1,162 research outputs found
Two barium stars in the Galactic bulge
Barium stars conserve important information on the s-process and the third
dredge-up in intermediate mass stars. Their discovery in various environments
is therefore of great help to test nucleosynthesis and mixing models. Our aim
is to analyse two stars with a very strong barium line detected in a large
survey of red giants in the Galactic bulge. Abundance analysis was done
comparing synthetic model spectra based on the COMARCS code with our medium
resolution spectra. Abundances of Ba, La, Y, and Fe were determined. Beside the
two main targets, the analysis was also applied to two comparison stars. We
confirm that both stars are barium stars. They are the first ones of this kind
identified in the Galactic bulge. Their barium excesses are among the largest
values found up to now. The elemental abundances are compared with current
nucleosynthesis and mixing models. Furthermore, we estimate a frequency of
barium stars in the Galactic bulge of about 1%, which is identical to the value
for disc stars.Comment: 4 pages, accepted for publication in A&
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
Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models
The heavy mass loss observed in evolved asymptotic giant branch stars is
usually attributed to a two-step process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. Detailed wind models suggest that the outflows of M-type
AGB stars may be triggered by photon scattering on Fe-free silicates with grain
sizes of about 0.1 - 1 m. Due to the low grain temperature, these Fe-free
silicates can condense close to the star, but they do not produce the
characteristic mid-IR features that are often observed in M-type AGB stars.
However, it is probable that the silicate grains are gradually enriched with Fe
as they move away from the star, to a degree where the grain temperature stays
below the sublimation temperature, but is high enough to produce emission
features. We investigate whether differences in grain temperature in the inner
wind region, which are related to changes in the grain composition, can be
detected with current interferometric techniques, in order to put constraints
on the wind mechanism. To investigate this we use radial structures of the
atmosphere and wind of an M-type AGB star, produced with the 1D
radiation-hydrodynamical code DARWIN. The spectral energy distribution is found
to be a poor indicator of different temperature profiles and therefore is not a
good tool for distinguishing different scenarios of changing grain composition.
However, spatially resolved interferometric observations have promising
potential. They show signatures even for Fe-free silicates (found at 2-3
stellar radii), in contrast to the spectral energy distribution. Observations
with baselines that probe spatial scales of about 4 stellar radii and beyond
are suitable for tracing changes in grain composition, since this is where
effects of Fe enrichment should be found.Comment: Accepted for publication in Section 8. Stellar atmospheres of
Astronomy and Astrophysics. The official date of acceptance is 07/09/2017. 9
pages, 7 figures, 4 figures in appendi
Exploring wind-driving dust species in cool luminous giants III. Wind models for M-type AGB stars: dynamic and photometric properties
Stellar winds observed in asymptotic giant branch (AGB) stars are usually
attributed to a combination of stellar pulsations and radiation pressure on
dust. Shock waves triggered by pulsations propagate through the atmosphere,
compressing the gas and lifting it to cooler regions, which create favourable
conditions for grain growth. If sufficient radiative acceleration is exerted on
the newly formed grains through absorption or scattering of stellar photons, an
outflow can be triggered. Strong candidates for wind-driving dust species in
M-type AGB stars are magnesium silicates (MgSiO and MgSiO). Such
grains can form close to the stellar surface, they consist of abundant
materials and, if they grow to sizes comparable to the wavelength of the
stellar flux maximum, they experience strong acceleration by photon scattering.
We use a frequency-dependent radiation-hydrodynamics code with a detailed
description for the growth of MgSiO grains to calculate the first
extensive set of time-dependent wind models for M-type AGB stars. The resulting
wind properties, visual and near-IR photometry and mid-IR spectra are compared
with observations.We show that the models can produce outflows for a wide range
of stellar parameters. We also demonstrate that they reproduce observed
mass-loss rates and wind velocities, as well as visual and near-IR photometry.
However, the current models do not show the characteristic silicate features at
10 and 18 m as a result of the cool temperature of MgSiO grains in
the wind. Including a small amount of Fe in the grains further out in the
circumstellar envelope will increase the grain temperature and result in
pronounced silicate features, without significantly affecting the photometry in
the visual and near-IR wavelength regions.Comment: 11 pages, 14 figure
Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up
The concomitant overabundances of C, N and s-process elements are commonly
ascribed to the complex interplay of nucleosynthesis, mixing and mass loss
taking place in Asymptotic Giant Branch stars. At low metallicity, the
enhancement of C and/or N may be up to 1000 times larger than the original iron
content and significantly affects the stellar structure and its evolution. For
this reason, the interpretation of the already available and still growing
amount of data concerning C-rich metal-poor stars belonging to our Galaxy as
well as to dwarf spheroidal galaxies would require reliable AGB stellar models
for low and very low metallicities. In this paper we address the question of
calculation and use of appropriate opacity coefficients, which take into
account the C enhancement caused by the third dredge up. A possible N
enhancement, caused by the cool bottom process or by the engulfment of protons
into the convective zone generated by a thermal pulse and the subsequent huge
third dredge up, is also considered. Basing on up-to-date stellar models, we
illustrate the changes induced by the use of these opacity on the physical and
chemical properties expected for these stars.Comment: 23 pages, 8 figures, accepted for publication in Ap
An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds
The purpose of this work is to present an extensive grid of dynamical
atmosphere and wind models for M-type AGB stars, covering a wide range of
relevant stellar parameters. We used the DARWIN code, which includes
frequency-dependent radiation-hydrodynamics and a time-dependent description of
dust condensation and evaporation, to simulate the dynamical atmosphere. The
wind-driving mechanism is photon scattering on submicron-sized MgSiO
grains. The grid consists of models, with luminosities from
to and
effective temperatures from 2200K to 3400K. For the first time different
current stellar masses are explored with M-type DARWIN models, ranging from
0.75M to 3M. The modelling results are radial atmospheric
structures, dynamical properties such as mass-loss rates and wind velocities,
and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of
condensed Si). We find that the mass-loss rates of the models correlate
strongly with luminosity. They also correlate with the ratio :
increasing by an order of magnitude increases the mass-loss rates by
about three orders of magnitude, which may naturally create a superwind regime
in evolution models. There is, however, no discernible trend of mass-loss rate
with effective temperature, in contrast to what is found for C-type AGB stars.
We also find that the mass-loss rates level off at luminosities higher than
, and consequently at pulsation periods longer
than days. The final grain radii range from 0.25 micron to 0.6
micron. The amount of condensed Si is typically between 10% and 40%, with
gas-to-dust mass ratios between 500 and 4000.Comment: Accepted to A&A, 17 pages, 15 figure
Oxygen isotopic ratios in intermediate-mass red giants
Context. The abundances of the three main isotopes of oxygen are altered in
the course of the CNO-cycle. When the first dredge-up mixes the burning
products to the surface, the nucleosynthesis processes can be probed by
measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red
giants of known mass we compare the isotope ratios with predictions from
stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were
derived from the K-band spectra of six red giants. The sample red giants are
open cluster members with known masses of between 1.8 and 4.5 Msun . The
abundance determination employs synthetic spectra calculated with the COMARCS
code. The effect of uncertainties in the nuclear reaction rates, the mixing
length, and of a change in the initial abundance of the oxygen isotopes was
determined by a set of nucleosynthesis and mixing models using the FUNS code.
Results. The observed 16O/17O ratios are in good agreement with the model
results, even if the measured values do not present clear evidence of a
variation with the stellar mass. The observed 16O/18O ratios are clearly lower
than the predictions from our reference model. Variations in nuclear reaction
rates and mixing length parameter both have only a very weak effect on the
predicted values. The 12C/13C ratios of the K giants studied implies the
absence of extra-mixing in these objects. Conclusions. A comparison with
galactic chemical evolution models indicates that the 16O/18O abundance ratio
underwent a faster decrease than predicted. To explain the observed ratios, the
most likely scenario is a higher initial 18O abundance combined with a lower
initial 16 O abundance. Comparing the measured 18 O/17 O ratio with the
corresponding value for the ISM points towards an initial enhancement of 17O as
well. Limitations imposed by the observations prevent this from being a
conclusive result.Comment: 9 pages, accepted for publication in Astronomy & Astrophysic
Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars
The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars
is usually attributed to a two-stage process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. The dust transfers momentum to the surrounding gas through
collisions and thereby triggers a general outflow. Radiation-hydrodynamical
models of M-type AGB stars suggest that these winds can be driven by photon
scattering -- in contrast to absorption -- on Fe-free silicate grains of sizes
0.1--1\,m. In this paper we study photometric constraints for wind-driving
dust species in M-type AGB stars, as part of an ongoing effort to identify
likely candidates among the grain materials observed in circumstellar
envelopes. To investigate the scenario of stellar winds driven by photon
scattering on dust, and to explore how different optical and chemical
properties of wind-driving dust species affect photometry we focus on two sets
of dynamical models atmospheres: (i) models using a detailed description for
the growth of MgSiO grains, taking into account both scattering and
absorption cross-sections when calculating the radiative acceleration, and (ii)
models using a parameterized dust description, constructed to represent
different chemical and optical dust properties. By comparing synthetic
photometry from these two sets of models to observations of M-type AGB stars we
can provide constraints on the properties of wind-driving dust species.
Photometry from wind models with a detailed description for the growth of
MgSiO grains reproduces well both the values and the time-dependent
behavior of observations of M-type AGB stars, providing further support for the
scenario of winds driven by photon scattering on dust.Comment: Accepted for publication in A&A. 15 pages, 14 figure
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&
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