Searching For X-ray Emission From AGB Stars

Details Searching For X-ray Emission From AGB Stars Searching For X-ray Emission From AGB Stars

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21943617m&link_type=abstract

American Astronomical Society, AAS Meeting #219, #436.17

Astronomy and Astrophysics

Astronomy

Scientific paper

Polarization of maser emission confirms the presence of magnetic fields in the circumstellar envelopes around stars on the asymptotic giant branch (AGB). Such large-scale fields may be linked to coronal activity above the stellar surface and, hence, may lead to X-ray emission. We have searched the data archives of X-ray satellite observatories for targeted and serendipitous observations of over 500 AGB stars. In addition to the (previously published) targeted observations of Mira, TX Cam, and T Cas, we have identified eight serendipitous observations of AGB stars by the XMM-Newton and Chandra X-ray Observatories. Spurious detections of four AGB stars were made by the XMM Slew survey but are likely due to optical loading. All but Mira are undetected in the remaining observations, from which we derive upper limits to the X-ray fluxes. Two ROSAT faint source catalog entries were found to lie in close proximity to the GALEX-detected AGB stars T Dra and R UMa. As Mira represents the only previously documented case of an AGB star unambiguously detected in X-rays, these two new potential X-ray detections of AGB stars are noteworthy. We present the evidence for X-ray emission from these AGB stars and compare the results of spectral fitting of all three AGB star X-ray sources. We conclude that the origin of the X-ray emission from T Dra and R UMa may be either accretion onto as-yet undetected, but suspected, binary companions, or coronal activity at the AGB stars or putative companions. We also explore the absorbing column density and temperature space which AGB stars might appear as X-ray sources, so as to better understand the dearth of ROSAT detections and evaluate the prospects for future Chandra and XMM detections of AGB stars.

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