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

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

Enrichment of the Galactic disc with neutron-capture elements: Gd, Dy, and Th

International audienceThe study of the origin of heavy elements is one of the main goals of nuclear astrophysics. In this paper, we present new observational data for the heavy r-process elements gadolinium (Gd, Z= 64), dysprosium (Dy, Z= 66), and thorium (Th, Z= 90) in a sample of 276 Galactic disc stars (–1.0 < [Fe/H] < + 0.3). The stellar spectra have a high resolution of 42 000 and 75 000, and the signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy, and Th have been determined by comparing the observed and synthetic spectra for three Gd lines (149 stars), four Dy lines (152 stars), and the Th line at 4019.13 Å (170 stars). For about 70 per cent of the stars in our sample, Gd and Dy are measured for the first time, and Th for 95 per cent of the stars. Typical errors vary from 0.07 to 0.16 dex. This paper provides the first extended set of Th observations in the Milky Way disc. Together with europium (Eu, Z= 63) data from our previous studies, we have compared these new observations with nucleosynthesis predictions and Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe] show the same behaviour of Eu. We study with GCE simulations the evolution of [Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu, either the Th production is metallicity dependent in case of a unique source of the r-process in the Galaxy, or the frequency of the Th-rich r-process source is decreasing with the increase in [Fe/H]

Enrichment of the Galactic disc with neutron-capture elements: Mo and Ru

We present new observational data for the heavy elements molybdenum (Mo, Z = 42) and ruthenium (Ru, Z= 44) in F-, G-, and K-stars belonging to different substructures of the Milky Way. The range of metallicity covered is -1.0 < [Fe/H] < + 0.3. The spectra of Galactic disc stars have a high resolution of 42 000 and 75 000 and signal-to-noise ratio better than 100. Mo and Ru abundances were derived by comparing the observed and synthetic spectra in the region of Mo I lines at 5506, 5533 Å for 209 stars and Ru I lines at 4080, 4584, 4757 Å for 162 stars using the LTE approach. For all the stars, the Mo and Ru abundance determinations are obtained for the first time with an average error of 0.14 dex. This is the first extended sample of stellar observations for Mo and Ru in the Milky Way disc, and together with earlier observations in halo stars it is pivotal in providing a complete picture of the evolution of Mo and Ru across cosmic time-scales

Stellar Parameters, Chemical composition and Models of chemical evolution

International audienc

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

Determinations of high-precision effective temperatures for giants based on spectroscopic criteria

Spectral lines with high and low excitation potentials respond differently to changes of the effective temperature (T eff), making the ratio of their depths (or equivalent widths) a very sensitive temperature indicator. We derive a set of 100 equations relating T eff to the line-depth ratios, calibrated against accurate (to within 1%) published temperature determinations for giants. These relations are used to determine very accurate temperatures for a sample of 110 giants with nearly solar metallicities based on high-resolution (R = 42 000) échelle spectra with high signal-to-noise ratios (SNRs). The calibration relations are valid for temperatures of 4000–7000 K (F2III–K4III). The internal errors of each of the calibration relations are below 95 K, and applying all these relations together to spectra with SNR = 100 reduces the errors to 5–25 K (1 ?). A major advantage of this technique is that it is independent from interstellar reddening, spectroscopic resolution, and line broadening due to rotation and microturbulence