Non-local thermodynamic equilibrium line formation for Si i–ii–iii in A–B stars and the origin of Si ii emission lines in ι Her

Abstract

A comprehensive model atom was developed for Si i-ii-iii using the most up-todate atomic data available so far. Based on the non-local thermodynamic equilibrium (NLTE) line formation for Si i, Si ii, and Si iii and high-resolution observed spectra, we determined the NLTE abundances for a sample of nine unevolved A9 to B3-type stars with well determined atmospheric parameters. For each star, NLTE reduces substantially the line-to-line scatter for Si ii compared with the LTE case and leads to consistent mean abundances from lines of different ionisation stages. In the hottest star of our sample, ι Her, Si ii is subject to overionisation that drives emission in the lines arising from the high-excitation doublet levels. Our NLTE calculations reproduced 10 emission lines of Si ii observed in ι Her. The same overionisation effect leads to greatly weakened Si ii lines, which are observed in absorption in ι Her. Large positive NLTE abundance corrections (up to 0.98 dex for 5055Å) were useful for achieving consistent mean abundances from lines of the two ionisation stages, Si ii and Si iii. It was found that the NLTE effects are overestimated for the Si ii 6347, 6371Å doublet in ι Her, while the new model atom works well for the cooler stars. At this stage, we failed to understand this problem. We computed a grid of the NLTE abundance corrections for lines of Si i, Si ii, and Si iii in the model atmospheres with effective temperatures and surface gravities characteristic of unevolved A-B type stars.

Observational constraints on the origin of the elements. IV: The standard composition of the Sun

“…Our NLTE abundances for all lines in the list are ∼ 0.03 to 0.05 dex lower compared to LTE. The most recent NLTE estimate of the solar Si abundance, based on the NLTE line-by-line spectrum synthesis, was presented in Mashonkina (2020). They find that NLTE abundances obtained from the solar Si I lines are on average 0.02 to 0.1 dex lower compared to LTE (their Table 2), fully in agreement with our findings.…”

Direct observation of the radial magnetic field gradient in HD58260 from spectropolarimetry of NLTE lines in emission

“…Systematic differences in the optical depth between observed and LTE equivalent widths in magnetic stars, as a class, are commonly interpreted as due to a vertical stratification of elements (Catanzaro et al 2016(Catanzaro et al , 2020. Nevertheless, for these stars the attempt to reproduce observed metal emission lines by taking into account the NLTE effect works in the direction to smooth abundance gradients (Alexeeva et al 2016(Alexeeva et al , 2019Mashonkina 2020;Mashonkina et al 2020). Remaining differences between observed and theoretical lines could be a matter of present limits of NLTE modeling that needs to be improved for treating the upper atmospheric regions, or the consequence of level populations distorted by the far-UV (Wahlgren & Hubrig 2004) and/or X-ray (Mitskevich & Tsymbal 1992) radiation from the magnetosphere wrapping this class of stars (Shore & Brown 1990).…”

“…These authors suggested that in this class of stars emission lines are produced by Non-Local Thermodynamic Equilibrium (NLTE) effects because of source functions rising with the atmosphere height without the necessity of a temperature inversion (chromosphere). A conclusion theoretically supported by Alexeeva et al (2016Alexeeva et al ( , 2018Alexeeva et al ( , 2019; Mashonkina (2020); Mashonkina et al (2020). From an analysis of line profiles of a sample of early B-type stars, Sadakane & Nishimura (2017) concluded that emission and absorption lines are equally broadened by the stellar rotation, supplying an observational proof that emission lines are from the outer stellar atmosphere and not from a circumstellar environment.…”

Inhomogeneous dust eclipses in young stars. The case of CQ Tauri

“…Departures from LTE can introduce more diversity into the limb darkening, and enhance individual differences in the line distortions; in some cases, non-LTE effects can also lead to appearance of the emission lines (Alexeeva et al 2016;Sitnova et al 2018;Mashonkina 2020). To illustrate how the diversity in the line limbdarkening can impact the distortions, we considered a simplest case of linear limb-darkening 1 − 𝜀 + 𝜀𝜇 with 𝜀 = 0, 1, −1; the corresponding profiles are shown in Fig.…”