II. The multiple wind of EP Aquarii
IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France e-mail: email@example.com
2 LERMA, UMR 8112, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
Accepted: 31 August 2007
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 with a spectral resolution of 0.1 km s-1 and with beams of and , respectively.
Results.The source is clearly resolved with a size of about (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 ~ and 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.
Key words: hydrodynamics / stars: mass-loss / stars: AGB and post-AGB / stars: winds, outflows / radio lines: stars / circumstellar matter
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).
© ESO, 2007