Current uncertainties in Red Giant Branch stellar models: Basti & the “Others”

Current uncertainties in Red Giant Branch stellar models:Basti & the “Others” Santi Cassisi INAF - Astronomical Observatory of Teramo, Italy

Huber et al. (2010) Stellar models & Asteroseismic analysis To assess the accuracy and reliability of the evolutionary scenario is mandatory! Kallinger et al. (2010) based on BaSTI models

Mup massive stars Setting the (evolutionary) “scenario” Intermediate-mass stars MHeF low-mass stars Intermediate-mass stars Low-mass stars Physical Properties: Microscopical Mechanisms: Macroscopical Mechanisms:

Input physics affecting models for RGB low-mass stars Input Evolutionary properties • Equation of State • Low Temperature Radiative Opacity • Efficiency of the convective energy transport • Boundary conditions • Abundances (He, Fe & -elements) Teff  RGB location & shape • Conductive Opacity • Neutrino energy losses • Atomic diffusion efficiency He core mass@RGB Tip  RGB Tip brightness He-burning stage luminosity

solar-calibrated ml The effect of the EOS • Models computed by using some of the most commonly adopted EOS show: • Different RGB slope • Even if the ml is calibrated on the Sun, differences in the Teff of the order of 100K exist

Low-temperature radiative opacity Ferguson et al. 05 Current sets of stellar models employ mainly the low-T opacity computations by Ferguson et al. (2005) The largest improvement in low-T opacity has been the proper treatment of molecular absorption… and grains… Ferguson et al. 2005 RGB models predict the same location and shape for the RGB until the Teff is larger than ~4000K; For lower Teff, computations based on the most updated opacity, predict cooler models (the difference is of the order of 100K);

Treatment of superadiabatic convection The mixing length is usually calibrated on the Sun: is this approach adequate for RGB stars? The solar-calibration of the ml guarantes that the models always predict the “right” Teff of at least solar-type stars; However, it is important to be sure that a solar ml is also suitable for RGB stars of various metallicities Ferraro et al. (2006) Basti models These results seem to point to the fact that the solar-calibrated ml is a priori adequate also for RGB stars

Outer Boundary conditions 1/2 What is the most adequate approach for fixing the boundary conditions? • The RGB based on model atmospheres shows a slightly different location with respect the models computed by using the Krishna-Swamy solar T() • The difference is of the order of 100K at solar metallicity

Outer Boundary conditions 2/2 What about at lower metallicities? Kurucz • The RGB based on model atmospheres shows a slightly different slope, crossing over models computed using the KS66 solar T() • …but… • The difference is always within ~50K or less

Outer Boundary conditions 3/2 T(τ) versus “model atmosphere”: structural predictions The trend of various thermodynamic quantities, opacity, convective velocity and the fraction of the total flux carried by convection in the subphotospheric layers of a solar model Solid line – model atmosphere Dashed line – evolutionary code integration but fixing the outer boundary conditions from the model atmosphere Vandenberg et al. (2008) Despite the significant differences in the two approaches quite similar results are obtained…

Red Giant Branch models: the state-of-the-art 200K Models from different libraries, based on a solar-calibrated ml, can show different RGB effective temperatures The difference in the RGB location can be also significantly larger (…up to 400 K…) when accounting from less updated model libraries This is probably due to some differences in the input physics, such EOS and/or boundary conditions which is not compensated by the solar recalibration of the ml

Input physics affecting the RGB models Input Evolutionary properties Teff~100K • Equation of State • Low Temperature Radiative Opacity • Efficiency of the convective energy transport • Boundary conditions • Abundances (He, Fe & -elements) Teff~150K Teff  RGB location & shape Solar calibrated ml Teff≤80K • Conductive Opacity • Neutrino energy losses • Atomic diffusion efficiency He core mass@RGB Tip  RGB Tip brightness He-burning stage luminosity

A crucial issue: the color – Teff relations Eclipsing binaries can represent an important benchmark for model libraries The case of V20 in the Galactic Open Cluster NGC6791 (Grundahl et al. 2008) (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)

The RGB luminosity function: the state-of-the-art Theoretical predictions about the RGB star counts appear a quite robust result! Bertelli et al. 08 (Padua) M13: Sandquist et al. (2010) Evolutionary lifetimes for the RGB stage are properly predicted; There is no “missing physics” in the model computations; What is present situation about the level of agreement between between theory and observations concerning the RGB bump brightness?

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)

The RGB bump brightness: an independent check In order to avoid any problem associated to the estimate of the HB luminosity level from both the theoretical and observational point of view, we decided to use the ΔV(Bump-Turn Off) parameter (see also Meissner & Weiss 06) BaSTI models Cassisi et al. (2010) a clear discrepancy between theory and observations is present, the theoretical RGB bump magnitudes being too bright by on average ~0.2 mag …any hint from asteroseismology…?

Input physics affecting the RGB models Input Evolutionary properties • Equation of State • Low Temperature Radiative Opacity • Efficiency of the convective energy transport • Boundary conditions • Abundances (He, Fe & -elements) Teff  RGB location & shape • Conductive Opacity • Neutrino energy losses • Atomic diffusion efficiency He core mass@RGB Tip  RGB Tip brightness He-burning stage luminosity

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 Being McHe@TRGB strongly dependent on the adopted “physical framework”, it has been often used as benchmark for testing “fundamental theory”

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

@TRGB 10% plasma neutrinos 4% radiative opacity 8% 3α reaction rate @ZAHB ΔMbol~0.1 mag The He core mass@TRGB Who is really governing the uncertainty in the McHe predictions? 42% conductive opacity 36% diffusion efficiency ΔMAXMcHe ≈ 0.01M Cassisi et al. (1998) – Michaud et al. (2010)

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…

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 seems to be 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

De Santis & Cassisi (1999) McHe & ZAHB brightness • 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]

Future perspectives for the BaSTI archive • to update the database, taking into account all the improvements in the physical framework; Pulsational models • to improve the parameter-space coverage…; • to check the accuracy & reliability by comparing the models with suitable empirical constraints such as eclipsing binaries, star clusters…; • collaborations with reseachers working in the asteroseismology field are very welcomed!; The BaSTI archive is available @ http://www.oa-teramo.inaf.it/BASTI

Current uncertainties in Red Giant Branch stellar models: Basti & the “Others”