Multiple Stellar Populations:the evolutionary framework Santi Cassisi

INAF - Astronomical Observatory of Teramo - Italy

The theoretical framework: a brief historical overviewThe study of Galactic globular clusters has always received a long-standing attention owing to its importance in retaining fundamental hints about the Galaxy formation and the age of the UniverseUntil the early 90, almost all sets of stellar models were computed by adopting scaled-solar abundances for the elements heavier than He:...but...

soon after..., the first sets of stellar models properly accounting for -enhanced heavy element mixtures came to light (Vandenberg et al. 92, Salaris et al. 93,..., Pietrinferni et al. 06, Dotter et al. 08) At fixed iron content, increasing the -element abundance makes the evolutionary tracks fainter and cooler with respect to the scaled-solar ones; so the isochrones...:The change in the radiative opacity contributes to about 60%;

The variation of the CNO-cycle efficiency - induced by the O abundance change - provides the difference;

The rescaling solutionIsochrones for -enhanced element abundances are well mimicked by those for scaled-solar mixtures of heavy elements, simply by requiring the total abundance of heavy elements (Z) to be the same:This had the implication than -enhanced stellar models are no mandatory for studying the stellar populations in GC!!!

The multiple stellar population chemistry 1/3light elements anti-correlationslight elements abundances can affect the radiative opacity evaluations and the H-burning rate via a change of the CNO-cycle efficiency + C - N and N - O anti-correlations

The multiple stellar population chemistry 2/3C+N+O enhancement (?)but there are somenotable exceptions+ NGC1851(still contradictory... ) Centauribeing C, N, and O the catalysts of the CNO-cyclea change of their sum affects the H-burning rate

The multiple stellar population chemistry 3/3He enhancementthe proofs:Direct spectroscopic measurements no possible, but for hot HB stars (but...), a small Teff window along the HB (Villanova, Piotto & Gratton 09), and RGB stars via the NIR He110830 transition (Dupree, Strader & Smith 11);Indirect estimates (Bragaglia, Carretta, Gratton et al. 10) by measuring differences in: effective temperature;[Fe/H];RGB bump luminosity level;

The multiple stellar population chemistry 3/3He enhancementthe proofs:Reliable direct spectroscopic measurements possible only in a small Teff range along the HB (Villanova, Piotto & Gratton 09), and RGB stars via the NIR He110830 transition (Dupree, Strader & Smith 11);Indirect estimates (Bragaglia et al. 2010) by measuring difference in: [Fe/H], Teff, RGB bump luminosity; Photometric evidence: Cen, NGC2808, NGC104, etc etc...

How these chemical patterns affect evolution of Stars?Evolutionary and structural propertiesStellar SpectraLet us consider separately the impact on:

The effect of a He enhancement on StarsWhen increasing the initial He content:radiative opacity decreases;As a consequence, for a given total mass and metallicity, the star becomes: brighter; bluer the H-burning rate increases;

YtH(Gyr)0.24611.30.3007.70.3505.50.4003.8

He-enhanced stellar populations: the theoretical scenarioHe-rich MS loci run almost parallel;At fixed luminosity, when increasing the He content by Y0.15 the MS Teff increases by 350K;the mass@Turn-off is 0.806M for Y0.25 and 0.610M for Y=0.40. Much smaller...;for a given age, the SGB loci overlap perfectly...;the bump becomes brighter;increasing the He abundancethe luminosity excursion V strongly decreases...;

He-enhanced Horizontal Branch StarsIn comparison with a He-normal ZAHB, He-rich ZAHBs are:

1) brighter for Teff20000K;

2) fainter at larger Teff;

He-enhanced HB Stars: HB morphology implications

The effects of light-element abundance changes:the general frameworkThere is a rich literature concerning the analysis of the effects on stellar evolution induced by a change of the heavy element abundances in the mixture: from Iben & Simoda (70).... to Salaris et al. (06), Dotter et al. (07), Pietrinferni et al. (09), Vandenberg et al. (12)O has the biggest impact; It is the most abundant metal; It affects both the opacity and the nucleosynthesis (as C & N);In order of decreasing influence, other important elements are: Si, Mg, Ne, S and C;The RGB location is affected only by the changes of Mg and Si (very efficient electron donors);

The impact of these selective enhancements on stellar isochronesThe different impact of the selective enhancement of the various elements on the MS/SGB and the RGB loci strongly suggests that:

a self-consistent comparison between theory and GC data has to take into account the (fine) details of the chemical pattern;

the horizontal method for GC age estimates could be affected by deceptive light-element abundance differences between GCs (see also Marin-Franch et al. 10);

The impact of light-element anti-correlationson the evolutionary frameworkextreme light-element anti-correlations versus a reference -enhanced mixture(Salaris et al. 06, Cassisi et al. 08, Ventura et al. 08, Pietrinferni et al. 09)The Teff changes on the MS and RGB are marginal (lower than 20K);

The effect on the SGB is mainly due to change in the nuclear network;

The change in the core H-burning lifetime is 1%

change (dex)/mixtureCNONa(C+N+O)(CNO)Na-0.6+1.44-0.8+0.8as in the reference mixture(CNO)extNa-0.6+1.8-0.8+0.8a factor of 2 larger

A 11Gyr-old CNO-enhanced isochrone is perfectly matched by an -enhanced isochrone with an age of 13Gyr...

there is an age-offset of about 1.5-2 GyrCNO-enhanced HB stars are brighter (0.12 mag on average) ...;

at fixed total mass, their ZAHB location is cooler;

they perform more extended blue loops during the core He-burning stage

The photometric appearance of multiple stellar populations: the fundamental rle of model atmospheresIn the H-R diagram, at fixed [Fe/H], a clear separation (split) of an evolutionary sequence can be obtained:for the MS, only as a consequence of a huge He-enhancement;for the SGB, only as a consequence of an increase of the (C+N+O) sum;in the case of the RGB, only as a consequence of an He increase;

The light-element changes affect mainly the portion of the spectra short of about 400 nm owing to the changes in molecular bands (...,NH, CN, and OH in the fainter MS stars...)

The impact on the color-Teff relationsThe magnesium abundance (and to a lesser extent oxygen and silicon) is mainly responsible for these differences;V-band and near-IR bands (...!) are not affected by chemical element varations, the same does not hold for U and B bands;

The impact on the color-Teff relationsAny photometric band bluer than the standard B band is hugely affected by the multiple populations chemical patterns... also (!!!) when the CNO sum is constant... (Sbordone et al. 2011, see also Cassisi et al. 2004);He enhancement - strongly affecting the stellar structures - is irrelevant for the atmospheric structure;

The impact on the color-Teff relationsAny photometric band bluer than the standard B band is hugely affected by the multiple populations chemical patterns... also (!!!) when the CNO sum is constant... (Sbordone et al. 2011, see also Cassisi et al. 2004);He enhancement - strongly affecting the stellar structures - is irrelevant for the atmospheric structure;

isochrones for multiple population: a self-consistent approach In the optical bands, a splitting of sequence along the MS up to the TO (and the RGB) can be achieved only in case of a huge He-enhancement; When bluer filters are used, CNONa anti-correlations and He differences can produce multiple sequences from the MS up to the RGB:This does not depend on the CNO sum;He-enhancements work in the opposite direction of light-element anti-correlations;

Strmgren photometric filters

The insensitivity of the optical bands on light-element anti-correlations: a lucky occurrence!!!The case of Centaurithe most accurate He-enhancement estimate in a MPs GCY=0.385

The near-infrared bands: a still largely unexplored windowThe H2O molecule has the strongest effect...

The Horizontal Branch: constraints from HB stellar modelsThe HB morphology in the optical bands has been used to constrain the He abundances of the various sub-populations, but...The case of NGC2808

The Horizontal Branch: constraints from evolutionary and pulsational modelsCombining photometric evidence and spectroscopy (for both RGB - Yong+ 09, Carretta+ 11-, SGB - Gratton+ 12. Lardo+ 12 -, and HB stars - Gratton+ 12) one can try to model the observed HB stellar distribution...... further constraints are required...

...NGC1851 hosts a quite rich population of RR Lyrae stars...

Final remarksThe evidence of multiple stellar populations in Galactic GCs severely challenges stellar model producers;In order to compute stellar models suitable for reproducing the properties of stars in a given GC, the (fine) details of the observed chemical pattern have to be taken into account:Accurate radiative opacity evaluations accounting for the various heavy element enhancements are mandatory;Appropriate color - Teff transformations and Bolometric Corrections are needed in order to perform a self-consistent comparison between theory and observations; Depending on the issue under scrutiny (age determination, mass function estimates, etc.), an appropriate set of photometric filters has to be selected in order to minimize the uncertainties associated with models (atmospheres...) and undetected peculiarities in the chemical patterns;