Asymptotic Giant Branch Stars

1. Asymptotic Giant Branch Stars Ashley Nord December 3, 2007

2. What are Asymptotoic Giant Branch (AGB) Stars? • Stars with masses ≤ 8M on the second ascent into the Red Giant Region • Often AGBs are Long-Period Variables • Can lose 50-70% of their mass during this period - major producer of interstellar dust

3. History of AGB Stars • “Bifurcation of the Red Giant Branch” (Arp, Baum, Sandage, 1953) • 1970’s: IRAS catalog- circumstellar dust envelopes • 1980’s: Radio observations- mass loss processes

5. Globular Cluster M5

6. http://www.noao.edu/outreach/press/pr03/sb0307.html

7. Main Sequence Red Giant Branch Asymptotic Giant Branch Horizontal Branch

8. The Early (E-AGB) Stage • Contraction of core and expansion of envelope lead to a rapid increase in luminosity. • He burning in the shell produces most of the energy. • Stellar envelope ~ 1013 cm • Envelope becomes pulsationally unstable

9. The Thermally Pulsing (TP-AGB) Stage • Once the AGB reaches about 3000L , the star is able to burn both He and H in shells. • Thin He layers burn rapidly into C, and falls onto the core • Produces “thermal pulse” or “He-shell flash” and a luminosity modulation • Between thermal pulses, the AGB again burns H. • Convection often carries C into the envelope.

10. The Atmosphere • The outer part of the envelope is cool enough to form molecules. • Pulsation causes shocks. At high enough altitudes, grain condensation occurs. • The AGB will eventually start to lose mass in the form of a slow wind. • The rate of ejection of matter is higher than the growth rate of the core.

11. Stellar Wind • As layers of the envelope blow away, they expose hotter layers- strengthens stellar wind • Faster winds collide with slower winds- produces dense shells of gas, some of which cool to form dust • The distribution of dust is not always uniform, as is the case with IRC+10216.

12. IRC+10216 at 2.2 micro-meter, evolution 1995-2001 (Weigelt et al. 2002, Astronomy and Astrophysics 392, p.131-141)

13. Why Asymmetric Winds? Freiburg, 2006

14. Dynamics of Stellar Winds • Dust grains form close to the star where the gas is dense and cool • Dust particles absorb stellar photons and accelerate outward, dragging gas with them • Further from the star, flow instabilities (e.g. Raleigh-Taylor) fragment outward moving shells, producing small-scale sub-structures

15. Woitke, Peter, 2006

16. Woitke, Peter, 2006. Astronomy and Astrophysics.

17. Woitke, Peter, 2006. Astronomy and Astrophysics.

18. Woitke, Peter, 2006. Astronomy and Astrophysics.

19. How Do We Recognize AGB Stars? • Often difficult to distinguish between AGB and RGB. • Stars more luminous than the tip of the RGB are usually AGB stars. • Thermal pulses cause an abundance of heavier elements in the outer atmosphere, compared to RGB. • Long-period Pulsations • Mass-loss

20. End Result • Once the entire outer shell has been expelled, a white dwarf remains. • The white dwarf ionizes the surrounding ejected matter, resulting in a planetary nebula. • The fossil AGB stellar wind can now be optically studied as spatial structures of gas and dust in the PN.

21. The Eskimo Nebula, Hubble Space Telescope,  WFPC2

23. Conclusion • Stars ≤ 8M will evolve into AGB stars. • These stars have an inert C-O core, surrounded by a He shell, a H shell, and a H envelope. • The envelope expands and becomes unstable • The star pulsates, causing shock waves which eject mass through stellar winds. • AGBs lose 50-70% of their mass, end as white dwarfs and planetary nebula.

24. References Asymptotic Giant Branch Stars. http://www.noao.edu/outreach/press/pr03/sb0307.html Clayton, Donald. Principles of Stellar Evolution and Nucleosynthesis. The University of Chicago Press, Chicago, IL. 1968. Harm and Olofsson, Hans. Asymptotic Giant Branch Stars. Springer-Verlag New York, Inc. 2004 http://www.astro.uu.se/~bf/publications/2006_06_12_Freiburg_RSG/agbmovie.htm Winters, et al. Mass loss from dust y, low outflow-velocity AGB Stars. II. A&A 475, 2, 559-568. Woitke, P. 2D Models for Dust-driven AGB Stars. A&A 452, 537-549