Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

Abstract

We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 2008–2009 using the CRIRES spectrograph mounted on the 8 m UT 1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He i <i>λ<i/>10833 extending up to -1900 km s<sup>-1<sup/> across the 2009.0 spectroscopic event. Analysis of archival <i>Hubble Space Telescope<i/>/Space Telescope Imaging Spectrograph ultraviolet and optical data identifies a similar high-velocity absorption (up to -2100 km s<sup>-1<sup/>) in the ultraviolet resonance lines of Si iv <i>λ<i/><i>λ<i/>1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si ii <i>λ<i/><i>λ<i/>1527, 1533 and C ii <i>λ<i/><i>λ<i/>1334, 1335, show absorption only up to -1200 km s<sup>-1<sup/>, indicating that the absorption with velocities -1200 to -2100 km s<sup>-1<sup/> originates in a region markedly more rapidly moving and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6 m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989–2009) also show high-velocity absorption in He i <i>λ<i/>10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than -900 km s<sup>-1<sup/> is present in the phase range 0.976 <i>≤<i/> <i>ϕ<i/> <i>≤<i/> 1.023 of the spectroscopic cycle, but absent in spectra taken at <i>ϕ<i/> <i>≤<i/> 0.94 and <i>ϕ<i/> <i>≥<i/> 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We propose that the high-velocity absorption component originates in shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is along the line-of-sight to the primary star only if the binary system is oriented in the sky such that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of <i>ω<i/> ~ 240°–270°. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range <i>i<i/> = 40°–60°.