Thermally-pulsing Asymptotic Giant Branch Phase Research Articles

Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Large Magellanic Cloud

ABSTRACT Reliable models of the thermally pulsing asymptotic giant branch (TP-AGB) phase are of critical importance across astrophysics, including our interpretation of the spectral energy distribution of galaxies, cosmic dust production, and enrichment of the interstellar medium. With the aim of improving sets of stellar isochrones that include a detailed description of the TP-AGB phase, we extend our recent calibration of the AGB population in the Small Magellanic Cloud (SMC) to the more metal-rich Large Magellanic Cloud (LMC). We model the LMC stellar populations with the trilegal code, using the spatially resolved star formation history derived from the VISTA survey. We characterize the efficiency of the third dredge-up by matching the star counts and the Ks-band luminosity functions of the AGB stars identified in the LMC. In line with previous findings, we confirm that, compared to the SMC, the third dredge-up in AGB stars of the LMC is somewhat less efficient, as a consequence of the higher metallicity. The predicted range of initial mass of C-rich stars is between Mi ≈ 1.7 and 3 M⊙ at Zi = 0.008. We show how the inclusion of new opacity data in the carbon star spectra will improve the performance of our models. We discuss the predicted lifetimes, integrated luminosities, and mass-loss rate distributions of the calibrated models. The results of our calibration are included in updated stellar isochrones publicly available.

22Ne and23Na ejecta from intermediate-mass stars: the impact of the new LUNA rate for22Ne(p, γ)23Na

We investigate the impact of the new LUNA rate for the nuclear reaction $^{22}$Ne$(p,\gamma)^{23}$Na on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim we use the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range $3.0\,M_{\odot} - 6.0\,M_{\odot}$, and metallicities $Z_{\rm i}=0.0005$, $Z_{\rm i}=0.006$, and $Z_{\rm i} = 0.014$. We find that the new LUNA measures have much reduced the nuclear uncertainties of the $^{22}$Ne and $^{23}$Na AGB ejecta, which drop from factors of $\simeq 10$ to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of $^{23}$Na, the uncertainties that still affect the $^{22}$Ne and $^{23}$Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available.

Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes – I. Hydrogen cyanide

We investigate the formation of hydrogen cyanide (HCN) in the inner circumstellar envelopes of thermally pulsing asymptotic giant branch (TP-AGB) stars. A dynamic model for periodically shocked atmospheres, which includes an extended chemo-kinetic network, is for the first time coupled to detailed evolutionary tracks for the TP-AGB phase computed with the COLIBRI code. We carried out a calibration of the main shock parameters (the shock formation radius and the effective adiabatic index) using the circumstellar HCN abundances recently measured for a populous sample of pulsating TP-AGB stars. Our models recover the range of the observed HCN concentrations as a function of the mass-loss rates, and successfully reproduce the systematic increase of HCN moving along the M-S-C chemical sequence of TP-AGB stars, that traces the increase of the surface C/O ratio. The chemical calibration brings along two important implications: i) the first shock should emerge very close to the photosphere, and ii) shocks are expected to have a dominant isothermal character in the denser region close to the star (within ~ 3-4 R), implying that radiative processes should be quite efficient. Our analysis also suggests that the HCN concentrations in the inner circumstellar envelopes are critically affected by the H-H2 chemistry during the post-shock relaxation stages.

The white dwarf population of NGC 6397

NGC 6397 is one of the most interesting, well observed and theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low metallicity progenitors,to determine its age and the percentage of unresolved binaries, and to assess other important characteristics of the cluster, like the slope of the initial mass function, or the fraction of white dwarfs with hydrogen deficient atmospheres. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do this using an up-to-date Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally-pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8+0.50-0.75 Gyr. Larger ages are also compatible with the observed data, but then realistic longer durations of the initial burst of star formation are needed to fit the luminosity function. We conclude that a correct modeling of the white dwarf opulation of globular clusters, used in combination with the number counts of main sequence stars provides an unique tool to model the properties of globular clusters.

THE CORE MASS GROWTH AND STELLAR LIFETIME OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS

We establish new constraints on the intermediate-mass range of the initial-final mass relation by studying white dwarfs in four young star clusters, and apply the results to study the evolution of stars on the thermally pulsing asymptotic giant branch (TP-AGB). We show that the stellar core mass on the AGB grows rapidly from 10% to 30% for stars with $M_{\rm initial}$ = 1.6 to 2.0 $M_\odot$. At larger masses, the core-mass growth decreases steadily to $\sim$10% at $M_{\rm initial}$ = 3.4 $M_\odot$. These observations are in excellent agreement with predictions from the latest TP-AGB evolutionary models in Marigo et al. (2013). We also compare to models with varying efficiencies of the third dredge-up and mass loss, and demonstrate that the process governing the growth of the core is largely the stellar wind, while the third dredge-up plays a secondary, but non-negligible role. Based on the new white dwarf measurements, we perform an exploratory calibration of the most popular mass-loss prescriptions in the literature. Finally, we estimate the lifetime and the integrated luminosity of stars on the TP-AGB to peak at $t$ $\sim$ 3 Myr and $E$ = 1.2 $\times$ 10$^{10}$ $L_\odot$ yr for $M_{\rm initial}$ $\sim$ 2 $M_\odot$ ($t$ $\sim$ 2 Myr for luminosities brighter than the RGB tip at $\log(L/L_{\odot})$ $>$ 3.4), decreasing to $t$ = 0.4 Myr and $E$ = 6.1 $\times$ 10$^{9}$ $L_\odot$ yr for stars with $M_{\rm initial}$ $\sim$ 3.5 $M_\odot$. The implications of these results are discussed with respect to general population synthesis studies that require correct modeling of the TP-AGB phase of stellar evolution.

CALIBRATING STELLAR POPULATION MODELS WITH MAGELLANIC CLOUD STAR CLUSTERS

Stellar population models are commonly calculated using star clusters as calibrators for those evolutionary stages that depend on free parameters. However, discrepancies exist among different models, even if similar sets of calibration clusters are used. With the aim of understanding these discrepancies, and of improving the calibration procedure, we consider a set of 43 Magellanic Cloud (MC) clusters taking age and photometric information from the literature. We carefully assign ages to each cluster based on up-to-date determinations ensuring that these are as homogeneous as possible. To cope with statistical fluctuations, we stack the clusters in five age bins deriving for each of them integrated luminosities and colors. We find that clusters become abruptly red in optical and optical-IR colors as they age from ~0.6 to 1 Gyr, which we interpret as due to the development of a well-populated thermally pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed this detection because of coarser age binnings. Maraston (2005; M05) and Girardi et al. (2010) models predict the presence of a populated TP-AGB at ~0.6 Gyr, with a correspondingly very red integrated color, at variance with the data; Bruzual & Charlot (2003) and Conroy et al. (2009) models run within the error bars at all ages. The discrepancy between the synthetic colors of M05 models and the average colors of MC clusters results from the now obsolete age scale adopted. Finally, our finding that the TP-AGB phase appears to develop between ~0.6-1 Gyr is dependent on the adopted age scale for the clusters and may have important implications for stellar evolution.

New colour–mass-to-light relations: the role of the asymptotic giant branch phase and of interstellar dust

Colour-M/L (mass-to-light) relations are a popular recipe to derive stellar mass in external galaxies. Stellar mass estimates often rely on near infrared (NIR) photometry, considered an optimal tracer since it is little affected by dust and by the "frosting" effect of recent star formation episodes. However, recent literature has highlighted that theoretical estimates of the NIR M/L ratio strongly depend on the modelling of the Asymptotic Giant Branch (AGB) phase. We use the latest Padova isochrones, with detailed modelling of the Thermally Pulsing AGB phase, to update theoretical colour-M/L relations in the optical and NIR and discuss the consequences for the estimated stellar masses in external galaxies. We also discuss the effect of attenuation by interstellar dust on colour-M/L relations in the statistical case of large galaxy samples.

Stellar Population Models

AbstractModelling stellar populations in galaxies is a key approach to gain knowledge on the still elusive process of galaxy formation as a function of cosmic time. In this review, after a summary of the state-of-art, I discuss three aspects of the modelling, that are particularly relevant to massive galaxies, the focus of this symposium, at low and high-redshift. These are the treatment of the Thermally-Pulsating Asymptotic Giant Branch phase, evidences of an unusual Initial Mass Function, and the effect of modern stellar libraries on the model spectral energy distribution.

THE CONTRIBUTION OF TP-AGB AND RHeB STARS TO THE NEAR-IR LUMINOSITY OF LOCAL GALAXIES: IMPLICATIONS FOR STELLAR MASS MEASUREMENTS OF HIGH-REDSHIFT GALAXIES

Using high spatial resolution HST WFC3 and ACS imaging of resolved stellar populations, we constrain the contribution of thermally-pulsing asymptotic giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 um near-infrared (NIR) luminosities of 23 nearby galaxies. The TP-AGB phase contributes as much as 17% of the integrated F160W flux, even when the red giant branch is well populated. The RHeB population contribution can match or even exceed the TP-AGB contribution, providing as much as 21% of the integrated F160W light. The NIR mass-to-light (M/L) ratio should therefore be expected to vary significantly due to fluctuations in the star formation rate over timescales from 25 Myr to several Gyr. We compare our observational results to predictions based on optically derived star formation histories and stellar population synthesis (SPS) models, including models based on the Padova isochrones (used in popular SPS programs). The SPS models generally reproduce the expected numbers of TP-AGB stars in the sample. The same SPS models, however, give a larger discrepancy in the F160W flux contribution from the TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 +/-0.8. This larger offset is driven by the prediction of modest numbers of high luminosity TP-AGB stars at young (<300 Myrs) ages. The best-fit SPS models simultaneously tend to under-predict the numbers and fluxes of stars on the RHeB sequence, typically by a factor of 2.0+/-0.6 for galaxies with significant numbers of RHeBs. Coincidentally, over-prediction of the TP-AGB and under-prediction of the RHeBs result in a NIR M/L ratio largely unchanged for a rapid star formation rate. However, the NIR-to-optical flux ratio of galaxies could be significantly smaller than AGB-rich models would predict, an outcome that has been observed in some intermediate redshift post-starburst galaxies. (Abridged)

The effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest-frame near-infrared galaxy luminosity function

We address the fundamental question of matching the rest-frame K-band luminosity function (LF) of galaxies over the Hubble time using semi-analytic models, after modification of the stellar population modelling. We include the Maraston evolutionary synthesis models, that feature a higher contribution by the Thermally Pulsating - Asymptotic Giant Branch (TP-AGB) stellar phase, into three different semi-analytic models, namely the De Lucia and Blaizot version of the Munich model, MORGANA and the Menci model. We leave all other input physics and parameters unchanged. We find that the modification of the stellar population emission can solve the mismatch between models and the observed rest-frame K-band luminosity from the brightest galaxies derived from UKIDSS data at high redshift. For all explored semi-analytic models this holds at the redshifts - between 2 and 3 - where the discrepancy was recently pointed out. The reason for the success is that at these cosmic epochs the model galaxies have the right age (~1 Gyr) to contain a well-developed TP-AGB phase which makes them redder without the need of changing their mass or age. At the same time, the known overestimation of the faint end is enhanced in the K-band when including the TP-AGB contribution. At lower redshifts (z<2) some of the explored models deviate from the data. This is due to too short merging timescales and inefficient 'radio-mode' AGN feedback. Our results show that a strong evolution in mass predicted by hierarchical models is compatible with no evolution on the bright-end of the K-band LF from z=3 to the local universe. This means that, at high redshifts and contrary to what is commonly accepted, K-band emission is not necessarily a good tracer of galaxy mass.

INTEGRATED STELLAR POPULATIONS: CONFRONTING PHOTOMETRY WITH SPECTROSCOPY

We investigate the ability of spectroscopic techniques to yield realistic star formation histories (SFHs) for the bulges of spiral galaxies based on a comparison with their observed broadband colors. Full spectrum fitting to optical spectra indicates that recent (within ~1 Gyr) star formation activity can contribute significantly to the V-band flux, whilst accounting for only a minor fraction of the stellar mass budget which is made up primarily of old stars. Furthermore, recent implementations of stellar population (SP) models reveal that the inclusion of a more complete treatment of the thermally pulsating asymptotic giant branch (TP-AGB) phase to SP models greatly increases the NIR flux for SPs of ages 0.2-2 Gyr. Comparing the optical--NIR colors predicted from population synthesis fitting, using models which do not include all stages of the TP-AGB phase, to the observed colors reveals that observed optical--NIR colors are too red compared to the model predictions. However, when a 1 Gyr SP from models including a full treatment the TP-AGB phase is used, the observed and predicted colors are in good agreement. This has strong implications for the interpretation of stellar populations, dust content, and SFHs derived from colors alone.

Hierarchical models of high-redshift galaxies with thermally pulsing asymptotic giant branch stars: comparison with observations

In a recent paper we presented the first semi-analytic model of galaxy formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of stellar evolution has been fully implemented. Here we address the comparison with observations, and show how the TP-AGB recipe affects the performance of the model in reproducing the colours and near-IR luminosities of high-redshift galaxies. We find that the semi-analytic model with the TP-AGB better matches the colour-magnitude and colour-colour relations at z ~ 2, both for nearly-passive and for star-forming galaxies. The model with TP-AGB produces star-forming galaxies with red V-K colours, thus revising the unique interpretation of high-redshift red objects as 'red & dead'. We also show that without the TP-AGB the semi-analytic model fails at reproducing the observed colours, a situation that cannot be corrected by dust reddening. We also explore the effect of nebular emission on the predicted colour-magnitude relation of star-forming galaxies, to conclude that it does not play a significant role in reddening their colours, at least in the range of star-formation rates covered by the model. Finally, the rest-frame K-band luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This indicates that the AGN feedback recipe that is adopted to regulate the high-mass end of the luminosity function should be sophisticated to take the effect of the stellar populations into account at high redshifts.

A near-infrared study of AGB and red giant stars in the Leo I dSph galaxy

A near-infrared imaging study of the evolved stellar populations in the dwarf spheroidal galaxy Leo I is presented. Based on JHK observations obtained with the WFCAM wide-field array at the UKIRT telescope, we build a near-infrared photometric catalogue of red giant branch (RGB) and asymptotic giant branch (AGB) stars in Leo I over a 13.5 arcmin square area. The V-K colours of RGB stars, obtained by combining the new data with existing optical observations, allow us to derive a distribution of global metallicity [M/H] with average [M/H] = -1.51 (uncorrected) or [M/H] = -1.24 +/- 0.05 (int) +/- 0.15 (syst) after correction for the mean age of Leo I stars. This is consistent with the results from spectroscopy once stellar ages are taken into account. Using a near-infrared two-colour diagram, we discriminate between carbon- and oxygen-rich AGB stars and obtain a clean separation from Milky Way foreground stars. We reveal a concentration of C-type AGB stars relative to the red giant stars in the inner region of the galaxy, which implies a radial gradient in the intermediate-age (1-3 Gyr) stellar populations. The numbers and luminosities of the observed carbon- and oxygen-rich AGB stars are compared with those predicted by evolutionary models including the thermally-pulsing AGB phase, to provide new constraints to the models for low-metallicity stars. We find an excess in the predicted number of C stars fainter than the RGB tip, associated to a paucity of brighter ones. The number of O-rich AGB stars is roughly consistent with the models, yet their predicted luminosity function is extended to brighter luminosity. It appears likely that the adopted evolutionary models overestimate the C star lifetime and underestimate their K-band luminosity.

Asymptotic-Giant-Branch Models at Very Low Metallicity

AbstractIn this paper we present the evolution of a low-mass model (initial mass M = 1.5 M⊙) with a very low metal content (Z = 5 × 10−5, equivalent to [Fe/H] = –2.44). We find that, at the beginning of the Asymptotic Giant Branch (AGB) phase, protons are ingested from the envelope in the underlying convective shell generated by the first fully developed thermal pulse. This peculiar phase is followed by a deep third dredge-up episode, which carries to the surface the freshly synthesized 13C, 14N and 7Li. A standard thermally pulsing AGB (TP-AGB) evolution then follows. During the proton-ingestion phase, a very high neutron density is attained and the s process is efficiently activated. We therefore adopt a nuclear network of about 700 isotopes, linked by more than 1200 reactions, and we couple it with the physical evolution of the model. We discuss in detail the evolution of the surface chemical composition, starting from the proton ingestion up to the end of the TP-AGB phase.

High-redshift galaxies and the TP-AGB phase

I discuss how galaxy fundamental parameters that are derived by comparing observational data with stellar population models, depend on the adopted models. The emphasis is on the treatment of the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) stellar phase, that is a source of major discrepancy between the various model renditions. In particular, I show that ages and stellar masses of galaxies decrease when models including an empirically-calibrated TP-AGB phase are considered. Illustrative examples are provided for a wide redshift range. Constraints to galaxy formation models need revision on the light of these results.

Evidence for TP‐AGB Stars in High‐Redshift Galaxies, and Their Effect on Deriving Stellar Population Parameters

We explore the effects of using different stellar population models on estimates of star formation histories, ages, and masses of high-redshift galaxies by fitting the SEDs with models by Maraston (hereafter M05) and by Bruzual & Charlot (hereafter BC03). We focus on the thermally pulsing asymptotic giant branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy. In this respect, BC03 models are representative of other models whose treatment of the TP-AGB phase is similar. Moreover, M05 and BC03 models adopt stellar tracks with different assumptions on convective overshooting. For our experiment we use a sample of high-z (1.4 z 2.7) galaxies, for which rest-frame UV spectroscopy and spectroscopic redshifts are available, along with Spitzer IRAC and MIPS photometry from GOODS. The mid-UV spectra of these galaxies exhibit features typical of A- or F-type stars, indicative of ages in the range ~0.2-2 Gyr, when the contribution of TP-AGB stars is expected to be maximum. We find that the TP-AGB phase plays a key role in the interpretation of the Spitzer data, where the rest-frame near-IR is sampled. Generally, M05 models give better fits than BC03 models and indicate systematically lower ages and lower masses (by ~60%, on average). Photometric redshifts derived using M05 models are also in better agreement with the spectroscopic ones, especially when the rest-frame near-IR fluxes from Spitzer IRAC are included in the fit. We argue that the different results are primarily a consequence of the different treatment of the TP-AGB phase, although other differences in the input stellar evolution also contribute. This work provides a first direct evidence for a strong contribution by TP-AGB stars to the SED of galaxies in the high-redshift universe (z ~ 2).

New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations

We present theoretical surface brightness fluctuation (SBF) amplitudes for single-burst stellar populations of young and intermediate age (25 Myr ≤ t ≤ 5 Gyr) and metallicities Z = 0.0003, 0.001, 0.004, 0.008, 0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expected in the Johnson-Cousins (UBVRIJHK) photometric system are provided. We pay attention to the contribution of thermally pulsating asymptotic giant branch (TP-AGB) stars. The sensitivity of the predicted SBF to changes in the mass-loss scenario along the TP-AGB phase is examined. Below 0.6–1 Gyr both optical and near-IR SBF models exhibit a strong dependence on age and mass loss. We also evaluate SBF amplitudes using Monte Carlo techniques to reproduce the random variation in the number of stars experiencing bright and fast evolutionary phases (red giant branch, AGB, TP-AGB). On these grounds we provide constraints on the faintest integrated flux of real stellar populations required to derive reliable and meaningful SBF measurements. We analyze a technique for deriving SBF amplitudes of star clusters from the photometry of individual stars and estimate the uncertainty due to statistical effects, which may impinge on the procedure. The first optical SBF measurements for 11 Large Magellanic Cloud (LMC) star-rich clusters—with ages ranging from a few megayears to several gigayears—are derived using Hubble Space Telescope observations. The measurements are compared to our SBF predictions, providing a good agreement with models of metallicity Z = 0.0001–0.01. Our results suggest that, for TP-AGB stars, a mass loss as a power-law function of the star luminosity is required in order to properly reproduce the optical SBF data of the LMC clusters. Finally, near-IR models have been compared to available data, thus showing that the general trend is well fitted. We suggest how to overcome the general problem of SBF models in reproducing the details of the near-IR SBF measurements of the Magellanic Cloud star clusters.

Probing AGB nucleosynthesis via accurate Planetary Nebula abundances

The elemental abundances of ten planetary nebulae, derived with high accuracy including ISO and IUE spectra, are analysed with the aid of synthetic evolutionary models for the TP-AGB phase. The accuracy on the observed abundances is essential in order to make a reliable comparison with the models. The advantages of the infrared spectra in achieving this accuracy are discussed. Model prescriptions are varied until we achieve the simultaneous reproduction of all elemental features, which allows placing important constraints on the characteristic masses and nucleosynthetic processes experienced by the stellar progenitors. First of all, it is possible to separate the sample into two groups of PNe, one indicating the occurrence of only the third dredge-up during the TP-AGB phase, and the other showing also the chemical signature of hot-bottom burning. The former group is reproduced by stellar models with variable molecular opacities (see Marigo 2002), adopting initial solar metallicity, and typical efficiency of the third dredge-up, λ ∼ 0.3−0.4. The latter group of PNe, with extremely high He content (0.15 ≤ He/H ≤ 0.20) and marked oxygen deficiency, is consistent with original sub-solar metallicity (i.e. LMC composition). Moreover, we are able to explain quantitatively both the N/H-He/H correlation and the N/H-C/H anti-correlation, thus solving the discrepancy pointed out long ago by Becker & Iben (1980). This is obtained only under the hypothesis that intermediate- mass TP-AGB progenitors (M > 4.5−5.0 M� ) with LMC composition have suffered a number of very efficient, carbon-poor, dredge-up events. Finally, the neon abundances of the He-rich PNe can be recovered by invoking a significant production of 22 Ne during thermal pulses, which would imply a reduced role of the 22 Ne(α, n) 25 Mg reaction as neutron source to the Planetary Nebulae (PNe) are assumed to consist of the gas ejected via stellar winds by low- and intermediate-mass stars (having initial masses 0.9 ≤ M/M� ≤ Mup, with Mup ∼ 5−8 Mdepending on model details) during their last evo- lutionary stages, the so-called Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase. PNe offer potentially a good possibility to test the results of stellar nucleosynthesis. This can be done in a reliable way by comparing the predicted abundances of the gas ejected close to the end of the AGB phase with the observed abundances because the expelled hot gas remains unaffected by interac- tion with the ISM or with previous shell ejection. Furthermore the ionized gas surrounding the central star shows lines of many elements from which accurate abundances can be de- rived. Also by the ejection of the outer layers PNe contribute to the enrichment of the interstellar medium (ISM) and therefore,

Production of Aluminium and the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars

AbstractWe investigate the production of aluminium and magnesium in asymptotic giant branch models covering a wide range in mass and composition. We evolve models from the pre-main sequence, through all intermediate stages, to near the end of the thermally-pulsing asymptotic giant branch phase. We then perform detailed nucleosynthesis calculations from which we determine the production of the magnesium and aluminium isotopes as a function of the stellar mass and composition. We present the stellar yields of sodium and the magnesium and aluminium isotopes. We discuss the abundance predictions from the stellar models in reference to abundance anomalies observed in globular cluster stars.

Parameterising the Third Dredge-up in Asymptotic Giant Branch Stars

AbstractWe present new evolutionary sequences for low and intermediate mass stars (1−6M⊙) for three different metallicities, Z = 0.02, 0.008, and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch phase. We have two sequences of models for each mass, one which includes mass loss and one without mass loss. Typically 20 or more pulses have been followed for each model, allowing us to calculate the third dredge-up parameter for each case. Using the results from this large and homogeneous set of models, we present an approximate fit for the core mass at the first thermal pulse, Mc1, as well as for the third dredge-up efficiency parameter, λ, and the core mass at the first dredge-up episode, Mcmin, as a function of metallicity and total mass. We also examine the effect of a reduced envelope mass on the value of λ.