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Register a new userThe axisymmetric envelopes of RS Cnc and EP Aqr
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We report on observations obtained at IRAM on two semi-regular variable Asymptotic Giant Branch (AGB) stars, RS Cnc and EP Aqr, undergoing mass loss at an intermediate rate of ~ 10^-7 solar mass per year. Interferometric data obtained with the Plateau-de-Bure interferometer (NOEMA) have been combined with On-The-Fly maps obtained with the 30-m telescope in the CO(1-0) and (2-1) rotational lines. The spectral maps of spatially resolved sources reveal an axisymmetric morphology in which matter is flowing out at a low velocity (~ 2 km/s) in the equatorial planes, and at a larger velocity (~ 8 km/s) along the polar axes. There are indications that this kind of morpho-kinematics is relatively frequent among stars at the beginning of their evolution on the Thermally-Pulsing AGB, in particular among those that show composite CO line profiles, and that it might be caused by the presence of a companion. We discuss the progress that could be expected for our understanding of the mass loss mechanisms in this kind of sources by increasing the spatial resolution of the observations with ALMA or NOEMA.
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Atacama Large Millimetre/sub-millimetre Array (ALMA) observations of the CO(1-0) and CO(2-1) emissions from the circumstellar envelope of the Asymptotic Giant Branch (AGB) star EP Aqr have been made with four times better spatial resolution than previously available. They are analysed with emphasis on the de-projection in space of the effective emissivity and flux of matter using as input a prescribed configuration of the velocity field, assumed to be radial. The data are found to display an intrinsic axi-symmetry with respect to an axis making a small angle with the line of sight. A broad range of wind configurations, from prolate (bipolar) to oblate (equatorial) has been studied and found to be accompanied by significant equatorial emission. Qualitatively, the effective emissivity is enhanced near the equator to produce the central narrow component observed in the Doppler velocity spectra and its dependence on star latitude generally follows that of the wind velocity with the exception of an omni-present depression near the poles. In particular, large equatorial expansion velocities produce a flared disc or a ring of effective emissivity and mass loss. The effect on the determination of the orientation of the star axis of radial velocity gradients and possibly competing rotation and expansion in the equatorial disc is discussed. In general, the flux of matter is found to reach a broad maximum at distances of the order of 500 au from the star. Arguments are given that may be used to prefer one wind velocity distribution to another. As a result of the improved quality of the data, a deeper understanding of the constraints imposed on morphology and kinematics has been obtained.
We present a phenomenological study of CO(1-0) and CO(2-1) emission from the circumstellar envelope (CSE) of the Asymptotic Giant Branch (AGB) star RS,Cnc. It reveals departures from central symmetry that turn out to be efficient tools for the exploration of some of the CSE properties. We use a wind model including a bipolar flow with a typical wind velocity of $sim$8 km,s$^{-1}$ decreasing to $sim$2 km,s$^{-1}$ near the equator to describe Doppler velocity spectral maps obtained by merging data collected at the IRAM Plateau de Bure Interferometer and Pico Veleta single dish radio telescope. Parameters describing the wind morphology and kinematics are obtained, together with the radial dependence of the gas temperature in the domain of the circumstellar envelope probed by the CO observations. Significant north-south central asymmetries are revealed by the analysis, which we quantify using a simple phenomenological description. The origin of such asymmetries is unclear.
We present a detailed study of the circumstellar gas distribution and kinematics of the semi-regular variable star RS Cnc on spatial scales ranging from ~1 (~150 AU) to ~6 (~0.25 pc). New modeling of CO1-0 and CO2-1 imaging observations leads to a revised characterization of RS Cncs previously identified axisymmetric molecular outflow. Rather than a simple disk-outflow picture, we find that a gradient in velocity as a function of latitude is needed to fit the spatially resolved spectra, and in our preferred model, the density and the velocity vary smoothly from the equatorial plane to the polar axis. In terms of density, the source appears quasi-spherical, whereas in terms of velocity the source is axi-symmetric with a low expansion velocity in the equatorial plane and faster outflows in the polar directions. The flux of matter is also larger in the polar directions than in the equatorial plane. An implication of our model is that the stellar wind is still accelerated at radii larger than a few hundred AU, well beyond the radius where the terminal velocity is thought to be reached in an asymptotic giant branch star. The HI data show the previously detected head-tail morphology, but also supply additional detail about the atomic gas distribution and kinematics. We confirm that the `head seen in HI is elongated in a direction consistent with the polar axis of the molecular outflow, suggesting that we are tracing an extension of the molecular outflow well beyond the molecular dissociation radius (up to ~0.05 pc). The 6-long HI `tail is oriented at a PA of 305{deg}, consistent with the space motion of the star. We measure a total mass of atomic hydrogen ~0.0055 solar mass and estimate a lower limit to the timescale for the formation of the tail to be ~6.4x10^4 years. (abridged)
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$pm$0.4)10$^{-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.
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.
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