We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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Theory of Star Formation

We discuss the properties of an accretion disk around a star with parameters typical of classical T Tauri stars (CTTSs) and with the average accretion rate for these disks The disk is assumed steady and geometrically thin The turbulent viscosity coefficient is expressed using the α prescription, and the main heating mechanisms considered are viscous dissipation and irradiation by the central star The energy is transported by radiation, turbulent conduction, and convection We find that irradiation from the central star is the main heating agent of the disk, except in the innermost regions, R 5 AU) becomes less dense, optically thin, and almost vertically isothermal, with a temperature distribution T ∝ R-1/2 The decrease in surface density at the outer disk decreases the disk mass by a factor of 4 with respect to a purely viscous case In addition, irradiation tends to make the outer disk regions stable against gravitational instabilities

https://iopscience.iop.org/article/10.1086/305702/pdf

Accretion Disks around Young Objects. I. The Detailed Vertical Structure

1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

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The Exoplanet Handbook

We construct detailed vertical structure models of irradiated accretion disks around T Tauri stars with interstellar medium dust uniformly mixed with gas. The dependence of the structure and emission properties on mass accretion rate, viscosity parameter, and disk radius is explored using these models. The theoretical spectral energy distributions (SEDs) and images for all inclinations are compared with observations of the entire population of classical T Tauri stars (CTTSs) and class I objects in Taurus. In particular, we find that the median near-infrared fluxes can be explained within the errors with the most recent values for the median accretion rates for CTTSs. We further show that the majority of the class I sources in Taurus cannot be class II sources viewed edge-on because they are too luminous and their colors would be consistent with disks seen only in a narrow range of inclinations. Our models appear to be too geometrically thick at large radii, as suggested by (1) larger far-infrared disk emission than in the typical SEDs of T Tauri stars, (2) wider dark dust lanes in the model images than in the images of HH 30 and HK Tau/c, and (3) a larger predicted number of stars extincted by edge-on disks than consistent with current surveys. The large thickness of the model is a consequence of the assumption that dust and gas are well mixed, suggesting that some degree of dust settling may be required to explain the observations.

https://arxiv.org/pdf/astro-ph/9907330.pdf

Accretion Disks around Young Objects. II. Tests of Well-mixed Models with ISM Dust

We investigate where magnetorotational instability operates in protoplanetary disks, which can cause angular momentum transport in the disks. We investigate the spatial distribution of various charged particles and the unstable regions for a variety of models for protoplanetary disks, taking into account the recombination of ions and electrons at grain surfaces, which is an important process in most parts of the disks. We find that for all the models there is an inner region that is magnetorotationally stable due to ohmic dissipation. This must make the accretion onto the central star nonsteady. For the model of the minimum-mass solar nebula, the critical radius, inside of which the disk is stable, is about 20 AU, and the mass accretion rate just outside the critical radius is 10-7-10-6 M☉ yr-1. The stable region is smaller in a disk of lower column density. Dust grains in protoplanetary disks may grow by mutual sticking and may sediment toward the midplane of the disks. We find that the stable region shrinks as the grain size increases or the sedimentation proceeds. Therefore, in the late evolutionary stages, protoplanetary disks can be magnetorotationally unstable even in the inner regions.

https://iopscience.iop.org/article/10.1086/317075/pdf

Magnetorotational Instability in Protoplanetary Disks. II. Ionization State and Unstable Regions

On a obtenu la vitesse de derive V B des champs magnetiques par rapport au fluide en resolvant les mouvements de nombreuses especes de particules chargees. Pour des densites d'hydrogene n H ≤10 11 cm −3 , V B est au moins dix fois plus faible que la vitesse de chute libre, et la dissipation du champ est donc inefficace

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Dissipation of magnetic fields in very dense interstellar clouds – I. Formulation and conditions for efficient dissipation

/pdf/the-densities-of-charged-particles-in-very-dense-2to4sh87kt.pdf

Toyoharu Umebayashi

Papers

Dissipation of magnetic fields in very dense interstellar clouds – I. Formulation and conditions for efficient dissipation

The Densities of Charged Particles in Very Dense Interstellar Clouds

Dissipation of magnetic fields in very dense interstellar clouds – II. Final phases of star formation and the magnetic flux of a newborn star