The temperature and chronology of heavy-element synthesis in low-mass stars

Neyskens, P.; Eck, S. Van; Jorissen, A.; Goriely, Stéphane; Siess, Lionel; Plez, B.

doi:10.1038/nature14050

Cited by 53 publications

(71 citation statements)

References 28 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…comm.). From a comparison of s-process element abundances with evolutionary models, the mass of intrinsic S stars has been estimated to lie between 1 and 3 M (Neyskens et al 2015). Here we adopted M = 1 M ; the impact of mass on the model structure is discussed in the next section.…”

Section: Ingredientsmentioning

confidence: 99%

A grid of MARCS model atmospheres for late-type stars

Eck

Neyskens

Jorissen

et al. 2017

A&A

View full text Add to dashboard Cite

S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing asymptotic giant branch. A grid of MARCS model atmospheres has been computed for S stars, covering the range 2700 ≤ T eff (K) ≤ 4000, 0.50 ≤ C/O ≤ 0.99, 0 ≤ log g ≤ 5, [Fe/H] = 0., −0.5 dex, and [s/Fe] = 0, 1, and 2 dex (where the latter quantity refers to the global overabundance of s-process elements). The MARCS models make use of a new ZrO line list. Synthetic spectra computed from these models are used to derive photometric indices in the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A method is proposed to select the model best matching any given S star, a non-trivial operation since the grid contains more than 3500 models covering a five-dimensional parameter space. The method is based on the comparison between observed and synthetic photometric indices and spectral band strengths, and has been applied on a vast subsample of the Henize sample of S stars. Our results confirm the old claim by Piccirillo (1980, MNRAS, 190, 441) that ZrO bands in warm S stars (T eff > 3200 K) are not caused by the C/O ratio being close to unity, as traditionally believed, but rather by some Zr overabundance. The TiO and ZrO band strengths, combined with V − K and J − K photometric indices, are used to select T eff , C/O, [Fe/H] and [s/Fe]. The Geneva U − B 1 and B 2 − V 1 indices (or any equivalent) are good at selecting the gravity. The defining spectral features of dwarf S stars are outlined, but none is found among the Henize S stars. More generally, it is found that, at T eff = 3200 K, a change of C/O from 0.5 to 0.99 has a strong impact on V − K (2 mag). Conversely, a range of 2 mag in V − K corresponds to a 200 K shift along the (T eff , V − K) relationship (for a fixed C/O value). Hence, the use of a (T eff , V − K) calibration established for M stars will yield large errors for S stars, so that a specific calibration must be used, as provided in the present paper. Using the atmospheric parameters derived by our method for the sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy among S stars reveals itself very clearly as a bimodal distribution in the effective temperatures. Moreover, the increase of s-process element abundances with increasing C/O ratios and decreasing temperatures is apparent among intrinsic stars, confirming theoretical expectations.

show abstract

Section: Ingredientsmentioning

confidence: 99%

A grid of MARCS model atmospheres for late-type stars

Eck

Neyskens

Jorissen

et al. 2017

A&A

View full text Add to dashboard Cite

show abstract

“…A new grid of MARCS model atmospheres dedicated to S stars (Van Eck et al 2011) has been used for that purpose. Neyskens et al (2015) obtained T eff = 3600 K, log g = 1.0, [Fe/H] = −0.5, C/O = 0.5, and [s/Fe] = +1.0. These parameters are adopted here.…”

Section: Atmospheric Parametersmentioning

confidence: 99%

“…Instead, for V1261 Ori, we adopted the parameters derived by Neyskens et al (2015), who took the specific chemical composition of S stars and its feedback on the model-atmosphere structure into account. A new grid of MARCS model atmospheres dedicated to S stars (Van Eck et al 2011) has been used for that purpose.…”

Section: Atmospheric Parametersmentioning

confidence: 99%

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

Merle

Jorissen

Eck

et al. 2016

A&A

Self Cite

View full text Add to dashboard Cite

Context. The enrichment in s-process elements of barium stars is known to be due to pollution by mass transfer from a companion formerly on the thermally pulsing asymptotic giant branch (AGB), now a carbon-oxygen white-dwarf (WD). Aims. We are investigating the relationship between the s-process enrichment in the barium star and the mass of its WD companion. It is expected that helium WDs, which have masses lower than about 0.5 M and never reached the AGB phase, should not pollute their giant companion with s-process elements. Therefore the companion should never turn into a barium star. Methods. Spectra with a resolution of R ∼ 86 000 were obtained with the HERMES spectrograph on the 1.2 m Mercator telescope for a sample of 11 binary systems involving WD companions of various masses. We used standard 1D local thermodynamical equilibrium MARCS model atmospheres coupled with the Turbospectrum radiative-transfer code that is embedded in the BACCHUS pipeline to derive the atmospheric parameters through equivalent widths of Fe i and Fe ii lines. Least-squares minimization between the observed and synthetic line shape was used to derive the detailed chemical abundances of CNO and s-process elements.Results. The abundances of s-process elements for the entire sample of 11 binary stars were derived homogeneously. The sample encompasses all levels of overabundances: from solar [s/Fe] = 0 to 1.5 dex in the two binary systems with S-star primaries (for which dedicated MARCS model atmospheres were used). The primary components of binary systems with a WD more massive than 0.5 M are enriched in s-process elements. We also found a trend of increasing [s/Fe] with [C/Fe] or [(C+N)/Fe]. Conclusions. Our results confirm the expectation that binary systems with WD companions less massive than 0.5 M do not host barium stars.

show abstract

“…It is generally acknowledged that the 13 C(α,n) 16 O reaction is the main neutron source in low-mass AGB stars (1−3 M ) (e.g. Straniero et al 1995;Gallino et al 1998;Mowlavi et al 1998;Abia et al 2002;Karakas & Lattanzio 2014;Neyskens et al 2015). It is also widely accepted that a 13 C-pocket is produced by the transport of protons from the convective envelope into the He-rich intershell.…”

Section: Introductionmentioning

confidence: 99%

Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Smedt

Winckel

Kamath

et al. 2016

A&A

Self Cite

View full text Add to dashboard Cite

Context. This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. The goal at large is to provide improved observational constraints to the models of the complex interplay between the AGB s-process nucleosynthesis and the associated mixing processes. Aims. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of s-process enriched post-AGB stars in the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. The determined upper limits based on spectral studies are much lower than what is predicted. In this paper, we focus specifically on the Pb abundance of 14 Galactic s-process enhanced post-AGB stars to check whether the same discrepancy is present in the Galaxy as well. Among these 14 objects, two were not yet subject to a detailed abundance study in the literature. We apply the same method to obtain accurate abundances for the 12 others. Our homogeneous abundance results provide the input of detailed spectral synthesis computations in the spectral regions where Pb lines are located. Methods. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of Galactic post-AGB stars. None of the sample stars display clear Pb lines, and we only deduced upper limits of the Pb abundance by using spectrum synthesis in the spectral ranges of the strongest Pb lines. Results. We do not find any clear evidence of Pb overabundances in our sample. The derived upper limits are strongly correlated with the effective temperature of the stars with increasing upper limits for increasing effective temperatures. We obtain stronger Pb constraints on the cooler objects. Moreover, we confirm the s-process enrichment and carbon enhancement of two unstudied 21 µm sources IRAS 13245-6428 and IRAS 14429-4539. The mildly s-process enhanced post-AGB star IRAS 17279-1119 is part of a binary system and may be the long sought precursor of extrinsic Ba stars. Conclusions. Stars with T eff > 7500 K do not provide strong constraints on the Pb abundance as the strongest line in the optical spectrum is only detectable at unrealistically high Pb atmospheric abundances. Combining the Pb abundance results from this study with abundances from our previous studies, we conclude that the discrepancy between theory and observation increases towards lower metallicities. The model predictions are consistent with the deduced upper limits on the Pb abundances for all stars with [Fe/H] > −0.7 dex. For stars with [Fe/H] < −0.7 dex, however, the model predictions overestimate the Pb abundances with respect to the other s-process elements. All objects, except IRAS 17279-1119, confirm the relation between neutron exposure [hs/ls] and third dredge-up efficiency [s/Fe], whereas no relation between metallicity and neut...

show abstract

The temperature and chronology of heavy-element synthesis in low-mass stars

Cited by 53 publications

References 28 publications

A grid of MARCS model atmospheres for late-type stars

A grid of MARCS model atmospheres for late-type stars

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

Detailed homogeneous abundance studies of 14 Galactics-process enriched post-AGB stars: In search of lead (Pb)

Contact Info

Product

Resources

About