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

¹ LERMA, UMR 8112, CNRS & Observatoire de Paris/PSL, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: nhung.pham@obspm.fr
² Department of Astrophysics, Vietnam National Satellite Center, VAST, 18 Hoang Quoc Viet, Ha Noi, Vietnam
³ IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 St.-Martin d’Hères, France

Received: 18 May 2015
Accepted: 6 August 2015

Abstract

The CO(1−0) and (2−1) emission of the circumstellar envelope of the asymptotic giant branch (AGB) star EP Aqr has been observed in 2003 using the IRAM Plateau-de-Bure Interferometer and in 2004 with the IRAM 30-m telescope at Pico Veleta. The line profiles reveal the presence of two distinct components centred on the star velocity, a broad component extending up to ~10 km s^-1 and a narrow component indicating an expansion velocity of only ~2 km s^-1. An early analysis of these data was performed under the assumption of isotropic winds. The present study revisits this interpretation, instead assuming 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^-1 at the equator to ~10 km s^-1 near the poles. The mass-loss rate is ~1.2 × 10^-7 M_⊙ yr^-1. The angular aperture of the bipolar outflow is ~ 45° with respect to the star axis, which makes an angle of ~ 13° 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 stellar latitude, smaller and steeper at the poles than at the equator at large distances from the star (>2′′ ≡ 1.0 × 10^-3 pc). Under the hypothesis of radial expansion of the gas and of rotation invariance about the star axis, the effective density was evaluated in space as a function of star coordinates (longitude, latitude, and distance from the star). 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. This variation is inconsistent with the assumption of a spherical wind and strengthens our interpretation in terms of an axisymmetric outflow. While the phenomenological model presented here reproduces well the general features of the observations, not only qualitatively but also quantitatively, significant differences are also revealed, which would require a better spatial resolution to be properly described and understood.