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

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.

Space reconstruction of the morphology and kinematics of axisymmetric radio sources

“…It gives evidence for a strong polar enhancement, at variance with a weak radial dependence up to r ∼ 8 ′′ . It provides a unified picture of the two models that have been proposed earlier, the model of Winters et al (2007) corresponding to V rad dependent on r but independent on α, that of Nhung et al (2015b) corresponding to V rad dependent on α but independent on r. The projections of the V rad distributions on the sin α and r axes illustrate the preference for the latter over the former.…”

“…The R-dependence is steeper for CO(2-1) than for CO(1-0) as expected, with respective power indices −1.2 and −0.9. The sky maps of the mean Doppler velocity display a small north-west/south-east asymmetry, interpreted by Nhung et al (2015b) as resulting from the small inclination of the star axis on the line of sight. In the θ = 0 approximation the stellar longitude ω is equal to the sky position angle ϕ of the pixel and the stellar latitude α is related to x by the relation x = R tan α.…”

The morpho-kinematics of the circumstellar envelope around the AGB star EP Aqr

“…Spatially resolved observations obtained at IRAM (Winters et al 2003;Nhung et al 2015a) on EP Aqr, and on other sources such as RS Cnc (Libert et al 2010;Hoai et al 2014;Nhung et al 2015b) and X Her (Kahane & Jura 1996) are generally interpreted as the superposition of a slowly expanding wind with a bipolar outflow. The presence of irregularities in the morpho-kinematics has been interpreted as suggesting an episode of increased mass loss rate (Knapp et al 1998;Nakashima 2006;Winters et al 2007).…”

How to disentangle geometry and mass-loss rate from AGB-star spectral energy distributions

“…Table 1 summarises its properties. There have been several recent investigations of the CSE around EP Aqr (Nhung et al 2015;Homan et al 2018;Tuyet Nhung et al 2019;Hoai et al 2019;Tuan-Anh et al 2019), most of them of the CO lines with ALMA. Tuan-Anh et al (2019) found jets extending from 25 to 1000 au.…”