The Expansion Proper Motions of the Planetary Nebula NGC 6302 from HST
  imaging
  Planetary nebulae expand on time scales of 10^3-10^4 yr. For nearby objects,
their expansion can be detected within years to decades. The pattern of
expansion probes the internal velocity field and provides clues to the nebula
ejection mechanism. In the case of non-symmetric nebulae, and bipolar nebulae
in particular, it can also provide information on the development of the
morphology. We have measured the expansion proper motions in NGC 6302 from two
epochs of HST imaging, separated by 9.43 years. This is used to determine the
expansion age and the structure of the velocity field. We use HST images in the
[N II] 6583{\AA} filter from HST WF/PC2 and WFC3. The proper motions were
obtained for a set of 200 individual tiles within 90" of the central star. The
velocity field shows a characteristic linear increase of velocity with radial
distance (a so-called Hubble flow). It agrees well with a previous
determination by Meaburn et al. (2008), made in a lobe further from the star,
which was based on a much longer time span. The pattern of proper motion
vectors is mostly radial and the origin is close to the position of the central
star directly detected by Szyszka et al. (2009). The results show that the
lobes of NGC 6302 were ejected during a brief event 2250 \pm 35yr ago. In the
inner regions there is evidence for a subsequent acceleration of the gas by an
additional 9.2 km/s, possibly related to the onset of ionization. The dense and
massive molecular torus was ejected over 5000yr, ending about 2900yr ago. The
lobes were ejected after a short interlude (the 'jet lag') of \sim 600 yr
during a brief event. The torus and lobes orig- inate from separate mass-loss
events with different physical processes. The delay between the cessation of
equatorial mass loss and the ejection of the lobes provides an important
constraint for explaining the final mass-loss stages of the progenitor stellar
system.
