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

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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 3

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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 22 Ne of an ONe white dwarf. 6

Chemical evolution of Super-AGB stars 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 7

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

Chemical evolution of Super-AGB stars From the main sequence to C-burning H- and He-burning 1 st dredge-up 9

Chemical evolution of Super-AGB stars From the main sequence to C-burning Abundances at the end of the 1 st dredge-up (Siess 2006): 10

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

Chemical evolution of Super-AGB stars From the main sequence to C-burning Efficient neutrino cooling of the central regions of the star. Carbon is ignited off-center. 12

Chemical evolution of Super-AGB stars Carbon burning Strong C flashes, L C of up to 10 7 L . 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

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars Carbon burning In massive models, the He-driven convective shell merges with the convective envelope (dredge-out). 16

Chemical evolution of Super-AGB stars Carbon burning Each carbon flash produces expansion and cooling of the region where it occurs. Efficient conduction, readjustment. 17

Chemical evolution of Super-AGB stars Carbon burning Key reactions during the C-burning phase 18

Chemical evolution of Super-AGB stars Carbon burning The burning front moves inwards and reaches the center during the second flash. 19

Chemical evolution of Super-AGB stars Carbon burning The flame speed agrees with the Timmes & Woosley (1994) theoretical calculation. 20

Chemical evolution of Super-AGB stars Carbon burning 21

Chemical evolution of Super-AGB stars Carbon burning 22 Abundances in the degenerate core:

Chemical evolution of Super-AGB stars Carbon burning 23 Abundances in the CO buffer: no Ne 22

Chemical evolution of Super-AGB stars Carbon burning Abundances at the end of the 2 nd dredge-up (Siess 2006): 24

Chemical evolution of Super-AGB stars Carbon burning Envelope mass fractions for a grid of metallicities can be found in Siess (2007): 25

Chemical evolution of Super-AGB stars The thermally pulsing phase 26 After the carbon burning phase the H burning shell is resuscitated. All the models but the 11 M  undergo the thermally pulsing SAGB phase.

Chemical evolution of Super-AGB stars The thermally pulsing phase 27 Mini-pulses. For the 10 M  : –L He max = L  –L He min = 10 2 L  –L H max = L  –L H min = 10 2 L  –T CSB = K –  =220 yr

Chemical evolution of Super-AGB stars The thermally pulsing phase 28 The mass overlap between successive convective shells is typically r=0.24.

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

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

Chemical evolution of Super-AGB stars The thermally pulsing phase HBB (Siess & Arnould 2008) 31

Chemical evolution of Super-AGB stars 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 32

Chemical evolution of Super-AGB stars Initial-final mass relationship Mass of the degenerate core vs. mass of the ignition point M ONe+CO M ONe M ZAMS 33 Mass of the core at the 1 st TP

Chemical evolution of Super-AGB stars Initial-final mass relationship Dobbie et al. (2009), Prasaepe, GD 50 & PG GD 50 PG

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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 36

Chemical evolution of Super-AGB stars Number of pulses Depending on the mass-loss rate the number of pulses can be very large (Izzard & Poelarends 2006): 37

Chemical evolution of Super-AGB stars Thermohaline instability Central carbon is not burned, the flame does not arrive to the center (Siess 2009): 38

Chemical evolution of Super-AGB stars Thermohaline instability The central carbon is ~ 4%, enough to completely disrupt the star (Gutiérrez, Canal & García-Berro 2005). 39

Chemical evolution of Super-AGB stars Convection Two sets of models: without overshooting, and with overshooting. –Eldridge & Tout (2004), Schröder, Pols & Eggleton (1997) –Set convection when 40

Chemical evolution of Super-AGB stars Convection For a Z=0 model (Gil-Pons et al. 2007): 41

Chemical evolution of Super-AGB stars Convection Diffusive treatment (Herwig 2000): 42

Chemical evolution of Super-AGB stars Convection Effects of overshooting and code selection (Z=10 -5, M=5 M  ): OVCodeX( 12 C)X( 14 N)X( 16 O)Z NoEVOLVE5.8× × × ×10 -5 f=0.0 LPCODE1.2× × × ×10 -5 f=0.002 LPCODE1.4× × × ×10 -5 f=0.004 LPCODE0.6× × × ×10 -5 f=0.008 LPCODE3.2× × × ×10 -5 f=0.016 LPCODE5.8× × × ×

Chemical evolution of Super-AGB stars Convection No overshooting (Z=0) 44 M ZAMS X(C)X(N)X(O)C:N:O Z env 5 M  5.9× × × : 0.3 : × M  1.2× × × : 0.03 : 4 × × M  2.8× × × : : × M  9.0× × × : : × M  2.1× × × : 0.01 : ×10 -4

Chemical evolution of Super-AGB stars Convection Overshooting 45 M ZAMS X(C)X(N)X(O)C:N:O Z env 5 M  3.0× × × : 0.07 : 3× × M  3.3× × × : 8×10 -4 : 4× × M  3.6× × × : 0.01 : ×10 -4

Chemical evolution of Super-AGB stars Final fate It depends on the adopted mass-loss rate and dredge-up efficiency (Poelarends 2008). 46

Chemical evolution of Super-AGB stars 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 47

Chemical evolution of Super-AGB stars 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

Chemical evolution of Super-AGB stars 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 C 12 (  )O 16 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

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