47 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
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
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&
Low temperature mean opacities for the carbon-rich regime
Asymptotic Giant Branch (AGB) stars undergo a change in their chemical
composition during their evolution. This in turn leads to an alteration of the
radiative opacities, especially in the cool layers of the envelope and the
atmosphere, where molecules are the dominant opacity sources. A key parameter
in this respect is the number ratio of carbon to oxygen atoms (C/O). In terms
of low temperature mean opacities, a variation of this parameter usually cannot
be followed in stellar evolution models, because up to now tabulated values
were only available for scaled solar metal mixtures (with C/O ~ 0.5). We thus
present a set of newly generated tables containing Rosseland mean opacity
coefficients covering both the oxygen-rich (C/O
1) regime. We compare our values to existing tabular data and investigate the
relevant molecular opacity contributors.Comment: 8 pages, 5 figures. To appear in the AIP Proceedings of the IXth
Torino Workshop on AGB Nucleosynthesi
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
The PTI Carbon Star Angular Size Survey: Effective Temperatures and Non-Sphericity
We report new interferometric angular diameter observations of 41 carbon
stars observed with the Palomar Testbed Interferometer (PTI). Two of these
stars are CH carbon stars and represent the first such measurements for this
subtype. Of these, 39 have Yamashita (1972,1975) spectral classes and are of
sufficiently high quality that we may determine the dependence of effective
temperature on spectral type. We find that there is a tendency for the
effective temperature to increase with increasing temperature index by ~120K
per step, starting at T_EFF ~= 2500K for C3,y, although there is a large amount
of scatter about this relationship. Overall, the median effective temperature
for the carbon star sample is found to be 2800 +- 270K, and the median linear
radius is 360 +- 100 R_SUN. We also find agreement on average within 15K with
the T_EFF determinations of Bergeat (2001,2002a,b), and a refinement of carbon
star angular size prediction based on V & K magnitudes is presented that is
good to an rms of 12%. A subsample of our stars have sufficient {u,v} coverage
to permit non-spherical modeling of their photospheres, and a general tendency
for detection of statistically significant departures from sphericity with
increasing signal-to-noise of the interferometric data is seen. The
implications of most - and potentially all - carbon stars being non-spherical
is considered in the context of surface inhomogeneities and a rotation-mass
loss connection.Comment: 59 pages, 15 figures, 6 tables; accepted for publication in Ap
Updated Low-temperature Gas Opacities with ÆSOPUS 2.0
This work introduces new low-temperature gas opacities in the range 3.2 < log(T/K) < 4.5 computed with the ae SOPUS code under the assumption of thermodynamic equilibrium. In comparison to the previous version, ae SOPUS 1.0, we updated and expanded molecular absorption to include 80 species, mostly using the recommended line lists currently available from the ExoMol and HITRAN databases. Furthermore, in light of a recent study, we revised the H- photodetachment cross section, added the free-free absorption of other negative ions of atoms and molecules, and updated the collision-induced absorption due to H-2/H-2, H-2/H, H2/He, and H/ He pairs. Using the new input physics, we computed tables of Rosseland mean opacities for several scaled-solar chemical compositions, including Magg et al.'s most recent one, as well as alpha-enhanced mixtures. The differences in opacity between the new ae SOPUS 2.0 and the original ae SOPUS 1.0 versions, as well as other sets of calculations, are discussed. The new opacities are released to the community via a dedicated webpage that includes both precomputed tables for widely used chemical compositions and a web interface for calculating opacities on the fly for any abundance distribution