The NLTE Analyses of Carbon Emission Lines in the Atmospheres of O and B type Stars

TL;DR

This study models non-LTE line formation of carbon in O-B stars, predicting emission lines at specific temperatures, and finds consistent carbon abundances with cosmic standards across multiple stars.

Contribution

It introduces an updated model atom for carbon and analyzes NLTE effects on emission lines in O-B star atmospheres, providing new insights into stellar carbon abundances.

Findings

Emission lines appear at specific effective temperatures.

Carbon abundances from emission and absorption lines agree.

Average carbon abundance aligns with cosmic standards.

Abstract

We present a model atom for C I - C II - C III - C IV using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium (NLTE) line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emission lines of C II 9903~\AA\ and 18535~\AA\ to appear at effective temperature \Teff~$\geq$~17\,500~K, those of C II 6151~\AA\ and 6461~\AA\ to appear at \Teff~$>$~25\,000~K, and those of C III 5695, 6728--44, 9701--17~\AA\ to appear at \Teff~$\geq$~35\,000~K (log~$g$=4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C II 9903 and 18535~\AA, the upper levels are mainly populated from C III reservoir through the Rydberg states. For C III 5695 and 6728--44~\AA, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426--28~\AA\ which become optically thin in the photosphere. We analysed the lines of C I, C II, C III, and C IV for twenty-two O-B-type stars with temperature range between 15\,800 $\leq$~\Teff~$\leq$ 38\,000~K. Abundances from emission lines of C I, C II and C III are in agreement with those from absorption ones for most of the stars. We obtained log~$\epsilon_{\rm C}$=8.36$\pm$0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15~Mon and HD~42088 from emission and absorption lines are 8.27$\pm$0.11 and 8.31$\pm$0.11, respectively.