A&A 366, 578–584 (2001) Astronomy DOI: 10.1051/0004-6361:20000293 & c ESO 2001 Astrophysics Asymptotic Giant Branch predictions: Theoretical uncertainties S. Cassisi1, V. Castellani2,3, S. Degl’Innocenti2,3,G.Piotto4, and M. Salaris5 1 Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy 2 Dipartimento di Fisica, Universit´a di Pisa, Piazza Torricelli 2, 56126 Pisa, Italy 3 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, via Livornese 582/A, S. Piero a Grado, 56010 Pisa, Italy 4 Dipartimento di Astronomia, Universit´a di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy 5 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead L41 1LD, UK Received 27 June 2000 / Accepted 9 November 2000 Abstract. In this paper we investigate the level of agreement between observations and “new” Asymptotic Giant Branch (AGB) models, as produced by updating the physical inputs adopted in previous stellar computations. One finds that the new physics increases the predicted luminosity of Horizontal Branch (HB) and AGB stellar structures by a similar amount, keeping unchanged the predictions about the difference in luminosity between these two evolutionary phases. The best fit of selected globular clusters appears rather satisfactory, disclosing the relevance of the assumption on the mass of the Red Giant Branch (RGB) progenitor in assessing the distance modulus of moderately metal rich clusters. The still existing uncertainties related either to the input physics or to the efficiency of some macroscopic mechanisms, like convection or microscopic diffusion, are critically discussed, ruling out the occurrence of the so called “breathing pulses” during the central He exhaustion, in agreement with earlier suggestions. Key words. stars: AGB – stars: evolution – stars: Hertzsprung-Russel diagram 1. Introduction phases of stellar evolution by Cassisi et al. (1998, here- inafter CCDW; 1999), and Castellani & Degl’Innocenti The capability of current stellar models to account for (1999), who discussed theoretical predictions concern- all the evolutionary phases observed in stellar clusters is ing central He-burning low-mass stars populating the undoubtedly an exciting achievement which crowns with Horizontal Branch of galactic globular clusters. success the development of stellar evolutionary theories In this paper we will discuss predictions concerning as pursued all along the second half of the last century. the evolutionary behaviour of Asymptotic Giant Branch Following such a success, one is often tempted to use evo- stars, devoting particular attention to two key observa- lutionary results in an uncritical way, i.e., taking these re- tional parameters, such as the luminosity of the predicted sults at their face values without allowing for theoretical AGB clump and the number ratio between HB and AGB uncertainties. However, theoretical uncertainties do exist, stars NAGB/NHB. These parameters appear of particu- as it is clearly shown by the not negligible differences still lar relevance since the AGB clump luminosity has been existing among evolutionary results provided by different proposed as an alternative distance indicator for old– theoretical groups. intermediate age stellar populations (Pulone 1992), while the ratio NAGB/NHB is an excellent tool for investigating The discussion of these theoretical uncertainties was the efficiency of mixing processes during the HB phase early addressed by Chaboyer (1995) in a pioneering pa- (Buonanno et al. 1985), being HB lifetimes extremely sen- per investigating the reliability of theoretical predictions sitive to the extension of the semiconvective region in the concerning H-burning structures presently evolving in stellar core. galactic globular clusters (GCs) and, in turn, on the In the two next sections we will first discuss the accuracy of current predictions about GC ages. More differences between “old” and “new” models, testing recently, such an investigation has been extended to later the most updated theoretical scenario on selected high- quality Color-Magnitude (CM) diagrams of galactic GCs. Section 4 will deal with an investigation on the uncertain- Send offprint requests to: S. Degl’Innocenti, ties still existing in current theoretical models. Concluding e-mail: [email protected] remarks will close the paper. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20000293 S. Cassisi et al.: AGB predictions: Theoretical uncertainties 579 2. Theoretical models 3.50 3.30 Theoretical predictions concerning AGB stars in galactic M=0.75 Mo globulars have been presented and discussed in a previ- 3.10 Z=0.0002 Y=0.23 ous paper (“old” models) about a decade ago (Castellani 2.90 et al. 1991, hereinafter CCP). According to a quite com- 2.70 mon procedure, the discussion was based on HB models as produced by Red Giant Branch (RGB) progenitors with 2.50 LogL/Lo an original mass of 0.8 M , in the assumption that, for 2.30 10 ages of the order of 10 years, differences in ages play 2.10 a minor role in defining the structure and the evolution of He-burning stars. On this basis it was shown that in 1.90 present work clusters with a well populated red HB theory predicts the 1.70 CCP occurrence of a clump of AGB stars with a rather well 1.50 defined low luminosity edge. Theoretical predictions were 0 102030405060708090100110120130 found in reasonable agreement with the two main obser- Age [Myr] vational parameters, as given by i) the luminosity of the Fig. 1. Time behaviour of the surface luminosity during the AGB clump with respect to the HB and, ii) the number central and shell He-burning phases for a model with the la- ratio of AGB to HB stars, the so-called R2 parameter. belled mass and chemical composition, computed by adopting However, since that time evolutionary models have been the most updated physics (solid line) or as in CCP (dashed progressively updated, following the availability of new line) and, hopefully, better physics and – in particular – of bet- ter neutrino energy losses, equation of state, opacities and nuclear cross sections. Thus the problem arises if a good effective temperatures in the observational (MV ,B − V ) fitting is preserved even in recent models. CM diagram, constraining the mixing length parameter by To address this question, Fig. 1 shows the time be- the requirement of reproducing the observed color of the haviour of the luminosity for a typical HB model through AGB branches. In cool stars absolute visual magnitudes and beyond the phase of central He-burning, i.e., along are indeed dependent on the adopted efficiency of the ex- both HB and AGB phases, as computed in CCP or ternal convection which influences the effective tempera- with the updated theoretical scenario presented in CCDW ture and, in turn, the bolometric correction; thus mean- (“new” models), which takes also into account the effi- ingful predictions for the luminosity of the AGB clump do ciency of element sedimentation in the RGB progenitors. require a suitable match of AGB colors. The most evident difference is the decrease of the HB life- Figure 2 shows the best fit of present HB and post-HB time, already discussed in CCDW. Consequently, the ra- evolutionary tracks to the CCD CM diagram of M 5 pre- tio of lifetimes in the two evolutionary phases, and thus sented by Sandquist et al. (1996). For the cluster metallic- the predicted star number ratio, is significantly different, ity we adopted from Sneden et al. (1992) [Fe/H] ≈−1.17 ∼ changing from τ(AGB)/τ(HB) 0.11 to about 0.15. As with [α/Fe] ≈ +0.2. By adopting the relation given by we will discuss later on in this paper, numerical exper- Salaris et al. (1993), and from the value of [α/Fe] one de- iments disclose that such differences in the He-burning rives Z =0.002 ([M/H] ≈−1.03). We also adopted Y = lifetimes are largely due to both the decreased efficiency 0.23. As expected, by keeping as in CCP E(B−V )=0.03, 12 16 of the C(α, γ) O nuclear reaction in the “new” models in agreement also with the recent estimates by Sandquist and to the change in radiative opacities. An additional, et al. (1996), one obtains a reasonable fit, provided that but secondary, contribution to the decrease of central He- the distance modulus estimated in CCP is increased by burning lifetime follows the larger luminosity of the new ∆(m − M)V =0.09 mag following the increased lu- ZAHB models. However the same figure shows that the minosity of the “new” models. This distance modulus luminosity of both HB and AGB clump is increased by appears in good agreement with the value provided by quite a similar amount, so that the difference in luminos- Sandquist et al. (1996, (m − M)V =14.50 0.07 mag). ity between these two observables (∆MV (AGB-HB)) as Sandquist et al. (1996) give for the observed number ratio predicted in CCP, does survive the change of the physical R2 = NAGB/NHB the value 0.169 0.06. Within the un- inputs. certainty this observational value appears consistent both with CCP (τAGB /τHB ≈ 0.11) and with present results (τ /τ ≈ 0.15) even if the central value appears in 3. Observational tests AGB HB better agreement with the new models. However, in the Taking advantage of the new and improved CM diagrams next section we will discuss the intrinsic weakness of such appeared in last years, one can repeat the analysis already a theoretical prediction. given in CCP to test the adequacy of the theoretical sce- To test theoretical models at larger metallicities, the nario. To this purpose, we used Castelli et al. (1997) model same fitting procedure has been applied to the high- atmospheres to translate bolometric luminosities and quality CM diagram for 47 Tuc (Sosin et al.