Rings and Filaments: the Remarkable Detached CO Shell of U Antliae
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Astronomy & Astrophysics manuscript no. aa30665-17 c ESO 2017 September 5, 2017 Rings and filaments: The remarkable detached CO shell of U Antliae?,??,??? F. Kerschbaum1, M. Maercker2, M. Brunner1, M. Lindqvist2, H. Olofsson2, M. Mecina1, E. De Beck2, M. A. T. Groenewegen3, E. Lagadec4, S. Mohamed5; 6; 10, C. Paladini7, S. Ramstedt8, W. H. T. Vlemmings2, and M. Wittkowski9 1 Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria e-mail: [email protected] 2 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden 3 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium 4 Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France 5 South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa 6 Astronomy Department, University of Cape Town, 7701, Rondebosch, South Africa 7 Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine C.P. 226, Boulevard du Triomphe, 1050 Brussels, Belgium 8 Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden 9 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany 10 National Institute for Theoretical Physics, Private Bag X1, 7602 Matieland, South Africa Received 20 February 2017 / Accepted 3 August 2017 ABSTRACT Aims. Our goal is to characterize the intermediate age, detached shell carbon star U Antliae morphologically and physically in order to study the mass-loss evolution after a possible thermal pulse. Methods. High spatial resolution ALMA observations of unprecedented quality in thermal CO lines allow us to derive first critical spatial and temporal scales and constrain modeling efforts to estimate mass-loss rates for both the present day as well as the ejection period of the detached shell. Results. The detached shell is remarkably thin, overall spherically symmetric, and shows a barely resolved filamentary substructure possibly caused by instabilities in the interaction zone of winds with different outflow velocities. The expansion age of the detached shell is of the order of 2700 yr and its overall width indicates a high expansion-velocity and high mass-loss period of only a few hun- −5 −1 dred years at an average mass-loss rate of ≈10 M yr . The post-high-mass-loss-rate-epoch evolution of U Ant shows a significant −8 −1 decline to a substantially lower gas expansion velocity and a mass-loss rate amounting to 4 × 10 M yr , at present being consistent with evolutionary changes as predicted for the period between thermal pulses. Key words. stars: AGB and post-AGB – stars: carbon – stars: evolution – stars: mass-loss 1. Introduction envelopes (CSEs) of AGB stars, the mass loss, and the mixing of material into the interstellar medium (ISM) is therefore crucial Mass loss on and shortly after the asymptotic giant branch to understanding early star formation and the evolution of galax- (AGB) is a major factor in the post-main-sequence evolution ies. Studies of AGB stars are also important for understanding of low- and intermediate-mass stars. The photospheres and ex- the future of our solar system. panding outflows show a rich molecular chemistry, and are the Mass loss of AGB stars takes the form of slow (typi- formation sites for microscopic dust particles. Mass loss plays cally 5–25 km s−1) winds with high mass-loss rates (of up to a crucial role within the cosmic cycle of matter, providing up −4 10 M /yr). Although mass loss has been studied since the to 70% of the interstellar dust (e.g. Schneider et al. 2014). Un- 1960s, many basic questions on its temporal evolution, geom- derstanding the chemistry in the atmospheres and circumstellar etry, and dynamics remain unanswered. There is growing evidence that mass loss on the AGB is ? This paper makes use of the following ALMA data: not a continuous process. One cause for variations are He shell ADS/JAO.ALMA2015.1.00007.S. ALMA is a partnership of ESO flashes (thermal pulses), which occur typically every 104 to (representing its member states), NSF (USA) and NINS (Japan), 105 yr (Vassiliadis & Wood 1993). They are thought to be re- together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI sponsible for the formation of detached shells observed around (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. carbon AGB stars (Olofsson et al. 1996). High spatial resolution ?? The reduced ALMA FITS data cubes are available at the CDS via observations of these objects have shown thin, clumpy, detached anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via shells, indicative of short phases of intense mass loss, collid- http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/vol/page ing with a previous, slower AGB wind (Lindqvist et al. 1999; ??? The movie is available at http://www.aanda.org Olofsson et al. 2000; Schöier et al. 2005; Maercker et al. 2012). Article published by EDP Sciences Article number, page 1 of8 A&A proofs: manuscript no. aa30665-17 The presence of a binary companion can further complicate of the spectral setup was the CO(1–0) line at 115.271 GHz, us- outflows from AGB stars (e.g. Mayer et al. 2011; Maercker et al. ing a spectral resolution of 244.019 kHz, which corresponds to 2012; Ramstedt et al. 2014, and references therein), and high a velocity resolution of 0.635 km s−1. In B6 the main target was spatial resolution observations of gas and dust (in mm-CO, far- the CO(2–1) line at 230.538 GHz, observed with a resolution of IR and visual scattered light) are needed to determine mor- 488.037 MHz to achieve the same velocity resolution as in B3. phology and mass-loss-rate evolution. The same holds for the In order to optimize the final data product quality, we re- clumpiness of the detached shells and the relative location of calibrated and re-imaged the data. The calibration was done with dust and gas (Kerschbaum et al. 2010; Olofsson et al. 2010; the Common Astronomy Software Application (CASA) pipeline Maercker et al. 2010, 2016), since they potentially hold crucial scripts as delivered to the principal investigator and no major ad- information on the mass-loss mechanism and the physical pro- justments needed to be done. Flux calibrators for the individual cesses predominating in the wind. data sets were Mars and Ganymede; phase calibrators were the The carbon star U Antliae provides a special opportunity quasars J1042-4143 and J1107-4449. The calibrated data sets of to study these questions in detail. Known already for its far- the MA and ACA were combined in the uv-plane with appropri- IR excess in the late 1980s and imaged using the Infrared As- ate weighting. tronomical Satellite (IRAS; Waters et al. 1994; Izumiura et al. The imaging process, which is critical for the final data qual- 1995), it was evident that this object (260 pc from Hipparcos) ity, was carried out with extended and custom python imaging had a spatially extended detached dust shell with a radius of scripts by our team; these scripts were executed in CASA. These about 5000. Single-dish mapping in the 12CO(1–0) and (2–1) scripts make use of an iterative and interactive imaging proce- lines revealed a detached shell of radius ∼4300 (Olofsson et al. dure with the CLEAN algorithm, using Briggs weighting (with 1996). Observations of scattered stellar light around U Ant robustness = 0.5), multi-scale cleaning, and iterative masking (González Delgado et al. 2001) revealed a complex shell struc- with decreasing thresholds to deconvolve the inter-shell struc- ture. The authors identified four shells at ∼2500, 3700, 4300, tures of the CSE. Compared to the imaging scripts of the de- and 4600, referred to as shell 1, 2, 3, and 4 below. This livered data products, our strategy employs more mask itera- structure was confirmed by studying scattered polarized light tions with finer thresholds and the CLEAN masks are carefully (González Delgado et al. 2003; Maercker et al. 2010). The mass adapted where necessary. of the outermost shell 4 (after Maercker et al.(2010) more at After the imaging process for the interferometric data, the 5000) is dominated by dust, while shell 3 at 4300 mainly consists TP single-dish data were added via feathering1. For the final data of gas. Radiative transfer models of single-dish CO data are con- products presented in Figs.1 and2, velocity binning to 1 km s −1 sistent with a thin CO shell at 4300 (Schöier et al. 2005). was applied during the imaging to increase the signal-to-noise The peak intensity of the thin (<300) dust shell seen us- ratio in the fainter parts of the shell. ing Herschel-PACS at 70 µm (Kerschbaum et al. 2010) coincides with shell 3, which is attributed to material expelled ∼3000 yr ago. This age places U Ant just between other detached-shell 3. Interpretation objects that were observed by mm-interferometers in the last 3.1. Morphology of the detached shell decade: U Cam (∼800 yr, observed with the Institut de ra- dioastronomie millimétrique Plateau de Bure interferometer A first inspection of the B3 and B6 maps close to the systemic (IRAM PdB); Lindqvist et al. 1999), R Scl (∼2300 yr, observed velocity of 24.5 km s−1 in Figs.1 and2 reveals a remarkably thin, with Atacama Large Millimeter/submillimeter Array (ALMA); overall spherically symmetric, detached CO shell of about 42.500 Maercker et al.