Eta Carinae's Declining Outflow Seen in the UV, 2002-2015

TL;DR

This study analyzes HST UV data from 2002-2015 to reveal eta Carinae's rapid changes in outflow characteristics, including emission line variations and density diagnostics, indicating complex and evolving stellar wind interactions.

Contribution

It provides the first direct gas density diagnostics from N III] line ratios and links UV spectral changes to episodic mass ejections, enhancing understanding of eta Carinae's outflow dynamics.

Findings

Confirmed rapid changes in UV absorption lines and extinction.

Identified N III] as a key luminosity and density diagnostic.

Detected anomalous high-velocity N III] emission possibly linked to mass ejection.

Abstract

Existing HST UV data offer many previously neglected clues to eta Car's behavior since 2000. Here we examine a subset of observations with diverse results. (1) The star's rapid change of state is confirmed by major changes in UV absorption lines, circumstellar extinction, and other features. (2) N III] 1750 is one of the two most luminous emission features in eta Car's observable spectrum, comparable to H-alpha. This and other semi-forbidden lines are useful because they have no P Cyg absorption. (3) N III] multiplet ratios provide the first direct diagnostic of gas densities in eta Car's outflow. They strongly suggest that high-excitation lines originate in condensations within the colliding-wind shocked region. The parameters imply that published models have not adequately represented the essential small size scales. (4) In 2002-2004, a very large amount of N III] emission had anomalous Doppler velocities from +400 to +1200 km/s. This is a mystery; we conjecture that it may have resulted from a burst of mass ejection in the 2003.5 periastron event. Various other effects are also difficult to explain and merit further investigation.