Element Enhancements Along the Entire Asymptotic Giant Branch Phase

Boer, E. C. Wylie-de; Cottrell, P. L.

doi:10.1088/0004-637x/692/1/522

Cited by 7 publications

(5 citation statements)

References 21 publications

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“…Our T eff , log g and [Fe/H] values match well those of de Boer & Cottrell (2009), but their C/O > 1 is larger than our estimate of C/O = 0.5, which is in agreement with that of Neyskens et al (2015). Our metallicity is also very similar to that of Neyskens et al (2015) ([Fe/H] = -0.30 and -0.31, respectively).…”

Section: Nq Pupsupporting

confidence: 83%

S stars and s-process in the Gaia era

Shetye

Eck

Jorissen

et al. 2021

A&A

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Context. 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) that are brought to their surface via the third dredge-up (TDU). The Tc-poor or extrinsic S stars gained their s-process overabundances via the accretion of s-process-rich material from an AGB companion that has since turned into a dim white dwarf. Aims. 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. Methods. We derived the stellar parameters using an iterative tool that 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 determined the heavy-element abundances by line synthesis. Results. 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 genuine 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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Section: Nq Pupsupporting

confidence: 83%

S stars and s-process in the Gaia era

Shetye

Eck

Jorissen

et al. 2021

A&A

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Section: Nq Pupsupporting

confidence: 84%

“…Our La ii, Nd ii, and Eu ii abundances agree with those from de Boer & Cottrell (2009) within error bars. We report lower [Y/Fe] and [Zr/Fe] values as compared to de Boer & Cottrell (2009). Our Zr abundance differs by 0.17 dex from that of Neyskens et al (2015) despite the fact that the same Zr lines have been used in both studies.…”

Section: Nq Pupsupporting

confidence: 82%

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S stars and s-process in the Gaia era I. Stellar parameters and chemical abundances in a sub-sample of S stars with new MARCS model atmospheres

Shetye,

Van Eck,

Jorissen

et al. 2018

Preprint

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Context. S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion which is now an undetected white dwarf, and since the epoch of the mass transfer, technetium has totally decayed. Aims. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally-pulsing AGB. Methods. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly-designed MARCS model atmospheres for S-type stars. Various stellar parameters impact the atmospheric structure of S stars, not only effective temperature, gravity, metallicity and microturbulence, but also C/O and [s/Fe]. We show that photometric data alone are not sufficient to disentangle these parameters. We present a new automatic spectral-fitting method that allows one to constrain the range of possible atmospheric parameters. Results. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung-Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. The Tc dichotomy between extrinsic and intrinsic S stars is seen as well in the Nb abundances: intrinsic, Tc-rich S stars are Nb-poor, whereas extrinsic, Tc-poor S stars are Nb-rich. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. All appear to be the cooler analogues of barium stars. Barium stars with masses smaller than 2.5 M turn into extrinsic S stars on the RGB, because only for those masses does the RGB tip extend to temperatures lower than ∼ 4200 K, which allows the ZrO bands distinctive of S-type stars to develop. On the contrary, barium stars with masses in excess of ∼ 2.5 M can only turn into extrinsic S stars on the E-AGB, but those are short-lived, and thus rare. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars. Although nucleosynthetic model predictions give a satisfactory distribut...

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“…Our La II, Nd II, and Eu II abundances agree with those from de Boer & Cottrell (2009) within error bars. We report lower [Y/Fe] and [Zr/Fe] values as compared to de Boer & Cottrell (2009). Our Zr abundance differs by 0.17 dex from that of Neyskens et al (2015) despite the fact that the same Zr lines have been used in both studies.…”

Section: Nq Pupcontrasting

confidence: 52%

S stars and s-process in the Gaia era

Shetye

Eck

Jorissen

et al. 2018

A&A

View full text Add to dashboard Cite

Context. S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion which is now an undetected white dwarf, and since the epoch of the mass transfer, technetium has totally decayed. Aims. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally pulsing AGB. Methods. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Various stellar parameters impact the atmospheric structure of S stars, not only effective temperature, gravity, metallicity and microturbulence but also C/O and [s/Fe]. We show that photometric data alone are not sufficient to disentangle these parameters. We present a new automatic spectral-fitting method that allows one to constrain the range of possible atmospheric parameters. Results. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung-Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. The Tc dichotomy between extrinsic and intrinsic S stars is seen as well in the Nb abundances: intrinsic, Tc-rich S stars are Nb-poor, whereas extrinsic, Tc-poor S stars are Nb-rich. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. All appear to be the cooler analogues of barium stars. Barium stars with masses smaller than 2.5 M turn into extrinsic S stars on the RGB, because only for those masses does the RGB tip extend to temperatures lower than ∼4200 K, which allows the ZrO bands distinctive of S-type stars to develop. On the contrary, barium stars with masses in excess of ∼2.5 M can only turn into extrinsic S stars on the E-AGB, but those are short-lived, and thus rare. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars. Although nucleosynthetic model predictions give a satisfactory distribution...

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Element Enhancements Along the Entire Asymptotic Giant Branch Phase

Cited by 7 publications

References 21 publications

S stars and s-process in the Gaia era

S stars and s-process in the Gaia era

S stars and s-process in the Gaia era I. Stellar parameters and chemical abundances in a sub-sample of S stars with new MARCS model atmospheres

S stars and s-process in the Gaia era

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