Current uncertainties in Stellar Evolution Models

1. Current uncertaintiesinStellar Evolution Models Santi Cassisi INAF - Astronomical Observatory of Teramo, Italy

2. The “ingredients” • Numerics • Boundary conditions • 1D versus 3D An evolutionary code • Equation of State • Radiative opacity • Conductive opacity • Nuclear reaction rates • Neutrino energy losses Physical inputs • Overshooting • Superadiabatic convection • Non-canonical processes Mixing treatment • Atomic diffusion • Radiative levitation Microscopic mechanism • Mass loss • Rotation • Magnetic field Additional mechanism

3. Some pieces of evidence… Very Low Mass stars: Segransan et al. (2003) Richer et al. (2008) King et al. (1998) Zoccali et al. (2000) ✓ Surface boundary conditions Equation of State Opacity ✓ ✓

4. Eclipsing binary: an important benchmark 1/2 The case of V69 in the Galactic GC 47Tuc (Thompson et al. 2010) Dartmouth BaSTI Victoria BaSTI YY Dartmouth ≈1.5Gyr When the differences (He content, heavy elements distribution, diffusion efficiency, etc…) are taken properly into account, the difference can be reduced to about 0.8Gyr,… Victoria Padua

5. The Age – Luminosity calibration: the clock A comparison among the various stellar model libraries suggests that an uncertainty of about 1Gyr (i.e. ≈10%) do exist at the older ages…

6. The case of V20 in the Galactic Open Cluster NGC6791 (Grundahl et al. 2008) Eclipsing binary: an important benchmark 2/2 (m-M)V=13.46 ± 0.10 E(B-V)=0.15 ± 0.02 Victoria-Regina (t=8.5Gyr) Photometry by Stetson et al. (2003) Kalirai et al.(2007)

7. Red Giant Branch Stars The location and slope are strongly dependent on the metallicity…; The RGB Tip brightness is one of the most important “primary” distance indicators; 47 Tuc HST Snapshot Piotto et al. (1999) • RGB star counts are quite important: • to check the inner chemical stratification; • being RGB stars among the brightest and cooler objects, their number (+ AGB stars)controls the integrated properties in the NIR bands; • the RGB/AGB number ratio provides hints on the Star Formation History of complex stellar populations (Greggio 2002); Accurate RGB modeling is mandatory for interpreting data of unresolved stellar systems using population synthesis tools as well as for estimating the properties of resolved systems by means of isochrone fitting techniques

8. The state-of-art of RGB models: the luminosity function Theoretical predictions about RGB star counts appear a quite robust result RGB bump M13: Sandquist et al. (2010) What is present situation about the level of agreement between between theory and observations concerning the RGB bump brightness?

9. The RGB bump brightness To overcome problems related to still-present indetermination on GC distance modulus and reddening, it is a common procedure to compare theory with observations by using the ΔV(Bump-HB) parameter Does it exist a real problem in RGB stellar models or is there a problem in the data analysis? Monelli et al. (2010)

10. The brightness of the Red Giant Branch Tip The I-Cousin band TRGB magnitude is one of the most important primary distance indicators: RGB tip • age independent for t>2-3Gyrs; • metallicity independent for [M/H]<−0.9 The TRGB brightness is a strong function of the He core mass at the He-burning ignition

11. TRGB: He core mass – luminosity ≈ 0.03M Salaris, Cassisi & Weiss (2001) These differences are – often but not always…- those expected when considering the different physical inputs adopted in the model computations

12. an update TRGB: He core mass & luminosity • last generations of stellar models agree – almost all – within ≈ 0.003M • a fraction of the difference in McHe is due to the various initial He contents – but in the case of the Padua models… • the difference in Mbol(TRGB) is of the order of 0.15 mag when excluding the Padua models…

13. The TRGB brightness as Standard Candle: theoretical calibrations ω Cen – Bellazzini et al. (2001) The I-band theoretical calibrations appear sistematically brighter by about 0.15 mag

14. The TRGB brightness: theory versus observations (an update) Updated RGB models are now in agreement with empirical data at the level of better than 0.5σ In the near-IR bands, the same calibration is in fine agreement with empirical constraints (but in the J-band…) • The reliability of this comparison would be largely improved by: • increasing the GC sample…; • reducing the still-existing uncertainties in the color-Teff transformations

15. The Horizontal Branch The brightness: • a primary standard candle • Star counts The R parameter The ZAHB luminosity is mainly fixed by the mass size of the He core@TRGB The color location along the HB DOES depend on the mass loss efficiency along the RGB Any physical inputs affecting the value of McHe, has a strong impact on the ZAHB luminosity The color distribution: • the 2° parameter problem Peculiar “patterns”: • rotation • surface chemical abundances

16. The ZAHB brightness: an update De Santis & Cassisi (1999) • The difference among the most recent models is about 0.15 mag • All models but the Dotter’s ones, predict the same dependence on [M/H]

17. High mass-loss efficiency low mass-loss efficiency Mass loss along the RGB: the impact on the HB The impact of mass loss phenomenon on the evolutionary properties of RGB stars is (…not always!...) negligible, but…it is very important for the Horizontal Branch Dorman, Rood & O’Connell (1993) The integrated magnitudes & colors of stellar systems can be largely affected by the HB morphology (see Conroy’s talk…)

18. These formulae are not able to reproduce the mass-loss rates measured by Origlia et al. (2002, 2007)… Mass loss on the RGB: not good news “Investigations of the impact of RGB mass loss upon the HB morphology have mostly relied on the Reimers’s (1975) formula, and it is widely used as a LAW” (Catelan 2005) But… • various prescriptions do exist • they predict quite different mass loss efficiency

19. HB stars show a number of peculiarities Discontinuities in the abundance ratios Diffusive processes (atomic diffusion + radiative levitation) are really at work in HB stars! What about stellar models…?

20. Various masses to cover range of Teff along the HB; In color from 15 to 30 Myr after ZAHB; Same turbulence model for all masses; Stellar model predictions(Michaud et al. 2007,2008) • Data for M15 by Behr (2003) • Stars with Teff < 11000 K have same metals as giants of cluster; • Stars with Teff > 11000 K have X100 overabundance; • Overabundances explained… but normal ones suggest something else also present since overbundances by X5 expected;

21. Field Stars Discontinuity in the rotation rates (Behr 00 + 03, Recio-Blanco et al. 02 + 04) Globular clusters • Some embarrassments: • they rotate… • some of them rotate fast… • it seems to exist a discontinuity…

22. Data from Behr et al. (1999, 2003) - black: M3; red: M13; green: M15; blue: M68; brown: NGC 288; Stars with Teff < 11000 K have [Fe/H] as RGB stars; Stars with Teff > 11000 K have [Fe/H] values from 10 to 100 times larger; Stars with anomalies have slow rotation; note star with arrow; [Fe/H] and rotation along the HB: an observational link Any clues from stellar models?

23. Dotted curve: ~max observed Vsin i; Circulation wipes out anomalies below about 11000 K; Dark gray region: no anomalies observed; White region: anomalies observed; Meridional circulation in HB stars • Trend with the Teff of the limiting rotational velocity for He settling in presence of meridional circulation for two cases: • Ciculation enters the He convection zone (dots); • Circulation does not enter into the convection zone (triangles); but NO clues on why HB stars rotate… Quievy et al. (2009)

24. Marigo et al. (2008) The Asymptotic Giant Branch • The AGB evolutionaryphaseisveryimportantformanyreasonsas: • NeutronCaptureNucleosynthesis • Populationtracers • Integratedpropertiesofresolved & unresolved stellar populations

25. But… it is in the age regime when AGB stars dominate the SED, where different population synthesis models give - quite - different results SED for SSP In some cases - as in Maraston (2005) - the AGB contribution to both the bolometric and near-IR light of a stellar population, is much larger (a factor of 2 or more…) than in other models… see the talks by Bruzual and Conroy

26. AGB stellar models: why a THORNY problem? pulsations Mass loss Nucleosynthesis Brightness Effective temperature scale  colors Evolutionary lifetime Initial – Final mass relation mixing opacity burning(s)

27. The TDU efficiency: an unsettled issue Problem: How to treat the mixing during the TDU? • Solution(s): • Bare Schwarzschild criterion • Envelope overshoot • Time dependent mixing • Diffusive process Free (!) parameter(s) The mixing efficiency during the TDU has important effects on: • the rate of surface C-enhancement; • the effective temperature scale and colors; • the mass loss efficiency and, in turn, the TP stage lifetime; • the amount of s-elements @ the stellar surface ;

28. The 2th problem: opacity for C-enhanced mixtures Long time ago, Scalo & Ulrich (1975) showed that: TiO and H2O are the most important molecules in the oxygen-rich regime (C/O<1), while carbon-bearing molecules (C2, CN, C2H2 and C3) dominate the opacity for C/O>1 A crucial issue! Fundamental further steps ahead have been NOW made (Lederer & Aringer 2008, Marigo & Aringer 2009, Weiss & Ferguson 2009) What is the impact on the AGB stars effective temperature scale?

29. The importance of an appropriate treatment of C-rich mixture opacity Direct effect: • huge decrease of the effective temperature Marigo & Girardi (2007) Indirect effect: • strong increase of the mass loss efficiency…

30. Fully evolutionary AGB models: is there a general consensus? Weiss & Ferguson (2009) versus Karakas (2003) and Wassiliadis & Wood (1993) No overshooting Overshooting + WF09 • A comparison among independent “fully AGB models” shows that: • relevant differences exist both in the TP lifetimes and TPs number; • sometime the differences have no explanation (as between K93 and WV93…); • significant differences do exist also for the He core mass predictions…;

31. Conclusions Can I trust stellar evolutionary models? If the models does not fit the data, maybe… this means that the data are wrong… Many thanks to the organizers for inviting me!