The Strontium Filament within the Homunculus of Eta Carinae

Abstract:
During a series of HST/STIS observations of Eta Carinae and associatedejecta, we noticed a peculiar emission filament located a few arcsecondsnorth of the central source. While bright in nebular standards, it issubmerged in a sea of scattered starlight until moderately highdispersion, long-slit spectroscopy with the STIS (R 8000) brings theemission lines out. The initial spectrum, centered on 6768A with theSTIS G750M grating, led to identification of twenty lines fromsingly-ionized species including [Sr II], [Fe II], [Ti II], [Ni II], [MnII], and [Co II] (Zethson, etal., 2001, AJ 122, 322). No Balmer emissionis detected from this filament and the Fe II 2507,9 lines, known to bepumped by Lyman alpha radiation in other regions near the centralsource, are not detected. Followup observations have led to detection ofhundreds more emission lines from iron group elements in neutral andsingly-ionized states. Thus far all are excited by less than 10 eV. Thispeculiar nebular emission is thought to be due to very intense stellarradiation, stripped of uv flux shortward of Lyman alpha, bathing aneutral structure. We are systematically identifying the many lines(over 90% identified) and measuring line intensities that will then bemodeled to determine excitation mechanisms, temperature and density. Two[Sr II] and two Sr II lines have now been measured. Bautista, etal. (inpreparation) have modeled the strontium flux ratios and find that largeradiation fluxes and/or high strontium abundances may account for thedetected emission. These observations were supported by STIS GTO fundingand GO funding through the STScI.