arXiv subjects

Lyudmila Mashonkina

Publications and source records attributed to Lyudmila Mashonkina.

24 records · Page 2Linked to original sources

arXiv2015

Atomic and Molecular Data: Division B / Commission 14

Commission 14 has been operating for almost a century. It was one of the 32 Standing IAU Commissions and established in 1919. The main purpose of Commission 14 is to foster interactions between the astronomical community and those conducting research on atoms, molecules, and solid state particles to provide data vital to reducing and analysing astronomical observations and performing theoretical investigations. Although Commission 14 terminated at the Honolulu General Assembly, the field Atomic and Molecular Data remains with the IAU. By call of the IAU, the Commission 14 OC initiated a proposal for a new Commission on Laboratory Astrophysics that was approved. Commission on Laboratory Astrophysics will be a natural evolution of Commission 14.

physics.atom-ph↗

arXiv2014

The Hamburg/ESO R-process Enhanced Star survey (HERES) IX. Constraining pure r-process Ba/Eu abundance ratio from observations of r-II stars

The oldest stars born before the onset of the main s-process are expected to reveal a pure r-process Ba/Eu abundance ratio. We revised barium and europium abundances of selected very metal-poor (VMP) and strongly r-process enhanced (r-II) stars to evaluate an empirical r-process Ba/Eu ratio. Our calculations were based on non-local thermodynamic equilibrium (NLTE) line formation for Ba II and Eu II in the classical 1D MARCS model atmospheres. Homogeneous stellar abundances were determined from the Ba II subordinate and resonance lines by applying a common Ba isotope mixture. We used high-quality VLT/UVES spectra and observational material from the literature. For most investigated stars, NLTE leads to a lower Ba, but a higher Eu abundance. The resulting elemental ratio of the NLTE abundances amounts, on average, log(Ba/Eu) = 0.78+-0.06. This is a new constraint to pure r-process production of Ba and Eu. The obtained Ba/Eu abundance ratio of the r-II stars supports the corresponding Solar System r-process ratio as predicted by recent Galactic chemical evolution calculations of Bisterzo, Travaglio, Gallino, Wiescher, and Kappeler. We present the NLTE abundance corrections for lines of Ba II and Eu II in the grid of VMP model atmospheres.

astro-ph.SR↗

arXiv2012

Astrophysical tests of atomic data important for stellar Mg abundance determinations

Magnesium abundances of cool stars with different metallicities are important for understanding the galactic chemical evolution. This study tests atomic data used in stellar magnesium abundance analyses. We evaluate non-local thermodynamical equilibrium (NLTE) line formation for Mg I using the most up-to-date theoretical and experimental atomic data available so far and check the Mg abundances from individual lines in the Sun, four well studied A-type stars, and three reference metal-poor stars. With the adopted gf-values, NLTE abundances derived from the Mg I 4703 A, 5528 A, and Mg Ib lines are consistent within 0.05 dex for each A-type star. The same four Mg I lines in the solar spectrum give consistent NLTE abundances at $\log N_{\rm Mg}/N_{\rm H} = -4.45$, when correcting the van der Waals damping constants inferred from the perturbation theory. Inelastic Mg+H collisions as treated by Barklem, Belyaev, Spielfiedel, Guitou, and Feautrier serve as efficient thermalizing process for the statistical equilibrium of Mg I in the atmospheres of metal-poor stars. The use of the Mg+H collision data improves Mg abundance determinations for HD 84937 and HD 122563, though does not remove completely the differences between different lines.

astro-ph.SR↗

arXiv2012

Non-LTE effects on the lead and thorium abundance determinations for cool stars

Knowing accurate Pb abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving Th abundances permits a nucleo-chronometric age determination of the star. We improve the calculation of the Pb I and Th II lines in stellar atmospheres based on non-LTE line formation and evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters. Comprehensive model atoms for Pb I and Th II are presented. The departures from LTE lead to systematically depleted total absorption in the Pb I lines and positive abundance corrections. Non-LTE removes the discrepancy between the solar and the meteoritic Pb abundance. With the Holweger & Mueller (1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001 and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results provide strong evidence for universal Pb/Eu relative r-process yields during course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars suggesting that the s-process synthesis of Pb started as early as the time when Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the -1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened Th II lines. Overall, the abundance correction does not exceed +0.2 dex when collisions with H I atoms are taken into account in non-LTE calculations.

astro-ph.GA↗

arXiv2009

Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Non-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and $β$ Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.

astro-ph.SR↗

arXiv2004

Magnesium abundances in mildly metal-poor stars from different indicators

We present magnesium abundances derived from high resolution spectra using several Mg I and two high excitation Mg II lines for 19 metal-poor stars with [Fe/H] values between -1.1 and +0.2. The main goal is to search for systematic differences in the derived abundances between the two ionisation state lines. Our analysis shows that the one-dimensional LTE and N-LTE study finds a very good agreement between these features. The [Mg/Fe] vs. [Fe/H] relationship derived, despite the small sample of stars, is also in agreement with the classical figure of increasing [Mg/Fe] with decreasing metallicity. We find a significant scatter however, in the [Mg/Fe] ratio at [Fe/H]$\sim -0.6$ which is currently explained as a consequence of the overlap at this metallicity of thick and thin disk stars, which were probably formed from material with different nucleosynthesis histories. We speculate on the possible consequences of the agreement found between Mg I and Mg II lines on the very well known oxygen problem in metal-poor stars. We also study the [O/Mg] ratio in the sample stars using oxygen abundances from the literature and find that the current observations and nucleosynthetic predictions from type II supernovae disagree. We briefly discuss some alternatives to solve this discrepancy.

astro-ph↗