post-agb evolution. learning outcome evolution from the tip of the agb to the wd stage object types...
Post on 20-Dec-2015
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Learning outcome
• evolution from the tip of the AGB to the WD stage
• object types along the post-AGB evolution
• basics about planetary nebulae
• basics about white dwarfs
OH/IR stars
• O-rich stars at the tip of the AGB
• Maximum of SED at 10µm
• Characteristic OH maser emission(photo-dissoziation of H2O)
• Optically thick envelope
• typically intermediatemass stars
The end of the AGB phase
• Mass loss removes most of the envelope
• H-burning shell finally reaches the surface
• This leads to an increase in temperature,the star moves to the left in the HRD
• Luminosity and timescale depend on core mass
Post-AGB objects
RV Tau / SRds
supergiants of spectral type
F-K
circumstellar shell
depletion of elements into
dust
Giridhar et al. 1999
Post-AGB objects
Protoplanetary Nebulae(PPN)
not clearly defined (resolved objects?)
star typically visible only inscattered light
e.g. Red Rectangle
Osterbart et al. 1997
Final Flash / Born Again AGB Stars
• star leaves AGB shortly before a TP
• final TP occurs already on the way to the WD stage
• Star returns to AGB within a very short time
• single or double loop scenario
FG Sge
• central star of PN He1-5
• Real-time stellar evolution
• C-star since 1981
• 1992 ejection of a dust shell
van Genderen 1994
R CrB stars
• only ~ 50 objects known in the Milky Way
• hydrogen deficient G-K supergiant exhibiting erratic variabilities
• infrared excess
• 2 scenarios:Double degenerate: fusion of two WDsFinal He flash: born again scenarioboth scenarios not totally satisfying
Planetary nebulae
Weinberger et al. 1994
Circumstellar material ejectedduring the AGB phase ionizedby the central star.
Spectrum characterized by emission lines of H and [O III]
detectable at large distances
Abundances from post-AGB stars and PNe
• allow insight in abundances returned to ISM
• in several objects s-process elements enhanced (dredge up)
• effect of depletion: elements selectively removed by dust formation ([Zn/Fe]=+3.1!)
White dwarfs
• Better: degenerated dwarfs
• Upper mass limit 1.44 Msun (nonrotating), but typically significantly lower (0.6 Msun)
• High log g and P values at the surface, thus highly broadened lines
• Cooling not totally understood, important for age determination