Spectral analysis by means of Non-LTE model-atmosphere techniques has arrived at a high level of sophistication: fully line-blanketed model atmospheres which consider opacities of all elements from H to Ni allow the reliable determination of photospheric parameters of hot, compact stars. Such models provide a crucial test of stellar evolutionary theory: recent abundance determinations of trace elements like, e.g., F, Ne, Mg, P, S, Ar, Fe, and Ni are suited to investigate on AGB nucleosynthesis. E.g., the strong Fe depletion found in hydrogen-deficient post-AGB stars is a clear indication of an efficient s-process on the AGB where Fe is transformed into Ni or even heavier trans iron-group elements. We present results of recent spectral analyses based on high-resolution UV observations of hot stars.
We study the formation of Li lines in a case study of an S-type AGB star in the SMC. AGB stars may be important providers of $^7$Li to the interstellar medium. Abundance analysis of these stars is difficult due to the heavy molecular blanketing, uncertainties in the model atmospheres and possible departures from LTE in the Li atomic level populations. We address here the latter problem and somewhat also that of background molecular line formation. We do not find non-LTE effects big enough to alone rule out the possibility that the reported super-Li-rich stars exist. There are, however, several…
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet (UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is performed, using non-local thermodynamic equilibrium (non-LTE) model atmospheres to estimate stellar atmospheric parameters and chemical composition. Abundances are derived for C, N, O, Mg, Si and S, and compared with those found previously by Moehler et al. We find a general metal underabundance relative to young B-type stars, consistent with the average metallicity …
We present circular polarization measurements of circumstellar water masers around 2 Mira variables and 4 supergaint stars. Both an LTE and non-LTE method are used to analyze the circular polarization and total intensity spectra and to obtain the magnetic field strength. The non-LTE method is found to be able to reproduce the observations best. Using this, we find fields from a few hundred milliGauss up to a few Gauss, indicating a solar-type r^(-2) dependence of the magnetic field on the distance, which leads to stellar surface magnetic fields of up to several hundred Gauss. No linear polariz…
New SiO multi-transition millimetre line observations of a sample of carbon stars, including J=8-7 observations with the APEX telescope, are used to probe the role of non-equilibrium chemistry and the influence of grains in circumstellar envelopes of carbon stars. A detailed radiative transfer modelling, including the effect of dust emission in the excitation analysis, of the observed SiO line emission is performed. A combination of low- and high-energy lines are important in constraining the abundance distribution. It is found that the fractional abundance of SiO in these C-rich environments …
With the ongoing AKARI infrared sky survey, of much greater sensitivity than IRAS, a wealth of post-AGB objects may be discovered. It is thus time to organize our present knowledge of known post-AGB stars in th galaxy with a view to using it to search for new post-AGB objects among AKARI sources. We searched the literature available on the NASA Astrophysics Data System up to 1 October 2006, and defined criteria for classifying sources into three categories: "very likely", "possible" and "disqualified" post-AGB objects. The category of "very likely" post-AGB objects is made up of several classe…
The `O-rich AGB sequence' is a sequence of colours describing the location of O-rich AGB stars in the IRAS two-colour diagram [12]--[25] vs [25]--[60]. We propose an evolutionary scenario for this sequence in which all stars, independent of their progenitor mass, start the AGB phase in the blue part of the `O-rich AGB sequence' and then evolve toward redder colors, although only the more massive stars would reach the very end of the `O-rich AGB sequence'. The sources located in the blue part of the sequence are mainly Mira variables, whose mean period is increasing with the IRAS colours. Most …
A self-consistent spectrophotometric modelling of intermediate age post-starburst requires accurate stellar ingredients taking into account a principal feature of the stars dominating the near-IR emission during this phase: the variability of the AGB stars. A new library of stellar spectra based on averages of the empirical spectra of variable AGB stars is presented. This library is designed for convenient use in the population synthesis models. We discuss meaningful ways to compute these averages, and the non-trivial connection with the theoretical stellar parameters. Our library covers the n…
Two evolutionary scenarios are proposed for the formation of extreme helium stars: a post-AGB star suffering from a late thermal pulse, or the merger of two white dwarfs. An identification of the evolutionary channel for individual objects has to rely on surface abundances. We present preliminary results from a non-LTE analysis of CNO, Mg and S for two unique objects, V652 Her and HD144941. Non-LTE abundance corrections for these elements range from negligible values to ~0.7 dex. Non-LTE effects typically lead to systematic shifts in the abundances relative to LTE and reduce the uncertainties.
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agree…
A parameter study of the spectral evolution of a typical post-AGB star, with particular emphasis on the evolution of the infrared colours, is presented. The models are based on the latest evolutionary tracks for hydrogen burning post-AGB stars. For such tracks the evolutionary rate is very dependent on the assumed mass loss rate as a function of time. We investigate this effect by modifying the mass loss prescription. The newly calculated evolutionary rates and density distributions are used to model the spectral evolution of a post-AGB star with the photo-ionization code Cloudy, including dus…
The formation of lithium lines in the atmosphere of C-rich giants is discussed. LTE and NLTE approximations are used to model lithium lines in the spectra of super Li-rich AGB stars. The system of equations of the statistical balance of neutral Li in plane-parallel model atmospheres is solved for a 20-level atom model. JOLA and line-by-line models of molecular absorption are used to compute synthetic spectra as well as the opacity in the frequencies of bound-bound and bound-free transitions of the lithium lines. Curves of growth and synthetic spectra are computed in LTE and NLTE for several mo…
The chemical compositions of the C-rich pAGB stars IRAS 05113+1347 and IRAS 22272+5424 are determined from high-resolution optical spectra using standard LTE model atmosphere-based techniques. The stars are C, N, and $s$-process enriched suggesting efficient operation of the third-dredge up in the AGB star following a first dredge-up that increased the N abundance. Lithium is present with an abundance requiring Li manufacture. With this pair, abundance analyses are now available for 11 C-rich pAGBs. A common history is indicated and, in particular, the $s$-abundances, especially the relative a…
We have discovered lines of highly ionized fluorine (Fv and Fvi) in the far-UV spectra of extremely hot (Teff=85,000--150,000 K) post-AGB stars. Our sample comprises H-rich central stars of planetary nebulae as well as H-deficient PG1159 stars. We performed non-LTE calculations and find strong F overabundances (up to 10**-4 by mass, i.e., 250 times solar) in a number of PG1159 stars, while F is essentially solar in the H-rich stars. Since PG1159 stars are believed to exhibit intershell matter of the preceding AGB phase on their surface, their chemical analyses allow for a direct insight into n…
We have obtained medium-resolution spectra of seven UV-bright stars discovered on images of four southern globular clusters obtained with the Ultraviolet Imaging Telescope (UIT). Effective temperatures, surface gravities and helium abundances are derived from LTE and non-LTE model atmosphere fits. Three of the stars have sdO spectra, including M4-Y453 (Teff = 58800 K, log g = 5.15), NGC 6723-III60 (Teff = 40600 K, log g = 4.46) and NGC 6752-B2004 (Teff = 37000 K, log g = 5.25). All seven stars lie along either post-extended horizontal branch (EHB) or post-early AGB evolutionary tracks. The pos…
Recent calculations of evolutionary tracks of TP-AGB stars of different mass and metallicity by Marigo et al. (2007) have been incorporated in the Bruzual & Charlot evolutionary population synthesis models. The mass of the stellar population in HUDF galaxies at z from 1 to 3 determined from fits to the spectro-photometric data of these galaxies is 50 to 80% lower than the mass determined from the BC03 models. The ages inferred for these populations are, with exceptions, 40 to 60% of the BC03 estimates.
We review the observed properties of extremely hot hydrogen-deficient post-AGB stars of spectral type [WC] and PG1159. Their H-deficiency is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse, laying bare interior stellar regions which are usually kept hidden below the hydrogen envelope. Thus, the photospheric element abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We summarize the state-of-the-art of stellar evolution models which simulate AGB evolution and the occurrence of a l…
We have used a detailed non-LTE radiative transfer code to model new APEX CO(J=3-2) data, and existing CO radio line data, on a sample of 40 AGB S-stars. The derived mass-loss-rate distribution has a median value of 2E-7 Msun/yr, and resembles values obtained for similar samples of M-stars and carbon stars. Possibly, there is a scarcity of high-mass-loss-rate (>=1E-5 Msun/yr) S-stars. The distribution of envelope gas expansion velocities is similar to that of the M-stars, the median is 7.5 km/s, while the carbon stars, in general, have higher gas expansion velocities. The mass-loss rate correl…
A unified evolutionary scheme that includes post-AGB systems, barium stars, symbiotics, and related systems, explaining their similarites as well as differences. Can we construct it? We compare these various classes of objects in order to construct a consistent picture. Special attention is given to the comparison of the barium pollution and symbiotic phenomena. Finally, we outline a `transient torus' evolutionary scenario that makes use of the various observational and theoretical hints and aims at explaining the observed characteristics of the relevant systems.
Based on evolutionary spectral synthesis models explicitly including the spectra of variable AGB stars, we select near-IR features that dentify the strong O-rich or C-rich AGB contributions to the near-IR light of post-starburst populations. AGB temperature scales and lifetimes remain major sources of uncertainties. We discuss applications and suggest massive post-starburst clusters as prime targets for observational tests.
This is the fourth in a series of papers that deal with angular momentum transport by internal gravity waves in stellar interiors. Here, we want to examine the potential role of waves in other evolutionary phases than the main sequence. We study the evolution of a 3Msun Population I model from the pre-main sequence to the early-AGB phase and examine whether waves can lead to angular momentum redistribution and/or element diffusion at the external convection zone boundary. We find that, although waves produced by the surface convection zone can be ignored safely for such a star during the main …
This paper is designed to explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially 2.7 M_sun star from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshooting, salt fingers and convection is considered. We employed the double-diffusive mixing-le…
The `O-rich AGB sequence' is a well defined area occupied by oxygen-rich AGB stars (OH/IR) in the IRAS two-colour diagram [12]-[25] vs [25]-[60]. This sequence is interpreted in terms of evolutionary stage and/or progenitor mass with the aim to link observed classes of AGB stars and Planetary Nebulae (PNe).
Two high galactic latitude B-type stars, PG1323-086 and PG1704+222, are analysed from low and medium resolution optical spectra and Stroemgren photometry. A differential abundance analysis for He, C, N, O, Mg, Al, and Si reveals that the He abundance is close to solar while metal underabundances relative to the solar value of typically 1.3 and 1.2 dex for PG1323-086 and PG1704+222, respectively, are found. For both stars, carbon is even more depleted. The atmospheric parameters are consistent with evolutionary tracks for stars evolving from the asymptotic giant branch (AGB) with a stellar mass…
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/$^{13}$C were measured and compared with our evolutionary models. Results: For the first time, we show the evolution of the C/O and $^{12}$C/$^{13}$C ratios along a cluster AGB. Our findings all…
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative a…
We have performed high spatial resolution observations of SiO line emission for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers. Detailed radiative transfer modelling suggests that there are steep chemical gradients of SiO in their circumstellar envelopes. The emerging picture is one where the radial SiO abundance distribution starts at an initial high abundance, in the case of M-stars consistent with LTE chemistry, that drastically decreases at a radius of ~1E15 cm. This is consistent with a scenario where SiO freezes out onto dust grains. The region of the wind with low …
Until recently synthetic AGB models had not taken into account the break-down of the core mass-luminosity (Mc-L) relation due to the occurrence of envelope burning in the most massive (M > 3.5 Msun) and luminous (Mbol > -6) stars. Marigo et al. (1998) made the first attempt to consistently include the related over-luminosity effect (i.e. above the Mc-L relation) in synthetic TP-AGB calculations. In this paper the reliability of the solution scheme is tested by comparison with the results of complete evolutionary calculations for a 7 Msun AGB star undergoing envelope burning (e.g. Bloecker & Sc…
The aim of this work is to use full evolutionary models to derive observational constraints on the mass loss rate of the upper Asymptotic Giant Branch (AGB) stars. The observations used to constrain the models are the relative number of luminous Lithium rich AGBs in the Magellanic Clouds and the s-process enhancement of the same sample. We find that we can put lower and upper limits to the mass loss rate during the AGB phase. The mass loss calibation obtained in this work implies that massive AGBs do not contribute significantly to the lithium enrichment of the interstellar medium.
Planetary nebulae (PN) represent the evolutionary fate of the asymptotic giant branch (AGB) stellar envelopes, thus are ideally suited to study the chemical impact of AGB stars. Stellar evolution predict elemental enrichment through the AGB evolution, and convective dredge-up episodes allow the products of stellar evolution to reach the stellar outer layers. Planetary nebulae are probes of these processes, and are also probes of the environment at the time of formation of their progenitors, through the elements not affected by AGB evolution. Ultimately PN may be used to test AGB stars as actor…
We reexamine the theoretical instability domain of pulsating PG1159 stars (GW Vir variables). We performed an extensive g-mode stability analysis on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG1159 stars have been considered. We found that pulsations in PG1159 stars are excited by the kappa-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the s…
Using objects found in a systematic survey of the galactic Plane in the 1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution, a red and a blue. We argue that the red and the blue groups separate by initial mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy distributions, outflow velocities and scaleheight. The higher-mass (blue) objects may have earlier AGB termination. The lower-mass (red) objects undergo very sudden reddening for IRAS colour R21\sim1.2; these sources must all undergo a very similar process at AGB termination. The transit…
We present a summary of the main sites for nucleosynthesis in intermediate mass Asymptotic Giant Branch (AGB) stars. We then discuss some detailed evolutionary models and how these have been used to create a synthetic evolution code which calculates the nucleosynthesis very rapidly, enabling us to investigate changes in some uncertain parameters in AGB evolution, such as mass-loss and dredge-up. We then present results for C, C/O, Mg and Al. We also discuss the changes due to the recent NACRE compilation of reaction rates.
95% of all stars end their lives as white dwarfs. About 20% of the hot post-AGB stars are hydrogen deficient. Most of these are the result of a late helium-shell flash, but the evolutionary status of a fraction of about 10-20% of the hottest hydrogen-deficient stars, namely four O(He) stars, is as yet unexplained. They could be the long-searched hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double degenerate merging process. More generally, understa…
We analyze the robustness of H--deficient post--AGB tracks regarding previous evolution of their progenitor stars and the constitutive physics of the remnants. Our motivation is a recent suggestion of Werner & Herwig (2006) that previous evolution should be important in shaping the final post--AGB track and the persisting discrepancy between asteroseismological and spectroscopical mass determinations. This work is thus complementary to our previous work (Miller Bertolami & Althaus 2006) and intends to shed some light on the uncertainty behind the evolutionary tracks presented there. We compute…
We present a critical analysis of the nature of the so-called Li-rich RGB stars. For a majority of the stars, we have used Hipparcos parallaxes to determine masses and evolutionary states by comparing their position on the Hertzsprung-Russell diagram with theoretical evolutionary tracks. Among the twenty Li-rich giants whose location on the HR diagram we were able to determine precisely, eight should be re-classified as Li-normal. For the remaining stars, the high Li abundance must be a result of fresh synthesis of this fragile element. We identify two distinct episodes of Li production which …
I discuss recent new models of post-Asymptotic Giant Branch stellar evolution. These models aim to clarify the evolutionary origin and status of a variety of hydrogen-deficient post-AGB stars such as central stars of planetary nebulae of Wolf-Rayet spectral type, PG1159 stars or Sakurai's object. Starting with AGB models with overshoot such stars can evolve through one of four distinct channels. Each of these channels has typical abundance patterns depending on the relative timing of the departure from the AGB and the occurrence of the last thermal pulse. I discuss the responsible mechanisms a…
We present the results of mid-resolution spectroscopy in the LiI 6708 AA spectral region of Asymptotic Giant Branch (AGB) stars belonging to young open clusters of the Large Magellanic Cloud. Most stars belong to the clusters NGC 1866 and NGC 2031, which have an age of ~ 150 Myr. Lithium lines of different strength are detected in the spectra of stars evolving along the AGB, not always in agreement with theoretical predictions. We also analyze the infrared luminosities (ISOCAM data) of these stars, to discuss if their evolutionary phase precedes or follows the lithium production stage.
Adpoting new s-process nucleosynthesis scenario and branch s-process path, we calculate the heavy-element abundances and C/O ratio of solar metallicity 3M_sun TP-AGB stars. The evolutionary sequence from M to S to C stars of AGB stars is explained naturally by the calculated results. Then combining the angular momentum conservation model of wind accretion with the heavy-element abundances on the surface of TP-AGB stars, we calculate the heavy-element overabundances of barium stars via successive pulsed accreting and mixing. Our results support that the barium stars with longer orbital period, …
In this letter, we report on our successful systematic search for lines of elements beyond the Ba-peak in spectra of s-process enriched post-AGB stars. Using newly released atomic data from both the VALD database and the D.R.E.A.M. project, we could derive abundances for several elements heavier than europium for three objects, on the base of high quality VLT+UVES spectra. The abundances of these elements are of particular interest since they turn out to be powerful constraints for chemical evolutionary AGB models. Their high abundances indicate that, also in only moderately metal deficient AG…
Spectral and photometric observations of nearby galaxies show a correlation between the strength of their mid-IR aromatic features, attributed to PAH molecules, and their metal abundance, leading to a deficiency of these features in low-metallicity galaxies. In this paper, we suggest that the observed correlation represents a trend of PAH abundance with galactic age, reflecting the delayed injection of carbon dust into the ISM by AGB stars in the final post-AGB phase of their evolution. AGB stars are the primary sources of PAHs and carbon dust in galaxies, and recycle their ejecta back to the …
We searched for Technetium (Tc) in a sample of bright oxygen-rich asymptotic giant branch (AGB) stars located in the outer galactic bulge. Tc is an unstable element synthesised via the s-process in deep layers of AGB stars, thus it is a reliable indicator of both recent s-process activity and third dredge-up. We aim to test theoretical predictions on the luminosity limit for the onset of third dredge-up. Using high resolution optical spectra obtained with the UVES spectrograph at ESO's VLT we search for resonance lines of neutral Tc in the blue spectral region of our sample stars. These measur…
A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 $μ$m. High-resolution (R$\sim 50000$) and high signal to noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon rich stars. Using spectral synthesis in LTE we derive for these stars fluorine abundances that are systematically lower by $\sim 0.8$ dex in average with re…
We present here a study of the Infrared properties of Asymptotic Giant Branch stars (hereafter AGB) based on existing databases, mainly from space-borne experiments. Preliminary results about C and S stars are discussed, focusing on the topics for which future Infrared surveys from Antarctica will be crucial. This kind of surveys will help in making more quantitative our knowledge of the last evolutionary stages of low mass stars, especially for what concerns luminosities and mass loss.
We present ground-based mid-infrared imaging for 27 M-, S- and C-type Asymptotic Giant Branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $\dot M$, for improving the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are obtained through distance compilations, and by applying previously-derived bolometric corrections; the variability is also studied, using data accumu…
We construct a data base of 125 post-AGB objects (including R CrB and extreme helium stars) with published photospheric parameters (effective temperature and gravity) and chemical composition. We estimate the masses of the post-AGB stars by comparing their position in the (log Teff, log g) plane with theoretical evolutionary tracks of different masses. We construct various diagrams, with the aim of finding clues to AGB nucleosynthesis. This is the first time that a large sample of post-AGB stars has been used in a systematic way for such a purpose and we argue that, in several respects, post-A…
The evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars. We focus on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars. It is shown that overshoot applied to all convection regions is a key ingredient to model these objects leading to intershell abundances already close to the surface abundances of Wolf-Rayet central stars. In contrast to standard evolutionary calculations, overshoot models do show dredge up for very low envelope masses and efficient dredge up was found even during the post-AGB stage. …
I present a short description of the STELIB, HNGSL, IndoUS, MILES, ELODIE, UVES-POP, and IRTF libraries of empirical stellar spectra and show some applications of their use in population synthesis models. When new calculations of the TP-AGB evolutionary phase for stars of different mass and metallicity are included in population synthesis models, the stellar mass in galaxies at z from 1 to 3 determined from spectro-photometric data can be up to 50% lower than the mass determined from the BC03 models. The ages inferred for these populations are considerably lower than the BC03 estimates.
We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000). Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the …
We present new evolutionary synthesis models for Single Stellar Populations covering a wide range in age and metallicity. The most important difference with existing models is the use of NLTE atmosphere models for the hot stars (O, B, WR, post-AGB stars, and planetary nebulae) that have an important impact in the stellar cluster's ionizing spectra.
Based on evolutionary computations of 90 stellar models, we have analysed the impact of initial composition and core overshooting on the post-He-burning evolution and the associated nucleosynthesis of Super-AGB stars, pointing particular attention on the C-burning phase. Moreover the possible link between the transition masses $M_{up}$, $M_{N}$ and $M_{mas}$ (defined as the critical initial mass above which C-burning ignites, the minimum initial mass for an electron-capture supernova and the minimum initial mass for the completion of all the nuclear burning phases respectively) and the propert…
Analytic formulae are presented to construct detailed secular lightcurves of both early asymptotic giant branch (AGB) and thermally pulsing AGB stars. They are based on an extensive grid of evolutionary calculations, performed with an updated stellar evolution code. Basic input parameters are the initial mass between 0.8 and 7 solar mass, metallicity of 0.0001, 0.008, 0.02, and the mixing length theory (MLT) parameter. The formulae allow for two important effects, namely that the first pulses do not reach the full amplitude, and hot bottom burning (HBB) in massive stars, which are both not acc…
New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including the effect of thermal dust grains in the excitation analysis. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical…
Despite its mean apparent magnitude of m(v)=6.39, the evolutionary status of HD190390 (HR7671), a luminous F-type supergiant at high galactic latitude, is still not very clear, but in most papers a post-AGB classification is assumed. New observational material has been obtained with four different instruments and is presented here. An extensive abundance analysis based on high resolution, high signal-to-noise NTT+EMMI spectra confirms the metal deficiency of this object ([Fe/H]=-1.6), together with a high lithium content (log(Li)=1.9). A variability analysis based on Geneva photometry over sev…
We present a model for the structure, temporal behavior, and evolutionary status of the bipolar nebula M2-9. According to this model the system consists of an AGB or post-AGB star and a hot white dwarf companion, with an orbital period of about 120 years. The white dwarf has undergone a symbiotic nova eruption about 1200 years ago, followed by a supersoft x-ray source phase. The positional shift of the bright knots in the inner nebular lobes is explained in terms of a revolving ionizing source. We show that the interaction between the slow, AGB star's wind, and a collimated fast wind from the …
We present a new set of theoretical evolutionary synthesis models, PopStar. This grid of Single Stellar Populations covers a wide range in both, age and metallicity. The models use the most recent evolutionary tracks together with the use of new NLTE atmosphere models for the hot stars (O, B, WR, post-AGB stars, planetary nebulae) that dominate the stellar cluster's ionizing spectra. The results of the models in VO format can be used through VOSpec.
Late asymptotic giant branch (AGB) and early post-AGB stars which are progenitors of planetary nebulae lose mass at extremely high rate, in what is termed a superwind. We show that the existence of this superwind during the post-AGB phase cannot be explained with models where the stellar effective temperature is the sole main physical parameter which determine the mass loss rate. Instead, we argue that the envelope structure, in particular the entropy and density gradients, should be among the main parameters which determine the mass loss rate on the tip of the AGB and the early post-AGB evolu…
Contrary to previous expectations, recent evolutionary models of zero-metallicity stars show that the development of mixing episodes at the beginning of the AGB phase allows low- and intermediate-mass stars to experience thermal pulses. If these stars, like their metal-rich counterparts, also experience partial mixing of protons from the H-rich envelope into the C-rich layers at the time of the third dredge-up, an extensive neutron capture nucleosynthesis leads to the production of s-process nuclei up to Pb and Bi. Nucleosynthesis calculations based on stellar AGB models are performed assuming…
Aims: We investigated the possible relationship between the evolutionary stage of post-AGB stars and planetary nebulae (PNe) and the presence of water masers in their envelopes. Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to search for the water maser transition at 22235.08 MHz, towards a sample of 105 sources with IRAS colour characteristic of post-AGB stars and PNe at declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15% PNe or PN candidates, while only 2% seem to be HII regions. Results: We have detected five water masers, of which four a…
Detailed atmospheric abundances have been calculated for a sample of A-G supergiant stars with IR fluxes and/or high galactic latitudes. HD 172481 and HD 158616 show clear indications of being post-AGB stars that have experienced third dredge-up. HD 158616 is carbon-rich while the abundance pattern of HD 172481 and its large Li enhancement gives support to the hot bottom burning scenario that explains paucity of carbon-rich stars among AGB stars. HD 172324 is very likely a hot post-AGB star that shows a strong carbon deficiency. HD 725, HD 218753 and HD 331319 also appear to be evolved objects…
