There is reasonable agreement between the measured and predicted s-process abundance profiles. We present AGB models producing TDU in those stars with in the range −0.25 to −0.5. This provides direct evidence for TDUs occurring in AGB stars with initial masses as low as ∼1 M ⊙ and at low luminosity, that is, at the start of the thermally pulsing AGB. We report on six Tc-rich S stars lying close to the 1 M ⊙ (initial mass) tracks of AGB stars of the corresponding metallicity and above the predicted onset of TDU, as expected. Stellar parameters were obtained using a combination of HERMES high-resolution spectra, accurate Gaia Data Release 2 ( Gaia-DR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Our aim is to derive parameters for a sample of low-mass, Tc-rich S stars and then, by comparing their location in the HR diagram with stellar evolution tracks, to derive their masses and to compare their measured s-process abundance profiles with recently derived STAREVOL nucleosynthetic predictions for low-mass AGB stars. Here we report on a population of low-mass, Tc-rich S stars previously unaccounted for by stellar evolution models.Īims. Exquisite Gaia parallaxes now allow for these stars to be precisely located in the Hertzsprung–Russell (HR) diagram. Intrinsic, technetium (Tc)-rich S stars are the first objects on the asymptotic giant branch (AGB) to undergo third dredge-up (TDU) events. Their overabundance pattern shows the signature of s-process nucleosynthesis. S stars are late-type giants with spectra showing characteristic molecular bands of ZrO in addition to the TiO bands typical of M stars. Institute of Astronomy and Astrophysics (IAA), Université libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, 1050 Bruxelles, BelgiumĮ-mail: of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, BelgiumĬontext. Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes
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