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.

/pdf/theory-of-star-formation-sol2jzngnw.pdf

Theory of Star Formation

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: ▪  monolithic collapse in isolated cores ▪  competitive accretion in a protocluster environment ▪  stellar collisions and mergers in very dense systems We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass ...

Toward Understanding Massive Star Formation

Understanding massive stars is essential for a variety of branches of astronomy including galaxy and star cluster evolution, nucleosynthesis and supernovae, pulsars, and black holes. It has become evident that massive star evolution is very diverse, being sensitive to metallicity, binarity, rotation, and possibly magnetic fields. Although the problem to obtain a good statistical observational database is alleviated by current large spectroscopic surveys, it remains a challenge to model these diverse paths of massive stars toward their violent end stage. I show that the main sequence stage offers the best opportunity to gauge the relevance of the various possible evolutionary scenarios. This also allows sketching the post-main-sequence evolution of massive stars, for which observations of Wolf-Rayet stars give essential clues. Recent supernova discoveries owing to the current boost in transient searches allow tentative mappings of progenitor models with supernova types, including pair-instability supernova...

Presupernova Evolution of Massive Single and Binary Stars

▪ Abstract Outflow activity is associated with all stages of early stellar evolution, from deeply embedded protostellar objects to visible young stars. Herbig-Haro (HH) objects are the optical manifestations of this powerful mass loss. Analysis of HH flows, and in particular of the subset of highly collimated HH jets, provides indirect but important insights into the nature of the accretion and mass-loss processes that govern the formation of stars. The recent recognition that HH flows may attain parsec-scale dimensions opens up the possibility of partially reconstructing the mass-ejection history of the newly born driving sources and, therefore, their mass-accretion history. Furthermore, HH flows are astrophysical laboratories for the analysis of shock structures, of hydrodynamics in collimated flows, and of their interaction with the surrounding environment. HH flows may be an important source of turbulence in molecular clouds. Recent technological developments have enabled detailed observations of outf...

/pdf/herbig-haro-flows-probes-of-early-stellar-evolution-uqtm8xvst8.pdf

HERBIG-HARO FLOWS: Probes of Early Stellar Evolution

We perform a large set of radiation hydrodynamic simulations of primordial star formation in a fully cosmological context. Our statistical sample of 100 First Stars shows that the first generation of stars has a wide mass distribution M popIII = 10 ~ 1000 M ☉. We first run cosmological simulations to generate a set of primordial star-forming gas clouds. We then follow protostar formation in each gas cloud and the subsequent protostellar evolution until the gas mass accretion onto the protostar is halted by stellar radiative feedback. The accretion rates differ significantly among the primordial gas clouds that largely determine the final stellar masses. For low accretion rates, the growth of a protostar is self-regulated by radiative feedback effects, and the final mass is limited to several tens of solar masses. At high accretion rates the protostar's outer envelope continues to expand, and the effective surface temperature remains low; such protostars do not exert strong radiative feedback and can grow in excess of 100 solar masses. The obtained wide mass range suggests that the first stars play a variety of roles in the early universe, by triggering both core-collapse supernovae and pair-instability supernovae as well as by leaving stellar mass black holes. We find certain correlations between the final stellar mass and the physical properties of the star-forming cloud. These correlations can be used to estimate the mass of the first star from the properties of the parent cloud or of the host halo without following the detailed protostellar evolution.

https://iopscience.iop.org/article/10.1088/0004-637X/781/2/60/pdf

One Hundred First Stars : Protostellar Evolution and the Final Masses

L'introduction presente les relations avec l'evolution stellaire et la structure du milieu interstellaire. On presente ensuite les equations a resoudre et les approximations possibles. Apres une discussion de la theorie «classique» de l'evolution des regions H II, on presente les resultats d'etudes recentes de la nature non classique. On compare la theorie aux resultats d'observations

The Dynamical Evolution of H II Regions—Recent Theoretical Developments

Molecular jets announce the successful birth of a protostar. We develop here a model for the jets and their environments, adapting a multi-dimensional hydrocode to follow themolecular-atomic transitions of hydrogen.We examine powerful outflows into dense gas. The cocoon which forms around a jet is a very low density cavity of atomic gas. These atoms originate from strong shocks which dissociate the molecules. The rest of the molecules are either within the jet or swept up into very thin layers. Pulsed jets produce wider cavities and molecular layers which can grow onto resolvable jet knots. Three-dimensional simulations produce shocked molecular knots, distorted and multiple bow shocks and arclike structures. The resemblance of simulated images of the 1-0 S(1) H2 emission to recently observed deeply embedded outflows in HH211, HH212, HH288 and L1634 is discussed. Spectroscopic and excitation properties of the hydrogen molecules and CO maps are calculated. In the infrared, strong emission is seen from shocks within the jet (when pulsed) as well as from discrete regions along the cavity walls. Excitation, as measured by line ratios, is not generally constant. Broad double-peaked, shifted emission lines are predicted. Low-J CO emission is limb-brightened but spread over the whole outflow region. Some of these signatures are shown to depend on the chosen jet conditions. We find that three-dimensional calculations are essential for numerical simulations of strong cooling jets.

http://aa.springer.de/papers/7318002/2300595.pdf

Multi-dimensional numerical simulations of molecular jets

The jet model for protostellar outflows is confronted with the constraints imposed by CO spectroscopic observations. From three dimensional simulations of a dense molecular medium being penetrated by a denser molecular jet, we simulate line profiles and construct position-velocity diagrams for the (low-J) CO transitions. We find (1) the profiles imply power law variation of integrated brightness with velocity over a wide range of velocities, (2) the velocity field resembles a 'Hubble Law' and (3) a hollow-shell structure at low velocities becomes an elongated lobe at high velocities. Moreover, the leading bow shock produces strong forward motion of the cool gas rather than the expected lateral expansion. We are thus able to satisfy the Lada and Fich (1996) criteria, employing NGC 2264G as an example. Deviations from the simple power law dependence of integrated brightness versus velocity occur at high velocities in our simulations. The curve first dips to a shallow minimum and then rises rapidly and peaks sharply. Reanalysis of the NGC 2264G and Cepheus E data confirm these predictions. We identify these two features with a jet-ambient shear layer and the jet itself. A deeper analysis reveals that the power-law index is an indicator of the evolutionary stage: a profile steepens with time. Also, the CO excitation temperature changes along the bow walls and thus a CO line intensity does not directly yield the mass distribution, as often assumed. Instead, the CO emission is enhanced near the excitation peaks.

Numerical hydrodynamic simulations of jet-driven bipolar outflows

We present the results of 13 mm dust continuum observations for a sample of 18 X-ray selected TTauri stars in the Ophiuchus star forming region performed with the MPIfR 7-channel bolometer array mounted at the IRAM 30 m Millimeter Radio Telescope We have detected cold dust emission from 3 of the objects and have derived 3σ upper limits (∼21 mJy) for the remaining objects These upper limits suggest that the disk masses (gas+dust) are less than 5·10−3 M⊙ Complementing our results with those obtained by Andre & Montmerle (1994) for a sample of 22 X-ray selected T Tauri stars (3 additional detections) we improved the statistical significance of our conclusions As the frequency of circumstellar disks and the disk properties might be dependent on the selection criteria as well as local conditions, comparison is made with 13 mm surveys of Hα selected T Tauri stars in Chamaeleon, Lupus, Ophiuchus, and Taurus-Auriga

1.3 MM Dust Continuum Observations of X-Ray Selected T Tauri Stars in Ophiuchus

We have performed 1.3 mm dust continuum observations for a sample of 18 X-ray selected pre-main sequence (PMS) stars in the Ophiuchus star forming region with the MPIfR 7-channel bolometer array mounted at the IRAM 30m Millimeter Radio Telescope (MRT). We have detected cold dust emission from 3 of the objects and have derived 3σ upper limits (∼21 mJy) for the remaining objects. These upper limits suggest that the cold dust masses are less than 5 · 10–5 M⊙.

H. W. Yorke

Papers

The Dynamical Evolution of H II Regions—Recent Theoretical Developments

Multi-dimensional numerical simulations of molecular jets

Numerical hydrodynamic simulations of jet-driven bipolar outflows

1.3 MM Dust Continuum Observations of X-Ray Selected T Tauri Stars in Ophiuchus

1.3 mm Dust Continuum Observations of Young X-ray Selected Stars in Ophiuchus