The bipolar jet of the symbiotic star R Aquarii: A study of its
  morphology using the high-resolution HST WFC3/UVIS camera
  R Aqr is a symbiotic binary system consisting of a Mira variable with a
pulsation period of 387 days and a hot companion which is presumably a white
dwarf with an accretion disk. This binary system is the source of a prominent
bipolar gaseous outflow. We use high spatial resolution and sensitive images
from the Hubble Space Telescope to identify and investigate the different
structural components that form the complex morphology of the R Aqr jet .
Methods. We present new high-resolution HST WFC3/UVIS narrow-band images of the
R Aqr jet obtained in 2013/14 in the light of the [OIII] 5007, [OI] 6300, [NII]
6583, and Ha emission lines. These images also allow us to produce detailed
maps of the jet flow in several line ratios such as [OIII]/[OI] and [NII]/[OI]
which are sensitive to the outflow temperature and its hydrogen ionisation
fraction. The new emission maps together with archival HST data are used to
derive and analyse the proper motion of prominent emitting features which can
be traced over 20 years with the HST observations. The images reveal the fine
gas structure of the jet out to distances of a few ten arcseconds from the
central region, as well as in the innermost region, within a few arcseconds
around the stellar source. They reveal for the first time the straight
highly-collimated jet component which can be traced to up to 900 AU in the NE
direction. Images in [OIII], [OI], and [NII] clearly show a helical pattern in
the jet beams which may derive from the small-scale precession of the jet. The
highly-collimated jet is accompanied by a wide opening angle outflow which is
filled by low excitation gas. The position angles of the jet structures as well
as their opening angles are calculated. Our measurements of the proper motions
of some prominent emission knots confirm the scenario of gas acceleration
during the propagation of the outflow.
