Mid-infrared Spectral Evolution of Post-AGB Stars

Kevin Volk, Gemini Observatory

Image from Ciardullo et al. (1999; AJ, 118, 488)

NGC7027

Late AGB: high dustshellsPN phase: low , newfeatures

Late AGB

O-rich C-rich

PN Stage

PPN Stage

PPN phase:somethingin between

High and T

PN heats/destroysthe dust grains

Dust cools,disperseslower and T

O-rich C-rich

Oxygen-rich Objects: the situation seems simple

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 post-AGB 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

O-rich post-AGB spectral evolution

Carbon-rich Objects: Complications

HR 4049: unusual dust features

Spitzer IRS data (Hrivnak & Volk, in prep.)

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

C-rich post-AGB spectral evolution

• 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?

Open Questions: C-rich post-AGB

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 mid-infrared 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 carbon-rich candidates are likely to be post-AGB objects. The C-rich 21 m feature candidates all seem to be post-AGB objects. For O-rich candidate objects the situation is often unclear.

The spectrum is similar to that of IRAS 17150-3224

Late AGB

PN Stage

PPN Stage

O-rich C-rich

Late AGB

O-rich C-rich

PN Stage

PPN Stage