A New Feature in the Spectrum of the Superluminous LMC Supergiant HDE 269896

TL;DR

The paper reports the discovery of strong N II emission and unusual ionization features in the spectrum of the LMC hypergiant HDE 269896, suggesting it may be in a pre-WNVL evolutionary state and providing insights into stellar atmospheres.

Contribution

It introduces new spectral features in HDE 269896 and discusses their implications for stellar evolution and atmospheric physics, highlighting the occurrence of different line behaviors in O-type spectra.

Findings

Detection of strong N II 5000A emission complex.

HDE 269896 may be in a pre-WNVL evolutionary stage.

Distinct line behaviors in O-type spectra are demonstrated.

Abstract

We have found strong selective emission of the N II 5000A complex in the spectrum of the LMC hypergiant HDE 269896, ON9.7 Ia$^+$. Since this object also has anomalously strong He II $\lambda$4686 emission for its spectral type, an unusually wide range of ionization in its extended atmosphere is indicated. The published model of this spectrum does not reproduce these emission features, but we show that increased nitrogen and helium abundances, together with small changes in other model parameters, can do so. The morphological and possible evolutionary relationships of HDE 269896, as illuminated by the new spectral features, to other denizens of the OB Zoo are discussed. This object may be in an immediate pre-WNVL (Very Late WN) state, which is in turn the quiescent state of at least some Luminous Blue Variables. More generally, the N II spectrum in HDE 269896 provides a striking demonstration of the occurrence of two distinctly different kinds of line behavior in O-type spectra: normal absorption lines that develop P Cygni profiles at high wind densities, and selective emission lines from the same ions that do not. Further analysis of these features will advance understanding of both atomic physics and extreme stellar atmospheres.