Mid-infrared Spectral Evolution of Post-AGB Stars
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DESCRIPTIONMid-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. Late AGB. PN phase: low , new features. PPN phase: something in between. Late AGB: high dust shells. PPN Stage. - PowerPoint PPT Presentation
Mid-infrared Spectral Evolution of Post-AGB StarsKevin Volk, Gemini Observatory
Image from Ciardullo et al. (1999; AJ, 118, 488)NGC7027
Late AGB: high dustshellsPN phase: low , newfeatures Late AGBO-richC-richPN StagePPN StagePPN phase:somethingin between
High and TPN heats/destroysthe dust grainsDust cools,disperseslower and TO-richC-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 weakO-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 (
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 AGBPN StagePPN StageO-richC-rich
Late AGBO-richC-richPN StagePPN Stage
NGC 7027 imageEvolution paths with imagesO-rich objectsO-rich PPN candidatesO-rich spectral evolution summaryC-rich objectsHR 404921 micron objects (1)Feature evolutionSpitzer IRS PPN spectraC2H2 band evolutionC-rich spectral evolution summaryOpen questions: C-rich PPN spectral featuresWhere do we go from here?SAGE LMC surveyCaveats for LMC objectsJ043722.71-682503.3 from Buchanan et al (2006)LMC11 HST image from Shaw et al. (2006)LMC 11 spectrum from Bernard-Salas et al. (2006)MSX SMC 029 spectrum from Kraemer et al (2006)EndIntroduction pageOH279.6-32.6 mid-IR imageOH 279.6-32.6 spectrum (T-ReCS)J043722.71-682503.3 IRS spectrum + photoemtryPPN evolution spare slideUIR features in the 21 micron objects Comparison with galactic objectsExample ISO spectraEvolution paths