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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
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
(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: urn:nbn:se:uu:diva-391308OAI: oai:DiVA.org:uu-391308DiVA, id: diva2:1344720
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-26Bibliographically approved
In thesis
1. Companion wind shaping in binaries involving an AGB star
Open this publication in new window or tab >>Companion wind shaping in binaries involving an AGB star
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
AGB and post-AGB stars, mass-loss, radio lines, binaries, wind shaping
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-391354 (URN)978-91-513-0728-2 (ISBN)
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

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