On the He<scp>ii</scp>Emission in η Carinae and the Origin of Its Spectroscopic Events

Abstract

We describe and analyze Hubble Space Telescope (HST) observations of transient emission near 4680 ˚A in η Car, reported earlier by Steiner &amp;amp; Damineli (2004). If, as seems probable, this is He II λ4687, then it is a unique clue to η Car’s 5.5-year cycle, because its excitation level is far higher than any other UV-to-IR spectral feature seen in this object. The emission appeared in early 2003, grew to a brief maximum during the 2003.5 spectroscopic event, and then abruptly disappeared. The peak brightness, substantially larger than Steiner &amp;amp; Damineli reported, is difficult to explain. According to our analyses this feature presents serious difficulties for an “eclipse ” model of the spectroscopic event, and strongly favors, instead, a mass ejection or wind disturbance model. The stellar wind may need to provide an unexpectedly large energy supply. However, we describe a special form of radiative amplification that may reduce the difficulty. The favored type of model has implications for the physical structure of η Car.