-fall time of $$\bar{\epsilon }_{\rm ff}\lesssim 0.05$$. These results favour a scenario of slow, quasi-equilibrium star cluster formation, regulated by magnetic fields and/or protostellar outflow feedback. 

ABSTRACT Most stars are born in the crowded environments of gradually forming star clusters. Dynamical interactions between close-passing stars and the evolving ultraviolet radiation fields from proximate massive stars are expected to sculpt the protoplanetary discs (PPDs) in these clusters, potentially contributing to the diversity of planetary systems that we observe. Here, we investigate the impact of cluster environment on disc demographics by implementing simple PPD evolution models within N-body simulations of gradual star cluster formation, containing 50 per cent primordial binaries. We consider a range of star formation efficiency per free-fall time, $$\epsilon _{\rm ff}$$, and mass surface density of the natal cloud environment, $$\Sigma _{\rm cloud}$$, both of which affect the overall duration of cluster formation. We track the interaction history of all stars to estimate the dynamical truncation of the discs around stars involved in close encounters. We also track external photoevaporation of the discs due to the ionizing radiation field of the nearby high- and intermediate-mass ($$\gt 5\,{\rm M}_\odot$$) stars. We find that $$\epsilon _{\rm ff}$$, $$\Sigma _{\rm cloud}$$, and the presence of primordial binaries have major influences on the masses and radii of the disc population. In particular, external photoevaporation has a greater impact than dynamical interactions in determining the fate of discs in our clusters. 

Abstract Determining the physical processes that control galactic-scale star formation rates is essential for an improved understanding of galaxy evolution. The role of orbital shear is currently unclear, with some models expecting reduced star formation rates and efficiencies with increasing shear, e.g., if shear stabilizes gas against gravitational collapse, while others predicting enhanced rates, e.g., if shear-driven collisions between giant molecular clouds trigger star formation. Expanding on the analysis of 16 galaxies by C. Suwannajak et al., we assess the shear dependence of star formation efficiency (SFE) per orbital time (ϵ_orb) in 49 galaxies selected from the PHANGS-ALMA survey. In particular, we test a prediction of the shear-driven giant molecular cloud​​​​​​ (GMC) collision model thatϵ_orb∝ (1–0.7β), where $\beta \equiv d\ln v_{\mathrm{circ}}/d\ln r$, i.e., SFE per orbital time declines with decreasing shear. We fit the functionϵ_orb=ϵ_orb,0(1 −α_CCβ) findingα_CC≃ 0.76 ± 0.16; an alternative fit withϵ_orbnormalized by the median value in each galaxy yields ${\alpha }_{\mathrm{CC}}^{*}=0.80\pm 0.15$. These results are in good agreement with the prediction of the shear-driven GMC collision theory. We also examine the impact of a galactic bar onϵ_orbfinding a modest decrease in SFE in the presence of a bar, which can be attributed to lower rates of shear in these regions. We discuss the implications of our results for the GMC life cycle and environmental dependence of star formation activity. 

Abstract 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+ 214μm 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 ofB-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 aB–nrelation 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 forB-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 theB-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. 

Abstract The “tuning-fork” (TF) analysis of CO and Hαemission has been used to estimate the lifetimes of molecular clouds in nearby galaxies. With simple model calculations, we show that this analysis does not necessarily estimate cloud lifetimes, but instead captures a duration of the cloud evolutionary cycle, from dormant to star-forming, and then back to a dormant phase. We adopt a hypothetical setup in which molecular clouds (e.g., traced in CO) live forever and form stars (e.g., Hiiregions) at some frequency, which then drift away from the clouds. The TF analysis still returns a timescale for the immortal clouds. This model requires drifting motion to separate the newborn stars from the clouds, and we discuss its origin. We also discuss the physical origin of the characteristic spatial separation term in the TF analysis and a bias due to systematic error in the determination of the reference timescale. 

ABSTRACT We study the formation and early evolution of star clusters that have a wide range of masses and background cloud mass surface densities, Σcloud, which help set the initial sizes, densities, and velocity dispersions of the natal gas clumps. Initial clump masses of 300, 3000, and 30 000 M⊙ are considered, from which star clusters are born with an assumed 50  per cent overall star formation efficiency and with 50  per cent primordial binarity. This formation is gradual, i.e. with a range of star formation efficiencies per free-fall time from 1 to 100  per cent, so that the formation time can range from 0.7 Myr for low-mass, high-Σcloud clumps to ∼30 Myr for high-mass, low-Σcloud clumps. Within this framework of the turbulent clump model, for a given Σcloud, clumps of higher mass are of lower initial volume density, but their dynamical evolution leads to higher bound fractions and causes them to form much higher density cluster cores and maintain these densities for longer periods. This results in systematic differences in the evolution of binary properties, degrees of mass segregation, and rates of creation of dynamically ejected runaways. We discuss the implications of these results for observed star clusters and stellar populations. 

Abstract We use a suite of 3D simulations of star-forming molecular clouds, with and without stellar feedback, magnetic fields, and driven turbulence, to study the compression and expansion rates of the gas as functions of density. We show that, around the mean density, supersonic turbulence promotes rough equilibrium between the amounts of compressing and expanding gas, consistent with continuous gas cycling between high- and low-density states. We find that the inclusion of protostellar jets produces rapidly expanding and compressing low-density gas. We find that the gas mass flux peaks at the transition between the lognormal and power-law forms of the density probability distribution function (PDF). This is consistent with the transition density tracking the post-shock density, which promotes an enhancement of mass at this density (i.e., shock compression and filament formation). At high densities, the gas dynamics are dominated by self-gravity: the compression rate in all of our runs matches the rate of the run with only gravity, suggesting that processes other than self-gravity have little effect at these densities. The net gas mass flux becomes constant at a density below the sink formation threshold, where it equals the star formation rate. The density at which the net gas mass flux equals the star formation rate is one order of magnitude lower than our sink threshold density, corresponds to the formation of the second power-law tail in the density PDF, and sets the overall star formation rates of these simulations. 

ABSTRACT Compression in giant molecular cloud (GMC) collisions is a promising mechanism to trigger the formation of massive star clusters and OB associations. We simulate colliding and non-colliding magnetized GMCs and examine the properties of pre-stellar cores, selected from projected mass surface density maps, including after synthetic ALMA observations. We then examine core properties, including mass, size, density, velocity, velocity dispersion, temperature, and magnetic field strength. After 4 Myr, ∼1000 cores have formed in the GMC collision, and the high-mass end of the core mass function (CMF) can be fit by a power-law dN/dlogM ∝ M−α with α ≃ 0.7, i.e. relatively top heavy compared to a Salpeter mass function. Depending on how cores are identified, a break in the power law can appear around a few $$\times 10\, \mathrm{M}_\odot$$. The non-colliding GMCs form fewer cores with a CMF with α ≃ 0.8–1.2, i.e. closer to the Salpeter index. We compare the properties of these CMFs to those of several observed samples of cores. Considering other properties, cores formed from colliding clouds are typically warmer, have more disturbed internal kinematics, and are more likely to be gravitational unbound, than cores formed from non-colliding GMCs. The dynamical state of the protocluster of cores formed in the GMC–GMC collision is intrinsically subvirial but can appear to be supervirial if the total mass measurement is affected by observations that miss mass on large scales or at low densities. 

Abstract We adopt the deep learning methodcasi-3d(convolutional approach to structure identification-3D) to infer the orientation of magnetic fields in sub-/trans-Alfvénic turbulent clouds from molecular line emission. We carry out magnetohydrodynamic simulations with different magnetic field strengths and use these to generate synthetic observations. We apply the 3D radiation transfer coderadmc-3dto model¹²CO and¹³CO (J = 1−0) line emission from the simulated clouds and then train acasi-3dmodel on these line emission data cubes to predict magnetic field morphology at the pixel level. The trainedcasi-3dmodel is able to infer magnetic field directions with a low error (≲10° for sub-Alfvénic samples and ≲30° for trans-Alfvénic samples). We further test the performance ofcasi-3don a real sub-/trans- Alfvénic region in Taurus. Thecasi-3dprediction is consistent with the magnetic field direction inferred from Planck dust polarization measurements. We use our developed methods to produce a new magnetic field map of Taurus that has a three times higher angular resolution than the Planck map. 

Context. We have studied the dense gas morphology and kinematics towards the infrared dark cloud (IRDC) G034.77-00.55, shock-interacting with the SNR W44, to identify evidence of early-stage star formation induced by the shock. Aims. We used high angular resolution N₂H⁺(1−0) images across G034.77-00.55, obtained with the Atacama Large Millimeter/sub-Millimeter Array. N₂H⁺is a well-known tracer of dense and cold material, optimal for identifying gas that has the highest potential to harbour star formation. Methods. The N₂H⁺emission is distributed in two elongated structures, one towards the dense ridge at the edge of the source and one towards the inner cloud. Both elongations are spatially associated with well-defined mass-surface density features. The velocities of the gas in the two structures (i.e. 38–41 km s⁻¹and 41–43 km s⁻¹) are consistent with the lowest velocities of the J- and C-type parts, respectively, of the SNR-driven shock. A third velocity component is present at 43–45.5 km s^–1. The dense gas shows a fragmented morphology with core-like fragments at scales consistent with the Jeans lengths, masses of ~1–20 M_⊙, densities of (n(H₂)≥10⁵cm^–3) sufficient to host star formation in free-fall timescales (a few 10⁴yr), and with virial parameters that suggest a possible collapse. Results. The W44 driven shock may have swept up the encountered material, which is now seen as a dense ridge, almost detached from the main cloud, and an elongation within the inner cloud, well constrained in both N₂H⁺emission and mass surface density. This shock compressed material may have then fragmented into cores that are either in a starless or pre-stellar stage. Additional observations are needed to confirm this scenario and the nature of the cores.