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
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
Globular Cluster M5
http://www.noao.edu/outreach/press/pr03/sb0307.html
Main Sequence
Red Giant Branch
Asymptotic Giant Branch
Horizontal Branch
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
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.
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.
Stellar Wind
• As layers of the envelope blow away, they
expose hotter layers- strengthens stellar wind
• Faster winds collide with slower windsproduces 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.
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
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
Woitke, Peter, 2006
Woitke, Peter, 2006.
Astronomy and Astrophysics.
Woitke, Peter, 2006.
Astronomy and Astrophysics.
Woitke, Peter, 2006.
Astronomy and Astrophysics.
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
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.
The Eskimo Nebula,
Hubble Space
Telescope, WFPC2
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.
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_Freib
urg_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