Detection of He ii λ4686 in η Carinae

We report the detection of the emission line He II 4686 A in eta Carinae. Theequivalent width of this line is ~100 mA along most of the 5.5-yr cycle andjumps to ~900 mA just before phase 1.0, followed by a brief disappearance. Thesimilarity between the intensity variations of this line and of the X-ray lightcurve is remarkable, suggesting that they are physically connected. We showthat the number of ionizing photons in the ultraviolet and soft X-rays,expected to be emitted in the shock wave from the colliding winds, is of theorder of magnitude required to produce the He II emission via photoionization. The emission is clearly blueshifted when the line is strong. The radialvelocity of the line is generally -100 Km/s, decreases steadily just before theevent, and reaches -400 Km/s at ph = 1.001. At this point, the velocitygradient suddenly changes sign, at the same time that the emission intensitydrops to nearly zero. Possible scenarios for explaining this emission arebriefly discussed. The timing of the peak of He II intensity is likely to beassociated to the periastron and may be a reliable fiduciary mark, importantfor constraining the orbital parameters.Comment: accepted in ApJLetters, 4 pages, 3 figure