DEATHSTAR: nearby AGB stars with the Atacama Compact Array

Abstract

Aims. We aim to constrain the sizes of, and investigate deviations from spherical symmetry in, the CO circumstellar envelopes (CSEs) of 16 S-type stars, along with an additional 7 and 4 CSEs of C-type and M-type AGB stars, respectively. Methods. We map the emission from the CO J = 2–1 and 3–2 lines observed with the Atacama Compact Array (ACA) and its total power (TP) antennas, and fit with a Gaussian distribution in the uv- and image planes for ACA-only and TP observations, respectively. The major axis of the fitted Gaussian for the CO(2–1) line data gives a first estimate of the size of the CO-line-emitting CSE. We investigate possible signs of deviation from spherical symmetry by analysing the line profiles and the minor-to-major axis ratio obtained from visibility fitting, and by investigating the deconvolved images. Results. The sizes of the CO-line-emitting CSEs of low-mass-loss-rate (low-MLR) S-type stars fall between the sizes of the CSEs of C-stars, which are larger, and those of M-stars, which are smaller, as expected because of the differences in their respective CO abundances and the dependence of the photodissociation rate on this quantity. The sizes of the low-MLR S-type stars show no dependence on circumstellar density, as measured by the ratio of the MLR to terminal outflow velocity, irrespective of variability type. The density dependence steepens for S-stars with higher MLRs. While the CO(2–1) brightness distribution size of the low-density S-stars is in general smaller than the predicted photodissociation radius (assuming the standard interstellar radiation field), the measured size of a few of the high-density sources is of the same order as the expected photodissociation radius. Furthermore, our results show that the CO CSEs of most of the S-stars in our sample are consistent with a spherically symmetric and smooth outflow. For some of the sources, clear and prominent asymmetric features are observed which are indicative of intrinsic circumstellar anisotropy. Conclusions. As the majority of the S-type CSEs of the stars in our sample are consistent with a spherical geometry, the CO envelope sizes obtained in this paper will be used to constrain detailed radiative transfer modelling to directly determine more accurate MLR estimates for the stars in our sample. For several of our sources that present signs of deviation from spherical symmetry, further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures. This will provide further insight into the mass-loss process and its related chemistry in S-type AGB stars.

Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Convection in 3D Simulations of Red Supergiants

“…This may apply to other cool stars, e.g., the ∼1 M e AGB star R Dor, which is reported to be rotating at ṽ i sin -1 km s 1 with ALMA (Figure 1 Ramstedt et al 2017;Doan et al 2020;De Ceuster et al 2022). This is particularly timely with the ongoing ALMA programs such as DEATHSTAR (Ramstedt et al 2020;Andriantsaralaza et al 2021) and ATOMIUM (Decin et al 2020;Gottlieb et al 2022;Decin et al 2023;Montargès et al 2023) that advance our understanding of chemistry, dust formation, planetary nebulae, binary interaction, and mass loss of these cool stars. 4.…”

Distance estimates for AGB stars from parallax measurements

“…The DEATHSTAR 1 sample consists of ∼ 200 nearby AGB stars. The first publications of the DEATHSTAR project present the CO observations of ∼ 70 southern sources (Ramstedt et al 2020;Andriantsaralaza et al 2021). The source selection and completeness of the full DEATHSTAR sample is discussed in Ramstedt et al (2020).…”

DEATHSTAR—CO Envelope Size and Asymmetry of Nearby AGB Stars

“…The generated CO cubes have a spectral resolution of 0.75 km s −1 in both bands and an angular resolution of ∼5 and ∼18 arcsec for the ACA and TP maps, respectively. The generated cubes and dust continuum maps are publicly available via the VizieR Online Data Catalog for the Cycle 4 [24] and Cycle 5 [25] data.…”

“…The aim of this work is to report the degree of sphericity of the CO-emitting CSEs of the southern AGB stars in the DEATHSTAR sample observed with the Atacama Compact Array (ACA). DEATHSTAR 1 (DEtermining Accurate mass-loss rate for THermally pulsing AGB stars; [19,20]) is a large project aimed at improving the accuracy of wind-parameter measurements towards the CSEs of nearby AGB stars, using CO observations from the ACA. These interferometric data will be used as constraints and combined with detailed radiative transfer modeling of the observed CO lines to obtain reliable mass-loss-rate estimates.…”

“…These interferometric data will be used as constraints and combined with detailed radiative transfer modeling of the observed CO lines to obtain reliable mass-loss-rate estimates. This present study is part of the first step of the DEATHSTAR project, which consists of constraining the size of the CO-emitting CSEs [19,20] and comparing them with the CO sizes obtained from photodissociation models [21,22], which are used in current mass-loss rate estimates.…”