Constraining cosmic scatter in the Galactic halo  through a differential analysis of metal-poor stars

Reggiani, Henrique; Meléndez, Jorge; Karakas, Amanda I.; Placco, Vinicius M.

doi:10.1051/0004-6361/201730750

Cited by 59 publications

(85 citation statements)

References 130 publications

(271 reference statements)

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“…two outliers). This would indicate that the decreasing abundance trend is mainly due to deviations from LTE, which are most strongly present at low metallicities; a similar result is shown in Reggiani et al (2017). Hence, the trend in Cr abundance is more so due to the simplified abundances assumptions made when computing Cr abundances, while the EWs seem to follow the metallicity well.…”

Section: Cr and Ni As Tracerssupporting

confidence: 66%

“…Kobayashi et al 2019). As is seen from observations, the [Ni/Fe] is flat with metallicity and has a scatter as low as 0.03 for very metal-poor stars (Reggiani et al 2017); it only has a slight scatter (0.1-0.2 dex) around the solar-scaled ratio.…”

Section: Cr and Ni As Tracersmentioning

confidence: 63%

“…For all 42 dwarfs, we adopted the stellar parameters from Reggiani et al (2017) and Reggiani et al in prep, see Table A.2. Finally, we included ten well-studied giants for comparison and calibration purposes.…”

Section: Observationmentioning

confidence: 99%

“…In total we have 38 giants (18 CEMP, 12 C-normal, eight stars above [Fe/H]=−2) and 18 dwarfs (six CEMP, ten Cnormal, two reference stars). The ten C-normal dwarf stars are fromReggiani et al (2017); we derived new C abundances for these stars, finding them to be C-normal (see open symbols inFig. 5andTable A.2).…”

mentioning

confidence: 99%

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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: Cr and Ni As Tracerssupporting

confidence: 66%

Section: Cr and Ni As Tracersmentioning

confidence: 63%

Section: Observationmentioning

confidence: 99%

mentioning

confidence: 99%

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An empirical metallicity tracer in CEMP and C-normal stars

Singh

Hansen

Byrgesen

et al. 2020

A&A

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“…This type of analysis provides high precision abundances, erasing most of the effects which blur typical chemical abundance procedures like unaccounted blends, effects of stellar evolution and poor atomic line characterization. This technique has also been applied to other stellar types, selecting a reference star which is as close as possible to the analyzed stars in terms of stellar parameters (Reggiani et al 2017;Hawkins et al 2016;Jofré et al 2015). In this case, the results tell us how much the abundances of the analyzed stars differ from those of the reference star, which is no longer necessarily the Sun.…”

Section: Differential Chemical Abundancesmentioning

confidence: 99%

Differential abundances of open clusters and their tidal tails: Chemical tagging and chemical homogeneity

Casamiquela

Tarricq

Soubiran

et al. 2020

A&A

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Context. Well studied Open Clusters (OCs) of the Solar neighbourhood are frequently used as reference objects to test galactic and stellar theories. For that purpose their chemical composition needs to be known with a high level of confidence. It is also important to clarify if each OC is chemically homogeneous and if it has a unique chemical signature. Aims. The aims of this work are (1) to determine accurate and precise abundances of 22 chemical species (from Na to Eu) in the Hyades, Praesepe and Rupecht 147 using a large number of stars at different evolutionary states, (2) to evaluate the level of chemical homogeneity of these OCs, (3) to compare their chemical signatures. Methods. We gathered ∼800 high resolution and high signal-to-noise spectra of ∼100 members in the three clusters, obtained with the latest memberships based on Gaia DR2 data. We build a pipeline which computes atmospheric parameters and strictly line-by-line differential abundances among twin stars in our sample. With this method we are able to reach a very high precision in the abundances (0.01-0.02 dex in most of the elements). Results. We find large differences in the absolute abundances in some elements, which can be attributed to diffusion, NLTE effects or systematics in the analysis. For the three OCs, we find strong correlations in the differential abundances between different pairs of elements. According to our experiment with synthetic data, this can be explained by some level of chemical inhomogeneity. We compare differential abundances of several stars from the Hyades and Praesepe tails: the stars that differ more in chemical abundances also have distinct kinematics, even though they have been identified as members of the tail. Conclusions. It is possible to obtain high precision abundances using a differential analysis even when mixing spectra from different instruments. With this technique we find that the Hyades and Preasepe have the same chemical signature when G dwarfs and K giants are considered. Despite a certain level of inhomogeneity in each cluster, it is still possible to clearly distinguish the chemical signature of the older cluster Ruprecht 147 when compared to the Hyades and Praesepe.

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High-precision stellar abundances of the elements: methods and applications

Nissen

Gustafsson

2018

Astron Astrophys Rev

102

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Efficient spectrographs at large telescopes have made it possible to obtain high-resolution spectra of stars with high signal-to-noise ratio and advances in model atmosphere analyses have enabled estimates of high-precision differential abundances of the elements from these spectra, i.e. with errors in the range 0.01 -0.03 dex for F, G, and K stars.Methods to determine such high-precision abundances together with precise values of effective temperatures and surface gravities from equivalent widths of spectral lines or by spectrum synthesis techniques are outlined, and effects on abundance determinations from using a 3D non-LTE analysis instead of a classical 1D LTE analysis are considered.The determination of high-precision stellar abundances of the elements have led to the discovery of unexpected phenomena and relations with important bearings on the astrophysics of galaxies, stars, and planets, i.e. i) Existence of discrete stellar populations within each of the main Galactic components (disk, halo, and bulge) providing new constraints on models for the formation of the Milky Way. ii) Differences in the relation between abundances and elemental condensation temperature for the Sun and solar twins suggesting dust-cleansing effects in proto-planetary disks and/or engulfment of planets by stars; iii) Differences in chemical composition between binary star components and between members of open or globular clusters showing that star-and cluster-formation processes are more complicated than previously thought; iv) Tight relations between some abundance ratios and age for solar-like stars providing new constraints on nucleosynthesis and Galactic chemical evolution models as well as the composition of terrestrial exoplanets.We conclude that if stellar abundances with precisions of 0.01 -0.03 dex can be achieved in studies of more distant stars and stars on the giant and supergiant branches, many more interesting future applications, of great relevance to stellar and galaxy evolution, are probable. Hence, in planning abundance surveys, it is important to carefully balance the need for large samples of stars against the spectral resolution and signal-to-noise ratio needed to obtain high-precision abundances. Furthermore, it is an advantage to work differentially on stars with similar atmospheric parameters, because then a simple 1D LTE analysis of stellar spectra may be sufficient. However, when determining high-precision absolute abundances or differential abundance between stars having more widely different parameters, e.g. metal-poor stars compared to the Sun or giants to dwarfs, then 3D non-LTE effects must be taken into account.

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Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars

Cited by 59 publications

References 130 publications

An empirical metallicity tracer in CEMP and C-normal stars

An empirical metallicity tracer in CEMP and C-normal stars

Differential abundances of open clusters and their tidal tails: Chemical tagging and chemical homogeneity

High-precision stellar abundances of the elements: methods and applications

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