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

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.