Dusty Disks Around Central Stars of Planetary Nebulae
(Mar 23, 2014)Abstract
Only a few percent of cool, old white dwarfs (WDs) have infrared excesses
interpreted as originating in small hot disks due to the infall and destruction
of single asteroids that come within the star's Roche limit. Infrared excesses
at 24 micron were also found to derive from the immediate vicinity of younger,
hot WDs, most of which are still central stars of planetary nebulae (CSPN). The
incidence of CSPN with this excess is 18\%. The Helix CSPN, with a 24 micron
excess, has been suggested to have a disk formed from collisions of Kuiper
belt-like objects (KBOs). In this paper, we have analyzed an additional sample
of CSPN to look for similar infrared excesses. These CSPN are all members of
the PG 1159 class and were chosen because their immediate progenitors are known
to often have dusty environments consistent with large dusty disks. We find
that, overall, PG 1159 stars do not present such disks more often than other
CSPN, although the statistics (5 objects) are poor. We then consider the entire
sample of CSPN with infrared excesses, and compare it to the infrared
properties of old WDs, as well as cooler post-AGB stars. We conclude with the
suggestion that the infrared properties of CSPN more plausibly derive from
AGB-formed disks rather than disks formed via the collision of KBOs, although
the latter scenario cannot be ruled out. We finally remark that there seems to
be an association between CSPN with a 24 micron excess and confirmed or
possible binarity of the central star.