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Register a new userS stars and s-process in the Gaia era II. Constraining the luminosity of the third dredge-up with Tc-rich S stars
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S stars are late-type giants that are transition objects between M-type stars and carbon stars on the asymptotic giant branch (AGB). They are classified into two types: intrinsic or extrinsic, based on the presence or absence of technetium (Tc). The Tc-rich or intrinsic S stars are thermally-pulsing (TP-)AGB stars internally producing s-process elements (including Tc) which are brought to their surface via the third dredge-up (TDU). Tc-poor or extrinsic S stars gained their s-process overabundances via accretion of s-process-rich material from an AGB companion which has since turned into a dim white dwarf. Our goal is to investigate the evolutionary status of Tc-rich S stars by locating them in a Hertzsprung-Russell (HR) diagram using the results of Gaia early Data Release 3 (EDR3). We combine the current sample of 13 Tc-rich stars with our previous studies of 10 Tc-rich stars to determine the observational onset of the TDU in the metallicity range [-0.7; 0]. We also compare our abundance determinations with dedicated AGB nucleosynthesis predictions. The stellar parameters are derived using an iterative tool which combines HERMES high-resolution spectra, accurate Gaia EDR3 parallaxes, stellar evolution models and tailored MARCS model atmospheres for S-type stars. Using these stellar parameters we determine the heavy-element abundances by line synthesis. In the HR diagram, the intrinsic S stars are located at higher luminosities than the predicted onset of the TDU. These findings are consistent with Tc-rich S stars being genuinely TP-AGB stars. The comparison of the derived s-process abundance profiles of our intrinsic S stars with the nucleosynthesis predictions provide an overall good agreement. Stars with highest [s/Fe] tend to have the highest C/O ratios.
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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.
Here we present the discovery of 895 s-process-rich candidates from 454,180 giant stars observed by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) using a data-driven approach. This sample constitutes the largest number of s-process enhanced stars ever discovered. Our sample includes 187 s-process-rich candidates that are enhanced in both barium and strontium, 49 stars with significant barium enhancement only and 659 stars that show only a strontium enhancement. Most of the stars in our sample are in the range of effective temperature and log g typical of red giant branch (RGB) populations, which is consistent with our observational selection bias towards finding RGB stars. We estimate that only a small fraction (0.5 per cent) of binary configurations are favourable for s-process enriched stars. The majority of our s-process-rich candidates (95 per cent) show strong carbon enhancements, whereas only five candidates (less than 3 percent) show evidence of sodium enhancement. Our kinematic analysis reveals that 97 percent of our sample are disc stars, with the other 3 percent showing velocities consistent with the Galactic halo. The scaleheight of the disc is estimated to be zh=0.634kpc, comparable with values in the literature. A comparison with the yields from asymptotic giant branch (AGB) models suggests that the main neutron source responsible for the Ba and Sr enhancements is the 13C(alpha,n)16O reaction. We conclude that the s-process-rich candidates may have received their overabundances via mass transfer from a previous ABG companion with an initial mass in the range 1-3Msun.
The recently discovered phosphorus-rich stars pose a challenge to stellar evolution and nucleosynthesis theory, as none of the existing models can explain their extremely peculiar chemical abundances pattern. Apart from the large phosphorus enhancement, such stars also show enhancement in other light (O, Mg, Si, Al) and heavy (e.g., Ce) elements. We have obtained high-resolution optical spectra of two optically bright phosphorus-rich stars (including a new P-rich star), for which we have deter-mined a larger number of elemental abundances (from C to Pb). We confirm the unusual light-element abundance pattern with very large enhancements of Mg, Si, Al, and P, and possibly some Cu enhancement, but the spectra of the new P-rich star is the only one to reveal some C(+N) enhancement.When compared to other appropriate metal-poor and neutron-capture enhanced stars, the two P-rich stars show heavy-element overabundances similar to low neutron density s-process nucleosynthesis,with high first- (Sr, Y, Zr) and second-peak (Ba, La, Ce, Nd) element enhancements (even some Pb enhancement in one star) and a negative [Rb/Sr] ratio. However, this s-process is distinct from the one occurring in asymptotic giant branch (AGB) stars. The notable distinctions encompass larger[Ba/La] and lower Eu and Pb than their AGB counterparts. Our observations should guide stellar nucleosynthesis theoreticians and observers to identify the P-rich star progenitor, which represents anew site for s-process nucleosynthesis, with important implications for the chemical evolution of our Galaxy.
We provide an individual analysis of 94 carbon enhanced metal-poor stars showing an s-process enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster toward the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Paper I. Several CEMP-s stars show an enhancement in both s and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r stars, we assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by Supernovae pollution. A general discussion and the method adopted in order to interpret the observations have been provided in Paper II. We present in this paper a detailed study of spectroscopic observations of individual stars. We consider all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls] and [Pb/hs]. The presence of an initial r-process contribution may be typically evaluated by the [La/Eu] ratio. We found possible agreements between theoretical predictions and spectroscopic data. In general, the observed [Na/Fe] (and [Mg/Fe]) provide information on the AGB initial mass, while [hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range of 13C-pocket strengths is required to interpret the observations. However, major discrepancies between models and observations exist. We highlight star by star the agreements and the main problems encountered and, when possible, we suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems ...
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