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T. Gull (LASP/GSFC/NASA), K. Ishibashi (NAS/NRC, LASP/GSFC/NASA), K. Davidson (U.MN)
STIS observations of Eta Carinae and immediate ejecta reveal narrow Balmer absorption lines in addition to the nebular-scattered broad P-Cygni absorption. The narrow absorption correlates with apparent disk structure that separates the two Homunculus lobes. We trace these features about half way up the Northern lobe until the scattered stellar Balmer line doppler-shifts redward beyond the nebular absorption feature. Three-dimensional data cubes, made by mapping the Homunculus at Balmer alpha and Balmer beta with the 52x0.1 arcsecond aperture and ~5000 spectral resolving power, demonstrate that the absorption feature changes slowly in velocity with nebular position.
We have monitored the stellar Balmer alpha line profile of the central source over the past four years. The equivalent width of the nebular absorption feature changes considerably between observations. The changes do not correlate with measured brightness of Eta Carinae. Likely clumps of neutral hydrogen with a scale size comparable to the stellar disk diameter are passing through the intervening light path on the timescales less than several months.
The excitation mechanism involves Lyman radiation and collisions leading to populating the 2S metastable state. Before the electron can jump to the ground state by two photon emission (lifetime ~1/8 second), a stellar Balmer photon is absorbed and the electron shifts to an NP level. We see the absorption feature in higher Balmer lines, and but not in Paschen lines. Indeed we see narrow nebular Paschen emission lines. At present, we do not completely understand the details of the absorption. Better understanding should lead to improved insight of the unique conditions around Eta Carinae that leads to these absorptions.
These observations should also lead to improved insight on circumstellar material conditions in early supernova stages.
Funding for this study is from NASA under the STIS GTO and STScI GO programs.
The author(s) of this abstract have provided an email address for comments about the abstract: gull@sea.gsfc.nasa.gov