ULTRAVIOLET EXCESS EMISSION FROM HIGHLY-EVOLVED SUN-LIKE STARS
Enmanuel Sanchez1, Rodolfo Montez Jr.2.
1Florida State University, Tallahassee, FL, 2Vanderbilt University, Nashville, TN.
Most of the components necessary for life (e.g., carbon, nitrogen, oxygen) are produced in stars, with the largest production occurring during a late stage of stellar evolution called the asymptotic giant branch (AGB) phase. Enriched material is liberated by the star during periods of heavy mass loss in the AGB phase. The heavy mass loss mechanism is uncertain, as AGB stars are difficult to observe because they are heavily enshrouded by circumstellar shells. We used archival ultraviolet observations performed by the Galaxy Evolution Explorer (GALEX) satellite to pierce these circumstellar shells. In our study, we have discovered numerous new sources of ultraviolet emission from AGB stars and expanded the number of known ultraviolet-emitting AGB stars by up to an order of magnitude. We studied the origin of this ultraviolet emission by comparing its characteristics to other properties of the AGB stars and to other ultraviolet-emitting stars. We find that it is difficult to reconcile the ultraviolet emission with currently understood spectral energy distributions of AGB stars. Alternative origins include binary companions, chromospheric emission, accretion, shocks in the circumstellar shells, and/or heretofore unknown physical processes. Ultimately, our results provide new avenues of exploration and potential constraints on the mass loss mechanism during the AGB phase in the evolution of a star like our sun.