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

Abstract

ALMA observations of CO(1-0) and CO(2-1) emissions of the circumstellar envelope of EP Aqr, an oxygen-rich AGB star, are reported. A thorough analysis of their properties is presented using an original method based on the separation of the data-cube into a low velocity component associated with an equatorial outflow and a faster component associated with a bipolar outflow. A number of important and new results are obtained concerning the distribution in space of the effective emissivity, the temperature, the density and the flux of matter. A mass loss rate of (1.6 ± 0.4)10 −7 solar masses per year is measured. The main parameters defining the morphology and kinematics of the envelope are evaluated and uncertainties inherent to de-projection are critically discussed. Detailed properties of the equatorial region of the envelope are presented including a measurement of the line width and a precise description of the observed inhomogeneity of both morphology and kinematics. In particular, in addition to the presence of a previously observed spiral enhancement of the morphology at very small Doppler velocities, a similarly significant but uncorrelated circular enhancement of the expansion velocity is revealed, both close to the limit of sensitivity. The results of the analysis place significant constraints on the parameters of models proposing descriptions of the mass loss mechanism, but cannot choose among them with confidence.

On the origin of high Doppler velocity wings in the spectra of O-rich AGB stars

“…It also disfavours an interpretation in terms of a spherical shell ejected by star pulsation at short distances from the star, as described in Winters et al [1] and McDonald & Zijlstra [19]. It favours instead an interpretation in terms of polar gas streams which is also consistent with the observations at large distances of CO emission which display clear bipolarity with a factor of 6 between polar and equatorial winds [6]. The high velocity streams are present in CO emission but weaker than in SiO emission (Figure 4), suggesting that they slow down and/or diverge at large distances from the star.…”

“…SiO emission gives evidence for a high velocity (~20 km s −1 ) wind close to the star. The P-V maps of Figures 2 show that the wind at short distances reaches a velocity well above the terminal velocity of ~12 km s −1 observed in CO emission at large distances from the star [6]. The absence of high velocities in SO 2 emission, if not the result of insufficient sensitivity, implies that the high velocity wings do not occur very close to the star.…”

“…Figure 1 shows the dependence on projected distance to the star, R, of the intensity multiplied by R. The beam sizes of these observations are 0.33×0.30 arcsec 2 , 0.29×0.25 arcsec 2 and 0.18×0.17 arcsec 2 for 12 CO(2-1), SiO(5-4) and SO 2 (16 6,10 -17 5,13 ) respectively. The CO emission at distances exceeding ~250 au [6] outflow expanding at a velocity increasing with stellar latitude α up to 10 to 12 km s −1 near the poles. A possible rotation velocity at the equator is shown not to exceed a third of the expanding velocity.…”

“…EP Aqr is an oxygen-rich star on the early phase of the AGB with no technetium in its spectrum [7] at a distance of 114±8 pc from the Sun [8][9]. It has a mass between 1 and 2 M O , a mass loss rate of ~1.6 10 −7 M⊙ yr −1 [6] and an effective temperature of ~ 3200 K [10].…”

“…Recently, the availability of high angular resolution ALMA (Atacama Large Millimeter/sub millimeter Array) observations of several oxygen rich AGB stars like EP Aqr, R Dor, L 2 Pup and Mira Ceti have shed a new light on this issue [3][4][5][6]. However, the origin of these high velocities is still unclear.…”