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Abstract

The supernova imposter Eta Carinae and its surrounding Homunculus reflection nebula have been of great interest for the last several decades. As the most massive star known in our Galaxy, this object is of particular importance in understanding high mass loss episodes and final stages in the evolution of other similarly massive stars in the later stages of their lives. Observations using the STIS CCD Instrument have been taken over the past 20 years for both the central star and the Homunculus as part of the Hubble Space Telescope (HST) Treasury Program on Eta Carinae. Recent changes in the spectra of the star from 2012 to 2018 confirm the continuation of certain trends in the star’s behavior, including both an overall brightening and the related decrease in the stellar wind density. Around 2004, Eta Carinae began a period of rapid change, where major stellar wind emission features decreased by factors of 2-4 over the rest of the decade. Here we find that this dramatic development seems to have slowed over the last 6 years, implying that the mass loss rate has not changed significantly since 2010; these findings are consistent over all visible wavelength regions available in our observations.

In addition to probing the behavior of the central star, we also consider the development of the Homunculus nebula over the past 18 years, using intensity tracings of the line-free continuum in order to determine a precise expansion rate of 11.9% ± 0.1% between the two epochs of March 2000 and February 2018. This is one of the longest baselines ever produced for this object using a single instrument. This expansion rate yields an ejection date (assuming ballistic motion) of 1849.2 ±1.7 yr, which is in good agreement with other recent studies. We also consider the change in brightness of the nebula over this baseline.

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