Current uncertaintiesin

Stellar Evolution Models

Santi CassisiINAF - Astronomical Observatory of Teramo, Italy

The “ingredients”

An evolutionary code • Numerics• Boundary conditions•1D versus 3D

Physical inputs

• Equation of State• Radiative opacity• Conductive opacity• Nuclear reaction rates• Neutrino energy losses

Mixing treatment• Overshooting• Superadiabatic convection• Non-canonical processes

Microscopic mechanism • Atomic diffusion• Radiative levitation

Additional mechanism • Mass loss• Rotation• Magnetic field

Some pieces of evidence…

Very Low Mass stars:

King et al. (1998) Richer et al. (2008)

Zoccali et al. (2000)

Surface boundary conditions

Equation of State

Opacity

✓ ✓ ✓

Segransan et al. (2003)

YYDartmouth

Victoria

BaSTI

Padua

≈1.5Gyr

Eclipsing binary: an important benchmark 1/2

The case of V69 in the Galactic GC 47Tuc (Thompson et al. 2010)

BaSTI DartmouthVictoria

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,…

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…

Eclipsing binary: an important benchmark 2/2

The case of V20 in the Galactic Open Cluster NGC6791(Grundahl et al. 2008)

Kalirai et al.(2007)

Victoria-Regina (t=8.5Gyr)

Photometry by Stetson et al. (2003)

(m-M)V=13.46 ± 0.10

E(B-V)=0.15 ± 0.02

Red Giant Branch StarsThe location and slope are strongly dependent on the metallicity…;

The RGB Tip brightness is one of the most important “primary” distance indicators;

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

47 TucHST Snapshot

Piotto et al. (1999)

The state-of-art of RGB models: the luminosity function

Theoretical predictions about RGB star counts appear a quite robust

result

M13: Sandquist et al. (2010)

RGB bump

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 brightness of the Red Giant Branch Tip

RGB tip

The I-Cousin band TRGB The I-Cousin band TRGB magnitude magnitude is one of the most is one of the most important primary distance important primary distance indicators:indicators:

• 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

TRGB: He core mass – luminosity

Salaris, Cassisi & Weiss (2001)

≈ 0.03M

These differences are – often but not always…- those expected when considering the different physical inputs adopted in the

model computations

TRGB: He core mass & luminosityan update

• 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 as Standard Candle: theoretical calibrations

The I-band theoretical calibrations appear sistematically brighter by about 0.15 mag

ω Cen – Bellazzini et al. (2001)

The TRGB brightness: theory versus observations (an update)

The reliability of this comparison would be largely improved by:• increasing the GC sample…;• reducing the still-existing uncertainties in the color-Teff transformations

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 Horizontal Branch

The brightness:• a primary standard candle

• the 2° parameter problem

• Star counts The R parameter

The ZAHB luminosity is mainly fixed by the mass size of the He core@TRGB

Any physical inputs affecting the value of McHe, has a strong impact on the ZAHB

luminosityThe color distribution:

Peculiar “patterns”:• rotation

• surface chemical abundances

The color location along the HB DOES depend on

the mass loss efficiency along the RGB

The ZAHB brightness: an update

• 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]

De Santis & Cassisi (1999)

The integrated magnitudes & colors of stellar systems can be largely affected by the HB morphology (see Conroy’s talk…)

Mass loss along the RGB: the impact on the HB

High mass-loss efficiency

low mass-loss efficiency

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)

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)

These formulae are not able to reproduce the mass-loss rates measured by Origlia et al. (2002, 2007)…

But…But…

various prescriptions do exist

they predict quite different mass loss efficiency

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…?

Stellar model predictions(Michaud et al. 2007,2008)

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;

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;

Discontinuity in the rotation rates (Behr 00 + 03, Recio-Blanco et al. 02 + 04)

Globular clusters Field Stars

Some embarrassments:•they rotate…

•some of them rotate fast…

•it seems to exist a discontinuity…

[Fe/H] and rotation along the HB: an observational link

• 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;

Any clues from stellar models?

Meridional circulation in HB stars

Dotted curve: ~max observed Vsin i;

Circulation wipes out anomalies below about 11000 K;

Dark gray region: no anomalies observed;

White region: anomalies observed;

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);

Quievy et al. (2009)

but NO clues on why HB stars rotate…

The Asymptotic Giant BranchThe Asymptotic Giant Branch

Marigo et al. (2008)

But… it is in the age regime when AGB stars dominate the SED, where different population synthesis models give - quite - different results

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…

SED for SSP

see the talks by Bruzual and Conroy

AGB stellar models: why a THORNY problem?AGB stellar models: why a THORNY problem?

Nucleosynthesis

Brightness

Effective temperature scale colors

Evolutionary lifetime

Initial – Final mass relation

Massloss

opacitymixing

burning(s)

pulsations

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 mass loss efficiency and, in turn, the TP stage lifetime;

• the amount of s-elements @ the stellar surface ;

• the effective temperature scale and colors;

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

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?

A crucial issue!

The importance of an appropriate treatment of C-rich mixture opacity

Marigo & Girardi (2007)

Direct effect:

• huge decrease of the effective temperature

• strong increase of the mass loss efficiency…

Indirect effect:

Fully evolutionary AGB models: is there a general consensus?

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…;

Weiss & Ferguson (2009) versus Karakas (2003) and Wassiliadis & Wood (1993)

No overshooting

Overshooting

+ WF09

Conclusions

Can I trust

stellar evolutio

nary

models?

If the models does not fit the data, maybe… this means that the data are wrong…