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the last nuclear burning phase of a low to intermediate - mass ( @xmath0 to 8@xmath1 ) star s life is the thermally - pulsing asymptotic giant branch ( tp - agb ) .
the outer envelope is lost by low velocity stellar winds during the agb , with the termination being the final ejection of the envelope .
the star then evolves through the brief post - agb and planetary nebula phases before ending its life as a white dwarf .
the gaseous nebula is the remnant of the envelope that once surrounded the core , that is now exposed as the central star of the illuminated nebula .
the abundances of the nebula can reveal information about stellar nucleosynthesis and mixing during the agb . for this reason
pn abundances could , in principle , be used to constrain stellar models .    in this proceedings
i summarize the evolution and nucleosynthesis during the agb .
i briefly discuss the differences between `` synthetic '' and `` detailed '' agb models , and compare yields from different authors .
the structure and evolution of low and intermediate mass stars prior to and during the agb has been previously discussed by @xcite ; see @xcite for a review of post - agb stars .
all stars begin their nuclear - burning life on the main sequence . following core h exhaustion the core contracts , the outer layers expand and the star becomes a red giant , characterized by an inert he core , an h - burning shell , and a deep convective envelope that extends to the stellar surface .
it is during the ascent of the giant branch that the inner edge of the convective envelope moves inward in mass and the first dredge - up ( fdu ) occurs , where partially h - processed material ( e.g. he , c , n ) is mixed to the surface .    after a phase of central he - burning , the core contracts and there is a structural re - adjustment to shell he burning .
the re - adjustment drives a strong expansion of the outer layers and the star becomes a red giant for the second time ; the star is now said to be on the agb . for stars with @xmath2 the convective envelope moves inward to regions where complete h burning ( mostly he and n ) had previously occurred , this is the second dredge up ( sdu ) .
, title="fig : " ] +    an agb star is characterized ( fig .  [ agb ] ) by two nuclear burning shells , one burning he above a degenerate c - o core and another burning h , below a deep convective envelope . in between lies the intershell region composed mostly of he .
the he - burning shell is thermally unstable , flashing every 10@xmath3  years or so .
the energy produced by the thermal pulse ( tp ) drives a convective pocket in the he - intershell which acts to homogenize abundances within that region . after the occurrence of a tp
the convective envelope may move inwards and mix products of partial he - burning ( mostly he left unburnt and c , see fig .
[ agb ] ) from the core to the stellar surface .
this is the third dredge - up ( tdu ) , and is the mechanism responsible for turning ( single ) stars into c stars , where c / o @xmath4 in the surface layers .
the tdu also mixes heavy elements produced by the @xmath5 process from the he - shell to the surface , where they were created during the previous interpulse @xcite .
following dredge - up , the star contracts , the h - shell is re - ignited and the star enters the interpulse phase where h - burning provides most of the luminosity .    hot bottom burning ( hbb ) can occur for stars with @xmath2 , when the base of the convective envelope dips into the top of the h - shell resulting in a thin layer hot enough to sustain proton - capture nucleosynthesis .
observational evidence for hbb includes the lack of bright c - rich agb stars in the large and small magellanic clouds ( lmc and smc ) @xcite ; many of these stars are also rich in li and @xmath5 process elements .
hbb converts c into n and will prevent the atmosphere from becoming c rich @xcite .
the copious amounts of n produced in this case will be primary owing to the primary c being dredged from the he - shell .
intermediate - mass evolution is sensitive to the initial composition and the mass - loss law used in the calculation @xcite , where the minimum stellar mass for hbb is pushed to lower mass in lower metallicity models .
there is evidence from agb stellar spectra @xcite , and from pre - solar grains @xcite that `` extra - mixing '' processes are also operating in low - mass ( @xmath6 ) agb stars , along with the tdu .
the physical mechanism(s ) responsible for the extra mixing are not known , although various processes have been proposed including rotation and thermohaline mixing .
the presence of a binary companion could have dramatic consequences for the evolution and nucleosynthesis , however i leave discussions to izzard , taam and podsiadlowski ( these proceedings ) .
owing to fact that calculating a tp - agb model is a computationally intensive task , synthetic agb models , which use fitting formulae to model the evolution quickly , have proved to be a successful approach for population syntheses studies that require @xmath7 stars .
historically this approach was validated by the fact that the stellar luminosity on the agb is nearly a linear function of the core mass @xcite , although this relation breaks down for stars undergoing hbb @xcite .
synthetic agb models have successfully been used to model agb populations @xcite , and compute stellar yields @xcite .
many of the parameterizations used in synthetic evolution studies are derived from detailed stellar models , such as the growth of the h - exhausted core with time , and as such are only accurate over the range in mass and metallicity of the stellar models they are based upon .
an example is provided by @xcite who compute agb yields for initial masses between 0.9 and 8@xmath1 whereas the interpulse - period - core mass relation @xcite they use was only derived for stars with initial masses between 1 and 3@xmath1 .
what affect this has on the yields is unclear since this relation will affect the number of tps during the tp - agb phase and hence the level of chemical enrichment .
recent improvements in computer power mean that grids of detailed agb models can now be produced in a reasonable time @xcite ; however producing yields from @xmath8 agb stars for any given metallicity range is still challenging .
for this reason synthetic models are still preferred for some applications .
an example of the difference between detailed models and the fits used in synthetic agb algorithms is shown in fig .
[ tau - ip ] for the interpulse - period core mass relation .
we show results from detailed agb models of solar composition against two commonly used fits .
the fits are a reasonable match to the detailed models with small core masses ( @xmath9 ) , but under - estimate the growth of the interpulse period for larger core masses .
years ) as a function of core mass ( @xmath1 ) for the @xmath10 stellar models ( colored dots ) .
the fits from @xcite ( dashed line ) and @xcite ( solid black dots ) are shown for comparison .
[ tau - ip],title="fig : " ] +
[ cols="^ " , ]     in @xcite we present yields for pn , where we integrate the amount of matter lost over the last two tps . in table
[ pnyield ] we show an example of the yields for a 3@xmath1 progenitor of solar composition .
the columns contain the species @xmath11 , the atomic weight @xmath12 , followed by the net yield ( in @xmath1 ; see @xcite for the definition ) , the average mass fraction of @xmath11 lost in the wind from the last two tps , @xmath13 , and the initial mass fraction @xmath14 .
the final column is the abundance of @xmath11 by number compared to the number of hydrogen atoms , @xmath15 , in the matter lost over the final two tps .
the online tables include yields for all 74 species included in the nuclear network .
these yields can be directly compared to the composition of pne and as such could be useful for comparisons to objects in the galaxy , lmc and smc .
this is because the initial c , n and o abundances used in the stellar models were taken from abundances derived from hii regions of the magellanic clouds , see @xcite for details .
in order to understand the composition of planetary nebulae we first need to study the nucleosynthesis occurring in the progenitor star during the thermally - pulsing asymptotic giant branch ( agb ) phase . 
 i present an overview of single agb evolution , with an emphasis on the mixing processes that alter the envelope composition , followed by a discussion of the stellar yields available from single agb stellar models .