Variable X-Ray and UV emission from AGB stars: Accretion activity associated with binarity

[摘要] Almost all of our current understanding of the late evolutionary stages of (1-8) Mostars is based on single-star models. However, binarity can drastically affect late stellar evolution, producing dramatic changes in the history and geometry of mass loss that occurs in stars as they evolve off the AGB to become planetary nebulae (PNe). A variety of binary models have been proposed, which can lead to the generation of accretion disks and magnetic fields, which in turn produce the highly collimated jets that have been proposed as the primary agents for the formation of bipolar and multipolar PNe. However, observational evidence of binarity in AGB stars is sorely lacking simply these stars are very luminous and variable, invalidating standard techniques for binary detection. Using an innovative technique of searching for UV emission from AGB stars with GALEX, we have identified a class of AGB stars with far- ultraviolet excesses (fuvAGB stars), that are likely candidates for active accretion associated with a binary companion. We have carried out a pilot survey for X-ray emission from fuvAGB stars. The X-ray fluxes are found to vary in a stochastic or quasi-periodic manner on roughly hour-long times-scales, and simultaneous UV observations show similar variations in the UV fluxes. We discuss several models for the X-ray emission and its variability and find that the most likely scenario for the origin of the X-ray (and FUV) emission involves accretion activity around a main-sequence companion star, with confinement by strong magnetic fields associated with the companion and/or an accretion disk around it.

[发布机构] Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena; CA; 91109, United States^1;Astrobiology Center (CSIC-INTA), ESAC Campus, Villanueva de la Canada, Madrid; E-28691, Spain^2

[效力级别] 天文学 [学科分类] 天文学(综合)

[关键词] Binary companion;Binary detection;Evolutionary stage;Far ultraviolet;Innovative techniques;Planetary nebulae;Stellar evolutions;Strong magnetic fields