Dust formation and inhomogeneous mass-loss from asymptotic giant branch stars

We examine the flow from asymptotic giant branch (AGB) stars when along a small solid angle the optical depth resulting from dust is very large. We consider two types of flows. In the first, small cool spots are formed on the surface of slowly rotating AGB stars. Large quantities of dust are expected to be formed above the surface of these cool spots. We propose that if the dust formation occurs during the last AGB phase when the mass-loss rate is high, the dust shields the region above it from the stellar radiation. This leads to both further dust formation in the shaded region and, owing to lower temperature and pressure, the convergence of the stream toward the shaded region, and the formation of a flow having a higher density than its surroundings. This density contrast can be as high as ∼4. A concentration of magnetic cool spots toward the equator will lead to a density contrast of up to a few between the equatorial and polar directions. This process can explain the positive correlation between high mass-loss rate and a larger departure from sphericity in progenitors of elliptical planetary nebulae. In the second type of flow, the high density in the equatorial plane is formed by a binary interaction, where the secondary star is close to, but outside the AGB envelope. The shielding of the radiation by dust results in a very slow and dense flow in the equatorial plane. We suggest this flow as an alternative explanation for the equatorial dense matter found at several hundred astronomical units around several post-AGB binary systems.     

Turbulent dynamo in asymptotic giant branch stars

Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo may amplify a large-scale magnetic field in the envelopes of asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB envelope can fix the symmetry axis, leading to the formation of an axisymmetric magnetic field structure. Unlike solar-type αω dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field; instead of an αω dynamo we propose an α ² ω . The magnetic field may reach a value of     , where B _e is the equipartition (between the turbulent and magnetic energy densities) magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots may regulate dust formation, and hence the mass-loss rate, leading to axisymmetric mass loss and the formation of elliptical planetary nebulae (PNe). Despite its role in forming cool spots, the large-scale magnetic field is too weak to play a dynamic role and directly influence the wind from the AGB star, as required by some models. We discuss other possible problems in models where the magnetic field plays a dynamic role in shaping the AGB winds, and argue that they cannot explain the formation of non-spherical PNe.     

Submillimetre photometry of post-asymptotic giant branch stars

Stars in the post-asymptotic giant branch (post-AGB) phase of evolution are surrounded by detached circumstellar envelopes containing dust which emits thermally in the mid- and far-infrared. Here we present 850-μm SCUBA photometry of nine candidate post-AGB stars. All targets are detected at 850 μm and we use these fluxes to estimate the envelope dust masses and, by comparison with the 100-μm IRAS fluxes, the dust emissivity index.     

Backflow in post-asymptotic giant branch stars

We derive the conditions for a backflow toward the central star(s) of circumstellar material to occur during the post-asymptotic giant branch (post-AGB) phase. The backflowing material may be accreted by the post-AGB star and/or its companion, if such exists. Such a backflow may play a significant role in shaping the descendant planetary nebula, by, among other things, slowing down the post-AGB evolution, and by forming an accretion disc which may blow two jets. We consider three forces acting on a slowly moving mass element: the gravity of the central system, radiation pressure, and fast wind ram pressure. We find that for a significant backflow to occur, a slow dense flow should exist, such that the relation between the total mass in the slow flow, M _i , and the solid angle it covers Ω, is given by     , where     . The requirement for both a high mass-loss rate per unit solid angle and a very slow wind, such that it can be decelerated and flow back, probably requires close binary interaction, hence this process is rare.     

The case for asymmetric dust around a C-rich asymptotic giant branch star

JHKL observations of the mass-losing carbon Mira variable IRAS 15194–5115 (II Lup) extending over about 18 yr are presented and discussed. The pulsation period is 575 d and has remained essentially constant over this time span. The star has undergone an extensive obscuration minimum during this time. This is complex and, like such minima in similar objects (e.g. R For), does not fit the model predictions of a simple long-term periodicity. Together with the high-resolution observations of Lopez et al., the results suggest that the obscuration changes are caused by the formation of dust clouds of limited extent in the line of sight. This is an R Coronae Borealis-type (RCB-type) model. The effective reddening law at J and H is similar to that found for R For.     

Optical properties of the carbon dust grains in the envelopes around asymptotic giant branch stars

We have investigated the optical properties of the carbon dust grains in the envelopes around carbon-rich asymptotic giant branch stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including IRAS Point Source Catalog and IRAS Low Resolution Spectrograph data. We have deduced an opacity function of amorphous carbon dust grains from model fitting with infrared carbon stars. From the opacity function, we have derived the optical constants of the AMC grains. The optical constants satisfy the Kramers–Kronig relation and produce the opacity function that fits the observations of infrared carbon stars better than previous works in the wide wavelength range 1–1000 μm. We have used simple mixtures of the AMC and silicon carbide grains for modelling. We have compared the contributions that AMC and SiC grains make to the opacity for the cases of simple mixtures of them and spherical core–mantle type grains consisting of a SiC core and an AMC mantle .     

100-yr mass-loss modulations on the asymptotic giant branch

We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models.
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch.     

Magnetic field, dust and axisymmetrical mass loss on the asymptotic giant branch

I propose a mechanism for axisymmetrical mass loss on the asymptotic giant branch (AGB) that may account for the axially symmetric structure of elliptical planetary nebulae. The proposed model operates for slowly rotating AGB stars, having angular velocities in the range of 10⁻⁴ω _Kep  ω  10⁻² ω_Kep, where ω_Kep is the equatorial Keplerian angular velocity. Such angular velocities could be gained from a planet companion of mass  0.1  M _Jupiter, which deposits its orbital angular momentum to the envelope at late stages, or even from single stars that are fast rotators on the main sequence. The model assumes that dynamo magnetic activity results in the formation of cool spots, above which dust forms much more easily. The enhanced magnetic activity towards the equator results in a higher dust formation rate there, and hence higher mass-loss rate. As the star ascends the AGB, both the mass-loss rate and magnetic activity increase rapidly, and hence the mass loss becomes more asymmetrical, with higher mass-loss rate closer to the equatorial plane.     

Stellar structure and mass loss on the upper asymptotic giant branch

We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early post-AGB phase. Because of the high mass-loss rate, the envelope mass decreases by more than an order of magnitude. This makes the density profile below the photosphere much shallower, and the entropy profile much steeper. We discuss the possible role of these changes in the profiles in the onset of the high mass-loss rate (superwind) and the large deviation from spherical mass loss at the termination of the AGB. We concentrate on the idea that the shallower density profile and steeper entropy profile allow the formation of cool magnetic spots, above which dust forms much more easily.     

The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars

We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities   Z = 10⁻⁴  and 10⁻⁸. We determine which stars undergo an episode of flash-driven mixing, where protons are ingested into the intershell convection zone, as they enter the thermally pulsing AGB phase and which undergo third dredge-up. We find that flash-driven mixing does not occur above a metallicity of   Z = 10⁻⁵  for any mass of star and that stars above  2 M_⊙  do not experience this phenomenon at any metallicity. We find carbon ingestion (CI), the mixing of carbon into the tail of hydrogen-burning region, occurs in the mass range  2 M_⊙  to around  4 M_⊙  . We suggest that CI may be a weak version of the flash-driven mechanism. We also investigate the effects of convective overshooting on the behaviour of these objects. Our models struggle to explain the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars that have both significant carbon and nitrogen enhancement. Carbon can be enhanced through flash-driven mixing, CI or just third dredge-up. Nitrogen can be enhanced through hot bottom burning and the occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio may be a good indicator of the mass of the primary AGB stars.