1. Episodic Mass Loss from Post- AGB Stars Angela Speck Angela Speck, Margaret Meixner, Maia Nenkova, Moshe Elitzur & Gill Knapp

2. Schematic of post-AGB dust shell dM / dt  10 -7 – 10 -6 M  yr -1 V exp  10-20 km/s

3. Understanding Intermediate Mass Stars and their Mass Loss What are the initial masses of the stars which evolve into planetary nebulae? What are the physical mechanisms for the intensive mass loss experienced by AGB stars?

4. Constant mass loss  1/r 2 Increasing mass loss  faster than 1/r 2 Episodic mass loss  discrete enhancements in dust density at certain radii Dust density distributions

5. Observational Data

7. Dust Shell Parameters Assume distance, constant outflow velocity Assume the bumps in emission are due to density enhancements  timescales Sourcet agb (yrs) t inner (yrs) t outer (yrs) Egg nebula90,00037,00074,000 AFGL 618160,00064,000117,000

8. Timescales for thermal pulses/enhanced mass loss M ms (M  ) t interpulse (10 3 yrs) t TP-AGB (10 3 yrs) T superwind (10 3 yrs) 0.9~1005700.4 1~1006500.8 1.5~1009400.8 2~9013406 2.5~7018304 3.5~303509 5~103603 t int 10 3 yrs t TP-AGB 10 3 yrs T swind 10 3 yrs Egg37>90<10 61864>160<10 From Vassialadis & Wood (1993)

9. R-T modeling with DUSTY Spherically Symmetric Input parameters from Skinner et al: – T star = 6500 K – L star = 2.67 x 10 4 L  – R in = 1.34 x 10 16 cm –  _9.7  m = 2.4 Includes heating from central star and ISRF (from Mathis et al. 1983) Variables: – Radial density profile – Grain size distribution

10. Model Results External heating by ISRF is always necessary. Separation of bumps must be > 100" Breadth of bumps is < 45" Best fit uses grain sizes up to 10  m. Underlying drop-off is ~ 1/r 2

11. Best Fit Models

12. Summary Dust shells around post-AGB stars Provide mass-loss histories Show evidence for enhanced mass loss on time-scale of thermal pulses Both stars come from progenitor stars of 3 M 

13. Summary II R-T modeling shows – ISRF heating always necessary – Large grains needed – Both theoretical models are consistent with observations – Constant underlying mass-loss rate on AGB!!

14. Future Work Further Radiative transfer modeling (with Moshe Elitzur @ Kentucky) Searching ISO archive (with Margaret Meixner @ STScI) Sub-mm Observations (with Darek Lis @ Caltech)

15. The End