HST and VLT observations of the symbiotic star Hen 2–147 *,**,***
Its nebular dynamics, its Mira variable and its distance
1
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: [miguelsg;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;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;mlivio]@stsci.edu
Received:
21
June
2006
Accepted:
9
January
2007
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 3D 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
kpc, which is a factor of two lower than the
distance of 
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: binaries: symbiotic / planetary nebulae: individual: Hen 2-147 / ISM: kinematics and dynamics
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 observations obtained at the 8 m VLT telescope of the European Southern Observatory in Chile; and on observations made at the South African Astronomical Observatory.
© ESO, 2007