Changes in the Na D-1 Absorption Components of eta Carinae Provide Clues on the Location of the Dissipating Central Occulter

The Na D absorption doublet in the spectrum of eta Carinae is complex, with multiple absorption features associated with the Great Eruption (1840s), the Lesser Eruption (1890s), and the interstellar clouds. The velocity profile is further complicated by the P Cygni profile originating in the system's stellar winds and blending with the He i lambda 5876 profile. The Na D profile contains a multitude of absorption components, including those at velocities of -145 km s(-1), -168 km s(-1), and +87 km s(-1), which we concentrate on in this analysis. Ground-based spectra recorded from 2008 to 2021 show significant variability of the -145 km s(-1) absorption throughout long-term observations. In the high-ionization phases of eta Carinae prior to the 2020 periastron passage, this feature disappeared completely but briefly reappeared across the 2020 periastron, along with a second absorption at -168 km s(-1). Over the past few decades, eta Carinae has been gradually brightening, which is shown to be caused by a dissipating occulter. The decreasing absorption of the -145 km s(-1) component, coupled with similar trends seen in absorptions of ultraviolet resonant lines, indicate that this central occulter was possibly a large clump associated with the Little Homunculus or another clump between the Little Homunculus and the star. We also report on a foreground absorption component at +87 km s(-1). Comparison of Na D absorption in the spectra of nearby systems demonstrates that this redshifted component likely originates in an extended foreground structure consistent with a previous ultraviolet spectral survey in the Carina Nebula.