Nitrogen and Oxygen Abundance Variations in the Outer Ejecta of Eta Carinae: Evidence for Recent Chemical Enrichment

We present optical spectra of the ionized ‘Outer Ejecta’ of η Carinae that reveal differences in chemical composition at various positions. In particular, young condensations just outside the dusty Homunculus Nebula show strong nitrogen lines and little or no oxygen — but farther away, nitrogen lines weaken and oxygen lines become stronger. The observed variations in the apparent N/O ratio may signify either that the various blobs were ejected with different abundances, or more likely, that the more distant condensations are interacting with normal-composition material. The second hypothesis is supported by various other clues involving kinematics and X-ray emission, and would suggest that η Car is enveloped in a “cocoon” deposited by previous stellar-wind mass loss. In particular, all emission features where we detect strong oxygen lines are coincident with or outside the soft X-ray shell. In either case, the observed abundance variations suggest that η Car’s ejection of nitrogen-rich material is a recent phenomenon — taking place in just the last few thousand years. Thus, η Carinae may be at a critical stage of evolution when ashes of the CNO cycle have just appeared at its surface. Finally, these spectra reveal some extremely fast nitrogen-rich material, with Doppler velocities up to 3200 km s, and actual space velocities that may be much higher. This is the fastest material yet seen in η Car’s nebula, but with unknown projection angles its age is uncertain. Subject headings: circumstellar matter — stars: individual (η Carinae)