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Distance estimates for AGB stars from parallax measurements
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, Observational Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, S-43992 Onsala, Sweden..
Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, S-43992 Onsala, Sweden..
2022 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 667, p. A74-, article id A74Article in journal (Refereed) Published
Abstract [en]

Context. Estimating the distances to asymptotic giant branch (AGB) stars using optical measurements of their parallaxes is not straightforward because of the large uncertainties introduced by their dusty envelopes, their large angular sizes, and their surface brightness variability. Aims. This paper aims to assess the reliability of the distances derived with Gaia DR3 parallaxes for AGB stars, and provide a new distance catalogue for a sample of similar to 200 nearby AGB stars. Methods. We compared the parallaxes from Gaia DR3 with parallaxes measured with maser observations with very long baseline interferometry (VLBI) to determine a statistical correction factor for the DR3 parallaxes using a sub-sample of 33 maser-emitting oxygen-rich nearby AGB stars. We then calculated the distances of a total of similar to 200 AGB stars in the DEATHSTAR project using a Bayesian statistical approach on the corrected DR3 parallaxes and a prior based on the previously determined Galactic distribution of AGB stars. We performed radiative transfer modelling of the stellar and dust emission to determine the luminosity of the sources in the VLBI sub-sample based on the distances derived from maser parallaxes, and derived a new bolometric period-luminosity relation for Galactic oxygen-rich Mira variables. Results. We find that the errors on the Gaia DR3 parallaxes given in the Gaia DR3 catalogue are underestimated by a factor of 5.44 for the brightest sources (G < 8 mag). Fainter sources (8 <= G < 12) require a lower parallax error inflation factor of 2.74. We obtain a Gaia DR3 parallax zero-point offset of -0.077 mas for bright AGB stars. The offset becomes more negative for fainter AGB stars. After correcting the DR3 parallaxes, we find that the derived distances are associated with significant, asymmetrical errors for more than 40% of the sources in our sample. We obtain a PL relation of the form Mbol = (- 3.31 +/- 0.24) [log P - 2.5]+(-4.317 +/- 0.060) for the oxygen-rich Mira variables in the Milky Way. A new distance catalogue based on these results is provided for the sources in the DEATHSTAR sample. Conclusions. The corrected Gaia DR3 parallaxes can be used to estimate distances for AGB stars using the AGB prior, but we confirm that one needs to be careful when the uncertainties on parallax measurements are larger than 20%, which can result in model-dependent distances and source-dependent offsets. We find that a RUWE (re-normalised unit weight error) below 1.4 does not guarantee reliable distance estimates and we advise against the use of only the RUWE to measure the quality of Gaia DR3 astrometric data for individual AGB stars.

Place, publisher, year, edition, pages
EDP Sciences EDP Sciences, 2022. Vol. 667, p. A74-, article id A74
Keywords [en]
stars: AGB and post-AGB, stars: distances, parallaxes, methods: statistical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-489341DOI: 10.1051/0004-6361/202243670ISI: 000880425800011OAI: oai:DiVA.org:uu-489341DiVA, id: diva2:1717104
Funder
Swedish National Space BoardSwedish Research Council, 2020-04044EU, European Research Council, 883867Available from: 2022-12-07 Created: 2022-12-07 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, Sofia

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