Abstract
Context: 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. Aims: 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. Methods: 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 arcsec × 100 arcsec with a spectral resolution of 0.1 km s-1 and with beams of 1.7 arcsec × 1.0 arcsec and 3.5 arcsec × 1.8 arcsec, respectively. Results: The source is clearly resolved with a size of about 15 arcsec (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 arcsec and 7.5 arcsec from the central star. The continuum level at 1.3 and 2.6 mm 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 ~100 yr timescale discovered by Winters et al. (2000), which show a layered structure in their extended circumstellar shells. Based on observations carried out with the IRAM Plateau de Bure Interferometer and the IRAM 30 m telescope on Pico Veleta. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). Data files (in FITS format) of the data presented in Figs. 6 and 7 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/475/559
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