Chemical evolution of Super-AGB stars

Chemical evolution of Super-AGB stars Chemical evolution of Super-AGB stars Enrique García-Berro1,2 1Universitat Politècnica de Catalunya 2Institut d’Estudis Espacials de Catalunya The Giant Branches Lorentz Center, May 2009

Overview • Introduction • Overview of the evolution • From the main sequence to carbon burning • Carbon burning in partially degenerate conditions • The thermally pulsing phase • Initial-final mass relationship • Open issues • Conclusions 2

Introduction • Stars which develop electron-degenerate cores made of matter which has experienced complete H-, He- and C-burning received little attention until recently. • Stars in a suitable mass range develop CO cores and TP at the AGB (TP-AGB phase). • Stars more massive (9 to 11 M) ignite carbon off-center in partially degenerate conditions and develop an ONe core. 4

Introduction • They also undergo a TP phase (TP-SAGB). • In early studies the emphasis was placed on the composition and growth of the degenerate core: • Miyaji et al. (1980) • Nomoto (1984, 1987) • Miyaji & Nomoto (1987) • The evolution of the envelope was completely disregarded. 5

Introduction • This range of masses is important for various reasons: • AIC: carbon-exhausted cores more massive than 1.37 M are expected to undergo electron-capture induced collapse (Nomoto, 1984). • Massive white dwarfs (M > 1.1 M) are presumably ONe white dwarfs (Liebert & Vennes, 2004). • Many novae show excesses of Ne, which can be due to mixing between the accreted matter and the underlying 22Ne of an ONe white dwarf. 6

From the main sequence to C-burning • HR diagram quite standard (9 M, Z) • 1st and 2nd ascent to the giant branch 8

From the main sequence to C-burning • H- and He-burning • 1st dredge-up 9

From the main sequence to C-burning • Abundances at the end of the 1st dredge-up (Siess 2006): 10

From the main sequence to C-burning • Envelope mass fractions for a grid of metallicities can be found in Siess (2007): 11

From the main sequence to C-burning • Efficient neutrino cooling of the central regions of the star. • Carbon is ignited off-center. 12

Carbon burning • Strong C flashes, LC of up to 107L. • Associated convective regions form. • The bulk of energy production occurs in the convective regions. • But the peak energy generation rate occurs at the base of the convective region. 13

Carbon burning • Expansion and cooling switch-off the He-burning shell. • H is extinguished. • The second dredge-up occurs when carbon has already processed the core. 14

Carbon burning • The evolution of the surface luminosity and of the radius are similar, the effective temperature controls the process. • Surface decoupled from the core. 15

Carbon burning • In massive models, the He-driven convective shell merges with the convective envelope (dredge-out). 16

Carbon burning • Each carbon flash produces expansion and cooling of the region where it occurs. • Efficient conduction, readjustment. 17

Carbon burning • Key reactions during the C-burning phase 18

Carbon burning • The burning front moves inwards and reaches the center during the second flash. 19

Carbon burning • The flame speed agrees with the Timmes & Woosley (1994) theoretical calculation. 20

Carbon burning 21

Carbon burning • Abundances in the degenerate core: 22

Carbon burning • Abundances in the CO buffer: no Ne22 23

Carbon burning • Abundances at the end of the 2nd dredge-up (Siess 2006): 24

Carbon burning • Envelope mass fractions for a grid of metallicities can be found in Siess (2007): 25

The thermally pulsing phase • After the carbon burning phase the H burning shell is resuscitated. • All the models but the 11 M undergo the thermally pulsing SAGB phase. 26

The thermally pulsing phase • Mini-pulses. • For the 10 M: • LHemax= 3 106L • LHemin= 102L • LHmax= 6 104L • LHmin= 102L • TCSB= 3.6 108 K • t=220 yr 27

The thermally pulsing phase • The mass overlap between successive convective shells is typically r=0.24. 28

The thermally pulsing phase • Temperatures are rather high. • A fraction of about 0.56 of 22Ne is burnt into 25Mg. • The mass fraction dredged-up is 0.07. 29

The thermally pulsing phase • At the end of the 2nd dredge-up the surface abundances are: (C:N:O)=(2.35:4.25:6.36) • When half of the initial 12C in the envelope has been burned: (C:N:O)=(1.17:5.43:6.26) 30

The thermally pulsing phase • HBB (Siess & Arnould 2008) 31

MZAMS MONe MONe+CO 9.3 1.00 1.07 10.0 1.05 1.09 10.5 1.14 1.15 11.0 1.21 1.22 11.5 1.30 1.31 12.0 1.33 1.33 Initial-final mass relationship • Mass of the degenerate core vs. mass of the ignition point. Mass of the core at the 1st TP 33

Initial-final mass relationship • Dobbie et al. (2009), Prasaepe, GD 50 & PG 0136+251 GD 50 PG 0136+251 34

Initial-final mass relationship • But: • If mass loss is not strong enough during the TP-SAGB the core may grow to beyond the Chandresekhar mass. • Depending on the metallicity, third dredge-up may inhibit core growth. It also depends on the numerical code. • Competition between these processes determines final fate. 35

Number of pulses • Depending on the mass-loss rate the number of pulses can be very large (Izzard & Poelarends 2006): 37

Thermohaline instability • Central carbon is not burned, the flame does not arrive to the center (Siess 2009): 38

Thermohaline instability • The central carbon is ~4%, enough to completely disrupt the star (Gutiérrez, Canal & García-Berro 2005). 39

Convection • Two sets of models: without overshooting, and with overshooting. • Eldridge & Tout (2004), Schröder, Pols & Eggleton (1997) • Set convection when 40

Convection • For a Z=0 model (Gil-Pons et al. 2007): 41

Convection • Diffusive treatment (Herwig 2000): 42

Convection • Effects of overshooting and code selection (Z=10-5, M=5 M): 43

Convection • No overshooting (Z=0) 44

Convection • Overshooting 45

Final fate • It depends on the adopted mass-loss rate and dredge-up efficiency (Poelarends 2008). 46

Conclusions • I have presented a summary of the state-of the-art of self-consistent calculations of the evolution of heavy-weight intermediate mass stars. • Solar metallicity stars with masses larger than ~8 M burn carbon off-center in partially degenerate conditions. • Solar metallicity stars with masses larger than ~10.5 M undergo electron captures. 48

Conclusions and future prospects • SAGB stars undergo a second dredge-up of variable extension. • A thermally pulse SAGB exists. • Via radiative winds, may loose their envelopes and form ONe white dwarfs. • The results are sensitive to the C12(a,g)O16 reaction rate, to the mass-loss rate, to metallicity, to convective prescription… • Stars in the upper range of masses could end up their evolution as EC-SNe. 49

Wish list • More detailed nucleosynthetic calculations during the TP-SAGB phase are needed. • Mass loss rates are needed as well, since the final fate depends sensitively on them. 50

Chemical evolution of Super-AGB stars