Searching for X-ray emission from AGB stars

• Published in: Astronomy and astrophysics (Berlin) Vol. 543; p. A147
• Main Authors: Ramstedt, S., Montez, R., Kastner, J., Vlemmings, W. H. T.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.07.2012
• Subjects: activity; Astronomy; circumstellar envelopes; dust formation; Earth, ocean, space; evolved stars; Exact sciences and technology; giant-branch stars; magnetic; mass-loss rates; period-luminosity relation; planetary-nebulae; radio line emission; stars: AGB and post-AGB; stars: magnetic field; white-dwarfs; X-rays: stars; Polarization; X ray observation; X-rays: stars; stars: magnetic field; Masers; Luminosity; stars: AGB and post-AGB; Magnetic stars; Magnetic fields; Asymptotic giant branch; Spectral analysis; X ray emission
• ISSN: 0004-6361; 1432-0746; 1432-0746
• DOI: 10.1051/0004-6361/201118516

Context. Magnetic fields have been measured around asymptotic giant branch (AGB) stars of all chemical types using maser polarization observations. If present, a large-scale magnetic field would lead to X-ray emission, which should be observable using current X-ray observatories. Aims. The aim is to search the archival data for AGB stars that are intrinsic X-ray emitters. Methods. We have searched the ROSAT, CXO, and XMM-Newton archives for serendipitous X-ray observations of a sample of ~500 AGB stars. We specifically searched for the AGB stars detected with GALEX. The data is calibrated, analyzed and the X-ray luminosities and temperatures are estimated as functions of the circumstellar absorption. Results. We identify 13 AGB stars as having either serendipitous or targeted observations in the X-ray data archives, however for a majority of the sources the detailed analysis show that the detections are questionable. Two new sources are detected by ROSAT: T Dra and R UMa. The spectral analysis suggests that the emission associated with these sources could be due to coronal activity or interaction across a binary system. Conclusions. Further observations of the detected sources are necessary to clearly determine the origin of the X-ray emission. Moreover, additional objects should be subject to targeted X-ray observations in order to achieve better constraints for the magnetic fields around AGB stars.