Secular Changes in Eta Carinae's Wind 1998-2011

TL;DR

Between 1998 and 2011, eta Carinae's stellar wind features decreased significantly, with changes depending on viewing angle, indicating evolving mass-loss and ionization structures in its wind over a decade.

Contribution

This study provides a detailed analysis of spectral changes in eta Carinae's wind over 13 years, highlighting the angle-dependent evolution of its wind properties.

Findings

Spectral features decreased by factors of 1.5-3 over 10 years.

Wind-emission lines became more similar at different viewing angles after 2009.

High-excitation absorption lines strengthened gradually.

Abstract

Stellar wind-emission features in the spectrum of eta Carinae have decreased by factors of 1.5-3 relative to the continuum within the last 10 years. We investigate a large data set from several instruments (STIS, GMOS, UVES) obtained between 1998 and 2011 and we analyze the progression of spectral changes in the direct view of the star, in the reflected polar-on spectra at FOS4, and at the Weigelt knots. We find that the spectral changes occurred gradually on a time scale of about 10 years and that they are dependent on the viewing angle. The line strengths declined most in our direct view of the star. About a decade ago, broad stellar wind-emission features were much stronger in our line-of-sight view of the star than at FOS4. After the 2009 event, the wind-emission line strengths are now very similar at both locations. High-excitation He I and N II absorption lines in direct view of the star strengthened gradually. The terminal velocity of Balmer P Cyg absorption lines now appears to be less latitude-dependent and the absorption strength may have weakened at FOS4. Latitude-dependent alterations in the mass-loss rate and the ionization structure of eta Carinae's wind are likely explanations for the observed spectral changes.