[en] Molecular clouds are supported by turbulence and magnetic fields, but quantifying their influence on cloud life cycle and star formation efficiency (SFE) remains an open question. We perform radiation magnetohydrodynamic simulations of star-forming giant molecular clouds (GMCs) with UV radiation feedback, in which the propagation of UV radiation via ray tracing is coupled to hydrogen photochemistry. We consider 10 GMC models that vary in either initial virial parameter (1 ≤ α _vir,0 ≤ 5) or dimensionless mass-to-magnetic flux ratio (0.5 ≤ μ _Φ,0 ≤ 8 and ∞ ); the initial mass 10⁵ M _⊙ and radius 20 pc are fixed. Each model is run with five different initial turbulence realizations. In most models, the duration of star formation and the timescale for molecular gas removal (primarily by photoevaporation) are 4–8 Myr. Both the final SFE (ε _*) and time-averaged SFE per freefall time (ε _ff) are reduced by strong turbulence and magnetic fields. The median ε _* ranges between 2.1% and 9.5%. The median ε _ff ranges between 1.0% and 8.0%, and anticorrelates with α _vir,0, in qualitative agreement with previous analytic theory and simulations. However, the time-dependent α _vir(t) and ε _ff,obs(t) based on instantaneous gas properties and cluster luminosity are positively correlated due to rapid evolution, making observational validation of star formation theory difficult. Our median ε _ff,obs(t) ≈ 2% is similar to observed values. We show that the traditional virial parameter estimates the true gravitational boundedness within a factor of 2 on average, but neglect of magnetic support and velocity anisotropy can sometimes produce large departures from traditional virial parameter estimates. Magnetically subcritical GMCs are unlikely to represent sites of massive star formation given their unrealistic columnar outflows, prolonged lifetime, and low escape fraction of radiation.

[en] A method for implementing cylindrical coordinates in the Athena magnetohydrodynamics (MHD) code is described. The extension follows the approach of Athena's original developers and has been designed to alter the existing Cartesian-coordinates code as minimally and transparently as possible. The numerical equations in cylindrical coordinates are formulated to maintain consistency with constrained transport (CT), a central feature of the Athena algorithm, while making use of previously implemented code modules such as the Riemann solvers. Angular momentum transport, which is critical in astrophysical disk systems dominated by rotation, is treated carefully. We describe modifications for cylindrical coordinates of the higher-order spatial reconstruction and characteristic evolution steps as well as the finite-volume and CT updates. Finally, we present a test suite of standard and novel problems in one, two, and three dimensions designed to validate our algorithms and implementation and to be of use to other code developers. The code is suitable for use in a wide variety of astrophysical applications and is freely available for download on the Web.

[en] In giant molecular clouds (GMCs), shocks driven by converging turbulent flows create high-density, strongly magnetized regions that are locally sheetlike. In previous work, we showed that within these layers, dense filaments and embedded self-gravitating cores form by gathering material along the magnetic field lines. Here, we extend the parameter space of our three-dimensional, turbulent MHD core formation simulations. We confirm the anisotropic core formation model we previously proposed and quantify the dependence of median core properties on the pre-shock inflow velocity and upstream magnetic field strength. Our results suggest that bound core properties are set by the total dynamic pressure (dominated by large-scale turbulence) and thermal sound speed c_s in GMCs, independent of magnetic field strength. For models with a Mach number between 5 and 20, the median core masses and radii are comparable to the critical Bonnor–Ebert mass and radius defined using the dynamic pressure for P_ext. Our results correspond to $M_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}=1.2{c_{\mathrm{s}}}^4{(G^3{\mathrm{\rho <!--\; ??\; -->}}_0{v_0}^2)}^{-<!--\; ???\; -->1/2}$ and $R_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}=0.34{c_{\mathrm{s}}}^2{(G{\mathrm{\rho <!--\; ??\; -->}}_0{v_0}^2)}^{-<!--\; ???\; -->1/2}$ for ${\mathrm{\rho <!--\; ??\; -->}}_0$ and v₀, the large-scale mean density and velocity. For our parameter range, the median $M_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}\sim <!--\; ???\; -->0.1-<!--\; ???\; -->1M_{\odot <!--\; ???\; -->}$, but a very high pressure cloud could have lower characteristic core mass. We find cores and filaments form simultaneously, and filament column densities are a factor of ∼2 greater than the surrounding cloud when cores first collapse. We also show that cores identified in our simulations have physical properties comparable to those observed in the Perseus cloud. Superthermal cores in our models are generally also magnetically supercritical, suggesting that the same may be true in observed clouds.

[en] Supernova (SN) explosions deposit prodigious energy and momentum in their environments, with the former regulating multiphase thermal structure and the latter regulating turbulence and star formation rates in the interstellar medium (ISM). However, systematic studies quantifying the impact of SNe in realistic inhomogeneous ISM conditions have been lacking. Using three-dimensional hydrodynamic simulations, we investigate the dependence of radial momentum injection on both physical conditions (considering a range of mean density n₀ = 0.1–$100\mathrm{c}{\mathrm{m}}^{-<!--\; ???\; -->3}$) and numerical parameters. Our inhomogeneous simulations adopt two-phase background states that result from thermal instability in atomic gas. Although the supernova remnant (SNR) morphology becomes highly complex for inhomogeneous backgrounds, the radial momentum injection is remarkably insensitive to environmental details. For our two-phase simulations, the final momentum produced by a single SN is given by $2.8\times <!--\; ??\; -->{10}^5{M}_{\odot <!--\; ???\; -->}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}{n}_0^{-<!--\; ???\; -->0.17}$. This is only 5% less than the momentum injection for a homogeneous environment with the same mean density, and only 30% greater than the momentum at the time of shell formation. The maximum mass in hot gas is also quite insensitive to environmental inhomogeneity. Strong magnetic fields alter the hot gas mass at very late times, but the momentum injection remains the same. Initial experiments with multiple spatially correlated SNe show a momentum per event nearly as large as single-SN cases. We also present a full numerical parameter study to assess convergence requirements. For convergence in the momentum and other quantities, we find that the numerical resolution Δ and the initial size of the SNR $r_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}$ must satisfy $\mathrm{\Delta <!--\; ??\; -->},r_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}}<<!--\; <\; -->r_{\mathrm{s}\mathrm{f}}/3$, where the shell formation radius is given by $r_{\mathrm{s}\mathrm{f}}=30\mathrm{p}\mathrm{c}{n}_0^{-<!--\; ???\; -->0.46}$ for two-phase models (or 30% smaller for a homogeneous medium).

[en] We investigate prestellar core formation and accretion based on three-dimensional hydrodynamic simulations. Our simulations represent local ∼1 pc regions within giant molecular clouds where a supersonic turbulent flow converges, triggering star formation in the post-shock layer. We include turbulence and self-gravity, applying sink particle techniques, and explore a range of inflow Mach numbers $\mathcal{M}=2-<!--\; ???\; -->16$. Two sets of cores are identified and compared: t₁ cores are identified from a time snapshot in each simulation and represent dense structures in a single cloud map; t_coll cores are identified at their individual time of collapse and represent the initial mass reservoir for accretion. We find that cores and filaments form and evolve at the same time. At the stage of core collapse, there is a well-defined, converged characteristic mass for isothermal fragmentation that is comparable to the critical Bonnor–Ebert mass at the post-shock pressure. The core mass functions (CMFs) of t_coll cores show a deficit of high-mass cores ($\gtrsim <!--\; ???\; -->7M_{\odot <!--\; ???\; -->}$) compared to the observed stellar initial mass function (IMF). However, the CMFs of t₁ cores are similar to the observed CMFs and include many low-mass cores that are gravitationally stable. The difference between t₁ cores and t_coll cores suggests that the full sample from observed CMFs may not evolve into protostars. Individual sink particles accrete at a roughly constant rate throughout the simulations, gaining one core mass per freefall time even after the initial mass reservoir is accreted. High-mass sinks gain proportionally more mass at later times than low-mass sinks. There are outbursts in accretion rates, resulting from clumpy density structures falling into the sinks.

[en] We investigate the roles of magnetic fields and ambipolar diffusion during prestellar core formation in turbulent giant molecular clouds, using three-dimensional numerical simulations. Our simulations focus on the shocked layer produced by a converging large-scale flow and survey varying ionization and the angle between the upstream flow and magnetic field. We also include ideal magnetohydrodynamic (MHD) and hydrodynamic models. From our simulations, we identify hundreds of self-gravitating cores that form within 1 Myr, with masses M ∼ 0.04-2.5 M _☉ and sizes L ∼ 0.015-0.07 pc, consistent with observations of the peak of the core mass function. Median values are M = 0.47 M _☉ and L = 0.03 pc. Core masses and sizes do not depend on either the ionization or upstream magnetic field direction. In contrast, the mass-to-flux ratio does increase with lower ionization, from twice to four times the critical value. The higher mass-to-flux ratio for low ionization is the result of enhanced transient ambipolar diffusion when the shocked layer first forms. However, ambipolar diffusion is not necessary to form low-mass supercritical cores. For ideal MHD, we find similar masses to other cases. These masses are one to two orders of magnitude lower than the value M _{mag, sph} = 0.007B ³/(G ^3/2ρ²) that defines a magnetically supercritical sphere under post-shock ambient conditions. This discrepancy is the result of anisotropic contraction along field lines, which is clearly evident in both ideal MHD and diffusive simulations. We interpret our numerical findings using a simple scaling argument that suggests that gravitationally critical core masses will depend on the sound speed and mean turbulent pressure in a cloud, regardless of magnetic effects.

[en] We present a unified model for molecular core formation and evolution, based on numerical simulations of converging, supersonic flows. Our model applies to star formation in giant molecular clouds dominated by large-scale turbulence, and contains four main stages: core building, core collapse, envelope infall, and late accretion. During the building stage, cores form out of dense, post-shock gas, and become increasingly centrally stratified as the mass grows over time. Even for highly supersonic converging flows, the dense gas is subsonic, consistent with observations showing quiescent cores. When the shock radius defining the core boundary exceeds R ∼ 4a(4πGρ_mean)^-1/2, where a is the isothermal sound speed, a wave of collapse propagates from the edge to the center. During the building and collapse stages, density profiles can be fitted by Bonnor-Ebert profiles with temperature 1.2-2.9 times the true value, similar to many observed cores. As found previously for initially static equilibria, outside-in collapse leads to a Larson-Penston density profile ρ ∼ 8.86a ²/(4πGr ²). The third stage, consisting of an inside-out wave of gravitational rarefaction leading to ρ ∝ r ^-3/2, v ∝ r ^-1/2, is also similar to that for initially static spheres, as originally described by Shu. We find that the collapse and infall stages have comparable duration, ∼t _ff, consistent with estimates for observed prestellar and protostellar (Class 0/I) cores. Core building takes longer, but does not produce high-contrast objects until shortly before collapse. The time to reach core collapse, and the core mass at collapse, decrease with increasing inflow Mach number. For all cases, the accretion rate is >> a ³/G early on but sharply drops off; the final system mass depends on the duration of late-stage accretion, set by large-scale conditions in a cloud.

[en] We introduce TIGRESS, a novel framework for multi-physics numerical simulations of the star-forming interstellar medium (ISM) implemented in the Athena MHD code. The algorithms of TIGRESS are designed to spatially and temporally resolve key physical features, including: (1) the gravitational collapse and ongoing accretion of gas that leads to star formation in clusters; (2) the explosions of supernovae (SNe), both near their progenitor birth sites and from runaway OB stars, with time delays relative to star formation determined by population synthesis; (3) explicit evolution of SN remnants prior to the onset of cooling, which leads to the creation of the hot ISM; (4) photoelectric heating of the warm and cold phases of the ISM that tracks the time-dependent ambient FUV field from the young cluster population; (5) large-scale galactic differential rotation, which leads to epicyclic motion and shears out overdense structures, limiting large-scale gravitational collapse; (6) accurate evolution of magnetic fields, which can be important for vertical support of the ISM disk as well as angular momentum transport. We present tests of the newly implemented physics modules, and demonstrate application of TIGRESS in a fiducial model representing the solar neighborhood environment. We use a resolution study to demonstrate convergence and evaluate the minimum resolution $\mathrm{\Delta <!--\; ??\; -->}x$ required to correctly recover several ISM properties, including the star formation rate, wind mass-loss rate, disk scale height, turbulent and Alfvénic velocity dispersions, and volume fractions of warm and hot phases. For the solar neighborhood model, all these ISM properties are converged at $\mathrm{\Delta <!--\; ??\; -->}x⩽<!--\; ???\; -->8\mathrm{p}\mathrm{c}$.

[en] We analyze the properties of steady and time-dependent C shocks under conditions prevailing in giant molecular clouds. For steady C shocks, we show that ionization equilibrium holds and uses numerical integration to obtain a fitting formula for the shock thickness mediated by ambipolar diffusion, L_shock∝n₀^–3/4 v₀^1/2 B₀^1/2χ_i0^–1. Our formula also agrees with an analytic estimate based on ion-neutral momentum exchange. Using time-dependent numerical simulations, we show that C shocks have a transient stage when the neutrals are compressed much more strongly than the magnetic field. The transient stage has a duration set by the neutral-ion collision time, t_AD ∼ L_shock/v_drift ∼ 0.1-1 Myr. This transient creates a strong enhancement in the mass-to-magnetic flux ratio. Under favorable conditions, supercritical prestellar cores may form and collapse promptly as a result of magnetic flux loss during the transient stage of C shocks.

[en] Radiation feedback from young star clusters embedded in giant molecular clouds (GMCs) is believed to be important to the control of star formation. For the most massive and dense clouds, including those in which super star clusters (SSCs) are born, pressure from reprocessed radiation exerted on dust grains may disperse a significant portion of the cloud mass back into the interstellar medium. Using our radiation hydrodynamics code, Hyperion, we conduct a series of numerical simulations to test this idea. Our models follow the evolution of self-gravitating, strongly turbulent clouds in which collapsing regions are replaced by radiating sink particles representing stellar clusters. We evaluate the dependence of the star formation efficiency (SFE) on the size and mass of the cloud and κ, the opacity of the gas to infrared (IR) radiation. We find that the single most important parameter determining the evolutionary outcome is κ, with $\mathrm{\kappa <!--\; ??\; -->}\gtrsim <!--\; ???\; -->15{\mathrm{c}\mathrm{m}}^2{\mathrm{g}}^{-<!--\; ???\; -->1}$ needed to disrupt clouds. For $\mathrm{\kappa <!--\; ??\; -->}=20-<!--\; ???\; -->40{\mathrm{c}\mathrm{m}}^2{\mathrm{g}}^{-<!--\; ???\; -->1}$, the resulting SFE $=50\mathrm{\%<!--\; \%\; -->}-<!--\; ???\; -->70\mathrm{\%<!--\; \%\; -->}$ is similar to empirical estimates for some SSC-forming clouds. The opacities required for GMC disruption likely apply only in dust-enriched environments. We find that the subgrid model approach of boosting the direct radiation force $L/c$ by a “trapping factor” equal to a cloud’s mean IR optical depth can overestimate the true radiation force by factors of $\sim <!--\; ???\; -->4-<!--\; ???\; -->5$. We conclude that feedback from reprocessed IR radiation alone is unlikely to significantly reduce star formation within GMCs unless their dust abundances or cluster light-to-mass ratios are enhanced.