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

Mariela A. Corti (1)
Nolan R. Walborn (2)
Christopher J. Evans (3)

(1) Instituto Argentino de Radioastronom'{i}a (IAR), Centro Cient'{i}fico Tecnol'ogico de La Plata (CCT--LP), CONICET, C.C. No. 5, 1894 Villa Elisa, Argentina; and Facultad de Ciencias Astron'omicas y Geof'{i}sicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
(2) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
(3) UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

We have found strong selective emission of the N~II 5000~AA 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.

Reference: PASP
Status: Manuscript has been accepted



Email: walborn@stsci.edu