Mono-enriched stars and Galactic chemical evolution

Hansen, C. J.; Koch, Andreas; Mashonkina, L.; Magg, Mattis; Bergemann, M.; Sitnova, T.; Gallagher, A. J.; Ilyin, I.; Caffau, E.; Zhang, Huawei; Strassméier, K. G.; Klessen, Ralf S.

doi:10.1051/0004-6361/202038805

Cited by 23 publications

(21 citation statements)

References 140 publications

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“…Stars with metallicities changing by more than 0.5 dex have very similar Be abundances of the order of log(Be/H) = −13.2 dex. Nevertheless, a number of upper limits (from this 7 We mention here that Hansen et al (2020) recently suggested that one of our sample stars, BD+09 2190 with [Fe/H] = −2.7, could actually be one such second-generation star. This observation seems to agree with our suggested interpretation of the Be and Fe abundance behaviour at the extremely metal-poor regime.…”

Section: Discussionmentioning

confidence: 82%

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

Smiljanic,

Zych,

Pasquini

2020

Preprint

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Context. Abundances of beryllium in metal-poor stars scale linearly with metallicity down to [Fe/H] ∼ −3.0. In the stars where Be has been detected at this extremely metal-poor regime, an increased abundance scatter has been previously reported in the literature. This scatter could indicate a flattening of the relation between Be abundances and metallicity. Aims. Our aim is to perform a new investigation of Be abundances in extremely metal-poor stars and try to clarify whether a Be abundance plateau exists. We revisited the Be abundances in a sample of nine dwarfs with metallicities close to [Fe/H] ∼ −3.0. Additionally, we analysed the Be lines in the spectra of stars BPS BS 16968-0061 and CD-33 1173 for the first time. Methods. We took advantage of Gaia DR2 parallaxes to refine values of the surface gravity of the stars. Updated values of surface gravity can have a significant impact on the determination of Be abundances. The other atmospheric parameters were computed using photometric and spectroscopic data. Abundances of Be were determined using spectrum synthesis and model atmospheres.Results. Some of the stars indeed suggest a flattening. Over about a 0.5 dex range in metallicity, between [Fe/H] ∼ −2.70 and −3.26, the Be abundances stay mostly constant at about log(Be/H) ∼ −13.2 dex. Nevertheless, for several stars, we could only place upper limits that are below that level. Most of the sample stars are consistent with having been formed at the progenitor of the so-called Gaia-Enceladus merger. Two out of the three stars likely formed in-situ are the ones that deviate the most from the linear relation. Conclusions. The mixed origin of these extremely metal-poor stars offers a clue to understanding the flattening. We suggest that our observations can be naturally understood as a consequence of the inhomogeneous star forming conditions in the early Galaxy. Without efficient mixing, the early interstellar medium would be characterised by a large scatter in Fe abundances at a given moment. Beryllium, on the other hand, because of its origins in cosmic-ray spallation, would have more homogeneous abundances (in a Galaxywide sense). We therefore suggest that the observed flattening of the Be-versus-metallicity relation reflects a stronger scatter in the Galactic Fe abundances at a given age.

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Section: Discussionmentioning

confidence: 82%

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

Smiljanic,

Zych,

Pasquini

2020

Preprint

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“…Unfortunately, Pb abundances are not available for most of our sample stars and the determination of Pb abundances has some difficulties: the Pb I line at 4057.8 Å, detectable in the optical wavelength, is often blended with CH and needs high-resolution, high signal-to-noise blue spectra for the derivation of the abundance. Additionally, the inclusion of non-LTE corrections is necessary to the most accurate comparison of the models with Pb and other elements [51][52][53]. Pb abundances from CEMP-s star HD 26 are indeed reproduced by our m3z1m3-bigpoc model, but larger statistical modeling is still prevented by the observational difficulties just mentioned.…”

Section: Discussionmentioning

confidence: 85%

“…m3z1m3-bigpoc is the only model consistent with first, second, and third s-process peak elements at the same time. Unfortunately, Pb abundances are not available for most of the sample stars we selected, and the determination of Pb abundances has some difficulties, often introducing big uncertainties [51][52][53]. Considering elements lighter than first-peak ones, m3z1m3 shows high enrichment, as shown in more detail in Figure 6, zoomed in the 30 < A < 41 region.…”

Section: Postprocessing Nucleosynthesis Calculationsmentioning

confidence: 99%

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

et al. 2021

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The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M⊙ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses Mini/M⊙=2 and 3 for initial metallicities Z=0.001 and 0.002. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the 13C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ∼3 × 10−4 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.

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“…We mention here thatHansen et al (2020) recently suggested that one of our sample stars, BD+09 2190 with [Fe/H] = −2.7, could actually be one such second-generation star. This observation seems to agree with our suggested interpretation of the Be and Fe abundance behaviour at the extremely metal-poor regime.…”

mentioning

confidence: 73%

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

Smiljanić

Zych

Pasquini

2021

A&A

View full text Add to dashboard Cite

Context. Abundances of beryllium in metal-poor stars scale linearly with metallicity down to [Fe/H] ∼ −3.0. In the stars where Be has been detected at this extremely metal-poor regime, an increased abundance scatter has been previously reported in the literature. This scatter could indicate a flattening of the relation between Be abundances and metallicity. Aims. Our aim is to perform a new investigation of Be abundances in extremely metal-poor stars and try to clarify whether a Be abundance plateau exists. We revisited the Be abundances in a sample of nine dwarfs with metallicities close to [Fe/H] ∼ −3.0. Additionally, we analysed the Be lines in the spectra of stars BPS BS 16968-0061 and CD-33 1173 for the first time. Methods. We took advantage of Gaia DR2 parallaxes to refine values of the surface gravity of the stars. Updated values of surface gravity can have a significant impact on the determination of Be abundances. The other atmospheric parameters were computed using photometric and spectroscopic data. Abundances of Be were determined using spectrum synthesis and model atmospheres. Results. Some of the stars indeed suggest a flattening. Over about a 0.5 dex range in metallicity, between [Fe/H] ∼ −2.70 and −3.26, the Be abundances stay mostly constant at about log(Be/H) ∼ −13.2 dex. Nevertheless, for several stars, we could only place upper limits that are below that level. Most of the sample stars are consistent with having been formed at the progenitor of the so-called Gaia-Enceladus merger. Two out of the three stars likely formed in-situ are the ones that deviate the most from the linear relation. Conclusions. The mixed origin of these extremely metal-poor stars offers a clue to understanding the flattening. We suggest that our observations can be naturally understood as a consequence of the inhomogeneous star forming conditions in the early Galaxy. Without efficient mixing, the early interstellar medium would be characterised by a large scatter in Fe abundances at a given moment. Beryllium, on the other hand, because of its origins in cosmic-ray spallation, would have more homogeneous abundances (in a Galaxy-wide sense). We therefore suggest that the observed flattening of the Be-versus-metallicity relation reflects a stronger scatter in the Galactic Fe abundances at a given age.

show abstract

Mono-enriched stars and Galactic chemical evolution

Cited by 23 publications

References 140 publications

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

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