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, the minor/major axes 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 C-stars,
which are larger, and M-stars, which are smaller, as expected due to the dierences 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 highdensity
sources are 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 and the physical processes behind these asymmetrical structures.
This will provide further insight into the mass-loss process and its related chemistry of S-type AGB stars.
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, the minor/major axes 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 C-stars,
which are larger, and M-stars, which are smaller, as expected due to the dierences 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 highdensity
sources are 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 and the physical processes behind these asymmetrical structures.
This will provide further insight into the mass-loss process and its related chemistry of S-type AGB stars.
| Original language | English |
|---|---|
| Journal | Astronomy & Astrophysics |
| Publication status | Accepted/In press - 4 Jun 2021 |