Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M_⊙

Abstract

Context. S stars are late-type giants with spectra showing characteristic molecular bands of ZrO in addition to the TiO bands typical of M stars. Their overabundance pattern shows the signature of s-process nucleosynthesis. Intrinsic, technetium (Tc)-rich S stars are the first objects, on the Asymptotic Giant Branch (AGB), to undergo third dredge-up (TDU) events. Gaia exquisite parallaxes now allow to precisely locate these stars in the Hertzsprung-Russell (HR) diagram. Here we report on a population of low-mass, Tc-rich S stars, previously unaccounted for by stellar evolution models. Aims. Our aim is to derive parameters of 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. Methods. The 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. Results. We report on 6 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. This provides direct evidence for TDUs occurring in AGB stars with initial masses as low as ∼1 M and at low luminosity, i.e. at the start of the thermally-pulsing AGB. We present AGB models producing TDU in those stars with [Fe/H] in the range −0.25 to −0.5 There is a reasonable agreement between the measured and predicted s-process abundance profiles. For 2 objects however (CD -29 • 5912 and BD +34 • 1698), the predicted C/O ratio and s-process enhancements do not match simultaneously the measured ones.

Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars

“…f over = 0.14, D min = 10 7 cm 2 s −1 , and p = 1/2, where f over controls the extent of the mixing, D min is the value of the diffusion coefficient at the innermost boundary of the diffusive region, and p is an additional free parameter describing the shape of the diffusion profile. As shown in Shetye et al (2019), this adopted set of diffusion parameters gives rise to early TDU episodes and s-process enrichments in stars with masses as low as 1 M and compatible with observations.…”

“…We follow Eq. (9) of Goriely & Siess (2018) and use the same diffusive mixing parameters in our simulations as in Shetye et al (2019), i.e. f over = 0.14, D min = 10 7 cm 2 s −1 , and p = 1/2, where f over controls the extent of the mixing, D min is the value of the diffusion coefficient at the innermost boundary of the diffusive region, and p is an additional free parameter describing the shape of the diffusion profile.…”

“…The large C overabundances predicted in AGB stars, but not confirmed by derived abundances, is a long-standing problem (see e.g. Shetye et al 2019). A figure displaying log C as a function of [Fe/H] was first presented by Spite et al (2013) and then further discussed by Hansen et al (2016c) and Yoon et al (2016).…”

Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars

“…First, if an appropriate statistics of bitrinsic stars were available, this would put interesting constraints on binary-synthesis evolution models, because the number of bitrinsic star depends, among other things, on the radius evolution during the RGB and AGB phases, which will determine whether/when a Roche-lobe overflow will occur, causing the disappearance of systems that would have become bitrinsic objects (Escorza et al 2020). A second constraint concerns the component masses: in order to exhibit s-process overabundances at their surface, AGB stars must have an initial mass larger than 1 M (as recently shown by Shetye et al 2019) and smaller than 4 − 5 M , above which the radiative s-process production is significantly reduced (Goriely & Siess 2004). Therefore, in the case of bitrinsic systems, both the primary and secondary components should satisfy this condition (along with M 1 > M 2 ).…”

“…The following sections present the detailed analysis of these stars. The analysis of the remaining of this sample is presented elsewhere (Shetye et al 2018(Shetye et al , 2019 and in a forthcoming paper (Shetye et al, in prep. ).…”