A large bubble around the AGB star R Dor detected in the UV

Abstract

Many asymptotic giant branch (AGB) and supergiant stars exhibit extended detached shells in the far-infrared, resembling rings or arcs. These structures have long been interpreted as the bow shock formed in the interface between the stellar wind and the interstellar medium, the astrosphere. To date, only a few AGB stars have been observed showing an extended shell in the ultraviolet: the cometary tail drifting away from o Ceti, and a bubble around IRC+10216, CIT6, and U Hya. This paper describes a search of UV extended shells around AGB stars using archival GALEX far-UV images. After inspecting visually 282 GALEX images, we identified the fourth discovery of a UV bubble around the AGB star R Dor. The bubble is seen as a 26′ × 29′ ring, corresponding to an actual diameter of 0.41 × 0.46 parsec2. The mass of the thin UV bubble is estimated to be ≃0.003 M⊙. The morphological asymmetry (less than ∼20%) and brightness variations of this shell are uncorrelated with the stellar proper motion and thus they can rather be ascribed to inhomogeneities in the ISM. Archival IRAS 60 and 100μm images reveal that the bubble is filled with cold (i.e. $\lesssim 32$K) dust. All UV bubbles known to date are limited to be within a distance $\lesssim 350$ pc and at high Galactic latitudes ($|b| \gtrsim 35^{\circ }$), which suggests that their detection is hampered in most cases by the strong UV interstellar extinction.