Stellar Wind Theories

In this lecture I describe the basic theory of stellar winds with momentum input due to a force or with energy input and I formulate the five laws of stellar winds. I review the different wind mechanisms and discuss the line driven wind model and the dust driven wind models. This revised version was published online in July 2006 with corrections to the Cover Date.     

The structure of circumstellar envelopes

Copious mass loss on the Asymptotic Giant Branch dominates the late stages of stellar evolution. Maps of extended circumstellar envelopes provide a history of mass loss and trace out anisotropic mass loss. This review concentrates on observations of millimeter wavelength molecular line emission, on high resolution maps of maser emission and on observations of submillimeter, millimeter and radio wavelength continuum emission. Radio continuum observations show that AGB stars are larger at radio than at optical wavelengths. The extended chromospheres indicated by these observations extend to distances from the star large enough for dust to form, thereby initiating mass loss. Molecular line maps have found time-variable mass loss for some stars, including detached shells indicating interrupted mass loss and evidence for a rapid increase in the mass loss rate at the end of the AGB phase. Maps of circumstellar envelopes show evidence of flattening, bipolar outflow and angular variations in both the mass loss rate and the outflow velocity. As stars evolve away from the AGB and planetary nebula formation begins, these structures become more pronounced, and fast bipolar molecular winds are observed. The time scales derived from the dynamical times of these winds and from the expansion rates of the central planetary nebulae are very rapid in some cases, about 100 years, in agreement with the predictions of stellar evolution theory.     

Cosmic Rays and Galactic Winds

Repeated SN-explosion provide large amounts of thermal energy as well as energetic particles through a 1. order Fermi-process. Both effects together with the generation of Alfvén-waves are considered to drive a large scale outflow from a galaxy. These so-called galactic windstransport stellar material enriched by heavy elements into the intergalactic space explaining also the large amount of metals found inthe intergalactic gas. The present contribution is focused on time-dependenteffects which originate from galactic winds driven by a star burst activity. Shock waves travelling through the galactic wind and radiative cooling within the expanding plasma lead to complex flow structures. Depending e.g. on theSFR of the galaxy galactic winds can remove almost all ISM into the galactic halo and therefore cease a subsequent star formation.     

Radiatively driven winds using Lagrangian hydrodynamics

The study of radiatively driven winds in O and B stars has been achieved mainly through analytic solutions of the equation of motion. These solutions can only be applied to simple geometries. Presented here are preliminary results of a particle based Lagrangian code (applied to a simple 1D radiatively driven wind test case), which will be able to resolve structure down to the smallest scales without becoming temporally prohibitive. This approach to modelling radiatively driven winds can be applied to the dispersal of massive young stellar objects' natal circumstellar material, Be star wind structure and CV disc winds.     

The stellar populations within theρ Oph cloud cores

We have obtained infrared colors and limiting magnitudes from 1.25–4.8µm for a sample of 26 of the cm continuum radio sources located in the core of the Oph molecular cloud. Their colors demonstrate that the majority of the sources appear to be heavily reddened objects surrounded by circumstellar accretion disks. In these cases the radio emission most likely diagnoses accretion driven energetic outflow phenomena: either ionized winds or possibly synchrotron emission from shocked gas associated with stellar jets.     

The angular momentum loss of low-mass close binary systems

The detailed evolution of low-mass main-sequence stars (M < 1M ) with a compact companion is studied. For angular momentum loss associated with magnetic braking it is found that about 10^–11–10^–12 M yr^–1 in stellar wind loss would be required. This wind is 10²–10³ times stronger than the solar wind, so we believe here magnetic stellar wind is insufficient. It is well known that there is mass outflow in low-mass close binary systems. We believe here that these outflows are centrifugal driven winds from the outer parts of the accretion disks. The winds extract angular momentum from these systems and therefore drive secular evolution. Disk winds are preferred to winds from the secondary, because of the lower disk surface gravity.     

Wind anisotropy revealed by dust formation around wolf-rayet stars

The presence of heated circumstellar dust around WC type Wolf-Rayet stars requires the episodic or persistent condensation of carbon grains in their stellar winds. In order to survive in the stars' strong ultraviolet radiation fields, the grains must be located at least 100AU from the stellar surfaces. The densities in isotropic winds at such large distances are too low to allow grain growth and anisotropies such as clumps, disks or wind-collision wakes in colliding-wind binary systems are required to provide grain nurseries. Observational evidence for such features in grain-forming W-R stars is examined.     

Formation of Dust in Hostile Environments — What We Learn from Observing Wolf-Rayet Stars

The formation of dust in Wolf-Rayet stellar winds presents challenges to our understanding on account of the stars' strong UV radiation fields. These would heat the dust grains to sublimation unless they were shielded or restricted to significant distances (∼ 100 AU) from the stars where the wind densities appear to be too low to allow dust formation. Valuable clues are provided by observations of episodic dust formation on different mass- and time-scales, especially major outbursts modulated by orbital motion in binaries. Wind inhomogeneities on all scales — global wind-compressed zones arising from stellar rotation, high-density wakes produced in colliding-wind binaries and smaller clumps all appear to be significant. The observational evidence for these effects is reviewed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Active galactic nuclei jet mass loading and truncation by stellar winds

Active galactic nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centres implies that there will be many jet/star interactions, which can mass load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass loading by stellar winds for a broader spectrum of wind mass-loss rates than has previously been considered. Given the observed stellar mass distributions in galactic centres, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses M _BH≲ 10⁴ M_⊙ are rapidly mass loaded and quenched by stellar winds. For  10⁴ M_⊙ < M _BH < 10⁸ M_⊙  , the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon-dominated composition on scales ≳2 kpc therein even if the jet is initially pair plasma dominated.     

Two-flow magnetohydrodynamical jets around young stellar objects

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. Our simulations show that the inner outflow is accelerated from the central object hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force. We also show that when a dense inner stellar wind occurs, the resulting disc-driven jet have a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass ejection rate in the disc-driven jet and a larger radial extension which is in better agreement with the observations besides being more consistent.     

The geometry of decoupled line driven winds

Bjorkman & Cassinelli (1993) have proposed a mechanism that is expected to produce strong equatorial focusing of the radiation-driven winds from rapidly-rotating B stars. Here the possibility of the decoupling of the stellar radiation field and the outflow is considered. We find that greater equator to pole density ratios may be generated than the standard model, bringing it more into line with results implied by observations.     

Dust in the disk winds from young stars as a source of the circumstellar extinction

We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～10⁴ K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? _a = 10^?8-10^?6 M _⊙yr^?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10^?6 M _⊙yr^?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images.     

Stochastic models of stellar winds in T Tauri-stars

We consider the conditions in which emission lines of hydrogen, ionized calcium and ionized magnesium are formed in the spectra of T Tauri-stars. It is well known that there is a problem in interpreting the two-component profiles which are frequently observed in H, Caii K, and Mgii k lines. This problem can be solved if we abandon an assumption which is customarily made in modelling stellar winds: namely, the approximation of a continuous medium. Our calculations indicate that line profiles similar to those which are observed can be obtained by making very simple assumptions about spherical-symmetry and quasistationary gas flow provided that the following conditions are also stratified: (i) the stellar wind consists of separate gas structure (blobs) with a volume-filling factorq of order 0.1; (ii) the blobs decelerate in the outer regions of the wind.The first of these conditions indicates that the outflow of the gas from the surface of a T Tauri-star is extremely inhomogeneous. At any instant, only about 1/10 of the stellar surface acts as a source of stellar wind. Although this conclusion is at first sight very unexpected, it is nevertheless entirely logical if the outflow of gas from the star originates in magnetic activity on the stellar surface.     

新单极高速分子外向流源IRAS19282+1814

^12CoJ=1-0成图观测表明在IRAS19282 1814附近存在着一个蓝向单极高速分子外流，计算了其基本参量并进行了分析，它的成协红外源IRAS19282 1814可能是大质量年轻星体，其IRAS波段色指数表明该源深埋于气体和尘埃物质之中，由60-100μm流量密度获得尘埃温度为30K,它的附近没有其他的源，所以IRAS19282 1814可能是外向流的驱动源。     

Mass loss from the inner regions of accretion discs due to centrifugally driven magnetic wind flows

Wind flows and collimated jets are believed to be a feature of a range of disc accreting systems. These include active galactic nuclei, T Tauri stars, X-ray binaries and cataclysmic variables. The observed collimation implies large-scale magnetic fields and it is known that dipole-symmetry fields of sufficient strength can channel wind flows emanating from the surfaces of a disc. The disc inflow leads to the bending of the poloidal magnetic field lines, and centrifugally driven magnetic winds can be launched when the bending exceeds a critical value. Such winds can result in angular momentum transport at least as effective as turbulent viscosity, and hence they can play a major part in driving the disc inflow.
It is shown here that if the standard boundary condition of vanishing viscous stress close to the stellar surface is applied, together with the standard connection between viscosity and magnetic diffusivity, then poloidal magnetic field bending increases as the star is approached with a corresponding increase in the wind mass loss rate. A significant amount of material can be lost from the system via the enhanced wind from a narrow region close to the stellar surface. This occurs for a Keplerian angular velocity distribution and for a modified form of angular velocity, which allows for matching of the disc and stellar rotation rates through a boundary layer above the stellar surface. The enhanced mass loss is significantly affected by the behaviour of the disc angular velocity as the stellar surface is approached, and hence by the stellar rotation rate. Such a mechanism may be related to the production of jets from the inner regions of disc accreting systems.     

Stellar wind velocities and the radiation-driven winds theory for O stars

In this paper we have carried out an analysis of the predictions of the radiatively driven stellar winds theories on 63 stars belonging to clusters or associations. The spectral types in our sample range from O3 to B0 and all classes of luminosity are considered. The study has been carried out starting from the relationship between the stellar-wind velocity (v _edge) obtained from the resonance doublet ofCIV for stars observed with the IUE, and the escape velocity. The stellar masses have been obtained from the evolutionary tracks of Maeder and Meynet. Results from recent NLTE analyses with blanketing of lines and winds have been used for the effective temperature.Based on data from the International Ultraviolet Explorer, de-archived from the Villafranca Data Archive of the European Space Agency.     

Dust in Hydrogen-Poor Stellar Winds, from Theory to Observations

We review the nature of dust in hydrogen-deficient stellar winds, in particular cool, carbon-rich Wolf-Rayet (WC) stars, and present new observations of WC objects taken with the Short Wavelength Spectrometer on board of the ISO satellite. Predictions from theoretical models of grain precursor formation are also presented and future directions in both observational and theoretical studies are outlined. This revised version was published online in September 2006 with corrections to the Cover Date.     

Oscillations in radiation-driven outflows

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.     

Circumstellar Shells of the Mass-Losing Asymptotic Giant Branch Stars: Limits for the Dust-Driven Winds

We investigated whether the radiation pressure on dust grains alone may account for driving winds in semiregular, irregular and non-variable AGB stars. A simple theoretical model is employed to calculate the limits for the radiatively driven winds and to compare theoretical predictions with observations. Present analysis indicates, that for most of the objects in the studied sample of 67 oxygen-rich and 40 carbon-rich mass-losing AGB stars radiation pressure on dust grains alone is not effective enough to drive the observed circumstellar outflows.     

Disc wind in the HH 30 binary models

Recent interferometric observations of the young stellar object (YSO) HH 30 have revealed a low-velocity outflow in the¹²CO J =1–2 molecule line. We present here two models of the low-velocity disc winds with the aim of investigating the origin of this molecular outflow. Following Andlada et al., we treated HH 30 as a binary system. Two cases have been considered: (i) the orbital period   P = 53 yr  and (ii)   P ≤ 1 yr  . Calculations showed that in the first case the outflow cone had a spiral-like structure due to summing the velocities of the orbital motion and the disc wind. Such a structure contradicts the observations. In the second case, the outflow cone demonstrates a symmetry relatively to the system axis and agrees well with the observations.