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D.R. Alves, H.E. Bond, D.R. Zurek (STScI)
The planetary nebula (PN) K~648 is a member of the Galactic globular cluster M15, and is the archetype of PNe produced by the oldest stellar populations. Due to the long transition lifetimes of single low-mass stars between the AGB and the hot nuclei of PNe, it has been proposed that PNe can arise in old populations only through binary-star mass-transfer processes. In order to test whether the central star of K~648 is a close binary, we obtained Hubble Space Telescope\/ observations with the WFPC2 camera and UByRI\/ filters. Frames were taken in December 1998 on 10 different HST\/ orbits, spaced so that we can test for variability over timescales from 45~minutes up to about 10~days.
Preliminary analysis indicates that no significant photometric variability of the central star of K~648 was detected. However this does not necessarily rule out the binary hypothesis, since the orbit could be viewed at low inclination, or the stars could have coalesced as a result of a common-envelope interaction.
We also obtained deep narrow-band images of K~648 in the light of H\alpha, [O~III], and [N~II], in order to study the nebular morphology. The PN shows an extended faint halo surrounding a bright inner elliptical shell. The shell exhibits a remarkable limb-brightening at one end of its major axis, suggestive of an interaction with the interstellar medium.
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