A Large Infrared Shell Associated with BI Crucis

Abstract

Spitzer IRAC and MIPS images reveal a large dust emission feature ~6' in diameter which appears to be a shell in close proximity to, and perhaps physically related to, the D-type Mira symbiotic BI Cru. Smaller optical lobes are already known to be emanating from some symbiotics including BI Cru. However, this is the first extended structure found in the IR which is associated with a symbiotic Mira system. The IR shell of BI Cru is more than 5 times larger in arc size than the star's optical lobe. Published distance estimates imply that the IR shell is ~4 to ~8 pc in diameter, which is larger than the largest optical lobe known to be associated with any Mira symbiotic system. The large disparity between its IR and optical shell sizes, along with what appear to be multiple intersecting arcs, suggest that BI Cru has undergone multiple mass-loss episodes. A trend of rapidly increasing brightness toward longer wavelengths, along with a much more diffuse structure at 70 μm than at shorter wavelengths, and suggests a greater abundance of relatively colder and older dust which may be the remnant of earlier mass outflows.