Setting the Stage for Evolution & Nucleosynthesis of Cluster AGB Stars Using Pulsation Analysis Devika Kamath Research School of Astronomy & Astrophysics Supervisors Prof Peter Wood [1] Dr Amanda Karakas [1] [1] Research School of Astronomy and Astrophysics

Objective To use the pulsation properties of AGB stars in NGC 1978 & NGC 419 to derive accurate masses and study mass loss on the AGB To use these results and recent AGB abundance determinations to constrain stellar evolution and nucleosynthesis models for the cluster AGB stars.

AGB stars in the HR Diagram 1 <~ M i <~ 8 M sun -3.6 <~ M bol <~ -7.1 Low mass AGB stars: M i <~ 2M sun For M i ~ 1.5 M sun : τ AGB ~ 8 * 10 6 yr When the envelope mass reduces to ~0.01, stars evolve to hotter T eff values (Post-AGB Phase)‏ Surface Enrichment, Mass Loss & Variability

Essential features of AGB Evolution Thermal pulses Surface abundance modifications (S and C stars)‏ Mass Loss AGB evolution is dominated by mass loss Termination of evolution on AGB Variability Enhances mass loss

Variability Owing to pulsations Pulsations : Radial & Non-Radial Typical time-scales : 20 ~ 2000 days Large amplitude MIRA variables: 200 ~ 800 days NOT THERMAL PULSES!

AGB Variables -> Long Period Variables – Miras – Semi-Regular variables – Irregular variables Seq A, B – 1 st, 2 nd, 3 rd overtone pulsators Seq C – Miras, Fundamental mode pulsators Seq D – Long secondary periods... ? Seq E - Binaries (Wood et al. 1999)‏

MASS LOSS Pulsations + Radiation pressure acting on dust grains Main mass losing interval : end of the TP-AGB phase Mass loss increases with luminosity (Vassiliadis & Wood 1993)‏

We aim to test whether the observed amounts of mass loss are consistent with mass loss prescriptions e.g. Vassiliadis & Wood Mass loss on the FGB: Modified Reimers mass loss Law M ~ LR/M (Reimers 1975)~<10 -8 M sun yr -1 Mass loss on the AGB: Mass loss prescriptions Blocker(1995), Vassiliadis & Wood (1993)‏, Groenewegen et al. (1998)‏ … Commonly used formulations of the mass loss rate (Vassiliadis & Wood 1993)‏ (Bl ö cker 1995)‏

Pulsation Modeling Step1: Initial static structure model Step 2: Linear, non-adiabatic stability analysis of static models Required parameters:  Luminosity  Mixing length  Core mass  An initial mass estimate

Linear Non-adiabatic Pulsation Models Works for small amplitude stars The Static Models solve for the the stellar structure T eff of the lower AGB gives Mixing Length We know R at a given L, If the periods don't match the observed ones for a given AGB luminosity, the Mass must be adjusted. (P ~ R 3/2 M -1/2 ) L=4π σ R 2 T eff 4

For large amplitude pulsators the linear and non-linear pulsation periods are different. We use : NON-LINEAR NON-ADIABATIC PULSATION MODELS Linear non-adiabatic period Non-linear non-adiabatic period (Wood 2007)‏

Without Mass loss: Incorrect linear periods for small amplitude stars With Mass loss: Correct linear periods for small amplitude stars Large amplitude variables show discrepancies as their periods are affected by non- linear effects Direct demonstration that mass loss has occurred on the FGB and AGB Lebzelter and Wood (2005)‏ An example of the Role Played by Mass Loss...

Pulsation Analysis of AGB Stars in Intermediate Age Clusters Target clusters: LMC-NGC 1978 & SMC-NGC 419 Only two clusters in the MCs with Mid-Infra- red Sources (MIR variables)‏ – These are stars that have superwind mass loss rates. They should have lost a lot of mass. Near-Infra-red sources (1 in each cluster)‏

NGC 1978 NGC 419 Massive, rich, luminous LMC cluster [Fe/H] = -0.4, Z= According to the isochrones from Girardi et al. (2000):  τ = 1.9 Gyr  Initial Mass of current AGB stars ~ 1.54 to 1.62 M sun  Current mass = 1.44 to 1.53 M sun (Scaled Reimers mass loss law)‏ Intermediate age SMC cluster [Fe/H] = -0.7, Z= According to the isochrones from Girardi et al. (2000):  τ = 1.4 Gyr  Initial mass of current AGB stars ~ 1.82 M sun  Current mass = 1.79M sun (Scaled Reimers mass loss law)‏

Light curves – MACHO (M B, M R ) & OGLE (V, I) & CASPIR (K,L)‏ Gives Periods Photometric data: – Near-IR Photometric data (CASPIR)- J(1.28μm), H(1.68μm), K(2.22μm), L(3.59μm)‏ – Spitzer Surveys: SAGE & S 3 MC (covering IRAC - 3.6μm, 4.5μm, 5.8μm and 8μm & MIPS – 24.0μm)‏ Gives Bolometric Luminosity Data & Observations

NGC 1978

Period Derivation Selected AGB Candidates in (NGC 1978) LMC and (NGC 419) SMC Analysed their light curves and extracted periods – Periods: Visual inspection PDM (IRAF) Fourier analysis Fourier fits from Period04 (Sperl98)‏ Target Clusters: NGC 1978: 12 AGB variables 1 MIR & 1NIR variable (large -amp)‏ Irregular periods, multi-periodicity NGC 419: 16 AGB variables 1 MIR & 1NIR variable (large-amp)‏ Irregular periods, multi-periodicity More C stars

The Observed HR Diagram The lower part of the CMD => M stars Transition from M to C stars Large J-K color stars – Opaque dust shells – Energy is emitted in IR – Indicative of high mass-loss rate

Preliminary Results for NGC 419 Fits to periods of M stars Mixing Length = C/O = M = 1.87 M sun NGC 419 Linear Periods for small amplitude Variables

Models Including TDU and C/O Change Fits to periods of a few C stars Mixing Length = C/O= increasing M = 1.87 M sun

A Non-linear Pulsation Model for NGC419 MIR1 Large amplitude variables  MIR1 and NIR1 Long term amplitude cycle can be observed, as in many dusty pulsating AGB stars M AGB ~ 1.6 M sun at M bol ~ 5.3 => Observed mass lost on AGB ~0.27 M sun Observed light curves:

Consequences of the MIR1 Modelling Groenewegen et al. (2007) => M ~ 1.7 x M sun yr -1, for MIR1 (P ~ 738) Vassiliadis & Wood (1993) =>M ~ 1.4 x M sun yr -1, for MIR1 (P ~ 738) Envelope mass ~ 1M sun Time needed to lose the envelope ~ 7 x 10 4 yr M bol ~ 0.07 Mag

Model with VW mass loss rate predicts the superwind phase starts at M bol ~ and all envelope mass is lost by M bol ~ However, MIR1 has M bol ~ -5.3 Problem: M reaches ~10 -5 M sun yr -1 at too short a period in VW mass loss prescriptions

Future Work Evolution & Nucleosynthesis Modelling This data will exist for 3 clusters: NGC 419, NGC 1978 & NGC 1846 (Lebzelter & Wood 2005)‏ NGC 1846 Lebzelter & Wood 2007 T eff =>Mixing length Mass => Mass loss rate M to C transition => Amount of third dredge-up

Cluster Details: NGC 1978 Mass: From Pulsation Models Z ~ NGC 419 Mass: From Pulsation Models Z ~ NGC 1846 Mass: From Pulsation studies by Lebzelter & Wood (2007) (~1.8M sun ) Z ~ Evolution and Nucleosynthesis of AGB Stars – More Abundance Constraints Lederer et al. (2009)‏

Summary Accurate masses & mass loss rates and T eff & mixing length values will be derived for AGB stars in NGC 1978 and NGC 419 We will use these results (& NGC 1846) to constrain evolution and nucleosynthesis models in order to try and reproduce the observed abundances of the cluster AGB stars.

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