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Companion wind shaping in binaries involving an AGB star
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stars of initial masses between 0.8-8 M☉ will become "asymptotic giant branch" (AGB) stars during the final stages of their evolution. During this phase, the stars are characterized by low velocity and high-density winds. An AGB star can lose a substantial fraction of its mass through the stellar wind and thereby avoid ending up as a supernova explosion. The AGB stars, therefore, play an important role in enriching the interstellar medium (ISM) with chemical elements and in contributing dust and gas to the ISM. The mass-loss rate on the AGB is a decisive parameter for the lifetime of this evolutionary phase and the fate of low- and intermediate-mass stars. An accurate mass-loss-rate estimation provides an important constraint for wind models aimed to better understand the wind-driving mechanism, as well as for stellar evolution.

The stellar wind is driven by radiation pressure on dust grains and blows away dust and gas from the central star. This creates an extended envelope which is expected to be spherical because of the isotropic radiation field of the central star, and the connection between the radiation field and the wind. However, there is growing observational evidence of asymmetrical morphology, e.g., torii, jets, bipolar outflows, in AGB circumstellar envelopes (CSEs). Moreover, proto-planetary nebulae (proto-PNe) and PNe, the next evolutionary phase after the AGB phase, show a wide range of asymmetrical morphologies. In many cases, an embedded binary system has been detected in the gas envelopes. This is pointing to the gravitational effect of the companion as important for the envelope shaping mechanism.

The work that this thesis is based on, studies two interesting examples of (post) AGB stars which show complex morphologies of their CSEs. The S-star π1 Gruis shows a CSE structure consisting of an equatorial low-velocity expanding spiral and a fast bipolar outflow. The circumstellar environment of the post-AGB (or post red giant branch, post-RGB) star HD 101584 shows an equatorial density enhancement and a high-velocity bipolar outflow. Same conclusions are drawn for both cases; that the radiation pressure on the dust cannot support the observed energetic outflows, and that interaction with the companions are proposed to shape the envelopes and accelerate the gas.

The thesis gives a brief introduction on AGB stars and wind shaping mechanisms of AGB CSEs. The thesis also presents the principles of interferometry, the data reduction methods, and the radiative transfer calculations used in the studies. Results from the included papers are also discussed.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1845
Keywords [en]
AGB and post-AGB stars, mass-loss, radio lines, binaries, wind shaping
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:uu:diva-391354ISBN: 978-91-513-0728-2 (print)OAI: oai:DiVA.org:uu-391354DiVA, id: diva2:1344764
Public defence
2019-10-09, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-09-17 Created: 2019-08-22 Last updated: 2019-10-15
List of papers
1. The extended molecular envelope of the asymptotic giant branchstar π1 Gruis as seen by ALMA: I. Large-scale kinematic structure and CO excitation properties
Open this publication in new window or tab >>The extended molecular envelope of the asymptotic giant branchstar π1 Gruis as seen by ALMA: I. Large-scale kinematic structure and CO excitation properties
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A28Article in journal (Refereed) Published
Abstract [en]

Context. The S-type asymptotic giant branch (AGB) star pi(1) Gru has a known companion at a separation of 2 ''.7 (approximate to 400 AU). Previous observations of the circumstellar envelope (CSE) show strong deviations from spherical symmetry. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive.

Aims. The aim is to make a 3D model of the CSE and to constrain the density and temperature distribution using new spatially resolved observations of the CO rotational lines.

Methods. We have observed the J = 3-2 line emission from (CO)-C-12 and (CO)-C-13 using the compact arrays of the Atacama Large Millimeter/submillimeter Array (ALMA). The new ALMA data, together with previously published (CO)-C-12 J = 2-1 data from the Submillimeter Array (SMA), and the (CO)-C-12 J = 5-4 and J = 9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL.

Results. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s(-1) along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s(-1); and a fast, bipolar outflow with velocity proportionally increasing from 14 km s(-1) at the base up to 100 km s(-1) at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7 x 10(-7) M-circle dot yr(-1). The total mass and linear momentum of the fast outflow are estimated at 7.3 x 10(-4) M-circle dot and 9.6 x 10(37) g cm s(-1), respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a (CO)-C-12/(CO)-C-13 abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed.

Keywords
stars: AGB and post-AGB; stars: mass-loss; stars: individual: pi(1) Gru; stars: general; radio lines: stars; binaries: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-327430 (URN)10.1051/0004-6361/201730703 (DOI)000412231200057 ()
Funder
EU, European Research Council, 614264
Available from: 2017-08-11 Created: 2017-08-11 Last updated: 2019-08-22Bibliographically approved
2. The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA: II. The spiral-outflow observed at high-angular resolution
Open this publication in new window or tab >>The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA: II. The spiral-outflow observed at high-angular resolution
(English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Other academic) Submitted
Abstract [en]

Context. This study follows up the previous analysis of lower-angular resolution data in which the kinematics and structure of the circumstellar envelope (CSE) around the S-type asymptotic giant branch (AGB) star π1 Gruis were investigated. The AGB star has a known companion (at a separation of 400 AU) which cannot explain the strong deviations from spherical symmetry of the CSE. Recently, hydrodynamic simulations of mass transfer in closer binary systems have successfully reproduced the spiral-shaped CSEs found around a handful of sources. There is growing evidence for an even closer, undetected companion complicating the case of π1 Gruis further.

Aims. The improved spatial resolution allows for the investigation of the complex circumstellar morphology and the search for imprints on the CSE of the third component.

Methods. We have observed the 12CO J=3-2 line emission from π1 Gruis using both the compact and extended array of AtacamaLarge Millimeter/submillimeter Array (ALMA). The interferometric data has furthermore been combined with data from the ALMA total power (TP) array. The imaged brightness distribution has been used to constrain a non-local, non-LTE 3D radiative transfer model of the CSE.

Results. The high-angular resolution ALMA data have revealed the first example of a source on the AGB where both a faster bipolar outflow and a spiral pattern along the orbital plane can be seen in the gas envelope. The spiral can be traced in the low- to intermediate-velocity (13–25 km s-1) equatorial torus. The largest spiral-arm separation is ≈5”.5 and consistent with a companion with an orbital period of 330 yrs and a separation of less than 70 AU. The kinematics of the bipolar outflow is consistent with it being created during a mass-loss eruption where the mass-loss rate from the system increased by at least a factor of 5 during 10-15 yrs.

Conclusions. The spiral pattern is the result of an undetected companion. The bipolar outflow is the result of a rather recent mass-loss eruption event.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-391308 (URN)
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-26Bibliographically approved
3. HD 101584: circumstellar characteristics and evolutionary status
Open this publication in new window or tab >>HD 101584: circumstellar characteristics and evolutionary status
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A153Article in journal (Refereed) Published
Abstract [en]

Context: There is growing evidence that red giant evolution is often affected by an interplay with a nearby companion, in some cases taking the form of a common-envelope evolution.

Aims: We have performed a study of the characteristics of the circumstellar environment of the binary object HD 101584, that provides information on a likely evolutionary scenario.

Methods: We have obtained and analysed ALMA observations, complemented with observations using APEX, of a large number of molecular lines. An analysis of the spectral energy distribution has also been performed.

Results: Emissions from 12 molecular species (not counting isotopologues) have been observed, and most of them mapped with angular resolutions in the range 0 ''.1-0 ''.6. Four circumstellar components are identified: (i) a central compact source of size approximate to 0 ''.15, (ii) an expanding equatorial density enhancement (a flattened density distribution in the plane of the orbit) of size approximate to 3 '', (iii) a bipolar high-velocity outflow (approximate to 150 km s(-1)), and (iv) an hourglass structure. The outflow is directed almost along the line of sight. There is evidence of a second bipolar outflow. The mass of the circumstellar gas is approximate to 0.5 [D/1 kpc](2) M-circle dot, about half of it lies in the equatorial density enhancement. The dust mass is approximate to 0.01 [D/1 kpc](2) M-circle dot, and a substantial fraction of this is in the form of large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is approximate to 770 [D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated gas are estimated to be 7 x 10(45) [D/1 kpc](2) erg and 10(39) [D/1 kpc](2) g cm s(-1), respectively.

Conclusions: We provide good evidence that the binary system HD 101584 is in a post-common-envelope-evolution phase, that ended before a stellar merger. Isotope ratios combined with stellar mass estimates suggest that the primary star's evolution was terminated already on the first red giant branch (RGB). Most of the energy required to drive the outflowing gas was probably released when material fell towards the companion.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
circumstellar matter, stars: individual: HD101584, stars: AGB and post-AGB, binaries: close, radio lines: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-381122 (URN)10.1051/0004-6361/201834897 (DOI)000462115700001 ()
Funder
Swedish Research CouncilEU, European Research Council, 614264
Available from: 2019-04-04 Created: 2019-04-04 Last updated: 2019-08-22Bibliographically approved
4. DEATHSTAR: Nearby AGB stars with ALMA ACA: I. A new hope for accurate mass-loss-rate estimates
Open this publication in new window or tab >>DEATHSTAR: Nearby AGB stars with ALMA ACA: I. A new hope for accurate mass-loss-rate estimates
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Context. This is the first publication from the DEATHSTAR project. The overall goal of the project is to reduce the uncertainties of the observational estimates of mass-loss rates from evolved stars on the Asymptotic Giant Branch (AGB).

Aims. The aim in this first publication is to constrain the sizes of the CO emitting region from the circumstellar envelopes around 21 M-type and 21 C-type stars using ALMA ACA. The symmetry of the mass-loss process is also investigated.

Methods. Line emission from CO J=2-1 and 3-2 from all the sources are mapped using ALMA ACA. In this initial analysis, the emission distribution is fitted to a Gaussian distribution in the uv-plane. Detailed radiative transfer analysis will be presented in a future publication. The major and minor axis of the best-fit Gaussian at the line center velocity of the CO(2-1) emission gives a first indication of the size of the emitting region. Furthermore, the fitting results, e.g., Gaussian major and minor axis, center position and goodness of fit across both lines constrains the symmetry of the emission distribution. For a subsample of sources, the measured emission distribution is compared to predictions from previous best-fit radiative transfer modelling results.

Results. The size of the CO(2-1) emitting region is essentially independent of the circumstellar density (measured by the mass-loss rate over the expansion velocity) for lower-mass-loss-rate irregular and semi-regular variables. For the M-type stars, there might be a weak correlation. Higher-mass-loss-rate Mira variables, on the other hand, show a strong correlation between the CO(2-1) size and the circumstellar density, although it appears to be slightly less steep than expected for the photodissociation radius. For a few lower-mass-loss-rate sources, the CO(2-1) size is found to be of the same order as (or even larger than) the predicted CO radius from photodissociation models. Strong asymmetries on large scales are found in about one third of the sources, which is consistent with previous investigations performed on northern sources of the same type. Smaller scale asymmetries are found in a larger fraction of sources.

Conclusions. Initial results on the envelope sizes and dynamics are presented and it is clear that further investigation of the data and the full sample will significantly improve our knowledge of AGB wind properties. For a large fraction of the sources, observations at higher spatial resolution will be necessary to deduce the nature and origin of the complex circumstellar dynamics revealed by the ACA observations presented in this paper.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-391352 (URN)
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-22

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  • Other locale
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Output format
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