NITROGEN AND OXYGEN ABUNDANCE VARIATIONS IN THE OUTER EJECTA OF η CARINAE: EVIDENCE FOR RECENT CHEMICAL ENRICHMENT

摘要

We present optical spectra of the ionized "outer ejecta" of η Car 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 in which 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 Car 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~(-1) 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.