First Detection of UV emission from a Detached Dust Shell: GALEX Observations of the Carbon AGB Star U Hya

Abstract

We present the discovery of an extended ring of ultraviolet emission surrounding the AGB star U Hya in archival observations performed by the Galaxy Evolution Explorer (GALEX). This is the third discovery of extended UV emission from a carbon AGB star and the first from an AGB star with a detached shell. From imaging and photometric analysis of the FUV and NUV images, we determined that the ultraviolet ring has a radius of $\sim 110^{\prime\prime}$, thus indicating that the emitting material is likely associated with the detached shell seen in the infrared. We find that scattering of the central point source of NUV and FUV emission by the dust shell is negligible. Moreover, we find that scattering of the interstellar radiation field by the dust shell can contribute at most $\sim10%$ of the FUV flux. Morphological and photometric evidence suggests that shocks caused by the star's motion through space and, possibly, shock-excited H$_2$ molecules are the most likely origins of the UV flux. In contrast to previous examples of extended UV emission from AGB stars, the extended UV emission from U Hya does not show a bow shock-like structure, which is consistent with a lower space velocity and lower interstellar medium density. This suggests the detached dust shell is the source of the UV emitting material and can be used to better understand the formation of detached shells.