The [o Iii] Veil: Astropause of Eta Carinae’s Wind?

We present narrowband images of η Carinae in the light of [O iii] λ5007 obtained with the Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2), as well as a groundbased image in the same emission line with a larger field of view. These images show a thin veil of [O iii] emission around η Car and its ejecta, confirming the existence of an oxygen-bearing “cocoon” inferred from spectra. This [O iii] veil may be the remnant of the pre-outburst wind of η Car, and its outer edge probably marks the interface where η Car’s ejecta meet the stellar wind of the nearby O4 V((f)) star HD 303308 or other ambient material in the Carina nebula – i.e., it marks the “astropause” in η Car’s wind. This veil is part of a more extensive [O iii] shell that appears to be shaped and ionized by HD 303308. A pair of HST images with a 10 yr baseline shows no significant proper motion, limiting the expansion speed away from η Car to 12±13 km s, or an expansion age of a few times 10 yr. Thus, this is probably the decelerated pre-outburst LBV wind of η Car. The [O iii] morphology is very different from that seen in [N ii], which traces younger dense knots of CNO-processed material; this represents a dramatic shift in the chemical makeup of material recently ejected by η Car. This change in the chemical abundances of the ejecta may have resulted from the sudden removal of the star’s outer envelope during η Car’s 19th century outburst or an earlier but similar event. Subject headings: circumstellar matter — H ii regions — ISM: bubbles — stars: individual (Eta Carinae) — stars: winds, outflows