3D radiative transfer simulations of Eta Carinae's inner colliding winds – II. Ionization structure of helium at periastron

Abstract

Spectral observations of the massive colliding wind binary Eta Carinae show phasedependent variations, in intensity and velocity, of numerous helium emission and absorption lines throughout the entire 5.54-year orbit. Approaching periastron, the 3D structure of the wind-wind interaction region (WWIR) gets highly distorted due to the eccentric (e ∼ 0.9) binary orbit. The secondary star (η B ) at these phases is located deep within the primary's dense wind photosphere. The combination of these effects is thought to be the cause of the particularly interesting features observed in the helium lines at periastron. We perform 3D radiative transfer simulations of η Car's interacting winds at periastron. Using the simplex radiative transfer algorithm, we post-process output from 3D smoothed particle hydrodynamic simulations of the inner 150 au of the η Car system for two different primary star mass-loss rates (Ṁ η A ). Using previous results from simulations at apastron as a guide for the initial conditions, we compute 3D helium ionization maps. We find that, for higherṀ η A , η B He 0+ -ionizing photons are not able to penetrate into the pre-shock primary wind. He + due to η B is only present in a thin layer along the leading arm of the WWIR and in a small region close to the stars. LoweringṀ η A allows η B 's ionizing photons to reach the expanding unshocked secondary wind on the apastron side of the system, and create a low fraction of He + in the pre-shock primary wind. With apastron on our side of the system, our results are qualitatively consistent with the observed variations in strength and radial velocity of η Car's helium emission and absorption lines, which helps better constrain the regions where these lines arise.

THE OPTICAL WIND LINE VARIABILITY OFηCARINAE DURING THE 2009.0 EVENT

“…We have continued this monitoring program through to the 2014 periastron passage and beyond. In future work we plan to make a detailed comparison of the wind line variations between these two events in order to investigate the issue of long-term variations in the mass loss rate (Mehner et al 2012) and to confront the predictions of hydrodynamical models and radiative transfer calculations for the ionization zones created by colliding winds (Clementel et al 2015a). …”

“…Madura et al (2012) estimate that the binary semimajor axis is 15.4 AU and the eccentricity is 0.9, so that the periastron separation is only 1.5 AU, similar to or smaller than the He I line forming region. This implies that the He I lines should form near the region of a wind-wind collision zone at periastron, and this expectation is borne out in detailed models of the wind ionization zones (Clementel et al 2015a). …”

“…More fundamentally, we have treated the secondary as a point source, whereas in fact it is a hot star with a fast wind that comes so close to the primary at periastron that the wind interaction zone reaches close to where the He I line formation zone occurs. The location and structure of the zones of singly ionized He in the wind of η Car A have recently been investigated in numerical models by Clementel et al (2015aClementel et al ( , 2015b. In particular, Clementel et al (2015a) have calculated the He ionization structure near periastron, and they find potential emission zones in the inner wind of A, the preshock wind of A near the apex of the wind-wind collision zone, and in post-shock wind gas that is also photoionized by B.…”

“…The location and structure of the zones of singly ionized He in the wind of η Car A have recently been investigated in numerical models by Clementel et al (2015aClementel et al ( , 2015b. In particular, Clementel et al (2015a) have calculated the He ionization structure near periastron, and they find potential emission zones in the inner wind of A, the preshock wind of A near the apex of the wind-wind collision zone, and in post-shock wind gas that is also photoionized by B. The combination of emission from all these zones might indeed cause an asymmetry in the wind emission in the sense of enhanced emission in the direction of companion.…”

“…Gull et al (2011) show that the spatially resolved spectroscopy of η Car is best explained through colliding winds and provide 2D models that explain the geometry of the different emission lines forming in the extended colliding-winds region. Madura et al (2012) and Clementel et al (2015aClementel et al ( , 2015b have further extended this modeling effort to show that the plane of the binary orbit is closely aligned to the plane of the Homunculus skirt, i.e., the bipolar ejecta of the Homunculus are nearly perpendicular to the binary plane. These models also show that η Car B passes behind η Car Aʼs thick wind across the observed minimum, consistent with the models of Okazaki et al (2008).…”