Observing the metal-poor solar neighbourhood: A comparison of galactic chemical evolution predictions

© 2017 The Authors. Atmospheric parameters and chemical compositions for 10 stars with metallicities in the region of -2.2 < [Fe/H] < -0.6 were precisely determined using high-resolution, high signal-tonoise, spectra. For each star, the abundances, for 14-27 elements, were derived using both local thermodynamic equilibrium (LTE) and non-LTE (NLTE) approaches. In particular, differences by assuming LTE or NLTE are about 0.10 dex; depending on [Fe/H], Teff, gravity and element lines used in the analysis. We find that the O abundance has the largest error, ranging from 0.10 and 0.2 dex. The best measured elements are Cr, Fe, and Mn; with errors between 0.03 and 0.11 dex. The stars in our sample were included in previous different observational work. We provide a consistent data analysis. The data dispersion introduced in the literature by different techniques and assumptions used by the different authors is within the observational errors, excepting for HD103095. We compare these results with stellar observations from different data sets and a number of theoretical galactic chemical evolution (GCE) simulations. We find a large scatter in the GCE results, used to study the origin of the elements. Within this scatter as found in previous GCE simulations, we cannot reproduce the evolution of the elemental ratios [Sc/Fe] , [Ti/Fe], and [V/Fe] at different metallicities. The stellar yields from core-collapse supernovae are likely primarily responsible for this discrepancy. Possible solutions and open problems are discussed

The Behavior of α-Elements Abundances in the Thin and Thick Disks of the Galaxy

We have carried out the detailed analysis of 350 high-resolution spectra of FGK dwarfs and giants. Abundances of Fe, Si, Ca and Ni have been determined under LTE approximation, whereas abundances of Mg have been determined under NLTE approximation. Spatial velocities with an accuracy better than 1 km/s, as well as orbits, have been computed for dwarfs (or all) stars. They have been used to define 2 subsamples kinematically representative of the thin disk and the thick disk in order to highlight their respective properties. A transition occurs at [Fe/H] = -0.3. Stars more metal-rich than this value have a flat distribution with Zmax<1 kpc and σW<20 km/s, and a narrow distribution of [α/Fe]. There exist stars in this metallicity regime which cannot belong to the thin disk because of their excentric orbits, neither to the thick disk because of their low scale height. Several thin disk stars are identified down to [Fe/H] = - 0.80. Their Mg enrichment is lower than thick disk stars with the same metallicity. Both the dwarfs and the giants show a decrease of [α/Fe] with [Fe/H] in the thick disk

Chemical composition of the clump giants

The atmospheric parameters (T_{eff}, log g, [Fe/H], V_{t}) and Li, C, N, O, Na, Mg, Si, Ca, Ni, Y, Ba, La, Ce, Pr, Nd, and Eu abundances in 177 clump giants of the Galactic disc were determined. The selection of the clump stars was made applying a colour-absolute magnitude window to nearby Hipparcos stars. The effective temperatures were estimated by the line-depth-ratio method. The surface gravities (log g) were determined by two methods - the ionization balance of iron and the method based on fitting of the wings of the Ca I 6162.17 A line. The abundances of carbon and nitrogen were obtained from the molecular spectral synthesis, Mg and Na abundances were derived using non-LTE approximation. For Ba and Eu, the hyperfine structure was taken into account. The underabundance of carbon, overabundance of nitrogen, and ``normal" abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their temperature, chemical composition and the evolutionary tracks was explored. The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behaviour of the dependencies of O, Mg, Ca, and Si (?-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. Clump giants and giants of the ascending branch do not show an essential difference between n-capture element abundances

Chemical composition of stars in kinematical substructures of the galactic disk

The Y, Zr, La, Ce, Nd , Sm and Eu abundances were found in LTE approach, and the abundance of Ba was computed in NLTE approximation for 280 FGK dwarfs in the region of metallicity of - 1<[Fe]< + 0.3. The selection of stars belonging to thin and thick disks and the stream Hercules was made on kinematic criteria. The analysis of enrichment of the different substructures of the Galaxy with α-element (Mg, Si), the iron peak (Ni) and neutron-capture elements was carried out

Elemental abundances in the atmosphere of clump giants

The aim of this paper is to provide the fundamental parameters and abundances for a large sample of local clump giants with a high accuracy. The selection of clump stars for the sample group was made applying a color - absolute magnitude window to nearby Hipparcos stars. The abundances of carbon and nitrogen were obtained from molecular synthetic spectrum, the Mg and Na abundances were derived using the non-LTE approximation. The "classical" models of stellar evolution without atomic diffusion and rotation-induced mixing were employed. The atmospheric parameters (Teff, log g, [Fe/H], Vt) and Li, C, N, O, Na, Mg, Si, Ca and Ni abundances in 177 clump giants of the Galactic disc were determined. The underabundance of carbon, overabundance of nitrogen and "normal" abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their chemical composition and the evolutionary tracks was explored. The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behavior of the dependencies of O, Mg, Ca, Si (alpha-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. This allows one to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy

Chemical composition of the clump giants

The atmospheric parameters (T_{eff}, log g, [Fe/H], V_{t}) and Li, C, N, O, Na, Mg, Si, Ca, Ni, Y, Ba, La, Ce, Pr, Nd, and Eu abundances in 177 clump giants of the Galactic disc were determined. The selection of the clump stars was made applying a colour-absolute magnitude window to nearby Hipparcos stars. The effective temperatures were estimated by the line-depth-ratio method. The surface gravities (log g) were determined by two methods - the ionization balance of iron and the method based on fitting of the wings of the Ca I 6162.17 A line. The abundances of carbon and nitrogen were obtained from the molecular spectral synthesis, Mg and Na abundances were derived using non-LTE approximation. For Ba and Eu, the hyperfine structure was taken into account. The underabundance of carbon, overabundance of nitrogen, and ``normal" abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their temperature, chemical composition and the evolutionary tracks was explored. The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behaviour of the dependencies of O, Mg, Ca, and Si (?-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. Clump giants and giants of the ascending branch do not show an essential difference between n-capture element abundances

Disturbance of meromixis in saline Lake Shira (Siberia, Russia): possible reasons and ecosystem response

Saline Lake Shira (Southern Siberia, Russia) was meromictic through the observation period 2002-2015. During the under-ice periods of 2015 and 2016, complete mixing of the water column was recorded for the first time, and hydrogen sulphide temporarily disappeared from the water column of the lake; i.e. in those years the lake turned to holomixis. In the summer of 2015, a sharp increase in chlorophyll a, organic carbon, zooplankton, and phytoflagellates was observed in the lake, which was probably due to the release of nutrients from the monimolimnion. Purple sulfur bacteria completely disappeared from the lake after the first mixing in 2015, and did not reappear despite the restoration of meromixis in 2017. Thus, it was demonstrated that purple sulfur bacteria are sensitive to the weakening of the stratification of Lake Shira. Based on the data of the seasonal monitoring of temperature and salinity profiles over the period 2002-2017, it was presumed that the main cause of deep mixing in 2015 was the weakening of the salinity gradient due to strong wind impact and early ice retreat in the spring of 2014. In addition, it was shown that in previous years a significant contribution to the maintenance of meromixis was made by an additional influx of fresh water, which caused a rise in the lake level in the period 2002-2007. Thus, we identified a relationship between the stratification regime of the lake and the change in its level, which provides valuable information both for the forecast of water quality and for reconstruction of the Holocene climate humidity in this region of Southern Siberia from the sediment cores of Lake Shira

Disturbance of meromixis in saline Lake Shira (Siberia, Russia): possible reasons and ecosystem response

Saline Lake Shira (Southern Siberia, Russia) was meromictic through the observation period 2002-2015. During the under-ice periods of 2015 and 2016, complete mixing of the water column was recorded for the first time, and hydrogen sulphide temporarily disappeared from the water column of the lake; i.e. in those years the lake turned to holomixis. In the summer of 2015, a sharp increase in chlorophyll a, organic carbon, zooplankton, and phytoflagellates was observed in the lake, which was probably due to the release of nutrients from the monimolimnion. Purple sulfur bacteria completely disappeared from the lake after the first mixing in 2015, and did not reappear despite the restoration of meromixis in 2017. Thus, it was demonstrated that purple sulfur bacteria are sensitive to the weakening of the stratification of Lake Shira. Based on the data of the seasonal monitoring of temperature and salinity profiles over the period 2002-2017, it was presumed that the main cause of deep mixing in 2015 was the weakening of the salinity gradient due to strong wind impact and early ice retreat in the spring of 2014. In addition, it was shown that in previous years a significant contribution to the maintenance of meromixis was made by an additional influx of fresh water, which caused a rise in the lake level in the period 2002-2007. Thus, we identified a relationship between the stratification regime of the lake and the change in its level, which provides valuable information both for the forecast of water quality and for reconstruction of the Holocene climate humidity in this region of Southern Siberia from the sediment cores of Lake Shira

Abundances of neutron-capture elements (Mishenina+, 2013)

table4.dat 276x64 The stellar parameters and {alpha}-element and; Ni abundances in studied stars ; table5.dat 276x60 n-capture element abundances in studied stars2013yCat..35529128M - VizieR On-line Data Catalog: J/A+A/552/A128. Originally published in: 2013A&A...552A.128MTables with abundances for 276 stars of the thick disk, thin disk, and Hercules stream. (2 data files)