We present two‐dimensional (2D) radiative transfer modelling of the Eta Carinae binary system accounting for the presence of a wind–wind collision (WWC) cavity carved in the optically thick wind of the primary star. By comparing synthetic line profiles with spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph near apastron, we show that the WWC cavity has a strong influence on multi‐wavelength diagnostics. This influence is regulated by the modification of the optical depth in the continuum and spectral lines. We find that Hα, Hβ and Fe ii lines are the most affected by the WWC cavity, since they form over a large volume of the stellar wind of the primary. These spectral lines depend on latitude and azimuth since, according to the orientation of the cavity, different velocity regions of a spectral line are affected. For 2D models with orientation corresponding to orbital inclination angle and longitude of periastron , the blueshifted and zero‐velocity regions of the line profiles are the most affected by the cavity. These orbital orientations are required to simultaneously fit the ultraviolet (UV) and optical spectrum of Eta Car around apastron, for a half‐opening angle of the cavity in the range of 50°–70°. We find that the excess P Cygni absorption seen in Hα, Hβ and optical Fe ii lines in 1D spherical models becomes much weaker or absent in the 2D cavity models, in agreement with the observations. The observed UV spectrum of Eta Car is strongly dominated by absorption of Fe ii lines that are superbly reproduced by our 2D models when the presence of the low‐density WWC cavity is taken into account. Small discrepancies still remain, as the P Cygni absorption of Hγ and Hδ is overestimated by our 2D models at apastron. We suggest that photoionization of the wind of the primary by the hot companion star is responsible for the weak absorption seen in these lines. Our cmfgen models indicate that the primary star has a mass‐loss rate of 8.5 × 10−4 M⊙ yr−1 and wind terminal velocity of 420 km s−1 around the 2000–2001 apastron.
The highly eccentric binary system, η Car, provides clues to the transition of massive stars from hydrogen-burning via the CNO cycle to a helium-burning evolutionary state. The fastmoving wind of η Car B creates a cavity in η Car A's slower, but more massive, stellar wind, providing an in situ probe. lines extend only 0.3 arcsec (700 au) from NE to SW and are blue shifted from −500 to +200 km s −1 . All observed spectral features vary with the 5.54-year orbital period. The highly ionized, forbidden emission disappears during the low state, associated with periastron passage. The high-ionization emission originates in the outer wind interaction region that is directly excited by the far-ultraviolet radiation from η Car B. The HST/STIS spectra reveal a time-varying, distorted paraboloidal structure, caused by the interaction of the massive stellar winds. The model and observations are consistent with the orbital plane aligned with the skirt of the Homunculus. However, the axis of the distorted paraboloid, relative to the major axis of the binary orbit, is shifted in a prograde rotation along the plane, which projected on the sky is from NE to NW.
Recent work suggests that the mass-loss rate of the primary star η A in the massive colliding wind binary η Carinae dropped by a factor of 2−3 between 1999 and 2010. We present results from large-(±1545 au) and small-(±155 au) domain, 3D smoothed particle hydrodynamics (SPH) simulations of η Car's colliding winds for three η A mass-loss rates (Ṁ ηA = 2.4, 4.8, and 8.5 × 10 −4 M ⊙ yr −1 ), investigating the effects on the dynamics of the binary wind-wind collision (WWC). These simulations include orbital motion, optically thin radiative cooling, and radiative forces. We find thatṀ ηA greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star η B switches from the adiabatic to the radiative-cooling regime during periastron passage (φ ≈ 0.985 − 1.02). This switchover starts later and ends earlier the lower the value ofṀ ηA and is caused by the encroachment of the wind of η A into the acceleration zone of η B 's wind, plus radiative inhibition of η B 's wind by η A . The SPH simulations together with 1D radiative transfer models of η A 's spectra reveal that a factor of two or more drop inṀ ηA should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop inṀ ηA was likely by a factor 2 and occurred after 2004. We speculate that most of the recent observed changes in η Car are due to a small increase in the WWC opening angle that produces significant effects because our line-of-sight to the system lies close to the dense walls of the WWC zone. A modest decrease inṀ ηA may be responsible, but changes in the wind/stellar parameters of η B , while less likely, cannot yet be fully ruled out. We suggest observations during η Car's next periastron in 2014 to further test for decreases inṀ ηA . IfṀ ηA is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.
We present a three‐dimensional (3D) dynamical model for the broad [Fe iii] emission observed in η Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS). This model is based on full 3D smoothed particle hydrodynamics simulations of η Car’s binary colliding winds. Radiative transfer codes are used to generate synthetic spectroimages of [Fe iii] emission‐line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA θ that the orbital plane projection of the line of sight makes with the apastron side of the semimajor axis and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3D orientation of the binary orbit. To simultaneously reproduce the blueshifted emission arcs observed at orbital phase 0.976, STIS slit PA =+38° and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i≈ 130° to 145°, θ≈−15° to +30° and an orbital axis projected on the sky at a PA ≈ 302° to 327° east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3D. The companion star, ηB, thus orbits clockwise on the sky and is on the observer’s side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses.
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