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Eta Carinae’s Change of State: HST 1998-2021

Presentation #227.04 in the session “Stars II”.

Published onJun 18, 2021
Eta Carinae’s Change of State: HST 1998-2021

The decades-old problem of giant stellar eruptions (a.k.a. supernova impostors) remains mostly unsolved. Eta Carinae is central to this mystery, because it is the only survivor of a recent eruption that is close enough to observe well. Its progressive changes show the post-eruption recovery process. Moreover, there is a close O4-type companion star in a highly eccentric 5.5-year orbit, causing the wind to vary almost like a lab experiment. Here we report HST data on the 2020.1 periastron passage and its aftermath. HST was required, because all ground-based spectroscopy is strongly contaminated by material located near r ~ 0.3 arcsec ~ 700 AU. (Periastron separation is roughly 3 AU.)

Eta’s 20-year spectroscopic change of state now seems to have practically reached completion. The 2020.1 periastron event resembled 2014.6, very different from preceding events observed with HST in 1998 and 2003. During the years 1820–2000, the primary star (120 Msun or more) was hidden in a dense opaque outflow driven by continuum radiation. But the flow rapidly abated after 2000, and now resembles an orthodox line-driven wind. Before 2004, the companion star’s ionizing UV was suppressed at periastron. That has ceased to be true; the 2014.6 and 2020.1 events were characterized instead by strong EUV. We suspect that Eta Car has nearly returned to the state that Halley saw 350 years ago — except that no one can replace the ejected 5-to-25-percent of the star’s mass!

Note also J.C. Martin’s talk in the same session, concerning Eta’s rapid apparent brightening since 1998.

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