Parameterising the Third Dredge-up in Asymptotic Giant Branch Stars

Abstract

We present new evolutionary sequences for low and intermediate mass stars (1−6M⊙) for three different metallicities, Z = 0.02, 0.008, and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch phase. We have two sequences of models for each mass, one which includes mass loss and one without mass loss. Typically 20 or more pulses have been followed for each model, allowing us to calculate the third dredge-up parameter for each case. Using the results from this large and homogeneous set of models, we present an approximate fit for the core mass at the first thermal pulse, Mc1, as well as for the third dredge-up efficiency parameter, λ, and the core mass at the first dredge-up episode, Mcmin, as a function of metallicity and total mass. We also examine the effect of a reduced envelope mass on the value of λ.

Asymptotic Giant Branch Evolution and the Initial-Final Mass Relation of Single CO White Dwarfs

“…We see that while in the former set the dependence on Z is not monotonic over the whole range of stellar masses, in the latter more massive white dwarfs are always expected at decreasing Z. This difference is likely due to differences in the description of mass loss and of molecular opacities † of C stars, coupled to the current surface C/O in Marigo & Girardi (2007), frozen to a scaled-solar mixture in Karakas et al (2002).…”

“…The semi-empirical IFMR seems to support a modest increase of the core mass during the TP-AGB. In fact, current AGB models predict values of the core mass at the first thermal pulse which are already inside the observed width ΔM f of the semi-empirical 42 P. Marigo IFMR (Karakas et al 2002;Marigo & Girardi 2007). Moreover, by assuming no dredgeup, we expect that intermediate mass stars would be able to reach the Chandrasekhar limit and thus explode as SN I 1/2 .…”

“…While λ values for low-mass stars are still quite heterogeneous, there is a general agreement among authors in predicting λ ≈ 1 for more massive AGB stars, with M i > 3−4M (Karakas et al 2002;Herwig 2004;Ventura & D'Antona 2005;Weiss & Ferguson 2009). A typical example is illustrated in Figure 4.…”

“…HBB produces two main evolutionary effects, namely i) it makes the stars brighter than expected by the classical M c − L relation Böcker & Scönberner 1991), and ii) it enriches the surface chemical composition with nuclei synthesized by the C-N-O, Ne-Na, and Mg-Al cycles (Forestini & Charbonnel 1997). Figure 5 shows the pre-flash quiescent luminosity as a function of the core mass for a few full stellar TP-AGB models of different mass and metallicity Karakas et al (2002). The Paczyński (1970) and Boothroyd & Sackmann (1988).…”

“…Dependence of the predicted IFMR on basic parameters, namely: efficiency of the third dredge-up (top left-hand side panel), the assumed mass-loss rates (top right-hand side panel), and metallicity (left bottom panel: Marigo & Girardi (2007); right bottom panel: Karakas et al (2002)). …”