NLTE spectral analysis of the intermediate helium-rich subdwarf B star CPD-20{\deg}1123

TL;DR

This paper analyzes the intermediate helium-rich subdwarf B star CPD-20{ extdegree}1123 using NLTE stellar atmosphere models, providing insights into its atmospheric composition and testing the NLTE modeling approach at high temperatures.

Contribution

It presents the first application of revised model atoms for light metals with the NLTE atmosphere code and tests the code's validity at high temperatures.

Findings

He/H ratio of 0.13 ± 0.05 in CPD-20{ extdegree}1123

Effective temperature of 25,500 ± 1,000 K and log g of 5.3 ± 0.3

Validation of NLTE models for intermediate He-rich sdB stars

Abstract

Subdwarf B (sdB) stars are core helium-burning stars with stratified atmospheres. Their atmospheres are dominated by hydrogen (H) while the helium (He) and metal abundances are shaped by an interplay of gravitational settling and radiative levitation. However, a small fraction of these show spectra dominated by He I absorption lines. In between these groups of He-deficient and extreme He-rich sdBs, some are found to have intermediate surface He abundances. These objects are proposed to be young "normal" (He-deficient) sdBs for which the dynamical stratification of the atmosphere is still ongoing. We present an analysis of the optical spectrum of such an intermediate He-rich sdB, namely CPD-20{\deg}1123, by means of non-local thermodynamic equilibrium (NLTE) stellar atmosphere models. It has a He-to-H number ratio of $\mathrm{He/H} = 0.13 \pm 0.05$ and its effective temperature of $T_\mathrm{eff} = 25\,500 \pm 1\,000$ together with a surface gravity of $\log\,(g\,/\,\mathrm{cm}/\mathrm{s}^2) = 5.3 \pm 0.3$ places the star close to the high-temperature edge until which it may be justified to use LTE model atmospheres. This work states a test of the T\"ubingen NLTE Model Atmosphere Package for this temperature regime. We present the first application of revised, elaborated model atoms of low ionization stages of light metals usable with this atmosphere code.