To $v_\infty$ and beyond! The He I absorption variability across the 2014.6 periastron passage of $\eta$ Carinae

TL;DR

This study presents detailed spectroscopic monitoring of $ heta$ Carinae during its 2014.6 periastron, revealing repeatable wind variability, a new high-velocity absorption component, and insights into wind-wind collision instabilities.

Contribution

It provides the first comprehensive, high-resolution spectral analysis of $ heta$ Carinae across multiple periastron passages, uncovering new wind features and variability mechanisms.

Findings

Discovery of a high-velocity absorption component in He I lines.

Variability in wind features is repeatable across cycles.

Variations are linked to instabilities in the wind-wind collision region.

Abstract

We have monitored the massive binary star $\eta$ Carinae with the CTIO/SMARTS 1.5~m telescope and CHIRON spectrograph from the previous apastron passage of the system through the recent 2014.6 periastron passage. Our monitoring has resulted in a large, homogeneous data set with an unprecedented time-sampling, spectral resolving power, and signal-to-noise. This allowed us to investigate temporal variability previously unexplored in the system and discover a kinematic structure in the P Cygni absorption troughs of neutral helium wind lines. The features observed occurred prior to the periastron passage and are seen as we look through the trailing arm of the wind-wind collision shock cone. We show that the bulk of the variability is repeatable across the last five periastron passages, and that the absorption occurs in the inner 230 AU of the system. In addition, we found an additional, high-velocity absorption component super-imposed on the P Cygni absorption troughs that has been previously un-observed in these lines, but which bears resemblance to the observations of the He~I $\lambda$10830 \AA\ feature across previous cycles. Through a comparison of the current smoothed particle hydrodynamical simulations, we show that the observed variations are likely caused by instabilities in the wind-wind collision region in our line-of-sight, coupled with stochastic variability related to clumping in the winds.