Michael T. Lederer

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES

The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, α, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M ☉), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the 13C(α, n)16O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ~ 1.0 × 108 K, with typical densities of (106-107) neutrons cm–3. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the 13C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ~ 1.5 × 108 K, thus producing larger neutron densities (up to 1011 neutrons cm–3). An additional neutron exposure, caused by the 22Ne(α, n)25Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like 60Fe, as a consequence of the anomalous first 13C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.INAF-Osservatorio Astronomico di Collurania, 64100 Teram o, Italy and R. Gallino2,3 Dipartimento di Fisica Generale, Universitá di Torino, 10 125 Torino, Italy Center for Stellar and Planetary Astrophysics, School of Ma thematical Sciences, Monash University, P.O. Box 28, Victoria 3800, Australia and L. Piersanti 1 INAF-Osservatorio Astronomico di Collurania, 64100 Teram o, Italy and I. Domı́nguez4 Departamento de Fı́sica Teórica y del Cosmos , Universidad de Granada, 18071 Granada, Spain and M.T. Lederer 5 Institut für Astronomie, Türkenschanzstraße 17, A-1180 Wien, Austria Received/ Accepted

Synthetic photometry for carbon rich giants I. Hydrostatic dust-free models ⋆

Context. Carbon rich objects represent an important phase during the late stages of evolution of low and intermediate mass stars. They contribute significantly to the chemical enrichment and to t he infrared light of galaxies. A proper description of their atmospheres is crucial for the determination of fundamental parameters su ch as effective temperature or mass loss rate. Aims. We study the spectroscopic and photometric properties of carbon stars. In the first paper of this series we focus on object s that can be described by hydrostatic models neglecting dynamical phenomena like pulsation and mass loss. As a consequence, the reddening due to circumstellar dust is not included. Our res ults are collected in a database, which can be used in conjunction with stellar evolution and population synthesis calculations i nvolving the AGB. Methods. We have computed a grid of 746 spherically symmetric COMARCSatmospheres covering effective temperatures between 2400 and 4000 K, surface gravities from log(g [cm/s 2 ]) = 0.0 to−1.0, metallicities ranging from the solar value down to one tenth of it and C/O ratios in the interval between 1.05 and 5.0. Subsequently, we used these models to create synthetic low resolution spectra and photometric data for a large number of filter systems. The tables including the results are electronically available . First tests of the application on stellar evolution calculations are show n. Results. We have selected some of the most commonly used colours in order to discuss their behaviour as a function of the stellar parameters. A comparison with measured data shows that down to 2800 K the agreement between predictions and observations of carbon stars is good and our results may be used to determine quantities like the effective temperature. Below this limit the synthetic colours are much too blue. The obvious reason for these problems is the neglect of circumstellar reddening and structura l changes due to pulsation and mass loss. Conclusions. The warmer carbon stars with weak pulsation can be successfully described by our hydrostatic models. In order to include also the cooler objects with intense variations, at least a proper treatment of the reddening caused by the dusty envelopes is needed. This will be the topic of the second paper of this seri es.

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 (AGB) stars. At low metallicity, the enhancement of C and/or N can 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. Based on up-to-date stellar models, we illustrate the changes induced by the use of these opacity coefficients on the physical and chemical properties expected for these stars.

Low temperature Rosseland opacities with varied abundances of carbon and nitrogen

Context. With certain assumptions, radiative energy transport can be modelled by the diffusion approximation. In this case, the Rosseland mean opacity coefficient characterises the interaction between radiation and matter. The opacity data are usually available in pre-tabulated form, and in the generation of the data one assumes a distinct heavy element mixture, which is usually a scaled solar one. Therefore, presently available data is unable to cover the full parameter range of some astrophysical problems, in which the chemical composition of the medium being considered varies. Aims. We attempt to produce low temperature opacity data incorporating the effects of varied abundances of the elements carbon and nitrogen. For our temperature range of interest, molecules represent the dominant opacity source. Our dataset covers a wide metallicity range and is meant to provide important input data for stellar evolution models and other applications. Methods. We conduct chemical equilibrium calculations to evaluate the partial pressures of neutral atoms, ions, and molecules. Based on a large dataset containing atomic line and continuum data and, most importantly, a plethora of molecular lines, we calculate Rosseland mean opacity coefficients not only for a number of different metallicities, but also for varied abundances of the isotopes 12 C and 14 N at each metallicity. The molecular data comprise the main opacity sources for either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Results. Due to the special role of the CO molecule, within a certain chemistry regime an alteration to the carbon abundance causes considerable changes in the Rosseland opacity. The transition from a scaled solar (i.e. oxygen-rich) mixture to a carbon-rich regime results in opacities that can, at low temperatures, differ by orders of magnitude from to the initial situation. The reason is that the mean opacity in either case is due to different molecular absorbers. Variations in the abundance of nitrogen have less pronounced effects but, nevertheless, cannot be neglected. Conclusions. In typical astrophysical applications, it is indispensable to take into account opacity variations due to chemistry changes. In this respect, the new data is superior to previous compilations, but is, however, still subject to uncertainties.

AGB stars of the intermediate-age LMC cluster NGC 1846 - II. Dredge up along the AGB

Aims. We investigate the change in the surface abundance of 12 C during the evolution along the AGB, aiming to constrain third dredge-up models. Methods. High-resolution, near-infrared spectra of a sample of AGB stars in the LMC cluster NGC 1846 were obtained. A cluster sample ensures a high level of homogeneity with respect to age, metallicity, and distance. The C/O ratio and the ratio of 12 C/ 13 Cw ere measured and compared with our evolutionary models. Results. For the first time, we show the evolution of the C/ Oa nd 12 C/ 13 C ratios along a cluster AGB. Our findings allow us to check the reliability of the evolutionary models and, in particular, the efficiency of the third dredge up. The increase in both C/ Oa nd 12 C/ 13 C in the observed O-rich stars is reproduced by the models well. However, the low carbon isotopic ratios of the two C-stars in our sample indicate the late occurrence of moderate extra mixing. The extra mixing affects the most luminous AGB stars and is capable of increasing the abundance of 13 C, while leaving unchanged the C/O ratio, which has been fixed by the cumulative action of several third dredge-up episodes. We find indications that the F abundance also increases along the AGB, supporting an in situ production of this element.

Synthetic photometry for carbon-rich giants: II. The effects of pulsation and circumstellar dust

Context. Red giant stars approaching the end of the evolutionary phase of the asymptotic giant branch (AGB) are, inter alia, characterised by (i) pulsations of the stellar interiors; and (ii) the development of dusty stellar winds. Therefore, such very evolved objects cannot be adequately described with hydrostatic dust-free model atmospheres. Aims. By using self-consistent dynamic model atmospheres which simulate pulsation-enhanced dust-driven winds we studied in detail the influence of the above mentioned two effects on the spectral appearance of long period variables with carbon-rich atmospheric chemistry. While the pulsations lead to large-amplitude photometric variability, the dusty envelopes (resulting from the outflows which contain dust particles composed of amorphous carbon) cause pronounced circumstellar reddening. Methods. Based on one selected dynamical model which is representative of C-type Mira variables with intermediate mass loss rates, we calculated synthetic spectra and photometry for standard broad-band filters (Johnson-Cousins-Glass system) from the visual to the near-infrared. The synthetic photometry was subsequently compared with observational results. Results. Our modelling allows to investigate in detail the substantial effect of circumstellar dust on the resultant photometry. The pronounced absorption of amorphous carbon dust grains (increasing towards shorter wavelengths; Qabs/a ∝ λ −β with β ≈ 1), leads to colour indices which are significantly redder than the corresponding ones based on hydrostatic dust-free models. Only if we account for this circumstellar reddening we get synthetic colours that are comparable to observations of evolved AGB stars. The photometric variations of the dynamical model were compared to observed lightcurves of the C-type Mira RU Vir which appears to be quite similar to the model (although the model is not a dedicated fit). We found good agreement concerning the principal behaviour of the BVRIJHKL lightcurves and also quantitatively fitting details (e.g. magnitude ranges, the amplitude decrease from visual to NIR, absolute magnitudes). The analysed model is able to reproduce the variations of RU Vir and other Miras in (J −H )v s. (H −K )d iagrams throughout the light cycle (ranges, loops). Contrasting the model photometry with observational data for a variety of galactic C-rich giants in such colour-colour diagrams proved that the chosen atmospheric model fits well into a sequence of objects with increasing mass loss rates, i.e., redder colour indices. Conclusions. The comparison of our synthetic photometry with observational results provides a further indication that the applied dynamic model atmospheres represent the outer layers of pulsating and mass-losing C-rich AGB stars reasonably well.

Low-mass lithium-rich AGB stars in the Galactic bulge: evidence for Cool Bottom Processing? ?

Context. The stellar production of the light element lithium is still a matter of debate. Aims. We report the detection of low-mass, Li-rich Asymptotic Giant Branch (AGB) stars located in the Galactic bulge. Methods. A homogeneous and well-selected sample of low mass, oxygen-rich AGB stars in the Galactic bulge has been searched for the absorption lines of Li. Using spectral synthesis techniques, we determine from high resolution UVES/VLT spectra the Li abundance in four out of 27 sample stars, and an upper limit for the remaining stars. Results. Two stars in our sample have a solar Li abundance or above; these stars seem to be a novelty, since they do not show any s-element enhancement. Two more stars have a Li abundance slightly below solar; these stars do show s-element enhancement in their spectra. Different scenarios which lead to an increased Li surface abundance in AGB stars are discussed. Conclusions. Of the different enrichment scenarios presented, Cool Bottom Processing (CBP) is the most likely one for the Li-rich objects identified here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass companion into the stars’ envelope is unlikely because the associated additional effects are lacking. Mass transfer from a former massive binary companion is a possible scenario, if the companion produced little s-process elements. A simple theoretical estimation for the Li abundance due to CBP is presented and compared to the observed values.

The puzzling dredge-up pattern in NGC 1978

Context. Low-mass stars are element factories that effi ciently release their products in the final stages of their ev olution by means of stellar winds. Since they are large in number, they contri bute significantly to the cosmic matter cycle. To assess this contribution quantitatively, it is crucial to obtain a detailed picture o f the stellar interior, particularly with regard to nucleos ynthesis and mixing mechanisms. Aims. We seek to benchmark stellar evolutionary models of low-mass stars. In particular, we measure the surface abundance of 12 C in thermally pulsing AGB stars with well-known mass and metallicity, which can be used to infer information about the ons et and effi ciency of the third dredge-up. Methods. We recorded high-resolution near-infrared spectra of AGB stars in the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich stars, and is well-constraine d in terms of the stellar mass, metallicity, and age. We deriv ed the C/O and 12 C/ 13 C ratio from the target spectra by a comparison to synthetic s pectra. Then, we compared the outcomes of stellar evolutionary models with our measurements. Results. The M stars in NGC 1978 show values of C/O and 12 C/ 13 C that can best be explained with moderate extra-mixing on the RGB coupled to a moderate oxygen enhancement in the chemical composition. These oxygen-rich stars do not seem to have undergone third dredge-up episodes (yet). The C stars show carbon-to- oxygen and carbon isotopic ratios consistent with the occurrence of the third dredge-up. We did not find S stars in this cluster. None o f the theoretical schemes that we considered was able to reproduce the observations appropriately. Instead, we discuss some non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.

Abundance analysis for long period variables: Velocity effects studied with O-rich dynamic model atmospheres

Context. Measuring the surface abundances of AGB stars is an important tool for studying the effects of nucleosynthesis and mixing in the interior of low- to intermediate mass stars during their final evolutionary phases. The atmospheres of AGB stars can be strongly affected by stellar pulsation and the development of a stellar wind, though, and the abundance determination of these objects should therefore be based on dynamic model atmospheres. Aims. We investigate the effects of stellar pulsation and mass loss on the appearance of selected spectral features (line profiles, line intensities) and on the derived elemental abundances by performing a systematic comparison of hydrostatic and dynamic model atmospheres. Methods. High-resolution synthetic spectra in the near infrared range were calculated based on two dynamic model atmospheres (at various phases during the pulsation cycle) as well as a grid of hydrostatic COMARCS models with effective temperatures Teff and surface gravities log g over an adequate range. Equivalent widths of a selection of atomic and molecular lines (Fe, OH, CO) were derived in both cases and compared with each other. Results. In the case of the dynamic models, the equivalent widths of all investigated features vary over the pulsation cycle. A consistent reproduction of the derived variations with a set of hydrostatic models is not possible, but several individual phases and spectral features can be reproduced well with the help of specific hydrostatic atmospheric models. In addition, we show that the variations in equivalent width that we found on the basis of the adopted state-of-the-art dynamic model atmospheres agree qualitatively with observational results for the Mira R Cas over its light cycle. Conclusions. The findings of our modelling form a starting point to deal with the problem of abundance determination in strongly dynamic AGB stars (i.e., long-period variables). Our results illustrate that some quantities such as the C/O ratio can probably still be determined to a reasonable accuracy, but the measurement of other quantities will be hampered by the dynamics. The qualitative agreement with observations of R Cas opens promising possibilities for a forthcoming quantitative comparison of our synthetic spectra with observed ones of AGB variables in the globular cluster 47 Tuc.

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.