Context :

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 \sim 90 km s ^ { -1 } .
A straightforward application of the expansion parallax method provides a distance of 1.5 \pm 0.4 kpc , which is a factor of two lower than the distance of 3.0 \pm 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 .

Conclusions :