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. Late AGB. PN phase: low , new features. PPN phase: something in between. Late AGB: high dust shells. PPN Stage.
Mid-infrared Spectral Evolution of Post-AGB Stars
Kevin Volk, Gemini Observatory
Image from Ciardullo et al. (1999; AJ, 118, 488)
PN phase: low , new
Late AGB: high dust
High and T
lower and T
the dust grains
the situation seems simple
O-rich post-AGB spectral evolution
Carbon-rich Objects: Complications
HR 4049: unusual dust features
Spitzer IRS data (Hrivnak & Volk, in prep.)
C-rich post-AGB spectral evolution
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