STScI Annual Report 2002: A Living Mission

26 science essays
blown off by the star, which act as crude mirrors scattering
starlight. The Hubble survey of 27 objects detected the
reflection nebulosity of 21 protoplanetary nebulae, all of
which showed an axisymmetry in the circumstellar environment.
This work supports the idea that the appearance
of axisymmetry in planetary nebulae precedes the protoplanetary
nebula phase and most likely originates during
the mass loss in the AGB stage.
The Hubble survey also discovered two new morphological
types of protoplanetary nebulae: ‘star-obvious
low-level-elongated’ (SOLE) nebulae and ‘dust-prominent
longitudinally extended’ (DUPLEX), which are illustrated in
Figure 1. The SOLE and DUPLEX protoplanetary nebulae
are probably the precursors to the elliptical and bipolar
planetary nebulae. Hubble’s excellent point spread function
was critical for revealing the detailed structure of these
nebulae and, for the SOLE nebulae, for improving the contrast
between the central star and the nebula.
Mid-infrared images of protoplanetary nebulae trace
the thermal emission from the circumstellar dust shell.
SOLE nebulae display optically-thin toroids perpendicular
to the elongated reflection nebulae, while DUPLEX nebulae
show optically-thick, compact cores and elongations
aligned with the bipolar reflection nebulae. These morphological
differences translate into spectral energy distribution
differences: SOLE nebulae have two peaks, one in the optical
due to the bright star and one in the infrared due to the
thermal dust emission; DUPLEX nebulae have one dominating
peak in the infrared from the dust shell and an optical excess
due to the reflection nebula. Radiative transfer model calculations
by the author, Ueta, and collaborators support
∆ DEC (arcsec)
∆ RA (arcsec)
2 0 -2 2 0 -2 2 0 -2
∆ RA (arcsec)
4 2 0 -2 -4 5 0 -5
2 0 -2
∆ RA (arcsec)
the conclusion that SOLE nebulae are physically distinct
from DUPLEX nebulae, with shells of lower optical depth
that permit the starlight to leak out to all viewing angles.
We believe that DUPLEX nebulae experienced higher massloss
rates, with higher equator-to-pole mass-loss ratios,
than SOLE nebulae. Interestingly, the DUPLEX nebulae
have a larger range of dust grain sizes (0.001 to 200
microns), which suggests more dust processing.
Independent studies suggest that the presence of longlived
disks can support grain growth in these sources.
Millimeter interferometry of protoplanetary nebulae
by Fong, the author, and collaborators reveals differences in
the kinematics of the molecular gas surrounding the central
star. SOLE nebulae appear to have simple expanding
envelopes, whereas DUPLEX nebulae have bipolar—or
multiple—collimated outflows of molecular gas that collide
with the slower moving, expanding wind. This difference
in kinematics suggests completely different shaping mechanisms
for the two types. AGB mass loss, which is probably
driven by radiation pressure on dust grains, shapes both
SOLE and DUPLEX nebulae. However, DUPLEX nebulae
appear to have some additional physical mechanism that
creates collimated mole- cular outflows that shape the nebulae.
The DUPLEX nebulae appear to have disks, which
suggests that the disk winds driving bipolar outflows in
star formation regions may do the same in these objects.
Stars that develop planetary nebulae distribute most
of the carbon in the universe—and in us. So, the next
time you view a planetary nebula, be in awe of its butterflylike
beauty and be thankful for its enrichments of space
and life. �
Figure 1: The morphological dichotomy of
protoplanetary nebulae shown by Hubble
images (color scale) and ground-based
mid-infrared images (contours). On top, the
SOLE nebulae, with prominent central
stars surrounded by elliptical, low-level
reflection nebulosity and by limb-brightened,
mid-infrared emission from dust tori.
On the bottom, the DUPLEX nebulae, with
exquisite, bipolar, reflection nebulosity and
centrally-peaked, mid-infrared emission
images. In each image, the (Infrared
Astronomical Satellite) IRAS names for
these sources are on top and the filters are
on the bottom (mid-infrared on left and
Hubble on right).