Neon and CNO Abundances for Extreme Helium Stars -- A Non-LTE Analysis

TL;DR

This study performs a non-LTE abundance analysis of extreme helium stars, focusing on neon and other elements, using advanced model atmospheres to better understand their composition and origins.

Contribution

It introduces a detailed non-LTE analysis of multiple elements in extreme helium stars, providing insights into their chemical composition and formation scenarios.

Findings

Neon abundances match predictions from cold merger models.

NLTE corrections improve the accuracy of neon abundance estimates.

Elemental abundances support the merger origin hypothesis.

Abstract

A non-LTE (NLTE) abundance analysis was carried out for three extreme helium stars (EHes): BD+10 2179, BD-9 4395, and LS IV+6 002, from their optical spectra with NLTE model atmospheres. NLTE TLUSTY model atmospheres were computed with H, He, C, N, O, and Ne treated in NLTE. Model atmosphere parameters were chosen from consideration of fits to observed He I line profiles and ionization equilibria of C and N ions. The program SYNSPEC was then used to determine the NLTE abundances for Ne as well as H, He, C, N, and O. LTE neon abundances from Ne I lines in the EHes: LSE 78, V1920 Cyg, HD 124448, and PV Tel, are derived from published models and an estimate of the NLTE correction applied to obtain the NLTE Ne abundance. We show that the derived abundances of these key elements, including Ne, are well matched with semi-quantitative predictions for the EHe resulting from a cold merger (i.e., no nucleosynthesis during the merger) of a He white dwarf with a C-O white dwarf.