Constraining the orbital orientation of Eta Carinae from H Paschen lines

TL;DR

This study uses hydrogen Paschen line spectra reflected by the Homunculus to constrain Eta Carinae's orbital inclination and phase, revealing a misalignment between the orbital plane and the Homunculus axis.

Contribution

First estimation of Eta Carinae's orbital inclination and phase based on spectral reflection analysis, clarifying the orbital geometry and its relation to the Homunculus.

Findings

Orbital inclination to the line of sight is approximately 60 degrees.

Orbital inclination with respect to the Homunculus axis is about 35 degrees.

Periastron occurs shortly after conjunction at phase ~ -40 degrees.

Abstract

During the past decade several observational and theoretical works provided evidences of the binary nature of Eta Carinae. Nevertheless, there is still no direct determination of the orbital parameters, and the different current models give contradictory results. The orbit is, in general, assumed to coincide with the Homunculus equator although the observations are not conclusive. Among all systems, Eta Carinae has the advantage that it is possible to observe both the direct emission of line transitions in the central source and its reflection by the Homunculus, which are dependent on the orbital inclination. In this work, we studied the orbital phase dependent hydrogen Paschen spectra reflected by the SE lobe of the Homunculus to constrain the orbital parameters of etaCar and determine its inclination with respect to the Homunculus axis. Assuming that the emission excess is originated in the wind-wind shock region we were able to model the latitude dependence of the spectral line profiles. For the first time, we were able to estimate the orbital inclination of etaCar with respect to the observer and to the Homunculus axis. The best fit occurs for an orbital inclination to the line of sight of i ~ 60 degrees, and i* ~ 35 degrees with respect to the Homunculus axis, indicating that the angular momenta of the central object and the orbit are not aligned. We were also able to fix the phase angle of conjunction as ~ -40 degrees, showing that the periastron passage occurs shortly after conjunction.