  The CO(1-0) and (2-1) emission of the circumstellar envelope of the AGB star
EP Aqr has been observed using the IRAM PdBI and the IRAM 30-m telescope. The
line profiles reveal the presence of two distinct components centered on the
star velocity, a broad component extending up to ~10 km/s and a narrow
component indicating an expansion velocity of ~2 km/s. An early analysis of
these data was performed under the assumption of isotropic winds. The present
study revisits this interpretation by assuming instead a bipolar outflow nearly
aligned with the line of sight. A satisfactory description of the observed flux
densities is obtained with a radial expansion velocity increasing from ~2 km/s
at the equator to ~10 km/s near the poles. The angular aperture of the bipolar
outflow is ~45 deg with respect to the star axis, which makes an angle of ~13
deg with the line of sight. A detailed study of the CO(1-0) to CO(2-1) flux
ratio reveals a significant dependence of the temperature on the star latitude,
smaller and steeper at the poles than at the equator at large distances from
the star. Under the hypothesis of radial expansion and of rotation invariance
about the star axis, the effective density has been evaluated in space as a
function of star coordinates. Evidence is found for an enhancement of the
effective density in the northern hemisphere of the star at angular distances
in excess of ~3" and covering the whole longitudinal range. The peak velocity
of the narrow component is observed to vary slightly with position on the sky,
a variation consistent with the model and understood as the effect of the
inclination of the star axis with respect to the line of sight. While the
phenomenological model presented here reproduces well the general features of
the observations, significant differences are also revealed, which would
require a better spatial resolution to be properly described.
