Setting the Stage for Evolution

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

• Devika Kamath
• Research School of Astronomy Astrophysics
• Supervisors
• Prof Peter Wood1 Dr Amanda Karakas1
• 1 Research School of Astronomy and Astrophysics

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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.

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AGB stars in the HR Diagram
Surface Enrichment, Mass Loss Variability

• 1 lt Mi lt 8 Msun
• -3.6 lt Mbol lt -7.1
• Low mass AGB stars Mi lt 2Msun
• For Mi 1.5 Msun tAGB 8 106 yr
• When the envelope mass reduces to 0.01 , stars
  evolve to hotter Teff values (Post-AGB Phase)?

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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

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Variability

• Owing to pulsations
• Pulsations Radial Non-Radial
• Typical time-scales 20 2000 days
• Large amplitude MIRA variables 200 800 days

NOT THERMAL PULSES!
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• AGB Variables -gt 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)?
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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)?
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Commonly used formulations of the mass loss rate
Mass loss on the FGB Modified Reimers mass loss
Law M LR/M (Reimers 1975)lt10-8Msunyr-1
Mass loss on the AGB Mass loss
prescriptions Blocker(1995), Vassiliadis Wood
(1993)?, Groenewegen et al. (1998)?
(Vassiliadis Wood 1993)?
(Blöcker 1995)?

• We aim to test whether the observed amounts of
  mass loss are consistent with mass loss
  prescriptions e.g. Vassiliadis Wood

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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

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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 )

L4p s R2Teff4
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• 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)?
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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)?
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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)?

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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)
• t 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)
• t 1.4 Gyr
• Initial mass of current AGB stars 1.82 Msun
• Current mass 1.79Msun (Scaled Reimers mass loss
  law)?

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Data Observations

• 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

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NGC 1978
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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
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The Observed HR Diagram

• The lower part of the CMD gt 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

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Preliminary Results for NGC 419
Linear Periods for small amplitude Variables
NGC 419

• Fits to periods of M stars
• Mixing Length 1.845
• C/O 0.311
• M 1.87 Msun

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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

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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 gt Observed mass
  lost on AGB 0.27 Msun
• Observed light curves

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Consequences of the MIR1 Modelling

• Groenewegen et al. (2007) gt M 1.7 x 10-5 Msun
  yr-1 , for MIR1 (P 738)
• Vassiliadis Wood (1993) gtM 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

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• 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

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Future Work Evolution Nucleosynthesis Modelling
NGC 1846
Lebzelter Wood 2007

• Teff gtMixing length
• Mass gt Mass loss rate
• M to C transition gt Amount of third dredge-up

• This data will exist for 3 clusters NGC 419, NGC
  1978 NGC 1846 (Lebzelter Wood 2005)?

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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)?
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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.

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• Thank you