Binary stellar systems form a large fraction of the Galaxys stars. They are useful as laboratories for studying the physical processes taking place within stars, and must be correctly taken into account when observations of stars are used to study th
e structure and evolution of the Galaxy. We present a sample of 12760 well-characterised double-lined spectroscopic binaries that are appropriate for statistical studies of the binary populations. They were detected as SB2s using a t-distributed stochastic neighbour embedding (t-SNE) classification and a cross-correlation analysis of GALAH spectra. This sample consists mostly of dwarfs, with a significant fraction of evolved stars and several dozen members of the giant branch. To compute parameters of the primary and secondary star ($T_{rm eff[1,2]}$, $log g_{[1,2]}$, [Fe/H], $V_{r[1,2]}$, $v_{rm mic[1,2]}$, $v_{rm broad[1,2]}$, $R_{[1,2]}$, and $E(B-V)$), we used a Bayesian approach that includes a parallax prior from Gaia DR2, spectra from GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. The derived stellar properties and their distributions show trends that are expected for a population of close binaries (a $<$ 10 AU) with mass ratios $0.5 leq q leq 1$. The derived metallicity of these binary stars is statistically lower than that of single dwarf stars from the same magnitude-limited sample.
Lithium abundances are presented for 91 dwarf and subgiant stars in the Galactic bulge. The analysis is based on line synthesis of the 7Li line at 6707 {AA} in high-resolution spectra obtained during gravitational microlensing events, when the bright
nesses of the targets were highly magnified. Our main finding is that the bulge stars at sub-solar metallicities, and that are older than about eight billion years, does not show any sign of Li production, that is, the Li trend with metallicity is flat (or even slightly declining). This indicates that no lithium was produced during the first few billion years in the history of the bulge. This finding is essentially identical to what is seen for the (old) thick disk stars in the Solar neighbourhood, and adds another piece of evidence for a tight connection between the metal-poor bulge and the Galactic thick disk. For the bulge stars younger than about eight billion years, the sample contains a group of stars at very high metallicities at [Fe/H]~+0.4 that have lithium abundances in the range A(Li)=2.6-2.8. In the Solar neighbourhood the lithium abundances have been found to peak at a A(Li)~3.3 at [Fe/H]~ +0.1 and then decrease by 0.4-0.5 dex when reaching [Fe/H]~+0.4. The few bulge stars that we have at these metallicities, seem to support this declining A(Li) trend. This could indeed support the recent claim that the low A(Li) abundances at the highest metallicities seen in the Solar neighbourhood could be due to stars from the inner disk, or the bulge region, that have migrated to the Solar neighbourhood.
The four main findings about the age and abundance structure of the Milky Way bulge based on microlensed dwarf and subgiant stars are: (1) a wide metallicity distribution with distinct peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41; (2) a high fra
ction of intermediate-age to young stars where at [Fe/H]>0 more than 35 % are younger than 8 Gyr, (3) several episodes of significant star formation in the bulge 3, 6, 8, and 11 Gyr ago; (4) the `knee in the alpha-element abundance trends of the sub-solar metallicity bulge appears to be located at a slightly higher [Fe/H] (about 0.05 to 0.1 dex) than in the local thick disk.
We present a detailed elemental abundance study of 90 F and G dwarf, turn-off and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (N
a, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. We find that the Galactic bulge has a wide metallicity distribution with significant peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41. We also find a high fraction of intermediate-age to young stars: at [Fe/H]>0 more than 35 % are younger than 8 Gyr. For [Fe/H]<-0.5 most stars are 10 Gyr or older. We have also identified several episodes when significant star formation in the bulge happened: 3, 6, 8, and 12 Gyr ago. We further find that the knee in the alpha-element abundance trends of the sub-solar metallicity bulge is located at about 0.1 dex higher [Fe/H] than in the local thick disk. The Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time there are additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. For instance, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy.
The study of the Milky Way relies on our ability to interpret the light from stars correctly. This calls for a reinvestigation of how reliably we can determine, e.g., iron abundances in such stars and how well they reproduce those of dwarf stars. Her
e we explore robust ways to determine the iron content of metal-rich giant stars. We aim to understand what biases and shortcomings widely applied methods suffer from. In this study we are mainly concerned with standard methods to analyse stellar spectra. This includes the analysis of individual lines to determine stellar parameters, analysis of the broad wings of certain lines (e.g., H$alpha$ and calcium lines) to determine effective temperature and surface gravity for the stars. For NGC 6528 we find that [Fe/H] = $+0.04$ dex with a scatter of $sigma=0.07$ dex, which gives an error in the derived mean abundance of 0.02 dex. Our work has two important conclusions for analysis of metal-rich red giant branch stars. 1) For spectra with S/N below about 35 per reduced pixel [Fe/H] become too high, 2) Determination of $T_{rm eff}$ using the wings of the H$alpha$ line results in [Fe/H] values about 0.1 dex higher than if excitational equilibrium is used. The last conclusion is perhaps not surprising as we expect NLTE effect to become more prominent in cooler stars and we can not use the the wings of the H$alpha$ line to determine $T_{rm eff}$ for the cool stars in our sample. We therefore recommend that in studies of metal-rich red giant stars care needs to be taken to obtain sufficient calibration data in order to be able to also use the cooler stars.
We present a summary of the splinter session Sun-like stars unlike the Sun that was held on 09 June 2016 as part of the Cool Stars 19 conference (Uppsala, Sweden). We discussed the main limitations (in the theory and observations) in the derivation o
f very precise stellar parameters and chemical abundances of Sun-like stars. We outlined and discussed the most important and most debated processes that can produce chemical peculiarities in solar-type stars. Finally, in an open discussion between all the participants we tried to identify new pathways and prospects towards future solutions of the currently open questions.
Several recent studies of Solar twins in the Solar neighbourhood have shown a tight correlation between various elemental abundances and age, in particular [Y/Mg]. If this relation is real and valid for other types of stars as well as elsewhere in th
e Galaxy it would provide a very powerful tool to derive ages of stars without the need to resort to determining their masses (evolutionary stage) very precisely. The method would also likely work if the stellar parameters have relatively large errors. The studies presented in the recent literature span a narrow range of [Fe/H]. By studying a larger sample of Solar neighbourhood dwarfs with a much larger range in [Fe/H], we find that the relation between [Y/Mg] and age depends on the [Fe/H] of the stars. Hence, it appears that the [Y/Mg] - age relation is unique to Solar analogues.
The origin of a new kinematically identified metal-poor stellar stream, the KFR08 stream, has not been established. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Ni, Zn, Sr, Y, Zr,
Ba, La, and Eu for 16 KFR08 stream members based on analysis of high resolution spectra. Based on the abundance ratios of 14 elements, we use the chemical tagging method to identify the stars which have the same chemical composition, and thus, might have a common birthplace, such as a cluster. Although three stars were tagged with similar elemental abundances ratios, we find that, statistically, it is not certain that they originate from a dissolved star cluster. This conclusion is consistent with the large dispersion of [Fe/H] ($sigma_{rm{[Fe/H]}} = 0.29$) among the 16 stream members. We find that our stars are $alpha$ enhanced and that the abundance patterns of the stream members are well matched to the thick disk. In addition, most of the stream stars have estimated stellar ages larger than 11 Gyr. These results, together with the hot kinematics of the stream stars, suggest that the KFR08 stream is originated from the thick disk population which was perturbed by a massive merger in the early universe.
The aim of this paper is to find lost siblings of the Sun by analyzing high resolution spectra. Finding solar siblings will enable us to constrain the parameters of the parental cluster and the birth place of the Sun in the Galaxy. The solar siblings
can be identified by accurate measurements of metallicity, stellar age and elemental abundances for solar neighbourhood stars. The solar siblings candidates were kinematically selected based on their proper motions, parallaxes and colours. Stellar parameters were determined through a purely spectroscopic approach and partly physical method, respectively. Comparing synthetic with observed spectra, elemental abundances were computed based on the stellar parameters obtained using a partly physical method. A chemical tagging technique was used to identify the solar siblings. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Ti, Cr, Fe, and Ni for 32 solar sibling candidates. Our abundances analysis shows that four stars are chemically homogenous together with the Sun. Technique of chemical tagging gives us a high probability that they might be from the same open cluster. Only one candidate HIP 40317 which has solar metallicity and age could be a solar sibling. We performed simulations of the Suns birth cluster in analytical Galactic model and found that most of the radial velocities of the solar siblings lie in the range $-10 leq mathrm{V_r}leq 10$ $mathrm{km~s^{-1}}$, which is smaller than the radial velocity of HIP 40317 $(mathrm{V_r} = 34.2~mathrm{km~s^{-1}})$, under different Galactic parameters and different initial conditions of the Suns birth cluster. The sibling status for HIP 40317 is not directly supported by our dynamical analysis.
ABRIDGED: METHODS: We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extreme
s...... The determination of stellar parameters and elemental abundances is based on a standard 1-D LTE analysis using equivalent width measurements in high-resolution (R=40000-110000) and high signal-to-noise (S/N=150-300) spectra obtained with..... RESULTS: .... Our data show that there is an old and alpha-enhanced disk population, and a younger and less alpha-enhanced disk population. While they overlap greatly in metallicity between -0.7<[Fe/H]<+0.1, they show a bimodal distribution in [alpha/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (Teff<5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. We furthermore find that the alpha-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the low-alpha stars mainly come from the outer Galactic disk........... We furthermore have discovered that a standard 1-D, LTE analysis, utilising ionisation and excitation balance of Fe I and Fe II lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off, and more evolved, stars, appears to be un-affected.
