chemical evolution of super-agb stars the giant branches lorentz center, may 2009 enrique...
TRANSCRIPT
Chemical evolution of Super-AGB stars
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The Giant Branches Lorentz Center, May 2009
Enrique García-Berro1,2
1Universitat Politècnica de Catalunya2Institut d’Estudis Espacials de Catalunya
Ch
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rs 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
• Conclusions2
Ch
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rs 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
• Conclusions3
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rs 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.
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rs 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.
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rs 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.
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Ch
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rs 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
• Conclusions7
Ch
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rs From the main sequence to C-burning
• HR diagram quite standard (9 M, Z)
• 1st and 2nd ascent to the giant branch
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Ch
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rs From the main sequence to C-burning
• H- and He-burning
• 1st dredge-up
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Ch
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rs From the main sequence to C-burning
• Abundances at the end of the 1st dredge-up (Siess 2006):
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Ch
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rs From the main sequence to C-burning
• Envelope mass fractions for a grid of metallicities can be found in Siess (2007):
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rs From the main sequence to C-burning
• Efficient neutrino cooling of the central regions of the star.
• Carbon is ignited off-center.
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rs Carbon burning
• Strong C flashes, LC of up to 107 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.
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• Expansion and cooling switch-off the He-burning shell.
• H is extinguished.
• The second dredge-up occurs when carbon has already processed the core.
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• The evolution of the surface luminosity and of the radius are similar, the effective temperature controls the process.
• Surface decoupled from the core.
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rs Carbon burning
• In massive models, the He-driven convective shell merges with the convective envelope (dredge-out).
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rs Carbon burning
• Each carbon flash produces expansion and cooling of the region where it occurs.
• Efficient conduction, readjustment.
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rs Carbon burning
• Key reactions during the C-burning phase
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rs Carbon burning
• The burning front moves inwards and reaches the center during the second flash.
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• The flame speed agrees with the Timmes & Woosley (1994) theoretical calculation.
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rs Carbon burning
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rs Carbon burning
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• Abundances in the degenerate core:
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rs Carbon burning
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• Abundances in the CO buffer: no Ne22
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rs Carbon burning
• Abundances at the end of the 2nd dredge-up (Siess 2006):
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rs Carbon burning
• Envelope mass fractions for a grid of metallicities can be found in Siess (2007):
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rs The thermally pulsing phase
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• After the carbon burning phase the H burning shell is resuscitated.
• All the models but the 11 M undergo the thermally pulsing SAGB phase.
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• Mini-pulses.
• For the 10 M:
– LHemax= 3 106 L
– LHemin= 102 L
– LHmax= 6 104 L
– LHmin= 102 L
– TCSB= 3.6 108 K
– =220 yr
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• The mass overlap between successive convective shells is typically r=0.24.
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• Temperatures are rather high.
• A fraction of about 0.56 of 22Ne is burnt into 25Mg.
• The mass fraction dredged-up is 0.07.
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rs The thermally pulsing phase
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• 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)
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rs The thermally pulsing phase
• HBB (Siess & Arnould 2008)
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Ch
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rs 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
• Conclusions32
Ch
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rs Initial-final mass relationship
• Mass of the degenerate core vs. mass of the ignition point.
1.331.3312.0
1.311.3011.5
1.221.2111.0
1.151.1410.5
1.091.0510.0
1.071.009.3
MONe+COMONeMZAMS
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Mass of the core at the 1st TP
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rs Initial-final mass relationship
• Dobbie et al. (2009), Prasaepe, GD 50 & PG 0136+251
GD 50
PG 0136+251
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rs 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.
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rs 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
• Conclusions36
Ch
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rs Number of pulses
• Depending on the mass-loss rate the number of pulses can be very large (Izzard & Poelarends 2006):
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rs Thermohaline instability
• Central carbon is not burned, the flame does not arrive to the center (Siess 2009):
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rs Thermohaline instability
• The central carbon is ~4%, enough to completely disrupt the star (Gutiérrez, Canal & García-Berro 2005).
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rs Convection
• Two sets of models: without overshooting, and with overshooting.– Eldridge & Tout (2004), Schröder, Pols &
Eggleton (1997)– Set convection when
δadrad 2
OV
16ζ20ζ2.5
δδ
0.12δOV gas
rad
P
Pζ
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• For a Z=0 model (Gil-Pons et al. 2007):
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rs Convection
• Diffusive treatment (Herwig 2000):
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0.0160.0f
fHH
rrz
νHD
H
2zexpDD
P
edge
P0
0OV
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• Effects of overshooting and code selection (Z=10-5, M=5 M):
OVOV Code X(12C) X(14N) X(16O) Z
NoNo EVOLVE 5.8×10-7 3.2×10-6 3.3×10-6 1.1010×10-5
ff=0.0=0.0 LPCODE 1.2×10-6 1.4×10-6 1.2×10-6 1.0219×10-5
ff=0.002=0.002 LPCODE 1.4×10-6 1.7×10-6 3.9×10-6 1.0654×10-5
ff=0.004=0.004 LPCODE 0.6×10-6 1.5×10-6 0.8×10-6 1.8361×10-5
ff=0.008=0.008 LPCODE 3.2×10-6 3.0×10-6 3.2×10-6 1.3200×10-5
ff=0.016=0.016 LPCODE 5.8×10-5 1.1×10-5 1.0×10-5 1.0272×10-5
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• No overshooting (Z=0)
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MMZAMSZAMS X(C)X(C) X(N)X(N) X(O)X(O) C:N:OC:N:O ZZenvenv
5 5 MM 5.9×10-9 2.0×10-9 1.1×10-11 1 : 0.3 : 0.02 8.0×10-9
6 6 MM 1.2×10-7 3.1×10-9 4.1×10-11 1 : 0.03 : 4 × 10-4 1.2×10-7
7 7 MM 2.8×10-6 5.7×10-9 5.2×10-9 1 : 0.002 : 0.002 2.8×10-6
8 8 MM 9.0×10-5 7.4×10-7 1.3×10-6 1 : 0.008 : 0.001 9.1×10-5
9 9 MM 2.1×10-4 2.0×10-6 2.9×10-6 1 : 0.01 : 0.015 2.1×10-4
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• Overshooting
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MMZAMSZAMS X(C)X(C) X(N)X(N) X(O)X(O) C:N:OC:N:O ZZenvenv
5 5 MM 3.0×10-7 2.0×10-8 9.4×10-11 1 : 0.07 : 3×10-4 3.2×10-7
6 6 MM 3.3×10-5 3.8×10-7 1.6×10-7 1 : 8×10-4 : 4×10-4 3.3×10-7
7 7 MM 3.6×10-4 2.7×10-6 6.0×10-6 1 : 0.01 : 0.02 2.7×10-4
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rs Final fate
• It depends on the adopted mass-loss rate and dredge-up efficiency (Poelarends 2008).
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rs 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
• Conclusions47
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rs 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.
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rs 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()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.
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rs 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.
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Chemical evolution of Super-AGB stars
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The Giant Branches Lorentz Center, May 2009
Enrique García-Berro1,2
1Universitat Politècnica de Catalunya2Institut d’Estudis Espacials de Catalunya