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Mid-infrared Spectral Evolution of
Post-AGB Stars
Kevin Volk, Gemini Observatory
NGC7027
Image from Ciardullo et al. (1999; AJ, 118, 488)
O-rich
C-rich
PPN
phase:low
PN phase:
Late
AGB:
high,new
dust Late AGB
features
something
shells
in between
PPN Stage
PN Stage
O-rich
C-rich
High  and T
Dust cools,disperses
lower  and T
PN heats/destroys
the dust grains
Oxygen-rich Objects:
the situation seems simple
O-rich post-AGB spectral evolution
We have a consistent picture of O-rich post-AGB
evolution:
• no new O-rich dust features seem to appear, the
silicate features decrease in optical depth and the
temperature goes down [and then maybe up again in
the PN phase]
• The grains may be smaller in the ionized regions than
in any external neutral shells
• The crystalline silicates may be enhanced in the postAGB evolution but this is not clearly established
• If the star becomes C-rich late in the evolution the
“PAH” features may appear, but are generally weak
Carbon-rich Objects: Complications
HR 4049: unusual dust features
Spitzer IRS data (Hrivnak & Volk, in prep.)
C-rich post-AGB spectral evolution
There is a significant evolution of the features in the
late-AGB/post-AGB phase for C-rich objects:
• The molecular bands weaken and disappear
• The UIR (“PAH”) bands usually appear early on, and
then generally remain strong thereafter
• Crystalline silicates may co-exist with the “PAH”
bands but not with the 21/30 m features
• A very few (<=3) C-rich post-AGB objects seem to
show the 11.3 m SiC feature but the shape is unusual
compared to that in carbon star spectra
• Some objects lack features for no known reason
• The 21 m feature may appear, cause unknown, and
disappear again
Open Questions: C-rich post-AGB
• why do some C-rich objects show weak “PAH”
features, or no features?
• why do some C-rich objects lack the 21 m feature?
• why is the 21 m feature so weak in PN/carbon star
spectra when it gets strong in PPN spectra?
• does the fading of the C2H2 band imply chemistry, or
are the molecules being broken down to atoms?
• are the PAHs always there and just not excited before
the end of the AGB, or are they created in the post-AGB
phase itself?
• what happens to the SiC feature in extreme C-stars
and post-AGB objects?
• where do the crystalline silicates come from?
• why do we not see 30 m features with silicates?
Where do we go from here?
New Spitzer results are starting to appear:
Garcia-Hernandez talk this afternoon
Corrigone talk this afternoon
For the future, new
observations of the
LMC and SMC by
Spitzer will be very
important for
finding new PPNs,
since the distance is
known and we will
have a complete
census. Work is
underway to find
PPN candidates.
Many of these can
be followed up from
the ground.
We need, however, to remember that the LMC is a
distinct environment where things are different than for
most of the Milky Way galaxy. This can be seen from
the Spitzer data we already have. A number of the
brightest AGB objects in the LMC have been observed
(see the Buchanan et al. (2006) paper) and the mid-IR
spectral properties are unusual compared to galactic
objects. See also work by Ciska Markwick-Kemper.
The SMC survey has been approved in the latest
round of Spitzer proposals (Karl Gordon, PI), but not
yet executed. These small galaxies probe a range of
sub-solar metallicity.
Planetary Nebula LMC11
The immediate progenitors of PNs are assumed to be
the AGB stars with high mass loss rates. Once the mass
loss ceases the spectral characteristics in the midinfrared evolve rapidly. This is one method of
identifing post-AGB objects. (Post-AGB  pre-PN)
Various famous post-AGB objects [the Egg Nebula,
the Red Rectangle] are atypical and will not be
discussed here.
One has to worry about confusion with massive
evolved stars. Abundance analysis is the only sure way
to identify post-AGB objects, although most carbonrich candidates are likely to be post-AGB objects. The
C-rich 21 m feature candidates all seem to be postAGB objects. For O-rich candidate objects the
situation is often unclear.
The spectrum is similar to that of IRAS 17150-3224
O-rich
C-rich
Late AGB
PPN Stage
PN Stage
O-rich
C-rich
Late AGB
PPN Stage
PN Stage