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Register a new userOn the validity of the core-mass luminosity relation for TP-AGB stars with efficient dredge-up
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F. Herwig
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We investigate the validity of the core mass - luminosity relation (CMLR), originally described by Paczynski (1970), for asymptotic giant branch stars under the presence of third dredge-up events. We find, that models with efficient third dredge-up with less massive cores than those associated with hot bottom burning (Bloecker and Schoenberner 1991) do not obey the linear CMLR. Complete evolutionary calculations of thermal pulse stellar models which consider overshoot according to an exponential diffusive algorithm show systematically larger third dredge-up for lower core masses (0.55 Msol < M_H < 0.8 Msol) than any other existing models. We present and discuss the luminosity evolution of these models.
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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 ESOs VLT we search for resonance lines of neutral Tc in the blue spectral region of our sample stars. These measurements allow us to improve the procedure of classification of stars with respect to their Tc content by using flux ratios. Synthetic spectra based on MARCS atmospheric models are presented and compared to the observed spectra around three lines of Tc. Bolometric magnitudes are calculated based on near infrared photometry of the objects. Among the sample of 27 long period bulge variables four were found to definitely contain Tc in their atmospheres. The luminosity of the Tc rich stars is in agreement with predictions from AGB evolutionary models on the minimum luminosity at the time when third dredge-up sets in. However, AGB evolutionary models and a bulge consisting of a single old population cannot be brought into agreement. This probably means that a younger population is present in the bulge, as suggested by various authors, which contains the Tc-rich stars here identified.
High dispersion near-infrared spectra have been taken of seven highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and SMC in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible.
As part of a reanalysis of Galactic Asymptotic Giant Branch stars (hereafter AGB stars) at infrared wavelengths, we discuss here two samples (the first of carbon-rich stars, the second of S stars) for which photometry in the near- and mid-IR and distance estimates are available. Whenever possible we searched also for mass-loss rates. The observed spectral energy distributions extended in all cases up to 20 $mu$m and for the best-observed sources up to 45 $mu$m. The wide wavelength coverage allows us to obtain reliable bolometric corrections, and hence bolometric magnitudes. We show that mid-IR fluxes are crucial for estimating bolometric magnitudes for stars with dusty envelopes and that the so-called luminosity problem of C stars (i.e. the suggestion that they are less luminous than predicted by models) does not appear to exist.
The spectro-photometric properties of galaxies in galaxy formation models are obtained by combining the predicted history of star formation and mass accretion with the physics of stellar evolution through stellar population models. In the recent literature, significant differences have emerged regarding the implementation of the Thermally-Pulsing Asymptotic Giant Branch phase of stellar evolution. The emission in the TP-AGB phase dominates the bolometric and near-IR spectrum of intermediate-age (~1 Gyr) stellar populations, hence it is crucial for the correct modeling of the galaxy luminosities and colours. In this paper for the first time, we incorporate a full prescription of the TP-AGB phase in a semi-analytic model of galaxy formation. We find that the inclusion of the TP-AGB in the model spectra dramatically alters the predicted colour-magnitude relation and its evolution with redshift. When the TP-AGB phase is active, the rest-frame V-K galaxy colours are redder by almost 2 magnitudes in the redshift range z~2-3 and by 1 magnitude at z~1. Very red colours are produced in disk galaxies, so that the V-K colour distributions of disk and spheroids are virtually undistinguishable at low redshifts. We also find that the galaxy K-band emission is more than 1 magnitude higher in the range z~1-3. This may alleviate the difficulties met by the hierarchical clustering scenario in predicting the red galaxy population at high redshifts. The comparison between simulations and observations have to be revisited in the light of our results.
We calculated models of massive AGB stars with a self-consistent coupling of time-dependent mixing and nuclear burning for 30 isotopes and 74 reactions. Overshoot with an exponentially declining velocity field was considered and applied during all stages of evolution and in all convective regions. Very efficient 3rd dredge-up was found even overcompensating the growth of the hydrogen-exhausted core after a few thermal pulses. Hot bottom burning occurs for M>4Msol within the sequences with overshoot. Carbon star formation in these more massive AGB stars is delayed or even prevented by hot bottom burning despite the very efficient dredge-up. With the simultaneous treatment of mixing and burning the formation of Li-rich AGB stars due to the Cameron-Fowler mechanism was followed. For a 6Msol model the maximum Li abundance was found to be epsilon(Li7)=4.4.
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