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D. J. Hillier (U. Pittsburgh), K. Davidson, K. Kazunori Ishibashi (U. Minnesota), T. Gull (GSFC)
Recent HST STIS observations have revealed the spectrum of the central star in Eta Carinae for the first time. These observations clearly show that the optical spectrum of the central star is dominated by emission lines of H, \ion{He}{1}, and \ion{Fe}{2}. Its spectrum is qualitatively similar to the extreme P Cygni star HDE\,316285.
Using the non-LTE atmosphere code of Hillier and Miller (1998) we have performed a spectroscopic investigation of the central star. Previously measured IR fluxes indicate that the luminosity of the system is approximately 5\times 106\,L\odot (for a distance of d=2.5\,kpc), which we adopt as the luminosity of the central star. An excellent fit is obtained to the emission line spectrum using a mass loss rate of 10-3M\odot\,yr-1, a terminal wind velocity of \approx 500\,kms-1, and a solar Fe abundance (by mass). Due to degeneracies in the model it is difficult to constrain both the effective temperature and the H/He abundance ratio.
The most significant discrepancy is that the strength of the P Cygni absorption components is overestimated, particularly on the \ion{Fe}{2}\ emission lines. These discrepancies may be due to asymmetries in the stellar wind and/or time variability. There may also be a problem for our models in matching the UV spectrum shortward of 1600Å, but STIS observations of the central star are not yet available, while earlier GHRS observations may be contaminated by nebula emission and/or a companion star.
We discuss the implications of our results for the current evolutionary status of \eta\,Car, and for the binary model of Damineli et\,al. (1997).
This research was supported in part by NASA through a grant from the STScI, which is operated by AURA, under NASA contract NAS5-26555. Additional support was provided by NASA grant NAG5-8211.