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1. STARFORGE: the effects of protostellar outflows on the IMF

   https://doi.org/10.1093/mnras/stab278  

   Guszejnov, Dávid; Grudić, Michael Y; Hopkins, Philip F; Offner, Stella S; Faucher-Giguère, Claude-André (February 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The initial mass function (IMF) of stars is a key quantity affecting almost every field of astrophysics, yet it remains unclear what physical mechanisms determine it. We present the first runs of the STAR FORmation in Gaseous Environments project, using a new numerical framework to follow the formation of individual stars in giant molecular clouds (GMCs) using the gizmo code. Our suite includes runs with increasingly complex physics, starting with isothermal ideal magnetohydrodynamics (MHD) and then adding non-isothermal thermodynamics and protostellar outflows. We show that without protostellar outflows the resulting stellar masses are an order of magnitude too high, similar to the result in the base isothermal MHD run. Outflows disrupt the accretion flow around the protostar, allowing gas to fragment and additional stars to form, thereby lowering the mean stellar mass to a value similar to that observed. The effect of jets upon global cloud evolution is most pronounced for lower mass GMCs and dense clumps, so while jets can disrupt low-mass clouds, they are unable to regulate star formation in massive GMCs, as they would turn an order unity fraction of the mass into stars before unbinding the cloud. Jets are also unable to stop the runaway accretion of massive stars, which could ultimately lead to the formation of stars with masses $${\gt}500\, \mathrm{M}_{\rm \odot }$$. Although we find that the mass scale set by jets is insensitive to most cloud parameters (i.e. surface density, virial parameter), it is strongly dependent on the momentum loading of the jets (which is poorly constrained by observations) as well as the temperature of the parent cloud, which predicts slightly larger IMF variations than observed. We conclude that protostellar jets play a vital role in setting the mass scale of stars, but additional physics are necessary to reproduce the observed IMF. 

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2. 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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3. Effects of the environment on the multiplicity properties of stars in the STARFORGE simulations

   https://doi.org/10.1093/mnras/stac3268  

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

   ABSTRACT Most observed stars are part of a multiple star system, but the formation of such systems and the role of environment and various physical processes is still poorly understood. We present a suite of radiation-magnetohydrodynamic simulations of star-forming molecular clouds from the STARFORGE project that include stellar feedback with varied initial surface density, magnetic fields, level of turbulence, metallicity, interstellar radiation field, simulation geometry and turbulent driving. In our fiducial cloud, the raw simulation data reproduces the observed multiplicity fractions for Solar-type and higher mass stars, similar to previous works. However, after correcting for observational incompleteness the simulation underpredicts these values. The discrepancy is likely due to the lack of disc fragmentation, as the simulation only resolves multiples that form either through capture or core fragmentation. The raw mass distribution of companions is consistent with randomly drawing from the initial mass function for the companions of $$\gt 1\, \mathrm{M}_{\rm \odot }$$ stars. However, accounting for observational incompleteness produces a flatter distribution similar to observations. We show that stellar multiplicity changes as the cloud evolves and anticorrelates with stellar density. This relationship also explains most multiplicity variations between runs, i.e. variations in the initial conditions that increase stellar density (increased surface density, reduced turbulence) also act to decrease multiplicity. While other parameters, such as metallicity, interstellar radiation, and geometry significantly affect the star formation history or the IMF, varying them produces no clear trend in stellar multiplicity properties. 

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4. The universal acceleration scale from stellar feedback

   https://doi.org/10.1093/mnrasl/slaa103  

   Grudić, Michael Y; Boylan-Kolchin, Michael; Faucher-Giguère, Claude-André; Hopkins, Philip F (July 2020, Monthly Notices of the Royal Astronomical Society: Letters)

   ABSTRACT It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale $$g_{\dagger }\sim 10^{-8}\, \rm {cm\, s^{-2}}$$, a scale promoted to a new fundamental constant in MOND. In recent years, theoretical and observational studies have shown that the star formation efficiency (SFE) of dense gas scales with surface density, SFE ∼ Σ/Σcrit with $$\Sigma _{\rm crit} \sim \langle \dot{p}/m_{\ast }\rangle /(\pi \, G)\sim 1000\, \rm {M_{\odot }\, pc^{-2}}$$ (where $$\langle \dot{p}/m_{\ast }\rangle$$ is the momentum flux output by stellar feedback per unit stellar mass in a young stellar population). We argue that the SFE, more generally, should scale with the local gravitational acceleration, i.e. that SFE $${\sim}g_{\rm tot}/g_{\rm crit}\equiv (G\, M_{\rm tot}/R^{2}) / \langle \dot{p}/m_{\ast }\rangle$$, where Mtot is the total gravitating mass and $$g_{\rm crit}=\langle \dot{p}/m_{\ast }\rangle = \pi \, G\, \Sigma _{\rm crit} \approx 10^{-8}\, \rm {cm\, s^{-2}} \approx \mathit{ g}_{\dagger }$$. Hence, the observed g† may correspond to the characteristic acceleration scale above which stellar feedback cannot prevent efficient star formation, and baryons will eventually come to dominate. We further show how this may give rise to the observed acceleration scaling $$g_{\rm obs}\sim (g_{\rm baryon}\, g_{\dagger })^{1/2}$$ (where gbaryon is the acceleration due to baryons alone) and flat rotation curves. The derived characteristic acceleration g† can be expressed in terms of fundamental constants (gravitational constant, proton mass, and Thomson cross-section): $$g_{\dagger }\sim 0.1\, G\, m_{\mathrm{ p}}/\sigma _{\rm T}$$. 

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5. 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; 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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