Mem. S.A.It. Vol. 77, 828c© SAIt 2006 Memorie della

Evolution and nucleosynthesis in super-AGBstars

C.L Doherty and J.C Lattanzio

Centre for Stellar and Planetary Astrophysics – Monash University, Australia e-mail:carolyn.doherty@sci.monash.edu.au

Abstract.Here we present results of an stellar evolutionary model for a 9.5 M�, solar compositionSuper Asymptotic Giant Branch (SAGB) star onto the thermally pulsing phase. We alsopresent preliminary nucleosynthesis results for the same star up to earlier stage of its evolu-tion, that of carbon burning. We find stellar conditions appropriate for Hot Bottom Burning(HBB) and Third Dredge Up (TDU), and mention the effect these would have on SAGBnucleosynthesis. Comparisons with previous studies by Ritossa et. al (1996); Garcia-Berroet. al (1997) & Siess (2006) are undertaken, resulting in a major difference due to our highefficiency of third dredge up.

Key words. Stars: evolution - Stars: AGB - Stars: Super-AGB - Stars: nucleosynthesis

1. Introduction

Super-AGB stars (SAGBs) are a class of starsin the mass range ≈ 7-12 M� which strad-dle the divide between high and intermedi-ate mass stars. What categorizes a SAGB staris the off centre carbon ignition which takesplace after core hydrogen and helium burning.After carbon burning these SAGBs end theirlife through a thermally pulsing (TP) stage, los-ing mass to form a ONe white-dwarf. SAGBstars may also end life more violently, under-going a SN explosion to become a neutron star.This occurs for a variety of reasons owing to ei-ther a slower mass loss rate, larger initial massor lower metallicity. For further discussion onthis mass cut-off between low and high massSAGB stars see Eldridge & Tout (2004) andreferences therein.

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As yet, only a few studies Garcia-Berro& Iben (1994); Ritossa et. al (1996); Garcia-Berro et. al (1997); Iben et. al. (1997); Ritossaet. al. (1999) and Siess (2006) have lookedinto these SAGB stars from a nucleosynthesispoint of view. Currently no set of nucleosyn-thetic yield calculations to the end of the TPstage are available. These SAGB yields wouldhave a role to play in Galactic chemical evo-lution models. In addition, the surface abun-dances could serve to find a possible obser-vational signature leading to identification ofSAGBs. Results for evolution and preliminarynucleosynthesis of a 9.5 M�, solar compositionSAGB star are presented. Section 2 briefly de-scribes the codes used and input physics. Theevolution and effects of carbon burning are de-scribed in Section 3. We show in Section 4,stellar conditions that are appropriate for HotBottom Burning (HBB) and Third Dredge Up

C.L Doherty & J.C Lattanzio: Evolution and Nucleosynthesis in SAGB stars 829

Fig. 1. HR Diagram for a 9.5 M� solar compositionSAGB star.

(TDU) and mention the expected effect thesewould have the nucleosynthesis.

2. Summary of the Evolution andNucleosynthesis Programs

The Monash Version of the Mount StromloStellar Evolution Program (MSSSEP) wasused for our stellar evolution calculation,whereas the nucleosynthesis is calculated us-ing a separate post-processing Monash StellarNucleosynthesis code (MOSN). A brief sum-mary of the major input physics for MSSSEPis as follows; neutrino losses Itoh et. al.(1996), electron screening from Graboske et.al. (1967), OPAL opacities, mass loss prescrip-tion from Vassiliadis & Wood. (1993). Thetreatment of convective boundaries in the studyof AGB stars is always a contentious subjectand for SAGBs there is no difference. TheMSSSEP code uses a neutral boundary condi-tion see Frost & Lattanzio (1996), which maylead to significant differences between code re-sults in latter stages of evolution. As we havetwo separate codes for evolution and nucle-osynthesis the evolution code needs only to in-clude the most energetic and structurally im-portant nuclear burning reactions reducing thecomputational time. In MSSSEP carbon burn-ing is approximated by the overall reaction12C+ 12C +16O = 220Ne which take into ac-count the two major carbon burning channels12C(12C, p)23Na and 12C(12C, α)20Ne and the

Fig. 2. Kippenhahn diagram of a 9.5 M� show-ing the convective features, core hydrogen burning,FDU, core helium burning until the onset of carbonburning.

absorption of the α by the 16O and the p by23Na. The MOSN code performs time depen-dent mixing in convective regions, and cur-rently uses a network of 74 species (Hydrogento Sulphur and including iron peak elements)and 506 nuclear reactions.

3. Results and Discussion

3.1. Evolution Prior to Carbon Burning

The evolution of SAGBs prior to carbon burn-ing (see Fig. 2) is very similar to that of lowermass AGB stars.

One small difference, due to the largertemperatures involved in the hydrogen burn-ing core in SAGBs, leads to a greater pro-duction of the NaNe chain products. Duringthe FDU (First Dredge Up) the surface is en-riched in these species. These enhancementswould make SAGBs - due to higher dilutionin their massive envelope, very hard to dis-tinguish from their lower mass counterparts.After FDU, SAGB stars undergo core heliumburning, and later shell helium burning whichcontinues past the onset of carbon burning.

We have run a 9.5 M� solar compositionmodel using the method outlined in Section 2until the onset of carbon burning. When com-pared with results by Siess (2006) Table 1 wefind strong similarities for these, the long lived

830 C.L Doherty & J.C Lattanzio: Evolution and Nucleosynthesis in SAGB stars

stages of the stellar evolution. Both studieshave very similar input physics, such as massloss rate and neutrino losses. When compar-isons are made to Garcia-Berro et. al (1997) fora 9 M� star similar trends are found. Why thestellar results between differing programs thenstart to diverge is still of major interest and isunder further investigation.

3.2. Carbon Burning

Carbon burning begins off centre when tem-perature in CO core exceeds ∼ 600 MK. Asseen in Fig.3 the journey of the carbon burn-ing region towards the core is a intricate one.For our 9.5 M� star the carbon burning pro-ceeds via an initial carbon shell flash and thena secondary flash, dubbed a “flame” as it prop-agates inwards reaching the core. These flashesinput energy into the region, creating convec-tive pockets exterior to the carbon burning.

In lower mass SAGB stars there can bea series of these initial carbon flashes beforea flame front develops. Conversely for moremassive SAGB stars only one flash (flame) oc-curs which burns all the way to the core.

When the carbon burning flame reaches thecore the degeneracy is lifted, letting the carbonburn in the core quiescently for a small perioduntil carbon exhaustion. After complete coreburning, contraction ensues and the core tem-perature increases leading to a series of flasheswhich burn outwards and remove the carbonrich regions left in the middle and outer regionsof the previous CO core.

Timmes & Woosley (1992) devised a the-oretical model which determines the speedof a flame front propagating inwards into adegenerate CO core. By assuming a “bal-anced power condition” in which the energyproduced in carbon burning flame convec-tive regions equals that of the neutrino losses,Timmes & Woosley (1992) then calculate theflame speed based only on local thermodynam-ical properties (excluding gravity). As withstudies by Garcia-Berro et. al (1997) and Siess(2006) we find close agreement to within a fac-tor of two between our model and the analyticapproximation.

Fig. 3. Kippenhahn diagram during the carbonburning phase, showing multiple convective carbonburning regions, as well as the helium burning shelland the base of the convective envelope.

Fig. 4. Luminosities for H, He and C burning.

Fig.4 shows the luminosity behavior at theonset of carbon burning. When the carbonflashes occur the large amount of energy in-jected causes a decrease in the helium luminos-ity. At the completion of carbon burning thehydrogen luminosity increases very quicklycorresponding to the re-ignition of the hydro-gen burning shell.

The core composition after the comple-tion of carbon burning comprises primarilyof oxygen (50%), neon (34%), sodium (3%),carbon (3%) (note the relative large carbonabundance) as well as aluminium, magnesium,other trace elements.

C.L Doherty & J.C Lattanzio: Evolution and Nucleosynthesis in SAGB stars 831

Table 1. Comparison of evolution prior to the onset of carbon burning with Siess (2006). MHB isthe maximum extent of convection during core H burning, MHeB maximum mass of convectivecore during helium burning, MFDU maximum inward penetration of convective envelope, MCOiis the core mass at the start of C burning, and the X[C], X[O] are the mass fraction of carbon andoxygen respectively at the completion of core He burning

MHB MHeB MFDU MCOi X[12C] X[16O]MSSSEP 2.902 0.947 2.091 1.150 0.623 0.350Siess 2.906 0.850 2.018 1.095 0.630 0.344

Fig. 5. Helium burning luminosities from onset ofthe thermally pulsing SAGB phase.

3.2.1. Second Dredge Up (SDU)

In intermediate mass AGB stars above ∼ 4 M�,SDU occurs following completion of core he-lium burning. For lower mass SAGB stars (∼8-9 M�) this is also the case. In more massivestars however, the ignition of carbon burningstalls the progress of the SDU so that it occursduring the carbon burning phase, or in moremassive stars still, after completion of carbonburning.

3.3. Thermally Pulsing SAGB Phase

During the thermally pulsing stage of evolu-tion there are multiple sites for nucleosynthe-sis. Here however we limit our investigationto the base of the convective envelope (HBB),leaving at this time, discussion of possible s-process nucleosynthesis. In Fig.5 we see the

evolution of TPs highlighted by the pulsed he-lium luminosity. As these SAGB stars are quitemassive their envelopes are significant there-fore even with very efficient mass loss theirjourney over to the white dwarf cooling trackwill involve many TPs. For our 9.5 M� modelwe expect in excess of 500 thermal pulses. Asthe mass loss rate dictates the number of TPs,this choice of mass loss rate greatly effectsthe composition obtained from both HBB andTDU.

As the interpulse period is inversely relatedto the mass of the hydrogen exhausted core,SAGB models have substantially lower inter-pulse periods, smaller convective pockets andshorter convective pocket lifetime than AGBstars. Our results for our 9.5 M� model find anaverage interpulse period of ∼ 350 years, con-vective pockets size of ∼ 10−4 M� and convec-tive pocket lifetimes (∼ 5 years).

4

Fig. 6. Temperatures at the base of the convectiveenvelope during the thermally pulsing SAGB phase.

832 C.L Doherty & J.C Lattanzio: Evolution and Nucleosynthesis in SAGB stars

Fig. 7. Mass of the hydrogen exhausted core, fromonset of thermally pulsing phase (same timescale asFig 5) highlighting the dredge up efficiency as wellas the lapse between the start of TPs and commence-ment of efficient TDU.

3.3.1. Hot Bottom Burning

During the thermally pulsing phase in SAGBstars the bottom of the convective envelopereaches into the hydrogen burning shell. If thetemperature at the base of this convective en-velope is large enough, nuclear burning begins,so this temperature is vital in determining theextent and end products of this nucleosynthe-sis. As the temperature involved in this regionof the star peaks at over 100 MK (see Fig.6) weexpect that both the NaNe and MgAl cycles aswell as the CNO cycle will be very efficient.

3.3.2. Third Dredge Up (TDU)

During TPs the peak helium luminosityreaches over 107 L� resulting in a decreasein the hydrogen luminosity due to the “extin-guishing” of the hydrogen burning shell. Thisallows the convective envelope to penetrateinto the regions that have undergone partial he-lium burning. This TDU is found with a dredgeup efficiency λ value of 0.71

This is most interesting when comparedwith results from published studies fromGarcia-Berro & Iben (1994) which find aboutλ ≈ 0.07 (for a 10 M� star) and Siess (2006)finds no TDU. However, Poelarends et al.; aspresented at this workshop, also finds an ef-ficient TDU. Why the efficiency (or lack) of

dredge up varies between different authors andmodels is currently under investigation.

For this 9.5 M� model the onset of TDUstarts at about the 10th pulse and increases withsubsequent pulses until maximum efficiency at∼ 18 pulse. The effect this TDU has on alteringthe surface composition and yield calculationcan currently be only speculated upon.

4. Conclusions

We find strong agreement between our currentSAGB model and work by others for bulk char-acteristics in the earlier stages of evolution.Why in the later stages of evolution the differ-ing stellar models diverge is now a subject ofinvestigation. Many complexities remain, such

1 Where λ = ∆Mdredge/∆MH.as the choice of mass loss and the very im-portant treatment of the convective boundaries.Results which will show the effects of HBBand TDU on the nucleosynthesis yields in theseSAGB are greatly anticipated. Models are cur-rently underway to compute a complete set ofyield calculations for a range of masses andmetallicities using the MSSSEP and MOSNcode (Doherty 2006).

Acknowledgements. Many thanks to the LOC fororganizing a great conference, and also for provid-ing local expenses for students.

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