EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited

TL;DR

This study revises the physical parameters of the peculiar hot subdwarf EC 11481-2303 using high-resolution optical and UV spectra, revealing a higher temperature, lower helium abundance, and enhanced iron-group elements compared to previous estimates.

Contribution

It provides a refined spectral analysis of EC 11481-2303 with non-LTE models, updating its temperature, gravity, and chemical composition based on new high-quality observations.

Findings

Effective temperature revised to 55,000 K

Helium abundance significantly lower at He/H=0.0025

Iron-group elements strongly enhanced by at least a factor of ten

Abstract

EC 11481-2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous optical spectroscopy revealed that it is a sdOB star with an effective temperature (Teff) of 41790 K, a surface gravity log(g)= 5.84, and He/H = 0.014 by number. We present an on-going spectral analysis by means of non-LTE model-atmosphere techniques based on high-resolution, high-S/N optical (VLT-UVES) and ultraviolet (FUSE, IUE) observations. We are able to reproduce the optical and UV observations simultaneously with a chemically homogeneous NLTE model atmosphere with a significantly higher effective temperature and lower He abundance (Teff = 55000 K, log (g) = 5.8, and He / H = 0.0025 by number). While C, N, and O appear less than 0.15 times solar, the iron-group abundance is strongly enhanced by at least a factor of ten.