Recent changes in the near-ultraviolet and optical structure of η carinae

Multiepoch HST/WFPC2 images of η Carinae are used to investigate the relationship between the photometric variability of the circumstellar nebula and the variability of the central star. In the past few years, the central star has brightened considerably, and the response of the reflection nebula to this brightening has been surprisingly complex. While the central star has brightened by a factor of 2 at near-ultraviolet and optical wavelengths, bright dust condensations in the bipolar lobes have increased by only a factor of ∼ 1.3, and dark lanes between these dust condensations have brightened by factors of 1.5-2. Certain regions of the nebula have brightened much more than the star itself (as much as a factor of 8), and others have actually faded, despite the brightening of the star. Some of the anomalous fading can be attributed to contributions of intrinsic [S III] and [N II] line emission. The variations of the equivalent width of [S III] λ6312 and [N II] λ6583 as measured in the WFPC2 F631N and F658N filters follow the same trend of other high-excitation lines observed in ground-based spectra during η Car's 5.5 yr spectroscopic cycle, and the amplitude of the change accounts for the total change measured in ground-based spectra for these same lines. The WFPC2 images indicate, however, that these high-excitation lines are emitted by circumstellar gas at distances of a few hundred to a few thousand AU from the star, probably located in the equatorial plane. Thus, periodic ionization of gas at large distances from the central star appears to dominate the spectroscopic changes that define the 5.5 yr cycle. The fact that the variable high-excitation emission is extended places important constraints on models for η Car's 5.5 yr spectroscopic variability.