Searching for X-ray emission from AGB stars

Abstract

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.

Single and Double Photoionization and Photodissociation of Toluene by Soft X-Rays in a Circumstellar Environment

“…The soft X-ray spectrum (0.2-2 keV) of the object shows emission centered at about 1 keV, observed by ROSAT (Ramstedt et al 2012). In order to obtain the photon flux as a function of the energy, a value of L X ≈3.2×10 31 erg s −1 was used for the X-ray luminosity, corresponding to a column density of N H ≈4,0×10 21 cm −2 (Ramstedt et al 2012). The main suggestions of Ramstedt et al (2012) regarding the origin of the X-ray emission of T Dra are that it is due: (1) to the mass accretion onto a compact white dwarf, in the case of binarity, or (2) to a coronal emission from a large-scale magnetic field.…”

“…T Dra, a carbon-rich AGB star, is at a distance of 610 pc, with a bolometric luminosity L=6300 L e , an effective temperature T * =1600 K, a velocity of expanding gas v exp =13.5 km s −1 , and a mass loss M 1.2 = ×10 −6 M e yr −1 (Schöier & Olofsson 2001). The soft X-ray spectrum (0.2-2 keV) of the object shows emission centered at about 1 keV, observed by ROSAT (Ramstedt et al 2012). In order to obtain the photon flux as a function of the energy, a value of L X ≈3.2×10 31 erg s −1 was used for the X-ray luminosity, corresponding to a column density of N H ≈4,0×10 21 cm −2 (Ramstedt et al 2012).…”

“…In order to obtain the photon flux as a function of the energy, a value of L X ≈3.2×10 31 erg s −1 was used for the X-ray luminosity, corresponding to a column density of N H ≈4,0×10 21 cm −2 (Ramstedt et al 2012). The main suggestions of Ramstedt et al (2012) regarding the origin of the X-ray emission of T Dra are that it is due: (1) to the mass accretion onto a compact white dwarf, in the case of binarity, or (2) to a coronal emission from a large-scale magnetic field. The gas density near the stellar surface should be sufficient to absorb the X-rays produced by the surface magnetic field, but morphological asymmetries could affect the photoabsorption and allow X-ray emission.…”

An edge-on translucent dust disk around the nearest AGB star, L2Puppis

“…This X-ray flux would be additionally reduced by the strong absorption by the dust disk. An X-ray emission of L 2 Pup was recently reported by Ramstedt et al (2012) from archival XMM data, but it was attributed by these authors to a leak of optical (red) photons and not to true emission from the star. So the X-ray emission does not provide a clear diagnostic on the existence of a companion of L 2 Pup.…”

A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars

“…The Sample of AGB Stars Our sample of AGB stars is derived from numerous AGB samples found in the literature and was originally compiled by Ramstedt et al (2012) to search for X-ray detections associated with AGB stars. As in Ramstedt et al (2012), there are a total of 468 unique AGB stars in our total sample; 286 M-type Miras from Little-Marenin & Little (1990); 171 AGB stars from the samples of Schöier & Olofsson (2001), González Delgado et al (2003), and Ramstedt et al (2006), plus 11 mixed type stars from the sample of Sahai et al (2008). The C-type (C/O>1) star sample is nearly complete out to 500 pc.…”