Abundances of metal-weak thick-disc candidates Bonifacio, P.; Centurion, M.; Molaro, P.
Monthly Notices of the Royal Astronomical Society,
10/1999, Letnik:
309, Številka:
2
Journal Article
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High-resolution spectra of five candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen are analysed to determine their chemical abundances. The low abundance of all the objects has ...been confirmed, with metallicity reaching Fe/H = −2.9. However, for three objects the astrometric data from the Hipparcos catalogue suggest they are true halo members. The remaining two, for which proper-motion data are not available, may have disc-like kinematics. It is therefore clear that it is useful to address properties of putative metal-weak thick-disc stars only if they possess full kinematic data. For CS 22894-19 an abundance pattern similar to those of typical halo stars is found, suggesting that chemical composition is not a useful discriminant between thick-disc and halo stars. CS 29529-12 is found to be C-enhanced with C/Fe = +1.0; other chemical peculiarities involve the s-process elements: Sr/Fe = −0.65 and Ba/Fe = +0.62, leading to a high Ba/Sr, considerably larger than that found in more metal-rich carbon-rich stars, but similar to those in LP 706-7 and LP 625-44, discussed by Norris et al. Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify three bona fide members, which support the existence of a metal-poor tail of the thick disc, at variance with a claim to the contrary by Ryan & Lambert.
We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly α absorbers (DLAs) obtained by means of high resolution ($FWHM \simeq 7$ km s-1) UVES/VLT spectra. In addition ...to these measurements we have compiled data from all DLAs with measurements of nitrogen and α-capture elements (O, S or Si) available in the literature, including all HIRES/Keck and UVES/VLT data for a total of 32 systems, i.e. the largest sample investigated so far. We find that N/α ratios are distributed in two groups: 75% of the DLAs show a mean value of $\rm N/\alpha = -0.87$ with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at $\rm N/\alpha = -1.45$ with an even lower dispersion of 0.05 dex. The high $\rm N/\alpha \simeq -0.9$ plateau is consistent with the one observed in metal-poor $\ion{H}{ii}$ regions of blue compact dwarf (BCD) galaxies ($\rm N/\alpha =-0.73 \pm 0.13$), while the $\rm N/\alpha \simeq -1.5$ values are the lowest ever observed in any astrophysical site. These low N/α ratios are real and are not due to ionization effects. They provide crucial evidence against the primary production of N by massive stars as being responsible for the plateau at $-0.9/$ -0.7 dex observed in DLAs and BCD galaxies. The transition between the low-N ($\rm N/\alpha\simeq-1.5$) and high-N ( $\rm N/\alpha \simeq -0.9$) DLAs occurs at a nitrogen abundance of $\rm N/H \simeq -2.8$, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The $\rm N/\alpha \simeq -1.5$ values and their low dispersion are consistent with a modest production of primary N in massive stars; however, due to the limited sample, specially for the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed.
The influence of Maillard reaction was studied on the recovery of free chitosan in cooked food prepared with this biopolymer and yeast. Formulations with different ingredients were prepared and ...heated. The percentage recovery of free chitosan was calculated. The formulations contained chitosan, yeast, and reducing sugars. In order to confirm the interaction between amino groups of chitosan and carbonyl groups of reducing sugars, theoretical model systems were prepared and x-ray diffraction studies were made on these samples. When chitosan was used as an ingredient in cooked food fermented with yeast and sucrose, it lost some of its functional properties because of the Maillard reaction.
Developing femtosecond resolution methods for directly observing structural dynamics is critical to understanding complex photochemical reaction mechanisms in solution. We have used two recent ...developments, ultrafast mega-electron-volt electron sources and vacuum compatible sub-micron thick liquid sheet jets, to enable liquid-phase ultrafast electron diffraction (LUED). We have demonstrated the viability of LUED by investigating the photodissociation of tri-iodide initiated with a 400 nm laser pulse. This has enabled the average speed of the bond expansion to be measured during the first 750 fs of dissociation and the geminate recombination to be directly captured on the picosecond time scale.
Absorption-line systems detected in quasar spectra can be used to compare the value of the fine-structure constant, alpha , measured today on Earth with its value in distant galaxies. In recent ...years, some evidence has emerged of small temporal and also spatial variations in a on cosmological scales. These variations may reach a fractional level of approximate10 ppm (parts per million). To test these claims we are conducting a Large Program of observations with the Very Large Telescope's Ultraviolet and Visual Echelle Spectrograph (UVES), and are obtaining high-resolution and high signal-to-noise ratio (S/N approximate100) UVES spectra calibrated specifically for this purpose. Here we analyse the first complete quasar spectrum from this programme, that of HE 2217-2818. The absorbers towards quasar HE2217-2818 reveal no evidence of any variation in a at the 3-ppm precision level. Our constraint of delta alpha / alpha = +1.3 + or - 2.4substat + or - l.0subsysppm is not inconsistent with this expectation.
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•Solution-state NMR spectroscopy suggests PDs and/or PCDs Cdots structures.•DFT calculations propose a core–shell structure which explains experimental features.•The methodology ...employed could structurally characterize other kinds of Cdots.
A combination of experimental and theoretical results was used for the structural characterization of Carbon dots (Cdots). The synthesis of the Cdots was performed using glucose as a precursor and via the microwave method (“bottom-up” route) as a simple and environmentally friendly method. The experimental data obtained by UV–visible, fluorescence, IR and solution-state NMR spectroscopy techniques were correlated with density functional theory calculations results. Particularly, the results obtained by solution-state NMR spectroscopy suggested Cdots structures of Polymer dots (PDs) and/or polymer Carbon dots (PCDs) kind. PDs are characterized by being formed only of polymer structures and PCDs by being formed of a small carbonized core surrounded by polymer structures. Calculations allow us to predict the UV–visible absorption and IR spectra behavior of the species detected by the NMR technique, and propose a structure that can explain the experimentally observed features and infer the presence or absence of a carbon core in the proposed structures. Density of states curves were used to analyze and interpret the theoretical spectra peaks. Additionally, a calculation of the IR spectra for the models proposed, was performed. The complement of experimental and theoretical results represents a useful tool for the structural characterization of Cdots.
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