Setting the Stage for Evolution & Nucleosynthesis of Cluster AGB Stars Using Pulsation Analysis

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

2. 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.

3. AGB stars in the HR Diagram Surface Enrichment, Mass Loss & Variability • 1 <~ Mi <~ 8 Msun • -3.6 <~ Mbol <~ -7.1 • Low mass AGB stars: Mi <~ 2Msun • For Mi ~ 1.5 Msun : τAGB ~ 8 * 106 yr • When the envelope mass reduces to ~0.01 , stars evolve to hotter Teff values (Post-AGB Phase)‏

4. 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

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

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

7. 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)‏

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

9. 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

10. Linear Non-adiabatic Pulsation Models Works for small amplitude stars The Static Models solve for the the stellar structure Teff 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 ~ R3/2M-1/2 ) L=4π σ R2Teff4

11. 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)‏

12. An example of the Role Played by Mass Loss... • 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)‏

13. 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)‏

14. NGC 1978 NGC 419 Massive, rich, luminous LMC cluster [Fe/H] = -0.4 , Z= 0.008 According to the isochrones from Girardi et al. (2000): τ= 1.9 Gyr Initial Mass of current AGB stars ~ 1.54 to 1.62 Msun Current mass = 1.44 to 1.53 Msun (Scaled Reimers mass loss law)‏ Intermediate age SMC cluster [Fe/H] = -0.7, Z= 0.004 According to the isochrones from Girardi et al. (2000): τ = 1.4 Gyr Initial mass of current AGB stars ~ 1.82 Msun Current mass = 1.79Msun (Scaled Reimers mass loss law)‏

15. Light curves MACHO (MB, MR)&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 & S3MC (covering IRAC - 3.6μm, 4.5μm, 5.8μm and 8μm & MIPS – 24.0μm)‏ Gives Bolometric Luminosity Data & Observations

16. NGC 1978

17. 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

18. 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

19. Preliminary Results for NGC 419 Linear Periods for small amplitude Variables • Fits to periods of M stars • Mixing Length = 1.845 • C/O = 0.311 • M = 1.87 Msun NGC 419

20. Models Including TDU and C/O Change • Fits to periods of a few C stars • Mixing Length = 1.845 • C/O= increasing • M = 1.87 Msun

21. 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 • MAGB ~ 1.6 Msun at Mbol~ 5.3 => Observed mass lost on AGB ~0.27 Msun • Observed light curves:

22. Consequences of the MIR1 Modelling Groenewegen et al. (2007) => M ~ 1.7 x 10-5 Msun yr-1 , for MIR1 (P ~ 738) Vassiliadis & Wood (1993) =>M ~ 1.4 x 10-5 Msun yr-1 , for MIR1 (P ~ 738) Envelope mass ~ 1Msun Time needed to lose the envelope ~ 7 x 104 yr Mbol ~ 0.07 Mag

23. Model with VW mass loss rate predicts the superwind phase starts at Mbol~ -5.05 and all envelope mass is lost by Mbol~ -5.14. However, MIR1 has Mbol ~ -5.3 • Problem: M reaches ~10-5 Msun yr-1 at too short a period in VW mass loss prescriptions

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

25. Evolution and Nucleosynthesis of AGB Stars – More Abundance Constraints Cluster Details: NGC 1978 Mass: From Pulsation Models Z ~ 0.008 NGC 419 Mass: From Pulsation Models Z ~ 0.004 NGC 1846 Mass: From Pulsation studies by Lebzelter & Wood (2007) (~1.8Msun ) Z ~ 0.006 Lederer et al. (2009)‏

26. Summary • Accurate masses & mass loss rates and Teff & 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.

27. Thankyou