Episodic Mass Loss from Post-AGB Stars

1. Episodic Mass Loss from Post-AGB Stars 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 Vexp 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. Dust density distributions • Constant mass loss  1/r2 • Increasing mass loss  faster than 1/r2 • Episodic mass loss  discrete enhancements in dust density at certain radii

5. Observational Data

7. Dust Shell Parameters • Assume distance, constant outflow velocity • Assume the bumps in emission are due to density enhancements  timescales

8. Timescales for thermal pulses/enhanced mass loss From Vassialadis & Wood (1993)

9. R-T modeling with DUSTY • Spherically Symmetric • Input parameters from Skinner et al: • Tstar = 6500 K • Lstar = 2.67 x 104 L • Rin = 1.34 x 1016 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/r2

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