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AG Carinae III. The 1990 hot phase of the star and the physical structure of the circumstellar environment

Long slit spectra of the region around the Luminous Blue Variable AG Car obtained in June 1990, just before the onset of its new brightening phase, indicate that AG Car was in the hottest phase so far recorded. The spectrum shows strong broad emissions of He II 468.6 nm, and [Fe III] 465.8-470.1 nm, and He I, N II, Si III, Al III and Fe III with a P Cygni profile. We have also identified weak lines of C II and O II. A faint continuum emission has been detected within the whole ring nebula. We also find that the spectrum of some regions is partially due to scattered star's light. This provides evidence of the presence of dust grains, and indicates that even in present times dust is condensing from AG Car wind. The sharp outer edge of the ring suggest the presence of a shock front. We have also identified an extended low density H II $halo$, which should be associated with the residual of the stellar wind of a previous cooler evolutionary phase of AG Car. In the halo the H$α$/[N II] ratio is strengthened with respect to the ring nebula, possibly because of a discontinuity in the N abundance. We suggest that part of the star's reddening originates in the ring nebula and diffuse halo, which could alter the present estimates of the star's distance. The extracted spectrograms are available as computer files on request to rossi@astrom.astro.it. Article in press in A\&A.