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DEATHSTAR: nearby AGB stars with the Atacama Compact Array II. CO envelope sizes and asymmetries: the S-type stars
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
Chalmers Univ Technol, Onsala Space Observ, Dept Space Earth & Environm, S-43992 Onsala, Sweden..
Katholieke Univ Leuven, Dept Phys & Astron, Inst Astron, Celestijnenlaan 200D, B-3001 Leuven, Belgium..
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2021 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 653, article id A53Article in journal (Refereed) Published
Abstract [en]

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 Cstars, 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.

Place, publisher, year, edition, pages
EDP SCIENCES S A EDP Sciences, 2021. Vol. 653, article id A53
Keywords [en]
stars: AGB and post-AGB, stars: mass-loss, stars: winds, outflows, circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-456520DOI: 10.1051/0004-6361/202140952ISI: 000693380000015OAI: oai:DiVA.org:uu-456520DiVA, id: diva2:1605521
Funder
Swedish Research Council, 2017-00648EU, Horizon 2020, 883867EU, Horizon 2020, 730562Available from: 2021-10-25 Created: 2021-10-25 Last updated: 2024-02-23Bibliographically approved
In thesis
1. Mass Loss of Evolved Stars: Improving Mass-Loss Rates and Distances
Open this publication in new window or tab >>Mass Loss of Evolved Stars: Improving Mass-Loss Rates and Distances
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the final stages of their lives, low to intermediate-mass stars enter the Asymptotic Giant Branch (AGB) phase, where they experience significant mass loss through dusty stellar winds. This mass loss is pivotal, not only for the chemical enrichment of the interstellar medium, setting the stage for new stars and planets to form, but also in dictating the evolution and the ultimate fate of the stars themselves. The study of mass loss in AGB stars is therefore of paramount importance. Analyses of emission from CO gas in these outflows allow for the determination of mass-loss rates, as CO serves as a primary tracer for the molecular gas in the circumstellar envelopes of AGB stars. 

However, one of the main uncertainties in mass loss estimations arises from the assumptions on the size of the CO envelope. This uncertainty can be removed using interferometry, which allows for direct measurements of the spatial extent of the CO emission. Using the compact array of the Atacama Large Millimeter/submillimeter Array (ALMA), we measured the extent and investigated the degree of sphericity of the CO-emitting regions around 69 AGB stars in the DEATHSTAR programme. Of those, 27 are presented in this thesis.

Another significant source of uncertainty in mass-loss rate estimates lies in the distance, a fundamental parameter which has been notoriously difficult to accurately determine for AGB stars, especially when relying on optical parallaxes like those from Gaia. To tackle this, we conducted a comparative analysis between Gaia DR3 parallaxes and the more robust parallaxes obtained from high-resolution interferometric observations of maser emissions. This approach enabled us to provide reliable distance estimates for a sample of 200 AGB stars, including the DEATHSTAR sample, offering a valuable resource for the AGB scientific community. 

Using the newly calculated distances and updated CO envelope size measurements, we performed radiative transfer modelling to derive the mass-loss rates of a sample of 27 carbon-rich AGB stars. The spatial information provided by ALMA observations acted as strong constraints for our models, ensuring a more accurate representation of flux contributions from various spatial scales.

By providing the measurement of CO envelope sizes, improving the distance determination to AGB stars, and presenting improved mass-loss rates, this thesis provides new insights into the complex nature of AGB stars and their mass loss.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2024. p. 90
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2367
Keywords
AGB stars, Mass loss, Distance, Circumstellar matter, Radio interferometry, Stellar evolution
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-523696 (URN)978-91-513-2043-4 (ISBN)
Public defence
2024-04-11, Heinz-Otto Kreiss (101195), Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2024-03-20 Created: 2024-02-23 Last updated: 2024-03-20

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Andriantsaralaza, MioraRamstedt, SofiaHöfner, Susanne

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