Eta Carinae's Brightness Variations Since 1998: HST Observations of the Central Star

Abstract

We have measured the brightness variations in Eta Carinae for the past six years using the Hubble Space Telescope Space Telescope Imaging Spectrograph and Advanced Camera for Surveys. Unlike ground-based data, observations by the HST allow direct measurement of the brightness of the central star by resolving it from the surrounding bright ejecta. We find interesting behavior during 2003 in the continuum and H$α$ emission. The data show that the established long term brightening trend of Eta Car continues, including regular events which coincide with the 5.5 year spectroscopic cycle and other more rapid and unexpected variations. In addition to the HST data, we also present ground-based data obtained from the AAVSO which show many of the same features. The dip in the apparent brightness of the central star at the time of the 2003.5 event is wavelength dependent with no decrease in the continuum. These observations cast doubt on asimple eclipse or occultation as the explanation for the dip and place constraints on the models for the event.