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

TL;DR

This study uses 3D radiative transfer simulations to analyze the ionization structure of helium in Eta Carinae's colliding winds at periastron, revealing how wind density affects helium ionization and observed spectral features.

Contribution

It provides new 3D ionization maps of helium in Eta Carinae's wind interaction region during periastron, considering different mass-loss rates and their impact on spectral line formation.

Findings

Higher primary wind mass-loss rates inhibit helium ionization in the primary wind.

Lower mass-loss rates allow ionizing photons to reach the secondary wind, matching observed spectral variations.

Results help constrain the regions producing helium emission and absorption lines in Eta Carinae.

Abstract

Spectral observations of the massive colliding wind binary Eta Carinae show phase-dependent 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 ($e \sim 0.9$) binary orbit. The secondary star ($\eta_{\mathrm{B}}$) 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 $\eta$ 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 $\eta$ Car system for two different primary star mass-loss rates ($\dot{M}_{\eta_{\mathrm{A}}}$). Using previous results from simulations at apastron as a guide for the initial conditions, we compute 3D helium ionization maps. We find that, for higher $\dot{M}_{\eta_{\mathrm{A}}}$, $\eta_{\mathrm{B}}$ He$^{0+}$-ionizing photons are not able to penetrate into the pre-shock primary wind. He$^{+}$ due to $\eta_{\mathrm{B}}$ is only present in a thin layer along the leading arm of the WWIR and in a small region close to the stars. Lowering $\dot{M}_{\eta_{\mathrm{A}}}$ allows $\eta_{\mathrm{B}}$'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 $\eta$ Car's helium emission and absorption lines, which helps better constrain the regions where these lines arise.