The Most Metal-poor Stars. V. The CEMP-no Stars in 3D and Non-LTE

Norris, John E.; Yong, David

doi:10.3847/1538-4357/ab1f84

Cited by 45 publications

(36 citation statements)

References 112 publications

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“…We assign the C-enhancement derived in Placco et al (2016) to noise and slight upturns (∼ 0.003 in normalised flux) in the spectra, which may be traces of the blaze function that could have left imprints in the spectrum that then affected the continuum placement (around 4280-4300 Å ), rendering it too high in their study. We do, however, note that when considering the large 3D, NLTE corrections (∼ 1 dex) versus the 1D, LTE abundances shown in Norris & Yong (2019), the values are in agreement with the 3D NLTE abundances from Amarsi et al (2019) and also with the 1D, LTE study by Placco et al (2016). If such corrections would be applied to our values, the two stars would end up having subsolar [C/Fe].…”

Section: Reference and Cemp Starssupporting

confidence: 89%

An empirical metallicity tracer in CEMP and C-normal stars

Singh

Hansen

Byrgesen

et al. 2020

A&A

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Context. Deriving the metallicity, [Fe/H], in low-resolution spectra of carbon-enhanced metal-poor (CEMP) stars is a tedious task that, owing to the large number of line blends, often leads to uncertainties on [Fe/H] exceeding 0.25 dex. The CEMP stars increase in number with decreasing [Fe/H] and some of these are known to be bona fide second generation halo stars. Hence, knowing their [Fe/H] is important for tracing the formation and chemical evolution of the Galaxy. Aims. Here, we aim to improve the [Fe/H] measurements in low-resolution spectra by avoiding issues related to blends. In turn, we improve our chemical tagging in such spectra at low metallicities. Methods. We developed an empirical way of deriving [Fe/H] in CEMP (and C-normal) stars that relates the equivalent width (EW) of strong lines, which remain detectable in lower-resolution, metal-poor spectra, such as X-Shooter spectra to [Fe/H].Results. The best [Fe/H] tracers are found to be Cr I and Ni I, which both show strong transitions in spectral regions that are free of molecular bands (between ∼ 5200 − 6800Å , a region accessible to most surveys). We derive different relations for dwarfs and giants. The relations are valid in the ranges ∼ −3 <[Fe/H]< −0.5 and 10 −3.2 and EW> 5 mÅ (Ni), depending on the trace element and line as well as the stellar evolutionary stage. Conclusions. The empirical relations are valid for both CEMP and C-normal stars and have been proven to be accurate tracers in a sample of ∼ 400 stars (mainly giants). The metallicities are accurate to within ± ∼ 0.2 dex depending on the sample and resolution, and the empirical relations are robust to within 0.05-0.1 dex. Our relations will improve the metallicity determination in future surveys, which will encounter a large number of CEMP stars, and will greatly speed up the process of determining [Fe/H] as the EWs only need to be measured in two or three lines in relatively clean regions compared to dealing with numerous blended Fe lines.

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Section: Reference and Cemp Starssupporting

confidence: 89%

An empirical metallicity tracer in CEMP and C-normal stars

Singh

Hansen

Byrgesen

et al. 2020

A&A

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“…The synthetic grid we used for the analysis follows the simple 1D/LTE assumptions, which are usually also adopted for the analysis of CEMP stars in the halo. Recently, Norris & Yong (2019) have cast doubts on the reported CEMP(-no) fractions in the literature.…”

Section: Abundance Variations and 3d/non-ltementioning

confidence: 99%

The Pristine Inner Galaxy Survey (PIGS) III: carbon-enhanced metal-poor stars in the bulge

Arentsen

Starkenburg

Aguado

et al. 2021

Monthly Notices of the Royal Astronomical Society

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The most metal-deficient stars hold important clues about the early build-up and chemical evolution of the Milky Way, and carbon-enhanced metal-poor (CEMP) stars are of special interest. However, little is known about CEMP stars in the Galactic bulge. In this paper, we use the large spectroscopic sample of metal-poor stars from the Pristine Inner Galaxy Survey (PIGS) to identify CEMP stars ($\rm {[C/Fe]} \geqslant +0.7$) in the bulge region and to derive a CEMP fraction. We identify 96 new CEMP stars in the inner Galaxy, of which 62 are very metal-poor ($\rm {[Fe/H]} < -2.0$); this is more than a ten-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42^{\, +14\, }_{\, -13} {{\ \rm per\ cent}}$ for stars with $\rm {[Fe/H]} < -3.0$, and decreases to $16^{\, +3\, }_{\, -3} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} < -2.5$ and $5.7^{\, +0.6\, }_{\, -0.5} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} < -2.0$. The PIGS inner Galaxy CEMP fraction for $\rm {[Fe/H]} < -3.0$ is consistent with the halo fraction found in the literature, but at higher metallicities the PIGS fraction is substantially lower. While this can partly be attributed to a photometric selection bias, such bias is unlikely to fully explain the low CEMP fraction at higher metallicities. Considering the typical carbon excesses and metallicity ranges for halo CEMP-s and CEMP-no stars, our results point to a possible deficiency of both CEMP-s and CEMP-no stars (especially the more metal-rich) in the inner Galaxy. The former is potentially related to a difference in the binary fraction, whereas the latter may be the result of a fast chemical enrichment in the early building blocks of the inner Galaxy.

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“…However, recent observations of CEMP-no stars indicate higher binary fraction compared to C-normal stars (Arentsen et al 2019). Furthermore, the general approach of measuring carbon through molecular bands, assuming 1D and local thermodynamic equilibrium (LTE), might significantly overestimate their C-abundances (Amarsi et al 2019a,b;Norris & Yong 2019).…”

Section: Introductionmentioning

confidence: 99%

Neutron-capture elements in dwarf galaxies

Skúladóttir

Hansen

Choplin

et al. 2020

A&A

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The slow (s) and intermediate (i) neutron (n) capture processes occur both in asymptotic giant branch (AGB) stars, and in massive stars. To study the build-up of the s- and i-products at low metallicity, we investigate the abundances of Y, Ba, La, Nd, and Eu in 98 stars, at −2.4 < [Fe/H] < −0.9, in the Sculptor dwarf spheroidal galaxy. The chemical enrichment from AGB stars becomes apparent at [Fe/H] ≈ −2 in Sculptor, and causes [Y/Ba], [La/Ba], [Nd/Ba] and [Eu/Ba] to decrease with metallicity, reaching subsolar values at the highest [Fe/H] ≈ −1. To investigate individual nucleosynthetic sites, we compared three n-rich Sculptor stars with theoretical yields. One carbon-enhanced metal-poor (CEMP-no) star with high [Sr, Y, Zr] > +0.7 is best fit with a model of a rapidly-rotating massive star, the second (likely CH star) with the i-process, while the third has no satisfactory fit. For a more general understanding of the build-up of the heavy elements, we calculate for the first time the cumulative contribution of the s- and i-processes to the chemical enrichment in Sculptor, and compare with theoretical predictions. By correcting for the r-process, we derive [Y/Ba]s/i = −0.85 ± 0.16, [La/Ba]s/i = −0.49 ± 0.17, and [Nd/Ba]s/i = −0.48 ± 0.12, in the overall s- and/or i-process in Sculptor. These abundance ratios are within the range of those of CEMP stars in the Milky Way, which have either s- or i-process signatures. The low [Y/Ba]s/i and [La/Ba]s/i that we measure in Sculptor are inconsistent with them arising from the s-process only, but are more compatible with models of the i-process. Thus we conclude that both the s- and i-processes were important for the build-up of n-capture elements in the Sculptor dwarf spheroidal galaxy.

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The Most Metal-poor Stars. V. The CEMP-no Stars in 3D and Non-LTE

Cited by 45 publications

References 112 publications

An empirical metallicity tracer in CEMP and C-normal stars

An empirical metallicity tracer in CEMP and C-normal stars

The Pristine Inner Galaxy Survey (PIGS) III: carbon-enhanced metal-poor stars in the bulge

Neutron-capture elements in dwarf galaxies

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