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. Al2_2O3_3 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 Al$_2$O$_3$ condenses at distances of about 2 stellar radii or less, prior to the formation of silicates. Al$_2$O$_3$ 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 Al$_2$O$_3$ and Fe-free silicates (Mg$_2$SiO$_4$). 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 Al$_2$O$_3$ 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 Al$_2$O$_3$ 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 Al$_2$O$_3$ 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

Constraining dust properties in circumstellar envelopes of C-stars in the Small Magellanic Cloud: optical constants and grain size of carbon dust

We present a new approach aimed at constraining the typical size and optical properties of carbon dust grains in circumstellar envelopes (CSEs) of carbon-rich stars (C-stars) in the Small Magellanic Cloud (SMC). To achieve this goal, we apply our recent dust growth description, coupled with a radiative transfer code to the CSEs of C-stars evolving along the thermally pulsing asymptotic giant branch, for which we compute spectra and colours. Then, we compare our modelled colours in the near- and mid-infrared (NIR and MIR) bands with the observed ones, testing different assumptions in our dust scheme and employing several data sets of optical constants for carbon dust available in the literature. Different assumptions adopted in our dust scheme change the typical size of the carbon grains produced. We constrain carbon dust properties by selecting the combination of grain size and optical constants which best reproduce several colours in the NIR and MIR at the same time. The different choices of optical properties and grain size lead to differences in the NIR and MIR colours greater than 2 mag in some cases. We conclude that the complete set of observed NIR and MIR colours are best reproduced by small grains, with sizes between ~0.035 and ~0.12 μm, rather than by large grains between ~0.2 and 0.7 μm. The inability of large grains to reproduce NIR and MIR colours seems independent of the adopted optical data set. We also find a possible trend of the grain size with mass-loss and/or carbon excess in the CSEs of these stars. © 2016 The Authors