Transcript Slide 1
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