The expansion proper motions of the planetary nebula NGC 6302 from Hubble Space Telescope imaging

Abstract

Planetary nebulae expand on time-scales of 10³- 10⁴ 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 Hubble Space Telescope (HST) imaging, separated by 9.43 yr. This is used to determine the expansion age and the structure of the velocity field. We use HST images in the [N II] 6583 Å filter from HST Wide Field Planetary Camera 2 and Wide Field Camera 3. The proper motions were obtained for a set of 200 individual tiles within 90 arcsec 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., made in a lobe further from the star, which was based on a much longer time-span, but ground-based imaging. 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. The results show that the lobes of NGC 6302 were ejected during a brief event 2250 ± 35 yr ago. In the inner regions there is evidence for a subsequent acceleration of the gas by an additional 9.2 km s^-1, possibly related to the onset of ionization. The dense and massive molecular torus was ejected over 5000 yr, ending about 2900 yr ago. The lobes were ejected after a short interlude (the 'jet lag') of ∼600 yr during a brief event. The torus and lobes originate 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.