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We explain the large variety of star formation laws in terms of one single, simple law that can be inferred from the definition of the star formation rate and basic algebra. The resulting equation, |${\rm SFR}= {\epsilon _\mathrm{ff}}\ M_\mathrm{collapse}/{\tau _{\rm ff}}$|⁠ , although it has been presented elsewhere, is interpreted in terms of clouds undergoing collapse rather than being turbulence supported, an idea that different groups have pursued this century. Under such assumption, one can explain the constancy of |${\epsilon _\mathrm{ff}}$|⁠ , the different intra-cloud correlations observed in Milky Way's molecular clouds, as well as the resolved and unresolved extragalactic relationships between SFR and a measurement of the mass in CO, HCN, and CO + HI. We also explain why the slope of the correlation changes when the orbital time |${\tau _{\rm orb}}$| is considered instead of the free-fall time, and why estimations of the free-fall time from extragalactic observations skew the correlation, providing a false sublinear correlation. We furthermore show that the apparent nearly linear correlation between the star formation rate and the dynamical equilibrium pressure in the mid-plane of the galaxies, |$P_{\rm DE}$|⁠ , is just a consequence of |$P_{\rm DE}$| values being dominated by the variation of the column density of molecular gas. All in all, we argue that the star formation law is driven by the collapse of cold, dense gas, which happens to be primarily molecular in the present Universe, and that the role of stellar feedback is just to shut down the star formation process, not to shape the star formation law. [ABSTRACT FROM AUTHOR]

The current kinematic state of young stellar clusters can give clues on their actual dynamical state and origin. In this contribution, we use Gaia DR3 data of the Lagoon Nebula Cluster (LNC) to show that the cluster is composed of two expanding groups, likely formed from different molecular cloud clumps. We find no evidence of massive stars having larger velocity dispersion than low-mass stars or being spatially segregated across the LNC, as a whole, or within the Primary group. However, the Secondary group, with 1/5th of the stars, exhibits intriguing features. On the one hand, it shows a bipolar nature, with an aspect ratio of ∼3:1. In addition, the massive stars in this group exhibit larger velocity dispersion than the low-mass stars, although they are not concentrated towards the centre of the group. This suggests that this group may have undergone dynamical relaxation, first, and some explosive event afterward. However, further observations and numerical work have to be performed to confirm this hypothesis. The results of this work suggest that, although stellar clusters may form by the global and hierarchical collapse of their parent clump, still some dynamical relaxation may take place. [ABSTRACT FROM AUTHOR]

We present a numerical study of the balance between the gravitational (E g), kinetic (E k), and magnetic (E m) energies of structures within a hub-filament system in a simulation of the formation and global hierarchical collapse (GHC) of a giant molecular cloud. For structures defined by various density thresholds, and at different evolutionary stages, we investigate the scaling of the virial parameter, α, with mass M , and of the Larson ratio , |${\cal {L}}_{\rm v}\equiv \sigma _{\rm v}/R^{1/2}$|⁠ , with column density Σ, where σ v is the 1D velocity dispersion, and R is an effective radius. We also investigate these scalings for the corresponding magnetic parameters αm and |${\cal {L}}_{\rm {m}}$|⁠. Finally, we compare our numerical results with an observational sample of massive clumps. We find that: 1) αm and |${\cal {L}}_{\rm {m}}$| follow similar α– M and |${\cal {L}}$| –Σ scalings as their kinetic counterparts, although the ratio E m/ E k decreases as | E g| increases. 2) The largest objects, defined by the lowest thresholds, tend to appear gravitationally bound (and magnetically supercritical), while their internal substructures tend to appear unbound (and subcritical). This suggests that the latter are being compressed by the infall of their parent structures, and supports earlier suggestions that the measured mass-to-magnetic flux ratio μ decreases inwards in a centrally-peaked cloud under ideal MHD. 3) The scatter in the α– M and |${\cal {L}}$| –Σ plots is reduced when E k and E m are plotted directly against E g, suggesting that the scatter is due to an ambiguity between mass and size. 4) The clumps in our GHC simulation follow the same trends as the observational sample of massive clumps in the |${\cal {L}}$| –Σ and α– M diagrams. We conclude that the main controlling parameter of the energy budget in the structures is E g, with the kinetic and magnetic energies being derived from it. [ABSTRACT FROM AUTHOR]

Using numerical simulations of the formation and evolution of stellar clusters within molecular clouds (MCs), we show that the stars in clusters formed within collapsing MC clumps exhibit a constant velocity dispersion regardless of their mass, as expected in violent relaxation processes. In contrast, clusters formed in turbulence-dominated environments exhibit an inverse mass segregated velocity dispersion, where massive stars exhibit larger velocity dispersions than low-mass cores, consistent with massive stars formed in massive clumps, which, in turn, are formed through strong shocks. We furthermore use Gaia EDR3 to show that the stars in the Orion Nebula Cluster exhibit a constant velocity dispersion as a function of mass, suggesting that it has been formed by collapse within one free-fall time of its parental cloud, rather than in a turbulence-dominated environment during many free-fall times of a supported cloud. Additionally, we have addressed several of the criticisms of models of collapsing star-forming regions: namely, the age spread of the ONC, the comparison of the ages of the stars to the free-fall time of the gas that formed it, the star formation efficiency, and the mass densities of clouds versus the mass densities of stellar clusters, showing that observational and numerical data are consistent with clusters forming in clouds undergoing a process of global, hierarchical and chaotic collapse, rather than being supported by turbulence. [ABSTRACT FROM AUTHOR]

We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H  ii regions in turbulent, magnetized, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics, and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionizing radiation eventually stops the accretion (through filaments) towards the massive star-forming regions. The new overpressured H  ii regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H  ii regions expand in a hybrid manner: they virtually do not expand towards the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionized regions grow anisotropically, and the ionizing stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H  ii -region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H  ii regions, producing a net expansion law (R ∝ t α, with α in the range of 0.93–1.47 and a mean value of 1.2 ± 0.17) that differs from any of those of the standard models. [ABSTRACT FROM AUTHOR]

We use numerical simulations of molecular cloud formation in the colliding flow scenario to investigate the reliability of observational estimates of the angular momenta of early-state, low-mass protostellar cores. We show that, with suitable corrections for projection factors, molecular line observations of velocity gradients in NH3 can be used to provide reasonable estimates of core angular momenta within a factor of two to three, with a few large underestimates due to unfavourable viewing angles. Our results differ from previous investigations which suggested that observations might overestimate true angular momenta by as much as an order of magnitude; the difference is probably due to the much smoother velocity field on small scales in our simulations, which result from allowing turbulence to decay and gravitational infall to dominate. The results emphasize the importance of understanding the nature of 'turbulent' velocities, with implications for the formation of protostellar discs during core collapse. [ABSTRACT FROM AUTHOR]

We explore the structure of magnetic field lines in and around filaments in simulations of molecular clouds undergoing global, multiscale gravitational collapse. In these simulations, filaments are not in a static equilibrium, but are long-lived flow structures that accrete gas from their environment and direct it towards clumps embedded in the filament or at the nodes at the conjunction with other filaments. In this context, the magnetic field is dragged by the collapsing gas, so its structure must reflect the flow that generates the filament. Around the filament, the gas is accreted on to it, and the magnetic lines must then be perpendicular to the filament. As the gas density increases, the gas flow changes direction, becoming almost parallel to the filament, and magnetic lines also tend to align with it. At the spine of the filament, however, magnetic lines become perpendicular again since they must connect to lines on the opposite side of the filament, resulting in 'U'-shaped magnetic structures, which tend to be stretched by the longitudinal flow along the filament. Magnetic diffusive processes, however, allow the gas to continue to flow. Assuming a stationary state in which the ram pressure of the flow balances the magnetic tension, the curvature of the field lines is determined by the diffusion rate. We derive an expression relating the curvature of the field lines to the diffusive coefficient, which may be used to observationally determine the nature of the diffusive process. [ABSTRACT FROM AUTHOR]

In previous contributions, we have presented an analytical model describing the evolution and star formation rate (SFR) of molecular clouds (MCs) undergoing hierarchical gravitational contraction. The cloud's evolution is characterized by an initial increase in its mass, density, SFR, and star formation efficiency (SFE), as it contracts, followed by a decrease of these quantities as newly formed massive stars begin to disrupt the cloud. The main parameter of the model is the maximum mass reached by the cloud during its evolution. Thus, specifying the instantaneous mass and some other variable completely determines the cloud's evolutionary stage. We apply the model to interpret the observed scatter in SFEs of the cloud sample compiled by Lada et al. as an evolutionary effect so that, although clouds such as California and Orion A have similar masses, they are in very different evolutionary stages, causing their very different observed SFRs and SFEs. The model predicts that the California cloud will eventually reach a significantly larger total mass than the Orion A cloud. Next, we apply the model to derive estimated ages of the clouds since the time when approximately 25 per cent of their mass had become molecular. We find ages from ∼1.5 to 27 Myr, with the most inactive clouds being the youngest. Further predictions of the model are that clouds with very low SFEs should have massive atomic envelopes constituting the majority of their gravitational mass, and that low-mass clouds (M ∼ 103–104M⊙) end their lives with a mini-burst of star formation, reaching SFRs ∼300–500M⊙ Myr−1. By this time, they have contracted to become compact (∼1 pc) massive star-forming clumps, in general embedded within larger giant molecular clouds. [ABSTRACT FROM AUTHOR]

We present magnetohydrodynamic simulations aimed at studying the effect of the magnetic suppression of turbulence (generated through various instabilities during the formation of molecular clouds by converging) on the subsequent star formation (SF) activity. We study four magnetically supercritical models with magnetic field strengths B = 0, 1, 2, and 3 μG (corresponding to mass–to–flux ratios of ∞, 4.76, 2.38, and 1.59 times the critical value), with the magnetic field, initially being aligned with the flows. We find that, for increasing magnetic field strength, the clouds formed tend to be more massive, denser, less turbulent, and with higher SF activity. This causes the onset of SF activity in the non-magnetic or more weakly magnetized cases to be delayed by a few Myr in comparison to the more strongly magnetized cases. We attribute this behaviour to the suppression of the non-linear thin shell instability (NTSI) by the magnetic field, previously found by Heitsch and coworkers. This result is contrary to the standard notion that the magnetic field provides support to the clouds, thus reducing their star formation rate. However, our result is a completely non-linear one, and could not be foreseen from simple linear considerations. [ABSTRACT FROM AUTHOR]

We analyse the morphology and kinematics of dense filamentary structures produced in a numerical simulation of a star-forming cloud of 1.4 × 104M☉ evolving under their own self-gravity in a magnetized media. This study is motivated by recent observations of velocitycoherent substructures ('fibres') in star-forming filaments. We find such 'fibres'ubiquitously in our simulated filament. We found that a fibre in one projection is not necessarily a fibre in another projection, and thus, caution should be taken into account when considering them as real objects. We found that only the densest parts of the filament (∼30 per cent of the densest volume, which contains ∼70 per cent of the mass) belongs to fibres in two projections. Moreover, it is quite common that they are formed by separated density enhancements superposed along the line of sight. Observations of fibres can yield insight into the level of turbulent substructure driven by gravity, but care should be taken in interpreting the results given the problem of line-of-sight superposition. We also studied the morphology and kinematics of the 3D skeleton (spine), finding that subfilaments accrete structured material mainly along the magnetic field lines, which are preferentially perpendicular to the skeleton. The magnetic field is at the same time dragged by the velocity field due to the gravitational collapse. [ABSTRACT FROM AUTHOR]

We use magnetohydrodynamical simulations of converging warm neutral medium flows to analyse the formation and global evolution of magnetized and turbulent molecular clouds subject to supernova feedback from massive stars. We show that supernova feedback alone fails to disrupt entire, gravitationally bound, molecular clouds, but is able to disperse smallsized (~10 pc) regions on time-scales of less than 1 Myr. Efficient radiative cooling of the supernova remnant as well as strong compression of the surrounding gas result in non-persistent energy and momentum input from the supernovae. However, if the time between subsequent supernovae is short and they are clustered, large hot bubbles form that disperse larger regions of the parental cloud. On longer time-scales, supernova feedback increases the amount of gas with moderate temperatures (T ≈ 300-3000 K). Despite its inability to disrupt molecular clouds, supernova feedback leaves a strong imprint on the star formation process. We find an overall reduction of the star formation efficiency by a factor of 2 and of the star formation rate by roughly factors of 2-4. [ABSTRACT FROM AUTHOR]