Setting the Stage for Evolution & Nucleosynthesis of Cluster AGB Stars

Using Pulsation Analysis

Devika KamathResearch School of Astronomy & Astrophysics

SupervisorsProf 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 <~ 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)

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 – 1st , 2nd , 3rd 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 LawM ~ LR/M (Reimers 1975)~<10-8M

sunyr-1

Mass loss on the AGB: Mass loss prescriptionsBlocker(1995), Vassiliadis & Wood (1993), Groenewegen et al. (1998) …

Commonly used formulations of the mass loss rate

(Vassiliadis & Wood 1993)

(Blöcker 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

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 R2Teff

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 AGBLebzelter 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= 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 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= 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)

• 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

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 variables1 MIR & 1NIR variable (large -amp)Irregular periods, multi-periodicity

NGC 419: 16 AGB variables1 MIR & 1NIR variable (large-amp)Irregular periods, multi-periodicityMore 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 = 1.845

C/O = 0.311

M = 1.87 Msun

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 = 1.845

C/O= increasing

M = 1.87 Msun

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:

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

Model with VW mass loss rate predicts the superwind phase starts at M

bol~ -5.05 and all envelope mass is lost by M

bol~ -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

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

• Teff

=>Mixing length• Mass => Mass loss rate• M to C transition =>

Amount of third dredge-up

Cluster Details:NGC 1978Mass: From Pulsation ModelsZ ~ 0.008

NGC 419Mass: From Pulsation ModelsZ ~ 0.004

NGC 1846Mass: From Pulsation studies by Lebzelter & Wood (2007) (~1.8M

sun )

Z ~ 0.006

Evolution and Nucleosynthesis of AGB Stars – More Abundance Constraints

Lederer et al. (2009)

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.

Thank you