43 research outputs found
Modelling polarized light from dust shells surrounding asymptotic giant branch stars
Winds of asymptotic giant branch (AGB) stars are commonly assumed to be
driven by radiative acceleration of dust grains. For M-type AGB stars, the
nature of the wind-driving dust species has been a matter of intense debate. A
proposed source of the radiation pressure triggering the outflows is photon
scattering on Fe-free silicate grains. This wind-driving mechanism requires
grain radii of about 0.1 - 1 micron in order to make the dust particles
efficient at scattering radiation around the stellar flux maximum. Grain size
is therefore an important parameter for understanding the physics behind the
winds of M-type AGB stars. We seek to investigate the diagnostic potential of
scattered polarized light for determining dust grain sizes. We have developed a
new tool for computing synthetic images of scattered light in dust and gas
shells around AGB stars, which can be applied to detailed models of dynamical
atmospheres and dust-driven winds. We present maps of polarized light using
dynamical models computed with the DARWIN code. The synthetic images clearly
show that the intensity of the polarized light, the position of the inner edge
of the dust shell, and the size of the dust grains near the inner edge are all
changing with the luminosity phase. Non-spherical structures in the dust shells
can also have an impact on the polarized light. We simulate this effect by
combining different pulsation phases into a single 3D structure before
computing synthetic images. An asymmetry of the circumstellar envelope can
create a net polarization, which can be used as diagnostics for the grain size.
The ratio between the size of the scattering particles and the observed
wavelength determines at what wavelengths net polarization switches direction.
If observed, this can be used to constrain average particle sizes.Comment: 9 page
Global 3D radiation-hydrodynamics models of AGB stars. Effects of convection and radial pulsations on atmospheric structures
Context: Observations of asymptotic giant branch (AGB) stars with increasing
spatial resolution reveal new layers of complexity of atmospheric processes on
a variety of scales. Aim: To analyze the physical mechanisms that cause
asymmetries and surface structures in observed images, we use detailed 3D
dynamical simulations of AGB stars; these simulations self-consistently
describe convection and pulsations. Methods: We used the CO5BOLD
radiation-hydrodynamics code to produce an exploratory grid of global
"star-in-a-box" models of the outer convective envelope and the inner
atmosphere of AGB stars to study convection, pulsations, and shock waves and
their dependence on stellar and numerical parameters. Results: The model
dynamics are governed by the interaction of long-lasting giant convection
cells, short-lived surface granules, and strong, radial, fundamental-mode
pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves
induce shocks on various scales in the atmosphere. Convection, waves, and
shocks all contribute to the dynamical pressure and, thus, to an increase of
the stellar radius and to a levitation of material into layers where dust can
form. Consequently, the resulting relation of pulsation period and stellar
radius is shifted toward larger radii compared to that of non-linear 1D models.
The dependence of pulsation period on luminosity agrees well with observed
relations. The interaction of the pulsation mode with the non-stationary
convective flow causes occasional amplitude changes and phase shifts. The
regularity of the pulsations decreases with decreasing gravity as the relative
size of convection cells increases. The model stars do not have a well-defined
surface. Instead, the light is emitted from a very extended inhomogeneous
atmosphere with a complex dynamic pattern of high-contrast features
Dynamic atmospheres and winds of cool luminous giants, I. AlO and silicate dust in the close vicinity of M-type AGB stars
High spatial resolution techniques have given valuable insights into the mass
loss mechanism of AGB stars, which presumably involves a combination of
atmospheric levitation by pulsation-induced shock waves and radiation pressure
on dust. Observations indicate that AlO condenses at distances of about
2 stellar radii or less, prior to the formation of silicates. AlO
grains are therefore prime candidates for producing the scattered light
observed in the close vicinity of several M-type AGB stars, and they may be
seed particles for the condensation of silicates at lower temperatures. We have
constructed a new generation of Dynamic Atmosphere & Radiation-driven Wind
models based on Implicit Numerics (DARWIN), including a time-dependent
treatment of grain growth & evaporation for both AlO and Fe-free
silicates (MgSiO). The equations describing these dust species are
solved in the framework of a frequency-dependent radiation-hydrodynamical model
for the atmosphere & wind structure, taking pulsation-induced shock waves and
periodic luminosity variations into account. Condensation of AlO at the
close distances and in the high concentrations implied by observations requires
high transparency of the grains in the visual and near-IR region to avoid
destruction by radiative heating. For solar abundances, radiation pressure due
to AlO is too low to drive a wind. Nevertheless, this dust species may
have indirect effects on mass loss. The formation of composite grains with an
AlO core and a silicate mantle can give grain growth a head start,
increasing both mass loss rates and wind velocities. Furthermore, our
experimental core-mantle grain models lead to variations of visual and near-IR
colors during a pulsation cycle which are in excellent agreement with
observations.Comment: Accepted for publication in Astronomy & Astrophysics (18 pages, 9
figures
Explaining the winds of AGB stars: Recent progress
The winds observed around asymptotic giant branch (AGB) stars are generally
attributed to radiation pressure on dust, which is formed in the extended
dynamical atmospheres of these pulsating, strongly convective stars. Current
radiation-hydrodynamical models can explain many of the observed features, and
they are on the brink of delivering a predictive theory of mass loss. This
review summarizes recent results and ongoing work on winds of AGB stars,
discussing critical ingredients of the driving mechanism, and first results of
global 3D RHD star-and-wind-in-a-box simulations. With such models it becomes
possible to follow the flow of matter, in full 3D geometry, all the way from
the turbulent, pulsating interior of an AGB star, through its atmosphere and
dust formation zone into the region where the wind is accelerated by radiation
pressure on dust. Advanced instruments, which can resolve the stellar
atmospheres, where the winds originate, provide essential data for testing the
models.Comment: Accepted for publication in "The Origin of Outflows in Evolved
Stars", Proceedings of IAU Symposium 366 (8 pages, 2 figures
Synthetic photometry for carbon-rich giants. IV. An extensive grid of dynamic atmosphere and wind models
The evolution and spectral properties of stars on the AGB are significantly
affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind
models are an essential tool for studying these evolved stars, both
individually and as members of stellar populations, to understand their
contribution to the integrated light and chemical evolution of galaxies.
This paper is part of a series testing state-of-the-art atmosphere and wind
models of carbon stars against observations, and making them available for use
in various theoretical and observational studies.
We have computed low-resolution spectra and photometry (in the wavelength
range 0.35-25 mu) for a grid of 540 dynamic models with stellar parameters
typical of solar-metallicity C-rich AGB stars and with a range of pulsation
amplitudes. The models cover the dynamic atmosphere and dusty outflow (if
present), assuming spherical symmetry, and taking opacities of gas-phase
species and dust grains consistently into account. To characterize the
time-dependent dynamic and photometric behaviour of the models in a concise way
we defined a number of classes for models with and without winds.
Comparisons with observed data in general show a quite good agreement for
example regarding mass-loss rates vs. (J-K) colours or K magnitudes vs. (J-K)
colours. Some exceptions from the good overall agreement, however, are found
and attributed to the range of input parameters (e.g. relatively high carbon
excesses) or intrinsic model assumptions (e.g. small particle limit for grain
opacities).
While current results indicate that some changes in model assumptions and
parameter ranges should be made in the future to bring certain synthetic
observables into better agreement with observations, it seems unlikely that
these pending improvements will significantly affect the mass-loss rates of the
models.Comment: 28 pages, 15 figures. Table B.1, an 11-page table, is only available
at CD
Abundance analysis for long-period variables II. RGB and AGB stars in the globular cluster 47\,Tuc
Asymptotic giant branch (AGB) stars play a key role in the enrichment of
galaxies with heavy elements. Due to their large amplitude variability, the
measurement of elemental abundances is a highly challenging task that has not
been solved in a satisfactory way yet.
Following our previous work we use hydrostatic and dynamical model
atmospheres to simulate observed high-resolution near-infrared spectra of 12
variable and non-variable red giants in the globular cluster 47 Tuc. The 47 Tuc
red giants are independently well-characterized in important parameters (mass,
metallicity, luminosity). The principal aim was to compare synthetic spectra
based on the dynamical models with observational spectra of 47 Tuc variables.
Assuming that the abundances are unchanged on the upper giant branch in these
low-mass stars, our goal is to estimate the impact of atmospheric dynamics on
the abundance determination.
We present new measurements of the C/O and 12C/13C ratio for 5 non-variable
red giants in 47Tuc. The equivalent widths measured for our 7 variable stars
strongly differ from the non-variable stars and cannot be reproduced by either
hydrostatic or dynamical model atmospheres. Nevertheless, the dynamical models
fit the observed spectra of long-period variables much better than any
hydrostatic model. For some spectral features, the variations in the line
intensities predicted by dynamical models over a pulsation cycle give similar
values as a sequence of hydrostatic models with varying temperature and
constant surface gravity.Comment: 16 pages, 12 figures; accepted for publication in A&
Synthetic photometry for carbon-rich giants. V. Effects of grain-size-dependent dust opacities
The properties and the evolution of asymptotic giant branch (AGB) stars are
strongly influenced by their mass loss through a stellar wind. This is believed
to be caused by radiation pressure due to the absorption and scattering of the
stellar radiation by the dust grains formed in the atmosphere. The optical
properties of dust are often estimated using the small particle limit (SPL)
approximation, and it has been used frequently in modelling AGB stellar winds
when performing radiation-hydrodynamics (RHD) simulations. We aim to
investigate the effects of replacing the SPL approximation by detailed Mie
calculations of the size-dependent opacities for grains of amorphous carbon
forming in C-rich AGB star atmospheres. We performed RHD simulations for a
large grid of carbon star atmosphere+wind models with different effective
temperatures, luminosities, stellar masses, carbon excesses, and pulsation
properties. Also, a posteriori radiative transfer calculations for many radial
structures (snapshots) of these models were done, resulting in spectra and
filter magnitudes. We find, when giving up the SPL approximation, the wind
models become more strongly variable and more dominated by gusts, although the
average mass-loss rates and outflow speeds do not change significantly; the
increased radiative pressure on the dust throughout its formation zone does,
however, result in smaller grains and lower condensation fractions (and thus
higher gas-to-dust ratios). The photometric K magnitudes are generally
brighter, but at V the effects of using size-dependent dust opacities are more
complex: brighter for low mass-loss rates and dimmer for massive stellar winds.
Given the large effects on spectra and photometric properties, it is necessary
to use the detailed dust optical data instead of the simple SPL approximation
in stellar atmosphere+wind modelling where dust is formed.Comment: 14 pages, 24 figures. Accepted for publication in Astronomy \&
Astrophysic