The Mira-type symbiotic star BI Crucis.

The nature of the emission line Mira-type variable BI Crucis is discussed on the basis of optical and IR observations. The yellow-red spectrum shows strong H-alpha emission, and prominent H I lines in agreement with the star being in a fairly high ionization stage. The red continuum is probably the highly reddened E(B - V) = 1.5 + or - 0.5 continuum of the hot component of the system. A minimum blackbody temperature of 26,500 K and a minimum radius and luminosity of 0.36 d R(solar) and 57 d-squared L(solar), respectively, at a distance d and a violet-shifted absorption extending from zero to - 300 km/s are obtained. H-alpha is probably formed in the outer parts of the cool giant's wind facing the hot component and ionized by its UV radiation. The optically thick Fe II emission lines are probably formed in the denser parts of the wind. The IR energy distribution is consistent with the combination of a Mira-type spectrum and thermal emission from a dust envelope with a large temperature gradient up to about 200 K and an estimated distance of 3.8 kpc.