12CO emission from EP Aqr: Another example of an axi-symmetric AGB wind?

Abstract

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.

accepted for publication in Astronomy & Astrophysics

References in corpus (2)

• The dust disk and companion of the nearby AGB star L2 Puppis - SPHERE/ZIMPOL polarimetric imaging at visible wavelengths
• The multi-scale environment of RS Cnc from CO and HI observations

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• How to disentangle geometry and mass-loss rate from AGB-star spectral energy distributions -- The case of EP Aqr
• ALMA observations of the circumstellar envelope around EP Aqr
• Molecules, shocks, and disk in the axi-symmetric wind of the MS-type AGB star RS Cancri
• De-projection of radio observations of axi-symmetric expanding circumstellar envelopes
• Morphology and kinematics of the gas envelope of the variable AGB star ${\pi }^1$ Gruis
• Probing sulfur chemistry in oxygen-rich AGB stars with ALMA