Multiple Stellar Populations: the evolutionary framework

Multiple Stellar Populations:the evolutionary framework Santi Cassisi INAF - Astronomical Observatory of Teramo - Italy

Castellani et al. (1989) • Chieffi & Straniero (1991) • Vandenberg & Bell (1984) • Vandenberg et al. (1990) • ... The theoretical framework: a brief “historical” overview The 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 Universe • Until the early 90’, almost all sets of stellar models were computed by adopting scaled-solar abundances for the elements heavier than He: ...but... Gratton, Sneden & Carretta (2004)

for a fixed [Fe/H] value Δ[α/Fe]=+0.3 →δt≃−1Gyr soon after..., the first sets of stellar models properly accounting for α-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 α-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 peculiar chemical patterns associated with the multiple population phenomenon in GCs complicates the evolutionary theoretical framework The “rescaling” solution Isochrones for α-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 α-enhanced stellar models are nomandatory for studying the stellar populations in GC!!!

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

M22 Marino et al. (2011) The multiple stellar population chemistry 2/3 • C+N+O enhancement (?) Carretta et al. (2005) but there are some notable exceptions • + • NGC1851(still contradictory... ) • ω Centauri In general, the CNO sum seems to be constant being C, N, and O the catalysts of the CNO-cycle a change of their sum affects the H-burning rate

The multiple stellar population chemistry 3/3 • He enhancement the proofs: Photometric evidence: ω Cen, NGC2808, NGC104, etc etc... King, Bedin, Cassisi et al. (2012) Piotto, Bedin, Anderson, et al. (2007) Indirect estimates (Bragaglia et al. 2010) by measuring difference in: [Fe/H], Teff, RGB bump luminosity; 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 Å transition (Dupree, Strader & Smith 11);

How these chemical patterns affect evolution of Stars? Let us consider separately the impact on: • Evolutionary and structural properties • Stellar Spectra

the mean molecular weight μ increases; so a ΔY≈0.1 implies since He The effect of a He enhancement on Stars When 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;

isochrones The RGB bump He-enhanced stellar populations: the theoretical scenario • He-rich MS loci run almost parallel; • At fixed luminosity, when increasing the He content by ΔY≈0.15 the MS Teff increases by ≈350K; • the mass@Turn-off is 0.806M⊙ for Y≈0.25 and 0.610M⊙ for Y=0.40. Much smaller...; • for a given age, the SGB loci overlap perfectly...; increasing the He abundance • the bump becomes brighter; • the luminosity excursion ΔV strongly decreases...;

He-enhanced HB stars perform more extended blue-loops during the core He-burning stage (1) ⇐ larger H-burning efficiency (2) ⇐ smaller He-core mass Cassisi & Salaris (2012) He-enhanced Horizontal Branch Stars In comparison with a “He-normal” ZAHB, He-rich ZAHBs are: 1)brighter for Teff≲20000K; 2) fainter at larger Teff;

Increasing the He-enhancement, in the optical bands the HB appears to be tilted: the luminosity increases moving from “red” to “blue”; The larger the He enhancement, the bluer the HB morphology, for any given assumption about the average mass-loss efficiency; He-enhanced HB Stars: “HB morphology” implications

∼100K Vandenberg et al. (2012) - by enhancing the abundances of several metals by 0.4 dex, in turn The effects of “light-element” abundance changes: the general framework There 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);

Vandenberg et al. (2012) The impact of these selective enhancements on stellar isochrones d(V-I)@2.5 18Gyr 14Gyr 10Gyr • The 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-correlations on the evolutionary framework “extreme” light-element anti-correlations versus a “reference” α-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%

The impact on the isochrones CNO-enhanced HB stars • A 11Gyr-old CNO-enhanced isochrone is perfectly matched by an α-enhanced isochrone with an age of 13Gyr... • there is an age-offset of about 1.5-2 Gyr • CNO-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

...but multi-band observations suggest that the changes in the stellar Spectral Energy Distribution induced by the peculiar chemical patterns are important...; The photometric appearance of multiple stellar populations: the fundamental rôle of model atmospheres In 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;

Sbordone et al. (2011) • black: reference mixture • red: (CNO)extNa RGB Teff=4476K, logg=1.2 TO MS Teff=6490K, logg=4.22 low MS Teff=4621K, logg=4.47 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...)

He-enhanced mixture Johnson-Cousin bands Strömgren bands The impact on the color-Teff relations (CNO)Na mixture (CNO)extNa mixture Johnson-Cousin bands Johnson-Cousin bands Strömgren bands Strömgren bands Any 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;

blue solid: reference mixture • red dash: CNONa • magenta dash: (CNO)enhNa - normal He • blue dot: (CNO)enhNa - Y=0.40 Marino et al. (2008) 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;

The insensitivity of the optical bands on light-element anti-correlations: a “lucky” occurrence!!! The case of ω Centauri Y=0.385 the most accurate He-enhancement estimate in a MPs GC King, Bedin, Cassisi et al. (2012)

This flux variation in the spectral window corresponding to the F160W@WFC3 filter makes the He-enhanced VLM sequence redder than the normal-He sequence by: Milone et al. (2012) bMS rMS Y=0.38 Y=0.25 Δ(MF110W-MF160W)=0.10 in agreement with the observations (≈0.06 mag) NGC2808 Milone, et al. (2012) The near-infrared bands: a still largely unexplored window • The H2O molecule has the strongest effect...

It is quite better to use UV photometric bands due to their quite larger sensitivity to Teff Dalessandro et al. (2011) D’Antona et al. (2005) The Horizontal Branch: constraints from HB stellar models The case of NGC2808 The HB morphology in the optical bands has been used to constrain the He abundances of the various sub-populations, but...

The case of NGC1851 The SGB splitting can be interpreted as due to an age difference or a (CNO) sum difference (Cassisi et al. 08) blue dot: <M>=0.67M⊙, Y=0.28 red dot: <M>=0.64M⊙, Y=0.265 red square: <M>=0.67M⊙, Y=0.248 cyan triangle: <M>=0.65M⊙, Y=0.248, CNO enh Gratton et al. (2012) The Horizontal Branch: constraints from evolutionary and pulsational models Combining 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...

Period/Amplitude distribution combining all together the hints coming from spectroscopy/photometry and pulsational properties of variable stars & comparing them with appropriate Synthetic HB models is “the best” approach for tracing the MPs properties Kunder et al. (2012) ...NGC1851 hosts a quite rich population of RR Lyrae stars...

Final remarks The 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;

Multiple Stellar Populations: the evolutionary framework