Detection of He II λ4686 in η Carinae

We report the detection of the emission line He II lambda4686 in eta Carinae. The equivalent width of this line is similar to 100 mAngstrom along most of the 5.5 yr cycle and jumps to similar to 900 mAngstrom just before phase 1.0, followed by a brief disappearance. The similarity between the intensity variations of this line and those of the X-ray light curve is remarkable, suggesting that they are physically connected. We show that the expected number of ionizing photons in the ultraviolet and soft X-rays, expected to be emitted in the shock front from the colliding winds, is of the order of magnitude required to produce the He II emission via photoionization. The emission is clearly blueshifted when the line is strong. The radial velocity of the line is generally - 100 km s(-1), decreases steadily just before the event, and reaches - 400 km s(-1) at phase 1.001. At this point, the velocity gradient suddenly changes sign, at the same time that the emission intensity drops to nearly zero. Possible scenarios for explaining this emission are briefly discussed. The timing of the peak of He II intensity is likely to be associated with the periastron and may be a reliable fiduciary mark, important for constraining the orbital parameters.