MID-CYCLE CHANGES IN ETA CARINAE

摘要

In late 2006, ground-based photometry of η Car plus the Homunculus showed an unexpected decrease in its integratedapparent brightness, an apparent reversal of its long-term brightening. Subsequent Hubble Space Telescope(HST)/WFPC2 photometry of the central star in the near-UV showed that this was not a simple reversal. This multiwavelengthphotometry did not support increased extinction by dust as the explanation for the decrease in brightness.A spectrum obtained with the Gemini Multi-Object Spectrograph on the Gemini-South telescope revealed subtlechanges mid-way in η Car’s 5.5 yr spectroscopic cycle when compared with HST/Space Telescope Imaging Spectrograph(STIS) spectra at the same phase in the cycle. At mid-cycle the secondary star is 20–30 AU from theprimary. We suggest that the spectroscopic changes are consistent with fluctuations in the density and velocity ofthe primary star’s wind, unrelated to the 5.5 yr cycle but possibly related to its latitude-dependent morphology. Wealso discuss subtle effects that must be taken into account when comparing ground-based and HST/STIS spectra.