SPHERE/ZIMPOL observations of the symbiotic system R Aquarii^⋆

I. Imaging of the stellar binary and the innermost jet clouds

¹ ETH Zurich, Institute for Astronomy, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
² European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
³ NOVA Optical Infrared Instrumentation Group at ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁴ Université Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange UMR 7293, CS 34229, 06304 Nice Cedex 4, France
⁷ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Dipartimento die Fisica e Astronomia “G. Galilei”, Universitá di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13388 Marseille, France
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹² LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322 Châtillon Cedex, France
¹⁵ Kiepenheuer-Institut für Sonnenphysik, Schneckstr. 6, 79104 Freiburg, Germany
¹⁶ European Southern Observatory, Karl Schwarzschild St, 2, 85748 Garching, Germany
¹⁷ Centre de Recherche Astrophysique de Lyon, CNRS/ENSL Université Lyon 1, 9 Av. Ch. André, 69561 Saint-Genis-Laval, France
¹⁸ Simcorp GmbH, Justus-von-Liebig-Strasse 1, 61352 Bad Homburg, Germany
¹⁹ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland

Received: 28 July 2016
Accepted: 14 February 2017

Abstract

Context. R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. Because of its proximity to the Sun, this object has been studied in much detail with many types of high resolution imaging and interferometric techniques. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT).

Aims. We describe SPHERE/ZIMPOL test observations of the R Aqr system taken in Hα and other filters in order to demonstrate the exceptional performance of this high resolution instrument. We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We use our data for a detailed characterization of the inner jet region of R Aqr.

Methods. We analyze the high resolution ≈ 25 mas images from SPHERE/ZIMPOL and determine from the Hα emission the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region <2′′ (<400 AU) of R Aqr. The data are compared to simultaneous HST line filter observations. The Hα fluxes and the measured sizes of the clouds yield Hα emissivities for many clouds from which one can derive the mean density, mass, recombination time scale, and other cloud parameters.

Results. Our Hα data resolve for the first time the R Aqr binary and we measure for the jet source a relative position 45 mas West (position angle −89.5°) of the mira. The central jet source is the strongest Hα component with a flux of about 2.5 × 10^-12 erg cm^-2 s^-1. North east and south west from the central source there are many clouds with very diverse structures. Within 0.5′′ (100 AU) we see in the SW a string of bright clouds arranged in a zig-zag pattern and, further out, at 1′′−2′′, fainter and more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure between 0.2′′−0.7′′ and faint perpendicular “wisps” further out. Some jet clouds are also detected in the ZIMPOL [O I] and He I filters, as well as in the HST-WFC3 line filters for Hα, [O III], [N II], and [O I]. We determine jet cloud parameters and find a very well defined correlation N_e ∝ r^-1.3 between cloud density and distance to the central binary. Densities are very high with typical values of N_e ≈ 3 × 10⁵ cm^-3 for the “outer” clouds around 300 AU, N_e ≈ 3 × 10⁶ cm^-3 for the “inner” clouds around 50 AU, and even higher for the central jet source. The high N_e of the clouds implies short recombination or variability timescales of a year or shorter.

Conclusions. Hα high resolution data provide a lot of diagnostic information for the ionized jet gas in R Aqr. Future Hα observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this jet outflow and its interaction with the wind of the red giant companion.