DENSITY EFFECTS ON Mg II EMISSION LINES OF MIRA AB

We present ultraviolet spectra of the Mira AB binary system taken by the International Ultraviolet Explorer during the period from 1979 to 1995. In this study, we concentrated on the Mg II k and h emission lines arising primarily from Mira B. The broad Mg II profiles from Mira B are enhanced on the red side of the profile and obscured on the blue side due to obscuration by the wind of Mira B. Small-scale variations in the flux of these lines are seen in these spectra. At some phases, the absorption and emission components appear in the blue side of the k and h lines and disappear at other phases. These components are not mentioned in previous studies. The absorption component originates from the circumstellar envelope, while the emission features likely arise from both Mira A and the accretion disk around Mira B. Overlying wind absorption is present on the red side of the Mg II lines, and this absorption extends out to -450 km s-1. There is a relation between the parameters of these lines (line flux, line width, line velocity) and phase, which we attribute to an increase and decrease of the density of matter around Mira B at different times. Mg II emission in single Mira variables often displays a stronger h line than k line, opposite to the values of their oscillator strengths, resulting from circumstellar absorption from neutral metals over the k line. Meanwhile, non-Mira red giant stars typically show the k line being stronger than the h line in accordance with their respective oscillator strengths. The Mg II k to h emission from Mira B more closely follows the relative strength of k and h as seen in the non-Mira red giants than in the single Mira-type red giants.