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Register a new userThird-dredge-up oxygen in planetary nebulae
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Authors
D. Pequignot
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The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf galaxy appear to result from nearly twin stars, except that third-dredge-up carbon is more abundant in He 2-436. A thorough photoionization-model analysis implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436, indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with respect to carbon is (7 +/- 4)%. Excess nitrogen in Wray 16-423 suggests third dredge-up of late CN-cycle products even in these low-mass, intermediate-metallicity stars.
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We study the relation between the chemical composition and the type of dust present in a group of 20 Galactic planetary nebulae (PNe) that have high quality optical and infrared spectra. The optical spectra are used, together with the best available ionization correction factors, to calculate the abundances of Ar, C, Cl, He, N, Ne, and O relative to H. The infrared spectra are used to classify the PNe in two groups depending on whether the observed dust features are representative of oxygen-rich or carbon-rich environments. The sample contains one object from the halo, eight from the bulge, and eleven from the local disc. We compare their chemical abundances with nucleosynthesis model predictions and with the ones obtained in seven Galactic H II regions of the solar neighbourhood. We find evidence of O enrichment (by $sim$ 0.3 dex) in all but one of the PNe with carbon-rich dust (CRD). Our analysis shows that Ar, and especially Cl, are the best metallicity indicators of the progenitors of PNe. There is a tight correlation between the abundances of Ar and Cl in all the objects, in agreement with a lockstep evolution of both elements. The range of metallicities implied by the Cl abundances covers one order of magnitude and we find significant differences in the initial masses and metallicities of the PNe with CRD and oxygen-rich dust (ORD). The PNe with CRD tend to have intermediate masses and low metallicities, whereas most of the PNe with ORD show higher enrichments in N and He, suggesting that they had high-mass progenitors.
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.
We investigate the formation of carbon stars as a function of the stellar mass and parent metallicity. Theoretical modelling is based on an improved scheme for treating the third dredge-up in synthetic calculations of thermally pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual criterion (based on a constant minimum core mass for the occurrence of dredge-up, M_c^min) is replaced by one on the minimum temperature at the base of the convective envelope, T_b^dred, at the stage of the post-flash luminosity maximum. Envelope integrations then allow determination of M_c^min as a function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus inferring if and when dredge-up first occurs. Moreover, the final possible shut down of the process is predicted. Extensive grids of TP-AGB models were computed using this scheme. We present and discuss the calibration of the two dredge-up parameters (lambda and T_b^dred) aimed at reproducing the carbon star luminosity function (CSLF) in the LMC. It turns out that the faint tail is almost insensitive to the history of star formation rate (SFR) in the parent galaxy (it is essentially determined by T_b^dred), in contrast to the bright wing which may be more affected by the details of the recent SFR. Once the faint end is reproduced, the peak location is a stringent calibrator of lambda. The best fit to the observed CSLF in the LMC is obtained with Z=0.008, lambda=0.50, log(T_b^dred)=6.4, and a constant SFR up to 5x10^8 yr ago. A good fit to the CSLF in the SMC is then easily derived from the Z=0.004 models, with a single choice of parameters, and a constant SFR over the entire significant age interval. The results are consistent with the theoretical expectation that the third dredge-up is more efficient at lower Zs.
Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type ($T_{rm eff}$ = 8250 $pm$ 250K) luminous (8200 $pm$ 700 $rm L_{odot}$), metal-poor ($textrm{[Fe/H]}$ = $- 1.18 pm$ 0.10), low-mass (M$_{rm initial}$ $approx$ 1.5 $-$ 2.0 $rm M_{odot}$) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] $<$ 0.50) nor enrichment of $s$-process elements. We derived an oxygen abundance of [O/Fe] = 0.29 $pm$ 0.1. For Fe and O, we took into account the effects of non-local thermodynamic equilibrium (NLTE). We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB and be carbon-enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogues which are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any third dredge-up episodes.
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