• 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

What are Asymptotoic Giant Branch (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

History of AGB Stars

Globular Cluster M5

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

Main Sequence Red Giant Branch

Horizontal Branch

Asymptotic Giant Branch

• 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

The Early (E-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.

The Thermally Pulsing (TP-AGB) Stage

• 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.

The Atmosphere

• 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.

Stellar Wind

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

Why Asymmetric Winds?

Freiburg, 2006

• 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

Dynamics of Stellar Winds

Woitke, Peter, 2006

Woitke, Peter, 2006. Astronomy and Astrophysics.

Woitke, Peter, 2006. Astronomy and Astrophysics.

Woitke, Peter, 2006. Astronomy and Astrophysics.

• 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

How Do We Recognize AGB Stars?

• 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.

End Result

The Eskimo Nebula, Hubble Space Telescope, WFPC2

• 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.

Conclusion

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

References