Red giants are sometimes surrounded by envelopes, theresult of the ejection of stellar matter at a large rate($dot M> 10^{-7}M_odot$/yr)and at a low velocity (10 km/s). In this reviewthe envelopes are discussed and the relation between stars andenvelope: what stars combine with what envelopes?The envelope emits radiation by various processes and has beendetected at all wavelengths between the visual and the microwaverange. I review the observations of continuum radiation emitted bydust particles and of rotational transitions of molecules, wherethese molecules have been excited by thermal or by non–thermal(“maser”) processes. I discuss mainly the oxygen–richstars, those of spectral type M, and only briefly the closely relatedcarbon–rich stars.By and large the density in the envelope is well described byspherically symmetric outflow at a constant velocity; on the timescale needed to flow from stellar surface to the outermost layers,i.e.$10^5$yr, the loss of mass is sometimes interrupted suddenlyafter which the envelope becomes “detached” from the star. Thetemperature decreases when moving outward; heat input is byfriction between dust particles and gas and cooling occurs by lineradiation by various molecules, especially byH$_2$O. The molecularcomposition is determined by formation in an equilibrium processdeep in the atmosphere and by destruction in the outer parts of theoutflow by interstellar UV radiation(H$_2$, CO, H$_2$O) or bydepletion due to condensation on dust grains (SiO); dust particlesof silicate material solidify where the radiation temperature isdecreased to about 1000 K, and this is at a few stellar radii.The various continuum spectra produced by the dust particles indifferent stars are well modelled by a simple model of the densityand dust temperature distribution plus the assumption that theparticles consist of “dirty silicate”, i.e. silicate with Fe and Alions added. A large range of optical depths,$tau_{9.7mu{rm m}}$, is observed:from 0.01 to 10. In envelopes with large optical depth the staritself can no longer be detected directly. Model calculations alsoshow that the momentum in the outflow, i.e.$dot M.v_{rm out}$isprovided by radiation pressure on the dust particles followed bythe complete transfer of this momentum to the gas. The mass–lossrate itself,$dot M$, is not determined by radiation pressure but bydynamic processes in the region below the dust condensation layer.When$tau_{9.7mu{rm m}}$is sufficiently large its measurement, that of thestellar luminosity,$L_*$and that of the outflow velocity,$v_{rm out}$,permit the determination of$dot M$, i.e. the totaloutflow rate, without making assumptions about the abundance of the dustparticles or of the molecular gases. Detached envelopes have beenseen in a few cases.Thermal molecular radiation is faint compared to the maser emissionbut has been measured in distant stars, e.g. in stars near thegalactic center. Different molecules outline different “spheres”around the star. The largest sphere (a radius of 0.1 pc) isoutlined by an emission line belonging to theCO($v=0, J=1to 0$)transition. Higher rotational transitions of CO give smallerdiameters. A comparison of CO($J=2to 1$) and ($J=1to 0$) fluxesin stars with very thick envelopes leads to the conclusion that anabrupt decrease in the mass–loss rate occurred some ten thousandyears ago.Three molecules produce each several maser lines: SiO,H$_2$O andOH. Several newH$_2$O lines have recently been discovered; theirexploration has hardly been started. The high intensity of themaser lines makes interferometry possible and hence detailedmapping. The SiO lines are formed deep in the envelope, below thedust condensation layer. OH maser lines are produced farthest out,H$_2$O lines inbetween. The excitation mechanisms for most maserlines is understood globally, but detailed models are lacking,largely because the problem is non–linear and the solution of theradiative transfer equation requires a highly anisotropic geometry.The geometrical and kinematical properties of the 1612 MHz OHmaser, which in many objects is very strong, are explained by athin shell of OH; because the angular diameter of the shell can bemeasured directly and the linear diameter can be determined fromthe difference in the time of maximum flux of blue and red maserpeaks, the distance of the shell and of the star can be measured.The presence or absence of individual maser lines appears to dependon the value of$tau_{9.7mu{rm m}}$and is well described by a sequence called“Lewis' chronology”.The central star is a long–period variable with a period of 300days or longer and with a large luminosity amplitude($Delta m_{rm bol}> 0.7^m$).Evidence is given that each star has themaximum luminosity it will reach during its evolution and that itis a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB)with a main–sequence mass between 1 and 6$M_odot$. Stars of the samemain–sequence mass,$M_{rm ms}$,have different mass–loss rates, insome cases by a factor of 10. The mass–loss rate probablyincreases with time, and the highest mass–loss rates are reachedtoward the end of the evolution. Stars with higher$M_{rm ms}$ultimatelyreach higher mass–loss rates. The calibration of themain–sequence mass is reviewed. Most Mira variables with mass losshave a mass between 1.0 and 1.2$M_odot$. OH/IR stars with periodsover 1000 days have no counterparts among the carbon stars and thushave$M_{rm ms}> 4.5M_odot$.Stars as discussed in this review have beenfound only in the thin galactic disk and in the bulge.Finally I review several recently proposed scenarios for TP–AGBevolution in which mass loss is taken into account. These scenariosrepresent the observations quite well; their major short–coming isthe lack of an explanation why the central stars are alwayslarge–amplitude, long–period var
展开▼
