2007A&A...475..559W

CO rotational lines are frequently used to trace the outflows from AGB stars. Some profiles are composite, with a narrow component super-imposed on a broader one. These profiles have been interpreted in different ways, calling for episodic mass loss, a bipolar flow, or a circumstellar disk. To investigate the structure of one of these outflows, we have obtained detailed ¹²CO(2-1) and ¹²CO(1-0) maps of EP Aqr, a prototypical source with composite CO profiles. Interferometric data were acquired with the IRAM interferometer and combined with on-the-fly maps obtained at the IRAM 30-m. The resulting maps in ¹²CO(2-1) and ¹²CO(1-0) cover a field of 100"x100" with a spectral resolution of 0.1km/s and with beams of 1.7"x1.0 " and 3.5"x1.8", respectively. The source is clearly resolved with a size of about 15" (FWHM). We do not observe any obvious departure from circular symmetry, but there is evidence of a ringed structure in the CO(2-1) map with enhanced intensity at ∼3.5" and 7.5" from the central star. The continuum level at 1.3 and 2.6mm is consistent with the star's photospheric emission. We modeled the spatio-kinematic structure with Monte-Carlo radiative transfer simulations assuming spherical symmetry. We reached a reasonable fit to the map-integrated spectra, but not to the imaging data, possibly because the circumstellar shell of EP Aqr presents inhomogeneities on a scale that is not, or is only barely, resolved in our maps. EP Aqr may be a proto-typical oxygen-rich source for the class of theoretical models exhibiting mass loss variations on a ∼100yr timescale discovered by Winters et al. (2000A&A...361..641W), which show a layered structure in their extended circumstellar shells.