Non-local Thermodynamic Equilibrium Abundances Research Articles

The Hamburg/ESO R-process Enhanced Star survey (HERES)

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.

STATISTICAL EQUILIBRIUM OF COPPER IN THE SOLAR ATMOSPHERE

Non-local thermodynamic equilibrium (NLTE) line formation for neutral copper in the one-dimensional solar atmospheres is presented for the atomic model, including 96 terms of Cui and the ground state of Cuii .T he accurate oscillator strengths for all the line transitions in model atom and photoionization cross sections were calculated using the R-matrix method in the Russell–Saunders coupling scheme. The main NLTE mechanism for Cui is the ultraviolet overionization. We find that NLTE leads to systematically depleted total absorption in the Cui lines and, accordingly, positive abundance corrections. Inelastic collisions with neutral hydrogen atoms produce minor effects on the statistical equilibrium of Cui in the solar atmosphere. For the solar Cui lines, the departures from LTE are found to be small, the mean NLTE abundance correction of ∼0.01 dex. It was found that the six low-excitation lines, with excitation energy of the lower level Eexc 1.64 eV, give a 0.14 dex lower mean solar abundance compared to that from the six Eexc > 3.7 eV lines, when applying experimental gf -values of Kock & Richter. Without the two strong resonance transitions, the solar mean NLTE abundance from 10 lines of Cui is log e� (Cu) = 4.19 ± 0.10, which is consistent within the error bars with the meteoritic value 4.25 ± 0.05 of Lodders et al. The discrepancy between Eexc = 1.39–1.64 eV and Eexc > 3.7 eV lines can be removed when the calculated gf -values are adopted and a mean solar abundance of log e� (Cu) = 4.24 ± 0.08 is derived.

NLTE analysis of Mn and Co in metal-poor stars

We analyse the statistical equilibrium of Mn and Co in the atmospheres of subdwarfs and subgiants of different metallicities. Significant departures from local thermodynamic equilibrium (LTE) level populations are found for the neutral ions. They are related to overionization in Co I; deviations from LTE in Mn I are due to the strong radiative processes in discrete transitions as long as the solar metallicities and temperatures are considered. In environments where the radiation field is amplified, such as metal-poor stars with solar or supersolar effective temperatures, overionization in Mn I is the dominant process. The ground states of the singly ionized species are almost unaffected by non-local thermodynamic equilibrium (NLTE). Differential analyses of Mn and Co is carried out for 18 stars in the metallicity range - 2.5<[Fe/H]<0. The abundances are derived by the method of spectrum synthesis. The NLTE abundances of Mn and Co in metal-poor stars are higher than the LTE abundances. The difference [El/Fe]NLTE- [El/Fe]LTE increases with decreasing [Fe/H] and reaches 0.4 dex for Mn and 0.6 dex for Co in the most metal-poor stars. [Mn/Fe] is slightly subsolar all over the metallicity range studied here; [Co/Fe] ratios steadily increase with decreasing Fe abundances in halo and thick-disc stars.

New insights into the nature of the peculiar star θ Carinae

Context. θ Carinae belongs to a group of peculiar early-type stars (OBN) with enhanced nitrogen and carbon deficiency. It is also known as a binary system, but it is not clear yet whether the chemical anomalies can be explained by mass transfer between the two components. On the basis of the previously reported spectral variability of a few metal lines it may be expected that θ Car possesses a weak magnetic field. Aims. A study of the physical nature of this hot massive binary which is furthermore a well-known blue straggler lying ∼2 mag above the turnoff of the young open cluster IC 2602 is important to understand the origin of its strong chemical anomalies. Methods. We acquired high resolution spectroscopic and low resolution spectropolarimetric observations to achieve the following goals: a) to improve the orbital parameters to allow a more in-depth discussion on the possibility of mass transfer in the binary system; b) to carry out a non-local thermodynamic equilibrium (NLTE) abundance analysis; and c) to search for the presence of a magnetic field. Results. The study of the radial velocities using CORALIE spectra allowed us to significantly improve the orbital parameters. A comparative NLTE abundance analysis was undertaken for θ Car and two other early B-type stars with recently detected magnetic fields, τ Sco and ξ 1 CMa. The analysis revealed significantly different abundance patterns: a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in θ Car, while the oxygen abundance is roughly solar. For the stars ξ 1 CMa and τ Sco the carbon abundance is solar and, while an N excess is also detected, it is of much smaller amplitude (0.4−0.6 dex). Such an N overabundance is typical of the values already found for other slowly-rotating (magnetic) B-type dwarfs. For θ Car, we attribute instead the chemical peculiarities to a past episode of mass transfer between the two binary components. The results of the search for a magnetic field using FORS 1 at the VLT consisting of 26 measurements over a time span of ∼1.2 h are rather inconclusive: only few measurements have a significance level of 3σ. Although we detect a periodicity of the order of ∼8.8 min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the Hα and Hβ lines, these results have to be confirmed by additional time-resolved magnetic field observations.