Astrophys. J., 810, 77 (2015/September-1)
A pilot deep survey for X-ray emission from fuvAGB stars.
SAHAI R., SANZ-FORCADA J., SANCHEZ CONTRERAS C. and STUTE M.
Abstract (from CDS):
We report the results of a pilot survey for X-ray emission from a newly discovered class of AGB stars with far-ultraviolet excesses (fuvAGB stars) using XMM-Newton and Chandra. We detected X-ray emission in three of six fuvAGB stars observed–the X-ray fluxes are found to vary in a stochastic or quasi-periodic manner on roughly hour-long timescales, and simultaneous UV observations using the Optical Monitor on XMM for these sources show similar variations in the UV flux. These data, together with previous studies, show that X-ray emission is found only in fuvAGB stars. From modeling the spectra, we find that the observed X-ray luminosities are ~(0.002-0.2) L☉ and the X-ray-emitting plasma temperatures are ~(35-160)x106 K. The high X-ray temperatures argue against the emission arising in stellar coronae, or directly in an accretion shock, unless it occurs on a WD companion. However, none of the detected objects is a known WD-symbiotic star, suggesting that if WD companions are present, they are relatively cool (<20,000 K). In addition, the high X-ray luminosities specifically argue against emission originating in the coronae of main-sequence companions. 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 companion star, with confinement by strong magnetic fields associated with the companion and/or an accretion disk around it.
binaries: general - binaries: symbiotic - circumstellar matter - stars: AGB and post-AGB - stars: individual: (EY, Hya, Y, Gem, CI, Hyi) - stars: mass-loss
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