Mid-Cycle Changes in Eta Carinae

TL;DR

This study investigates unexpected mid-cycle brightness and spectroscopic changes in Eta Carinae, suggesting wind fluctuations of the primary star as a possible cause, based on multi-wavelength observations and spectral analysis.

Contribution

It provides new insights into Eta Carinae's mid-cycle behavior, highlighting wind density and velocity fluctuations as key factors, and discusses observational effects influencing spectral comparisons.

Findings

Brightness decrease not caused by dust extinction.

Spectroscopic changes indicate wind fluctuations.

Secondary star's position remains consistent during observations.

Abstract

In late 2006, ground-based photometry of $\eta$ Car plus the Homunculus showed an unexpected decrease in its integrated apparent brightness, an apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry of the central star in the near-UV showed that this was not a simple reversal. This multi-wavelength photometry did not support increased extinction by dust as the explanation for the decrease in brightness. A spectrum obtained with GMOS on the Gemini-South telescope, revealed subtle changes mid-way in $\eta$ Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the primary. We suggest that the spectroscopic changes are consistent with fluctuations in the density and velocity of the primary star's wind, unrelated to the 5.5 yr cycle but possibly related to its latitude-dependent morphology. We also discuss subtle effects that must be taken into account when comparing ground-based and HST/STIS spectra.