There has been an incredibly large investment in obtaining high-resolution stellar spectra for determining chemical abundances of stars. This information is crucial to answer fundamental questions in ...astronomy by constraining the formation and evolution scenarios of the Milky Way as well as the stars and planets residing in it.
We have just entered a new era, in which chemical abundances of FGK-type stars are being produced at industrial scales, and in which the observations, reduction, and analysis of the data are automatically performed by machines. Here, we review the latest human efforts to assess the accuracy and precision of such industrial abundances by providing insights into the steps and uncertainties associated with the process of determining stellar abundances.
We also provide a description of current and forthcoming spectroscopic surveys, focusing on their reported abundances and uncertainties. This allows us to identify which elements and spectral lines are best and why. Finally, we make a brief selection of main scientific questions the community is aiming to answer with abundances.
Uncertainties in abundances need to be disentangled into random and systematic components.
Precision can be increased by applying differential or data-driven methods based on accurate data.
High-resolution and signal-to-noise spectra provide fundamental data that can be used to calibrate lower-resolution and signal-to-noise spectra of millions of stars.
Different survey calibration strategies must agree on a common set of reference stars to create data products that are consistent.
Data products provided by individual groups must be published using standard formats to ensure straightforward applicability.
Abstract
The age–metallicity relation (AMR) is fundamental to study the formation and evolution of the disk. Observations have shown that this relation has a large scatter, which cannot be explained ...by observational errors only. That scatter is hence attributed to the effects of radial migration in which stars tracing different chemical evolution histories in the disk get mixed. However, the recent study of Nissen et al., using high-precision observational data of solar-type stars, found two relatively tight AMRs. One sequence of older and metal-richer stars probably traces the chemical enrichment history of the inner disk while the other sequence of younger and metal-poorer stars traces the chemical enrichment history of the outer disk. If uncertainties in age measurements increase, these sequences mix, explaining the scatter of the one relation observed in other studies. This work follows up on these results by analyzing an independent sample of red clump giants observed by APOGEE. Because ages for red giants are significantly more uncertain, the C/N ratios are considered as a proxy for age. This larger data set is used to investigate these relations at different Galactic radii, finding that these distinct sequences exist only in the solar neighborhood. The APOGEE data set is further used to explore different abundance and kinematical planes to shed light on the nature of these populations.
ABSTRACT
We exploit the Mg/Mn-Al/Fe chemical abundance plane to help identify nearby halo stars in the 14th data release from the APOGEE survey that have been accreted on to the Milky Way. Applying a ...Gaussian Mixture Model, we find a ‘blob’ of 856 likely accreted stars, with a low disc contamination rate of ∼7 per cent. Cross-matching the sample with the second data release from Gaia gives us access to parallaxes and apparent magnitudes, which place constraints on distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this enables us to estimate new ages for the accreted stars, with typical uncertainties of ∼20 per cent. This does not account for systematic uncertainties. Our new catalogue is further supplemented with estimates of orbital parameters. The blob stars span Fe/H between −2.5 to −0.5, and Mg/Fe between −0.1 to 0.5. They constitute ∼30 per cent of the metal-poor (Fe/H < −0.8) halo at Fe/H ∼ −1.4. Our new ages mainly range between 8 to 13 Gyr, with the oldest stars the metal-poorest, and with the highest Mg/Fe abundance. If the blob stars are assumed to belong to a single progenitor, the ages imply that star formation lasted 5 Gyr after which the system merged with our Milky Way around 8 Gyr ago. Dynamical arguments suggest that such a single progenitor would have had a total mass of $\sim 10^{11}\, \mathrm{M}_{\odot }$, similar to that found by other authors using chemical evolution models and simulations.
The physical processes driving chemical evolution in the Milky Way can be probed using the distribution of abundances in low-mass FGK type stars in space phase at different times. During their final ...stages of evolution, stars experience nucleosynthesis several times, each at different timescales and producing different chemical elements. Finding abundance ratios that have simple variations across cosmic times therefore remains a challenge. Using the sample of 80 solar twins for which ages and abundances of 30 elements have been measured with high precision, we searched for all possible abundance ratio combinations that show linear trends with age. We found 55 such ratios, all combining an
n
-capture element and another element produced by different nucleosynthesis channels. We recovered the ratios of Y/Mg, Ba/Mg, and Al/Y that have been reported previously in the literature, and found that C/Ba depends most strongly on age, with a slope of 0.049 ± 0.003 dex Gyr
−1
. This imposes constraints on the magnitude of the time dependency of abundance ratios in solar twins. Our results suggest that
s
-process elements, in lieu of Fe, should be used as a reference for constraining chemical evolution models of the solar neighbourhood. Our study illustrates that a wide variety of chemical elements measured in high-resolution spectra is key to meeting the current challenges in understanding the formation and evolution of our Galaxy.
Abstract
Using 17 chemical elements as a proxy for stellar DNA, we present a full phylogenetic study of stars in the solar neighbourhood. This entails applying a clustering technique that is widely ...used in molecular biology to construct an evolutionary tree from which three branches emerge. These are interpreted as stellar populations that separate in age and kinematics and can be thus attributed to the thin disc, the thick disc and an intermediate population of probable distinct origin. We further find six lone stars of intermediate age that could not be assigned to any population with enough statistical significance. Combining the ages of the stars with their position on the tree, we are able to quantify the mean rate of chemical enrichment of each of the populations, and thus show in a purely empirical way that the star formation rate in the thick disc is much higher than that in the thin disc. We are also able to estimate the relative contribution of dynamical processes such as radial migration and disc heating to the distribution of chemical elements in the solar neighbourhood. Our method offers an alternative approach to chemical tagging methods with the advantage of visualizing the behaviour of chemical elements in evolutionary trees. This offers a new way to search for ‘common ancestors’ that can reveal the origin of solar neighbourhood stars.
The Consistency of Chemical Clocks among Coeval Stars Espinoza-Rojas, Francisca; Chanamé, Julio; Jofré, Paula ...
Astrophysical journal/The Astrophysical journal,
10/2021, Letnik:
920, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The abundance ratios of some chemical species have been found to correlate with stellar age, leading to the possibility of using stellar atmospheric abundances as stellar age indicators. ...These chemical clocks have already been calibrated with solar twins and open clusters, but it remains to be seen whether they can be effective at identifying coeval stars in a population that spans a broad parameter space (i.e., the promise of chemical tagging). Since the components of wide binaries are known to be stars of common origins, they constitute ideal laboratories for testing the usefulness of chemical clocks for the age dating of field stars. Using a combination of our new measurements and literature data on wide binaries, we show for the first time that chemical clocks are even more consistent among the components of wide binaries than their individual abundances. Moreover, the special case of HIP 34426/HIP 34407 may indicate that chemical clocks are consistent for coeval stars even when those individual abundances are not. If the assumption that chemical clocks are reliable age indicators is correct, this would constitute the first statistically significant evidence that the components of wide binaries are indeed coeval, validating a large body of published work that relies on that to be the case. Furthermore, our results provide strong evidence that chemical clocks indeed carry important information about stellar birthplaces and chemical evolution, and thus we propose that including them in chemical tagging efforts may facilitate the identification of now-dissolved stellar groups.
ABSTRACT Stellar elemental abundances are important for understanding the fundamental properties of a star or stellar group, such as age and evolutionary history, as well as the composition of an ...orbiting planet. However, as abundance measurement techniques have progressed, there has been little standardization between individual methods and their comparisons. As a result, different stellar abundance procedures determine measurements that vary beyond the quoted error for the same elements within the same stars. The purpose of this paper is to better understand the systematic variations between methods and offer recommendations for producing more accurate results in the future. We invited a number of participants from around the world (Australia, Portugal, Sweden, Switzerland, and the United States) to calculate 10 element abundances (C, O, Na, Mg, Al, Si, Fe, Ni, Ba, and Eu) using the same stellar spectra for four stars (HD 361, HD 10700, HD 121504, and HD 202206). Each group produced measurements for each star using (1) their own autonomous techniques, (2) standardized stellar parameters, (3) a standardized line list, and (4) both standardized parameters and a line list. We present the resulting stellar parameters, absolute abundances, and a metric of data similarity that quantifies the homogeneity of the data. We conclude that standardization of some kind, particularly stellar parameters, improves the consistency between methods. However, because results did not converge as more free parameters were standardized, it is clear there are inherent issues within the techniques that need to be reconciled. Therefore, we encourage more conversation and transparency within the community such that stellar abundance determinations can be reproducible as well as accurate and precise.
ABSTRACT
Planets and stars ultimately form out of the collapse of the same cloud of gas. Whilst planets, and planetary bodies, readily loose volatiles, a common hypothesis is that they retain the ...same refractory composition as their host star. This is true within the Solar system. The refractory composition of chondritic meteorites, Earth, and other rocky planetary bodies are consistent with solar, within the observational errors. This work aims to investigate whether this hypothesis holds for exoplanetary systems. If true, the internal structure of observed rocky exoplanets can be better constrained using their host star abundances. In this paper, we analyse the abundances of the K-dwarf, G200-40, and compare them to its polluted white dwarf companion, WD 1425+540. The white dwarf has accreted planetary material, most probably a Kuiper belt-like object, from an outer planetary system surviving the star’s evolution to the white dwarf phase. Given that binary pairs are chemically homogeneous, we use the binary companion, G200-40, as a proxy for the composition of the progenitor to WD 1425+540. We show that the elemental abundances of the companion star and the planetary material accreted by WD 1425+540 are consistent with the hypothesis that planet and host-stars have the same true abundances, taking into account the observational errors.
Abstract Phylogenetic methods have long been used in biology and more recently have been extended to other fields—for example, linguistics and technology—to study evolutionary histories. Galaxies ...also have an evolutionary history and fall within this broad phylogenetic framework. Under the hypothesis that chemical abundances can be used as a proxy for the interstellar medium’s DNA, phylogenetic methods allow us to reconstruct hierarchical similarities and differences among stars—essentially, a tree of evolutionary relationships and thus history. In this work, we apply phylogenetic methods to a simulated disk galaxy obtained with a chemodynamical code to test the approach. We found that at least 100 stellar particles are required to reliably portray the evolutionary history of a selected stellar population in this simulation, and that the overall evolutionary history is reliably preserved when the typical uncertainties in the chemical abundances are smaller than 0.08 dex. The results show that the shapes of the trees are strongly affected by the age–metallicity relation, as well as the star formation history of the galaxy. We found that regions with low star formation rates produce shorter trees than regions with high star formation rates. Our analysis demonstrates that phylogenetic methods can shed light on the process of galaxy evolution.
ABSTRACT We report the discovery of K2-56b, a high-density sub-Neptune exoplanet, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission, ground-based radial velocity (RV) ...follow-up from HARPS and high-resolution lucky and adaptive optics imaging obtained using AstraLux and MagAO, respectively. The host star is a bright (V = 11.04, Ks = 9.37), slightly metal-poor (Fe/H = −0.15 0.05 dex) solar analogue located at pc from Earth, for which we find a radius of and a mass of . A joint analysis of the K2 photometry and HARPS RVs reveal that the planet is in a 42 day orbit around its host star, has a radius of , and a mass of . Although the data at hand put the planet in the region of the mass-radius diagram where we could expect planets with a pure rock (i.e., magnesium silicate) composition using two-layer models (i.e., between rock/iron and rock/ice compositions), we discuss more realistic three-layer composition models which can explain the high density of the discovered exoplanet. The fact that the planet lies in the boundary between "possibly rocky" and "non-rocky" exoplanets makes it an interesting planet for future RV follow-up.
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