He II λ4686 emission from the massive binary system in η Car : constraints to the orbital elements and the nature of the periodic minima

Abstract

Eta Carinae (η Car) is an extremely massive binary system in which rapid spectrum variations occur near periastron. Most notably, near periastron the He ii λ4686 line increases rapidly in strength, drops to a minimum value, then increases briefly before fading away. To understand this behavior, we conducted an intense spectroscopic monitoring of the He ii λ4686 emission line across the 2014.6 periastron passage using ground- and space-based telescopes. Comparison with previous data confirmed the overall repeatability of the line equivalent width (EW), radial velocities, and the timing of the minimum, though the strongest peak was systematically larger in 2014 than in 2009 by 26%. The EW variations, combined with other measurements, yield an orbital period of 2022.7 ± 0.3 days. The observed variability of the EW was reproduced by a model in which the line flux primarily arises at the apex of the wind?wind collision and scales inversely with the square of the stellar separation, if we account for the excess emission as the companion star plunges into the hot inner layers of the primary?s atmosphere, and including absorption from the disturbed primary wind between the source and the observer. This model constrains the orbital inclination to 135°?153°, and the longitude of periastron to 234°?252°. It also suggests that periastron passage occurred on {T}0=2456874.4quad (+/- 1.3 days). Our model also reproduced EW variations from a polar view of the primary star as determined from the observed He ii λ 4686 emission scattered off the Homunculus nebula.