High-excitation emission lines near eta Carinae, and its likely companion star

TL;DR

This study analyzes high-excitation emission lines around eta Carinae over a 6-year period, revealing their variability, spatial distribution, and implications for the properties of its likely companion star.

Contribution

It provides new insights into the spatial and temporal behavior of high-excitation lines and constrains the properties of eta Carinae's companion star using photoionization modeling.

Findings

High-excitation fluxes vary systematically over the 5.5-year cycle.

A secondary maximum occurred before the 2003.5 event.

Emission lines are concentrated at the Weigelt knots.

Abstract

In order to study the distribution of gas and ionizing radiation around eta Car and their implications for its likely companion star, we have examined high-excitation emission lines of [Ne III], [Fe III], etc., in spectra obtained with the HST/STIS instrument during 1998-2004. Our principal results, some of them unexpected, are: (1) The high-excitation fluxes varied systematically and non-trivially throughout eta Car's 5.5-year spectroscopic cycle. Instead of rising to a plateau after the 1998 "event," they changed continuously with a maximum in mid-cycle. (2) At one significant location a brief, strong secondary maximum occurred just before the 2003.5 spectroscopic event. (3) These emission lines are strongly concentrated at the "Weigelt knots" several hundred AU northwest of the star. With less certainty, [Ne III] appears to be somewhat more concentrated than [Fe III]. (4) A faster, blue-shifted component of each feature appears concentrated near the star and elongated perpendicular to the system's bipolar axis. This structure may be related to the equatorial outflow and/or to dense material known to exist along our line of sight to the star. (5) Using the photoionization program Cloudy, we estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the ejecta. Teff = 39000 K and L = 400000 Lsun, for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested.