ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars

Abstract

This overview paper presents ATOMIUM, a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of ATOMIUM is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (∼0″​​.025–0″​​.050), medium (∼0″​​.13–0″​​.24), and low (∼1″) angular resolution. The sensitivity per ∼1.3 km s−1 channel was 1.5–5 mJy beam−1, and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO2, and HCN; secondary molecules include SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result – the wind kinematics of the ATOMIUM sources. Our analysis suggests that the ATOMIUM sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12CO v = 0 J = 2 − 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The ATOMIUM data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models.

Molecules, shocks, and disk in the axi-symmetric wind of the MS-type AGB star RS Cancri

“…15. A similar feature was seen in high-spatialresolution observations of other oxygen-rich, low-mass-loss-rate (Decin et al 2020;Gottlieb et al 2022), calling for a common mechanism causing high-velocity wings in this type of object. In the case of EP Aqr, where the bipolar outflow axis almost coincides with the line of sight (with an inclination angle of i ≈ 10 • ), the highvelocity wings were interpreted in terms of narrow polar jets.…”

Evolution and Mass Loss of Cool Ageing Stars: a Daedalean Story

“…The proposed evolutionary scheme for AGB wind morphologies can explain various AGB, post-AGB, and PNe phenomena, including for instance why post-AGB star binaries can have nonzero eccentricities (Decin et al 2020). The binary scenario gets support from an analysis of the kinematic wind properties (Gottlieb et al 2020). Moreover, it was shown that early-type AGB stars with a low mass-loss rate are prime candidates for detecting planets.…”

“…This conclusion has a serious repercussion on any empirically retrieved mass-loss rate relation discussed in Section 2.2.4. Building on the discussion in Section 4.2, we conjecture that most empirically retrieved mass-loss rates yield mass-loss rate measures that are too high for application in single-star evolution models, since samples of stars will be flawed by a large fraction of stars that experience binary interaction (Gottlieb et al 2020). This brings us back to one of the pitfalls of retrieval approaches outlined in Section 2.2, and the caution expressed there about unrecognised bias effects in sample selections and the elemental difference between correlation and causal effects.…”

Multi-line diagnostics of the axi-symmetric wind around the MS-type AGB star RS Cancri

“…Temporal variations of SiO masers are quite common for pulsating AGB stars, but that the maser completely disappeared came as a surprise. A striking feature of the SiO lines is their broad wings that indicate velocities almost twice as high as the maximum velocities seen in CO: this has been seen in many other similar stars, like EP Aqr ( [5]), R Dor ( [6]), and in most of the stars covered by the ALMA large program ATOMIUM on AGB stars ( [7]). The map in the right plot of Fig.…”