The role of AGB stars in stellar populations

The role of AGB stars in stellar populations Alessandro Bressan International School for Advanced Studies (SISSA) Trieste A. Nanni (SISSA) L. Fan (SISSA) L. Danese (SISSA) P. Marigo (UNIPD) L. Girardi (OAPD)

Outline Importance of AGB stars Basic stellar evolution during the AGB phase Synthetic AGB Evolution: the ingredients Circum-stellar dusty envelopes Synthetic C-M Diagrams in the NIR-MIR NAOC, Beijing, 20 Dec 2011

After Central Helium Burning Low Mass Stars & Intermediate Mass stars enter a common phase The ASYMPTOTIC GIANT BRANCH (AGB) The name comes from the asymptotic approach to the RGB In the Colour-Magnitude diagram Buonanno et al. 1987

Importance of AGB stars Very Luminous Tracers of Intermediate Stellar Populations Strong NIR and MIR light contributors Information on the SFR history in resolved populations Enrichment of the ISM Carbon and Nitrogen in non massive single stars S-process elements (Herwig 2004 ARA&A; Church et al. 2009) Dust in their circum-stellar envelopes Crucial for the interpretation of high-z galaxies (t < 1Gyr) Break the age-metallicity degeneracy in early-type galaxies Site of many stellar processes and dependence on M,Z: convection, dredge-up, hot-bottom burning, mass loss, pulsation Not yet well understood: Mixing processes Mass loss

INFRARED PHOTOMETRY OF MAGELLANIC CLOUD STARS near-ir: 2MASS mid-ir: Spitzer-SAGE* LMC contains ~ 30 000 TP-AGB stars (above TRGB) ~ 9 000 are C-type stars ~ 1 400 are luminous M-type stars ~ 2 000 are optically-obscured August 200 are 03-11, in star 2009 clusters Rio de Janeiro - IAU Symp. 262 A GALAXY'S EVOLUTION 7 most are Long Period Variables *Meixner et al. 2006 SURVEYING THE AGENTS OF

NGC 7424 HAWK-I VLT (Grosbol & Dottori 2010) D ~ 12 Mpc Data reduction by Divakara Mayya MS Turn-Off Not reproduced by models

High Redshift ULIRGs L. Fan et al. 2011 Age Distribution z = 1.6 Log L FIR /L = 12.2 AGE = 1.5 Gyr A V = 2.4 AGB

Integrated Colours (LMC) Cluster Ages from Girardi et al 1995 Integrated colours from compilations by Persson et al.(1983) Kyeong et al. (2003) Pessev et al. (2006) LMC Black: average in age bins Green: our models Blue: Maraston et al. 2005

What is the effect of metallicity? Z 0.05 0.02 0.008 0.0004

Importance of AGB stars Very Luminous Tracers of Intermediate Stellar Populations Strong NIR and MIR light contributors Information on the SFR history in resolved populations Enrichment of the ISM Carbon and Nitrogen in non massive single stars S-process elements (Herwig 2004 ARA&A; Church et al. 2009) Dust in their circum-stellar envelopes Crucial for the interpretation of SEDs of high-z galaxies especially in the early phases (t < 1Gyr) Break the age-metallicity degeneracy in early-type galaxies (Bressan et al 2006) Site of many stellar processes and dependence on M,Z: convection, dredge-up, hot-bottom burning, mass loss, pulsation Not yet well understood: Mixing processes Mass loss

Importance of AGB stars Basic stellar evolution during the AGB phase Synthetic AGB Evolution: the ingredients Circum-stellar dusty envelopes Synthetic C-M Diagrams in the NIR-MIR

BASIC STELLAR EVOLUTION ALONG THE AGB (e.g. Iben 1991) Two Distinct Phases The EARLY AGB The TP- AGB

Mass The Early AGB After central He exhaustion a thick He shell ignites above the contracting Carbon-Oxygen (CO) core T C increases due to gravitational contraction (Virial Theorem) Luminosity grows and drives the star toward the RGB (asymptotically) External Convection penetrates inward II Dredge UP for masses above 3 Mo: CNO processed material to the surface: t EAGB Initial C:N:O ~ 1/2:1/6:1 I DgUP C:N:O ~ 1/3:1/3:1 IIDgUP C:N:O ~ 0.3:0.5 :0.9 10 7 2 Max(2, M i ) 3.64 Yr Neutrino Losses become important. For Mi < 5-6 Mo heating of the core balanced by neutrino losses: CO core becomes e-degenerate He H Time C & O

The Thermally Pulsing AGB He shell moves outwards rapidly while H discontinuity may possibly moves inwards because of external convection When the two discontinuities reach almost in contact the H shell reignites and the star begins the double shell phase commonly called: TP-AGB The name from the fact that the He-shell undergoes PERIODIC FLASHES of NUCLEAR BURNING (Thermal Pulses) alternated to long intervals of inactivity (interpulse period) where the only nuclear source is shell H burning

Internal Structure of a TP-AGB star e - Degenerate CORE RAD ADI CONVECTIVE ENVELOPE r/cm Herwig 2004

Evolution along the AGB The (Core)Mass-Luminosity relation (Paczynski 1970) As the mass of the Hydrogen exhausted core (M H ) increases due to H-shell burning the luminosity changes according to 4 RAD RAD ADI ADI kp 4 T 0.25 L M L 1.23 10 ( M 0.46) M 5 2 0.19 H Contribution from CNO burning at the base of convective envelope Hot Bottom Burning (HHB) H at the base of the convective envelope (M H ) 0.4 The star climbs the AGB possibly reaching a maximum luminosity (setting M H =1.4 M, Chandrasekhar mass) L L 5.9 10 ( M H 0.495) Modern model computations show that the relation is more complex with periodic maxima and dips L M MAX BOL L 5.3 10 4 7mag The M-L relation is a basic ingredient for synthetic AGB modeling L

Mass Time Thermal Pulses & III Dredge-UP Efficiency l H rich conv.envelope H Shell III Dredge-UP M H M D l = M D M H He-rich Intershell Convection DM He l=0 No D-UP l=1 No Growth He Disc He Pulse Burning n Pulse 12 C Pulse n +1 C-O Core

Effects of III DredgeUP: C-STARS Elements produced by 3a process and other a-capture reactions are mixed up to the photosphere of the star Flash is rapid: no time to produce Oxygen via 12 C(a,g) 16 O Surface 12 C increases as the number of pulses increase 12 C/ 16 O increases until > 1 The high bond energy of the CO molecule leads to three different spectral types C/O < 1: all carbon bound in CO, excess oxygen is available for the other molecules C (TiO) M Stars; C/O 1: almost all carbon and oxygen atoms bound in CO; molecules from less abundant el. S Stars C/O > 1: all oxygen is bound in CO, excess carbon mainly in hydrocarbon molecules Carbon Stars O C CO C-O CO O CO Hoefner 2009

NOT THE FULL STORY!!! HOT BOTTOM BURNING at the base of the convective envelope

T bottom [10 6 K] HOT Bottom Burning In more massive AGB stars CN cycle active at the base of the convective envelope. Bloecker et al 2000 C 12 transformed into N 14 Primary N 14 production Inhibits C-Star Formation

Outline Importance of AGB stars Basic stellar evolution during the AGB phase Synthetic AGB Evolution: the ingredients Circum-stellar dusty envelopes Synthetic C-M Diagrams in the NIR-MIR

Synthetic AGB Evolution A Pulse + Interpulse cycle requires at least 5000 computer models m=10-6 M and t a fraction of a Yr Full computations needed for a detailed investigation of physical parameters But massive calculations not feasible!!! tracks for Population Synthesis at several compositions exploratory analysis of dependence on parameters opacities with consistent chemistry of abundant elements effect of the III DgUP efficiency mass-loss rate dust production Synthetic AGB Evolution

Simple Synthetic AGB Evolution Structure of the Star makes possible a semi-analytic approach The rate of evolution along the AGB is given by the outward displacement of the H-shell The growth of the core mass is (e H ~ 6.3 10 18 erg/g) dm L H 12 H 9.6 10 M dt X / Inserting the core-luminosity relation with L~L H Rate of brightening Since number of stars in any PMS phase n ~ t AGB star luminosity function should be FLAT M H =M H (t) & L=L(t) T EFF =T EFF (L,Z,M) provided by stellar models yr dlogl dt dm 6 1.4 10 BOL 1 yr 2 10 6 Mag / dt X yr 10 7 M / 7 5.7 10 1 L(t) & T EFF (t) describe the evolution on the HR diagram M H =M H (t) 4 L 5.9 10 ( M H 0.495) In principle the star can be followed until M H =M CH X yr yr dt dm BOL for L~10 4 L 5 10 5 yr / Mag L

The END of the AGB PHASE i.e. another important ingredient in model computations: the AGB Mass Loss

Mfinal (M ) Observed rates Bright tail of AGB LF M BOL Initial Final mass relation (Weidemann 1987) 6 + M-Type AGB Olofsson et al. 02, Gonzalez Delgado et al. 02 Marigo & Girardi 07 Models by Hoefner 08 Mass-Loss already known in RGB stars M LR h M 13 M /yr h~0.4 4 10 Reimers formulation not good for AGB h at least 5 times larger to reproduce Minitial (M )

Brief review of ingredients used in the most recent models of the Synthetic AGB Phase Marigo, Bressan, Girardi & al. 2011 Marigo et al. 2008 Marigo & Girardi 2007, A&A, 469, 239 Bertelli et al. 2008, A&A, 484, 815

Log L/Lo Luminosity & Teff L: from Mass Luminosity relation must be corrected for complex luminosity variations due to thermal pulses and HBB Important to explain: faint C stars below the RGB tip brightest AGB stars From complete models: Groenewegen & Wagenhuber 1998 Izzard et al. 2004 Time Teff: from envelope integrations

III Dup & HBB THE THIRD DREDGE-UP: (M c min, λ) primary 12C; formation of carbon stars HOT-BOTTOM BURNING (M > 4 Mo) CNO-cyle, brightest M stars; primary 14N (Karakas et al. 2002)

III Dup & Low Temperature Opacities (Marigo & Aringer 09) C/O CN, C2, C3, HCN, C2H2

Log Teff C/O III Dup & Low Temperature Opacities Molecular opacities consistent with the current surface C/O ratio, in place of tables valid for solarscaled compositions (Marigo 2002) NEW IMPROVEMENT: ÆSOPUS (Marigo & Aringer 09) Time Photospheric cooling effect in C-rich AGB models Consistent with observed Teffs and colors of carbon stars

Mass Loss: Current Status (see e.g. Hoefner 2009) MIR observations indicate presence of dust Dust is a good candidate for momentum transfer from radiation to gas Photosphere T EFF (~3000K) larger than dust condensation temperatures Expansion velocities smaller than escape velocities at the photosphere Some mechanism must levitate matter up to where dust may form (~3R) Good correlation between mass-loss and pulsation period (Wood 93) pulsations give rise to shock waves that levitate matter to outer regions dust forms radiation pressure on dust grains accelerate matter outwards current models solve coupled hydrodynamics and radiation transfer equations but still need initial velocity set by shocks (pulsations or convection) (Hoefner 09,,Winters et al 03, Elitzur & Ivezic 01, Willson 00, Bowen & Willson 91) Dust depends on C/O ratio (e.g. Ferrarotti & Gail 2006, Nanni et al. 2011) O rich AGB: silicates (olivine- and pyroxene-type materials) (apparently not enough opacity to accelerate matter) C- stars : amorphous carbon (no problems)

Outline Importance of AGB stars Basic stellar evolution during the AGB phase Synthetic AGB Evolution: the ingredients Circum-stellar dusty envelopes Synthetic C-M Diagrams in the NIR-MIR

Why Circum-Stellar dusty envelopes? Optical-NIR CM Star counts ~ stellar lifetimes & SFR & Z SFR & Z derived from selected features of the CMD lifetimes set by evolution & mass-loss (remember the case of C-stars) MIR CM Dusty CS emit in the MIR Star counts ~ dust mass loss rate

Circum-stellar dusty envelopes The case of GC 47 Tuc (12 Gyr) Variable stars observed with Spitzer InfraRed Spectrograph (IRS) Long period variable stars (Lebzelter et al. 2005)

Kmag I ov. Spitzer IRS MIR spectra f. m. Dust Features 9.7 m: amorphous Mg-Fe Silicate 11.5 m: amorphous Al 2 O 3 13. m: MgAl 2 O 4 (Spinel) Log (Period) l m l m from Lebzelter et al. 2006

Not only different species of dust but also nice evidence of dust evolution along the AGB! Silicates Al 2 O 3 No dust

Modeling circum-stellar (CS) dusty envelopes Spherically symmetric stationary wind outside the dust condensation radius r in run of the dust density: dust/gas ratio dm/dt mass loss rate V exp gas velocity r ( r) Optical depth of the envelope is thus (integrating dt=k r dr): d M 4 V exp 1 r (Habing 94, Ivezic & Elitzur 95) 2 t n 4 M V exp k n r in k n is the opacity of the dust mixture Bressan, Granato & Silva 98 Marigo et al. 2008

Modeling circum-stellar (CS) dusty envelopes A number of Circum-Stellar spectra are pre-computed: solve the radiative transfer problem (the dust emission may be self-absorbed) For: given dust mixture different values of t at a reference wavelength (e.g.1 m), To find a correspondence between the envelope and the AGB star one has to express, dm/dt, r in and V exp as a function of basic stellar parameters a a 2.3 10 9.8 10 9 9 for silicates mixture for carbonacous mixture accuracy 5% for T EFF 2500K to 4000K

Parameters of Variable V1 in 47 Tuc Spitzer IRS + Contemp. Photometry IRS Modelling CS dusty envelopes Bressan et al 98, Marigo et al 08

Ongoing Work (A. Nanni et al.) Dust Formation in Stellar Environment (Ferrarotti & Gail 06)

C/O =1 Carbon Dust fc fc = condensation fraction

Outline Importance of AGB stars Basic stellar evolution during the AGB phase Synthetic AGB Evolution: the ingredients Circum-stellar dusty envelopes Synthetic C-M Diagrams in the NIR-MIR

The red tail of C stars: the rôle of molecular opacities 2MASS towards the LMC (all simulations by Girardi & Marigo 09) Simulation: scaled-solar κ mol K Core He-burn + RGB +E-AGB O-rich TP-AGB stars C-stars Milky Way disk Milky Way Halo J-K

The red tail of C stars: the rôle of molecular opacities 2MASS towards the LMC Simulation with variable κ mol K Core He-burn + RGB +E-AGB O-rich TP-AGB stars C-stars Milky Way disk Milky Way Halo J-K

Near- & mid-ir CMDs for the LMC [8.0] vs J-[8.0] NO DUST 2MASS + Spitzer data for inner LMC MW foreground LMC O-rich LMC C-rich

Near- & mid-ir CMDs for the LMC [8.0] vs J-[8.0] Marigo, Girardi, Bressan et al. 08 WITH DUST 2MASS + Spitzer data for inner LMC MW foreground LMC O-rich LMC C-rich

The new Padova Stellar Evolution Models PARSEC PAdova TRieste Stellar Evolution Code (Bressan, Marigo, Girardi, Nanni et al. 2011 in prep.) + Synthetic AGB Evolution (Marigo, Bressan, Girardi et al. 2011 in prep.) + Dust Formation along the AGB (Nanni, Bressan, Marigo, Girardi & Danese 2011 in prep.)

Log L/Lo Z=0.07 Y=0.389 ENHANCED MIXTURE PARSEC Log Teff All input physics updated to 2011 Opacity Nuc. Reac. EOS Diffusion Pre Main Sequence V-Low Mass Stars Hot HB stars Z sets for 3 Partitions: Solar (Caffau 09) a Enhanced a Depressed

Tracks Isochrones

New Tracks & Isochrones with PARSEC + AGB