Post-AGB evolution

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

IRC+10216

Leao et al.2006

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-evolution

Blöcker 1995time steps in units of 10³ yrs

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

Post-AGB objects

van Winckel 2003

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

V605 Aql

Hinkle et al. 2008

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

R CrB stars

result of ejection ofa dust shell

Clayton 1996

Planetary Nebulae (PN)

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

Distances to PNe

Hajan et al. 1993

time diff. 5.5 yr

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!)

Abundances from post-AGB stars and PNe

van Winckel 2003

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

Types of White Dwarfs

• DC: barely lines• DO: He II lines dominate• DB: He I strong, no H• DA: H present, no He

• Surface composition influenced by diffusion (radiation pressure, gravity) and accretion of ISM