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1. A model for the formation of stellar associations and clusters from giant molecular clouds

   https://doi.org/10.1093/mnras/stab1894  

   Grudić, Michael Y; Kruijssen, J M; Faucher-Giguére, Claude-André; Hopkins, Philip F; Ma, Xiangcheng; Quataert, Eliot; Boylan-Kolchin, Michael (January 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   Abstract We present a large suite of MHD simulations of turbulent, star-forming giant molecular clouds (GMCs) with stellar feedback, extending previous work by simulating 10 different random realizations for each point in the parameter space of cloud mass and size. It is found that once the clouds disperse due to stellar feedback, both self-gravitating star clusters and unbound stars generally remain, which arise from the same underlying continuum of substructured stellar density, ie. the hierarchical cluster formation scenario. The fraction of stars that are born within gravitationally-bound star clusters is related to the overall cloud star formation efficiency set by stellar feedback, but has significant scatter due to stochastic variations in the small-scale details of the star-forming gas flow. We use our numerical results to calibrate a model for mapping the bulk properties (mass, size, and metallicity) of self-gravitating GMCs onto the star cluster populations they form, expressed statistically in terms of cloud-level distributions. Synthesizing cluster catalogues from an observed GMC catalogue in M83, we find that this model predicts initial star cluster masses and sizes that are in good agreement with observations, using only standard IMF and stellar evolution models as inputs for feedback. Within our model, the ratio of the strength of gravity to stellar feedback is the key parameter setting the masses of star clusters, and of the various feedback channels direct stellar radiation (photon momentum and photoionization) is the most important on GMC scales. 

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2. The lifecycle of molecular clouds in nearby star-forming disc galaxies

   https://doi.org/10.1093/mnras/stz3525  

   Chevance, Mélanie; Kruijssen, J M; Hygate, Alexander P; Schruba, Andreas; Longmore, Steven N; Groves, Brent; Henshaw, Jonathan D; Herrera, Cinthya N; Hughes, Annie; Jeffreson, Sarah M; et al (April 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT It remains a major challenge to derive a theory of cloud-scale ($$\lesssim100$$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $$10\!-\!30\,{\rm Myr}$$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $$\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $$\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $$1\!-\!5\,{\rm Myr}$$ once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $$100\!-\!300\,{{\rm pc}}$$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles. 

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3. The dynamics and outcome of star formation with jets, radiation, winds, and supernovae in concert

   https://doi.org/10.1093/mnras/stac526  

   Grudić, Michael Y.; Guszejnov, Dávid; Offner, Stella S. R.; Rosen, Anna L.; Raju, Aman N.; Faucher-Giguère, Claude-André; Hopkins, Philip F. (March 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse the first giant molecular cloud (GMC) simulation to follow the formation of individual stars and their feedback from jets, radiation, winds, and supernovae, using the STARFORGE framework in the GIZMO code. We evolve the GMC for $$\sim 9 \rm Myr$$, from initial turbulent collapse to dispersal by feedback. Protostellar jets dominate feedback momentum initially, but radiation and winds cause cloud disruption at $$\sim 8{{\ \rm per\ cent}}$$ star formation efficiency (SFE), and the first supernova at $$8.3\, \rm Myr$$ comes too late to influence star formation significantly. The per-free-fall SFE is dynamic, accelerating from 0 per cent to $$\sim 18{{\ \rm per\ cent}}$$ before dropping quickly to <1 per cent, but the estimate from YSO counts compresses it to a narrower range. The primary cluster forms hierarchically and condenses to a brief ($$\sim 1\, \mathrm{Myr}$$) compact ($$\sim 1\, \rm pc$$) phase, but does not virialize before the cloud disperses, and the stars end as an unbound expanding association. The initial mass function resembles the Chabrier (2005) form with a high-mass slope α = −2 and a maximum mass of 55 M⊙. Stellar accretion takes $$\sim 400\, \rm kyr$$ on average, but $$\gtrsim 1\,\rm Myr$$ for >10 M⊙ stars, so massive stars finish growing latest. The fraction of stars in multiples increase as a function of primary mass, as observed. Overall, the simulation much more closely resembles reality, compared to previous versions that neglected different feedback physics entirely. But more detailed comparison with synthetic observations will be needed to constrain the theoretical uncertainties. 

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4. The elephant in the room: the importance of the details of massive star formation in molecular clouds

   https://doi.org/10.1093/mnras/stz1820  

   Grudić, Michael Y; Hopkins, Philip F (September 2019, Monthly Notices of the Royal Astronomical Society)

   Abstract Most simulations of galaxies and massive giant molecular clouds (GMCs) cannot explicitly resolve the formation (or predict the main-sequence masses) of individual stars. So they must use some prescription for the amount of feedback from an assumed population of massive stars (e.g. sampling the initial mass function, IMF). We perform a methods study of simulations of a star-forming GMC with stellar feedback from UV radiation, varying only the prescription for determining the luminosity of each stellar mass element formed (according to different IMF sampling schemes). We show that different prescriptions can lead to widely varying (factor of ∼3) star formation efficiencies (on GMC scales) even though the average mass-to-light ratios agree. Discreteness of sources is important: radiative feedback from fewer, more-luminous sources has a greater effect for a given total luminosity. These differences can dominate over other, more widely recognized differences between similar literature GMC-scale studies (e.g. numerical methods, cloud initial conditions, presence of magnetic fields). Moreover the differences in these methods are not purely numerical: some make different implicit assumptions about the nature of massive star formation, and this remains deeply uncertain in star formation theory. 

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5. Can magnetized turbulence set the mass scale of stars?

   https://doi.org/10.1093/mnras/staa1883  

   Guszejnov, Dávid; Grudić, Michael Y; Hopkins, Philip F; Offner, Stella S; Faucher-Giguère, Claude-André (August 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the gizmo code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs ($$\mathcal {M} \sim 10\!-\!50$$). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M50 becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find $$M_\mathrm{50} \,\, \buildrel\propto \over \sim \,\,M_\mathrm{0} \mathcal {M}^{-3} \alpha _\mathrm{turb}\, \mathrm{SFE}^{1/3}$$ for GMC mass M0, sonic Mach number $$\mathcal {M}$$, virial parameter αturb, and star formation efficiency SFE = M⋆/M0. This fit agrees well with previous IMF results from the ramses, orion2, and sphng codes. Although M50 has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M50 to be $$\gt 20 \, \mathrm{M}_{\odot }$$, an order of magnitude larger than observed ($$\sim 2 \, \mathrm{M}_\odot$$), together with an excess of brown dwarfs. Moreover, M50 is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF. 

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