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

As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and nonmagnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit in 2002, and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by about a factor of 30-50 when compared with the nonmagnetic counterparts. Angular momentum in that part of the presupernova star destined to become a neutron star is an increasing function of main-sequence mass. That is, pulsars derived from more massive stars rotate faster and rotation plays a more important role in the star's explosion. The final angular momentum of the core has been determined—to within a factor of 2—by the time the star ignites carbon burning. For the lighter stars studied, around 15 M☉, we predict pulsar periods at birth near 15 ms, though a factor of 2 range is easily tolerated by the uncertainties. Several mechanisms for additional braking in a young neutron star, especially by fallback, are explored.

https://iopscience.iop.org/article/10.1086/429868/pdf

Presupernova evolution of differentially rotating massive stars including magnetic fields

Magnetic fields are present in a wide variety of stars throughout the HR diagram and play a role at basically all evolutionary stages, from very-low-mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved giants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a few μG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in nondegenerate stars in particular, they feature large-scale topologies varying from simple nearly axisymmetric dipoles to complex nonaxsymmetric structures, and from mainly poloidal to mainly toroidal topologies. After recalling the main techniques of detecting and modeling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot, and young nondegenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Magnetic Fields of Nondegenerate Stars

Magnetic fields are present in a wide variety of stars through out the HR diagram and play a role at basically all evolutionary stages, from very-low- mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved gi- ants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a fewG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in non-degenerate stars in particular, they feature large-scale topologies varying f rom simple nearly-axisymmetric dipoles to complex non-axsymmetric structures, and from mainly poloidal to mainly toroidal topology. After recalling the main techniques of detecting and modelling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot and young non-degenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Magnetic fields of non-degenerate stars

Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively if it is to be successfully included in broader astrophysical applications such as galactic and cosmic evolution and ionization. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on developments in the past decade, concentrating on the winds from OB-stars, with some excursions to the winds from Luminous Blue Variables (including super-Eddington, continuum-driven winds), winds from Wolf–Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After recapitulating the 1-D, stationary standard model of line-driven winds, extensions accounting for rotation and magnetic fields are discussed. Stationary wind models are presented that provide theoretical predictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results from corresponding campaigns (including the VLT-flames survey of massive stars) with theoretical predictions, featuring the mass loss-metallicity dependence. Subsequently, we concentrate on two urgent problems, weak winds and wind-clumping, that have been identified from various diagnostics and that challenge our present understanding of radiation driven winds. We discuss the problems of “measuring” mass-loss rates from weak winds and the potential of the NIR Br
 α
 -line as a tool to enable a more precise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model. Wind-clumping, conventionally interpreted as the consequence of a strong instability inherent to radiative line-driving, has severe implications for the interpretation of observational diagnostics, since derived mass-loss rates are usually overestimated when clumping is present but ignored in the analyses. Depending on the specific diagnostics, such overestimates can amount to factors of 2 to 10, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present-day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars. Such emission is thought to originate both from magnetically confined winds and from non-magnetic winds, in the latter case related to the line-driven instability and/or clump-clump collisions. We highlight as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping.

http://www.usm.uni-muenchen.de/people/puls/papers/review_mdot.pdf

Mass loss from hot massive stars

We report the detection, through spectropolarimetric observations, of a strong dipolar magnetic field of presumably fossil origin at the surface of the very young O star theta (1) Ori C. The Stokes V signatures we detect are variable with time, the variations being consistent with rotational modulation. A detailed modelling of our observations indicates that this dipole field has an intensity of 1.1+/-0.1 kG and is inclined at 42degrees+/-6degrees with respect to the rotation axis (assumed to be inclined at 45degrees to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Halpha emission component observed on this star reaches maximum strength. This discovery represents the first definite detection of a magnetic field in an O star, as well as the first detection of a fossil field in a very young star.We also investigate in this paper the magnetic confinement of the radiatively driven wind of theta (1) Ori C in the context of the magnetically confined wind-shock model of Babel & Montmerle. In the case of theta (1) Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2-3R *), consisting of a very hot post-shock region (with temperatures in excess of 10 MK and densities of about 10(10) -10(11) cm(-3) ) generated by the strong collision of the wind streams from both stellar magnetic hemispheres, as well as a dense cooling disc forming in the magnetospheric equator. We find that this model includes most of the physics required to obtain a satisfactory level of agreement with the extensive data sets available for theta (1) Ori C in the literature (and, in particular, with the recent X-ray data and the phase-resolved spectroscopic observations of ultraviolet and optical wind lines) provided that the mass-loss rate of theta (1) Ori C is at least 5 times smaller than that predicted by radiatively driven wind models. We finally show how new observations with the XMM or Chandra spacecraft could help us constrain this model much more tightly and thus obtain a clear picture of how magnetic fields can influence the winds of hot stars.

The magnetic field and wind confinement of θ1 Orionis C

An account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, σ 2 CrB, II Peg and UX Ari

Zeeman-Doppler imaging of active stars. III: Instrumental and technical considerations

Recently Semel (1989) has introduced the technique of Zeeman-Doppler Imaging (ZDI) which essentially consists of Doppler Imaging using the Stokes V (circular polarization) profile. A code which uses maximum entropy principles to recover the distribution of brightness, abundance and magnetic fields across the surface of stars, is presented. For brightness and abundance mappings, the code is found to produce results comparable to those found in the literature. The first reconstructions of magnetic test structures from the Stokes V parameter alone show that this quantity contains enough spatial information to recover spotted images but not dipolar ones

Zeeman-Doppler imaging of solar-type and AP stars. IV. Maximum entropy reconstruction of 2D magnetic topologies.

Erratum - Detection of a Magnetic Region on HR:1099

We applied the new technique or Zeeman-Doppler Imaging (measuring circularly polarized spectra to detect magnetic structures on rapidly rotating stars) on 9 active RS CVn binary systems. In 4 systems (HR 1099, σ 2 CrB, II Peg and UX Ari), we detected a magnetic field on the cool active component; in each or them, the corresponding Stokes V signature is found to vary with rotational phase. Moreover, on HR 1099, we have observational evidence that the magnetic structure varied significantly within a period of 1.3 yr

M. Semel

Papers

The magnetic field and wind confinement of θ1 Orionis C

Zeeman-Doppler imaging of active stars. III: Instrumental and technical considerations

Zeeman-Doppler imaging of solar-type and AP stars. IV. Maximum entropy reconstruction of 2D magnetic topologies.

Erratum - Detection of a Magnetic Region on HR:1099

Circularly polarized spectroscopic observations of RS CVn systems