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The Extended Molecular Envelope of the Asymptotic Giant Branch Star $\Pi

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The Extended Molecular Envelope of the Asymptotic Giant Branch Star $\Pi Astronomy & Astrophysics manuscript no. paper2_fin c ESO 2019 November 28, 2019 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 L. Doan1, S. Ramstedt1, W. H. T. Vlemmings2, S. Mohamed3, 4, 5, S. Höfner1, E. De Beck2, F. Kerschbaum6, M. Lindqvist2, M. Maercker2, C. Paladini7, and M. Wittkowski8 1 Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden e-mail: [email protected] 2 Department of Earth and Space Sciences, Chalmers University of Technology, SE-43992 Onsala, Sweden 3 South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa 4 Astronomy Department, University of Cape Town, University of Cape Town, 7701, Rondebosch, South Africa 5 National Institute for Theoretical Physics, Private Bag X1, Matieland, 7602, South Africa 6 Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria 7 European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile 8 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany ABSTRACT 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 Atacama Large 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. Key words. stars: AGB and post-AGB – stars: mass-loss – binaries: general - radio lines: stars - stars: general – stars: individual: π1 Gru 1. Introduction transfer will form an accretion wake and the companion’s orbital arXiv:1911.10756v2 [astro-ph.SR] 27 Nov 2019 motion can shape the circumstellar envelope into a spiral mor- The spiral structure of an extended AGB envelope was for phology.Chen et al. (2017) studied mass transfer throughBondi- the first time discovered in the carbon star AFGL 3068 Hoyle accretion in binary systems with a low mass-loss-rate pri- (Mauron & Huggins 2006). The unexpected morphology was mary and reproduced the spiral structures found from observa- explained when a companion was detected orbiting the pri- tions. Kim et al. (2017) modelled recent detailed ALMA obser- mary (Morris et al. 2006). Similar circumstellar structures have vations of the CSE around AFGL 3068 including an eccentric since then been revealed around several AGB stars (e.g., binary orbit. The outcomes of these simulations are affected by Maercker et al. 2012; Ramstedt et al. 2014, 2017; Kim et al. the accuracy of the equation of state, the opacity profile, and the 2015). There have been a number of attempts to model the cir- grid resolution (Chen et al. 2017). cumstellar morphology induced in a binary system with a mass- losing primary. Mohamed & Podsiadlowski (2012) proposed a The role of a companion in shaping the AGB outflow mass transfer mechanism, referred to as ’wind Roche-lobe over- has been investigated with our ALMA project where we flow’ (wRLOF), in a detached binary system. If the dust conden- have observed four well studied binary systems: R Aquarii, sation radius is comparable to the Roche-lobe radius, the mass Mira (o Ceti), W Aquilae, and π1 Gruis. All four systems show Article number, page 1 of 12 A&A proofs: manuscript no. paper2_fin Table 1. Summary of the ALMA observations used in this study. Nant is This study presents the data analysis of the rotational line the number of antennas emission, 12CO J=3-2 , from π1 Gruis observed with the ALMA full array. The low-angular resolution data used in this study has Array On-source Nant Baseline already been discussed in Paper I. The data is used to constrain time[min] [m] a 3D radiative transfer model to study the morphology, the tem- ALMA-ACA 23.7 7 11-49 perature distribution, and the gas kinematics. Here the low- and ALMA12-m 7.06 31 15-438 high-angular resolution interferometric data, and the total power 7.06 41 15-348 data are combined to investigate the circumstellar envelope with ALMA-TP 67.1 3 the hitherto highest angular resolution in this frequency range. The content of the paper will be presented in the following or- Table 2. The spectral resolution ∆υ and the rms noise levels of the final der: Observations and data reduction in Sect. 2; Observational images. results in Sect. 3; Discussion in Sect. 4; and Summary in Sect. 5. LSR velocity ∆υ rms km s−1 km s−1 Jy beam−1 2. Observations and data reduction [h−40, 16]i h 2i h 0.022 i We have observed π1 Gruis using both the compact array, ACA, < −40 4 0.006 and the 12-m main array. ALMA total power (TP) observations > 16 4 0.005 were also performed to complement the interferometric obser- vations. A summary of the observations is given in Table 1. The observations were done with four spectral windows that have a the imprint of companions on the AGB circumstellar envelopes. width of 2 GHz each, centered on 331, 333, 343, and 345 GHz. The smallest separation (∼10 AU) binary system of the Mira The data reduction was done using the Common Astron- star R Aquarii and its white-dwarf companion shows a ring- omy Software Application (CASA 4.3.0). The interferometric like structure aligned with the orbital plane (Ramstedt et al. data was first calibrated and combined separately, then combined 2018; Bujarrabal, V. et al. 2018). One possible explanation is with the TP data. All the data was checked and adjusted for that the structure is due to mass-loss variations. The CSE of pointing positions to verify that the observations have the same the Mira (o Ceti) system, which has a moderate separation (∼60 phase center. The ratio between the ACA (7-m) array and the AU), presents a complex morphology of bubbles and spiral arcs main (12-m) array weights was scaled to be 0.18. The combined resulting from several dynamical processes during the evolution data recovers ∼90% of the flux observed by APEX in 2005. of the system. (Ramstedt et al. 2014). The CSE of W Aquilae The combined visibilities were finally imaged and decon- and its main-sequence companion at a separation of ∼100 AU volved using the CLEAN package using an iterative procedure shows two weak spiral patterns with different inter-arm spac- with a decreasing threshold parameter. The initial spectral reso- ings (Ramstedt et al. 2017). The larger-spacing pattern, which lution of 0.5kms−1 has been binned to 2 and 4kms−1 for the is brighter (denser) on the west side, can be reproduced from low- and high-velocity channels, respectively, to improve the the known companion using an eccentric hydrodynamicwRLOF signal-to-noiseratio. The synthesized beam size is 0′′.84×0′′.56 at model of mass transfer. However, the smaller-spacing pattern is a position angle (PA) of −81◦. The properties of the final image formed by an unknown process. The diverse morphologies seen cubes are summarized in Table 2. in the binary systems reflect the complicated dynamic processes of AGB wind shaping. The outcome will depend on many pa- rameters, e.g., the binary separation, the mass ratio, the wind 3. Observational results speed of the mass-losing star, and the evolutionary stage of the 3.1. Images companion. The largest separation (∼400 AU) system in the project is The velocity channel maps of the 12CO J=3-2 emission are the S-type AGB star π1 Gruis and its main-sequence companion. given in Fig. 1 and 2. The emission at low-offset velocities (rel- −1 The photosphereof the star appearsroundwith no sign of Roche- ative to the systemic velocity, vsys = −12kms ) is dominat- Lobe shape (Paladini et al. 2018). The inner circumstellar enve- ing and shown separately from the high-offset velocity channels: lope (within 5 stellar radii) probed by VLTI/MIDI appears very the low-offset velocities (Fig.
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