arXiv:astro-ph/0701404v1 14 Jan 2007 Astronomy & Astrophysics manuscript no. santander c  ESO 2018 October 21, 2018 HST and VLT observations of the Symbiotic Star Hen 2–147 ⋆ Its Nebular Dynamics, its Mira Variable and its Distance M. Santander-Garc´ ıa 1 , R. L. M. Corradi 2,1 , P. A. Whitelock 3,4 , U. Munari 5 , A. Mampaso 1 , F. Marang 3 , F. Boffi 6 , and M. Livio 6 1 Instituto de Astrof´ ısica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: miguelsg@iac.es; amr@iac.es 2 Isaac Newton Group of Telescopes, Ap. de Correos 321, 38700 Sta. Cruz de la Palma, Spain e-mail: rcorradi@ing.iac.es 3 South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa e-mail: paw@saao.ac.za; fm@saao.ac.za 4 Department of Mathematics and Applied Mathematics and Department of Astronomy, University of Cape Town, South Africa 5 INAF Osservatorio Astronomico di Padova, via dell’Osservatorio 8, 36012 Asiago (VI), Italy e-mail:munari@pd.astro.it 6 Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: boffi@stsci.edu, mlivio@stsci.edu October 21, 2018 ABSTRACT Aims. We investigate the dynamics of the nebula around the symbiotic star Hen 2–147, determine its expansion parallax, and compare it with the distance obtained via the Period-Luminosity relation for its Mira variable. Methods. A combination of multi-epoch HST images and VLT integral field high-resolution spectroscopy is used to study the nebular dynamics both along the line of sight and in the plane of the sky. These observations allow us to build a 3-D spatio–kinematical model of the nebula, which together with the measurement of its apparent expansion in the plane of the sky over a period of 3 years, provides the expansion parallax for the nebula. Additionally, SAAO near-infrared photometry obtained over 25 years is used to determine the Mira pulsation period and derive an independent distance estimation via the Period-Luminosity relationship for Mira variables. Results. The geometry of the nebula is found to be that of a knotty annulus of ionized gas inclined to the plane of sky and expanding with a velocity of ∼ 90 km s −1 . A straightforward application of the expansion parallax method provides a distance of 1.5±0.4 kpc, which is a factor of two lower than the distance of 3.0 ± 0.4 kpc obtained from the Period-Luminosity relationship for the Mira (which has a pulsation period of 373 days). The discrepancy is removed if, instead of expanding matter, we are observing the expansion of a shock front in the plane of the sky. This shock interpretation is further supported by the broadening of the nebular emission lines. Key words. symbiotic stars: Hen 2-147 – planetary nebulae – interstellar medium: kinematics and dynamics 1. Introduction The study of nebulae around symbiotic stars, and in particular those containing Mira variables, is key to understanding sev- eral aspects of their binary interactions, outburst properties and timescales. Such studies provide information about the geom- Send offprint requests to: M. Santander-Garc´ ıa ⋆ Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555; on obser- vations obtained at the 8m VLT telescope of the European Southern Observatory in Chile; and on observations made at the South African Astronomical Observatory etry, dynamics, and history of mass loss from these systems over the last few thousand years. It is generally believed that in these binaries some percent of the Mira wind is accreted by the white dwarf companion (Kenyon & Webbink 1984), re- sulting in stable nuclear H-burning at the surface of the white dwarf for some systems (e.g. Sokoloski 2002), and sometimes causing thermonuclear outbursts (e.g. Kenyon 1986, Livio et al. 1989) lasting for hundreds of years (e.g. Munari 1997). The ejecta and fast wind produced during and after such an out- burst collide with the slowly expanding circum-binary gas re- sulting from the non-accreted Mira wind. This results in a com- plex circum-binary nebula whose ionization mechanism (radia-