Changes in the Na D₁ Absorption Components of η Carinae Provide Clues on the Location of the Dissipating Central Occulter

Abstract

The Na D absorption doublet in the spectrum of η 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 λ5876 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 η 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, η 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.

The Long-term Spectral Changes of Eta Carinae: Are they Caused by a Dissipating Occulter as Indicated by cmfgen Models?

“…The evolution of these two spectral lines shows that the amplitude of both emission and absorption variations seem to have slowed down in the last three cycles, consistent with the almost complete disappearance of many circumstellar absorption lines in the UV (Gull et al 2021(Gull et al , 2023, and with the disappearance of the blueshifted narrow absorption of NaD (Pickett et al 2022).…”

Eta Carinae: The Dissipating Occulter Is an Extended Structure

“…Most resonant absorption lines include a component from a thermally cold, foreground cloud near 0 km s −1 , which is marked by the vertical dashed lines in Figure 3. An additional absorption velocity, +87 km s −1 , also is present as identified by Walborn et al (2002) in the Carina Nebula and more recently in Na D λ5898 by Pickett et al (2022).…”

“…Damineli et al (2021) predicted that by the 2030s, the occulter will have completely dissipated. Recently Pickett et al (2022) showed that the longterm evolution of the Na D absorption from the Little Homunculus was consistent with a disappearing occulter.…”

Long-term Evolution in Ionization of Ejecta Illuminated by Eta Carinae

“…Hillier & Allen (1992) first suggested that an occulter must be in our line of sight (LOS) to η Car. Its impact on the photometric and spectroscopic evolution of η Car was further discussed by Damineli et al (2019Damineli et al ( , 2021Damineli et al ( , 2023 and Pickett et al (2022). In addition, Gull et al (2023) showed that, between 2004 and 2018, many singly ionized absorption lines disappeared across an extended portion of the Homunculus (within ±1 5 of the central stellar core), indicating that the dissipating occulter extended well beyond our LOS to η Car.…”

“…Using multiple ground-based spectrographs, we gathered spectra with a signal-to-noise ratio of 100-500 and spectral resolving power ranging from R ∼ 90,000 to 22,000 (see Table 1 for the origin of the spectra). Many of these spectra have been used in several recent papers by our group which detail the reductions of the data (see, e.g., Damineli et al 2008;Richardson et al 2010Richardson et al , 2015Richardson et al , 2016Teodoro et al 2016;Damineli et al 2019Damineli et al , 2021Pickett et al 2022;Damineli et al 2023;Strawn et al 2023). The spectra were normalized by using a linear fit through the pseudo-continuum in the range 4550-4750 Å.…”