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AGB - AGB - Asymptotic Giant BranchAsymptotic Giant Branch

Ryszard Szczerba

Centrum Astronomiczne im. M. Kopernika, Toruń

szczerba@ncac.torun.pl

(56) 62 19 249 ext. 27

http://www.ncac.torun.pl/~szczerba/

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„„Asymptotic Giant Branch”Asymptotic Giant Branch”

Harm Habing, Hans Olofsson (Eds.)

A&A Library, 2004 Springer-Verlag

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What I will talk about?• Stellar evolution of low- and intermediate-mass stars;• phase when stars reach their largest luminosities and radii;• about nucleosynthesis during AGB;• mass loss and the end of evolution during AGB;• molecules and dust formation;• dynamics and instabilities in dusty winds;• circumstellar envelopes;• post-AGB stellar evolution;• AGB evolution in binary systems;• AGB stars in other galaxies;

• observations and theory related to this phase of stellar evolution;• about results from IRAS, ISO, MSX, HST, SST, ...

• about future of the Sun.

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AGB Stars: History• What is important ... before 1985: Iben & Renzini (1983); IRAS results.

• At the beginning of the XX centuary dwarfs and giants were discovered in the Henry Draper (HD) catlogue. (Hertzsprung 1911, Russell 1914=> H-R diagram). • The reason what causes a star to be either a dwarf or a giant was unknown until 1960’s. • Many AGB stars are Long Period Variables (LPVs):

M-stars:1596 Fabricius discoverd that one 3rd magnitude star disappeared! (note that Tycho Brahe – discovered „Tycho’s supernova” in 1572);Fabricius -> Brahe -> published by Kepler.In 1638 the star re-appeared (seen by Dutch astronomer Holwarda). He established period of this phenomena for about 1 year! (Stella Mira „the wonderful star” Hewelius)

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AGB Stars: History• Classification based on the appearance of the light curves: Miras, Semiregulars, Irregulars does not allow to understand physical reasons of the variablility.• Feast (1963) showed that Vr’s of Miras are different for stars of different periods => he showed that statistically: Miras with shorter P are older and less massive than the Miras with longer periods.

• Glass & Lloyd Evans (1981) discovered a linear relation between K-mag and log(P) in Mira variables.

C-stars: Kirchoff and Bunsen (1860) had published correct interpretation of spectral lines; 1868 - Father Secchi (Vatican Observatory) clasified spectra of ~4000 stars. He recognized a small group of very red stars with spectra „similar” to that of the ligth in carbon arcs.

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AGB Stars: History• Why there are 2 very different classe of red stars (C- and M-type: O-rich)? The question answered in 1934.• Russell (1934) showed that high binding energy of CO molecule (11.09 eV) leads to: M-type spectra when O > C C-type spectra when C > O

Stellar models (Main Sequence):• Eddington (1926) „The internal constitution of the Stars” – he stated that H->He is (probably) the source of the stellar energy! but he didn’t know how the mechanism works.• It was assumed that the atomic composition of the Sun was the same as that of the Earth- ~TRUE! if one ignore H and He. • Payne-Gaposchkin (1925) had found the large relative abundances of H and He, but she rejected this result!• Russel (1929) draw the correct conclussion about chemical composition of the Sun.

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AGB Stars: History• Bethe (1939) shows that pp – reactions works in ~1 Mo stars (T< 15 milion K), while in more massive stars the CNO-cycle dominates.

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AGB Stars: History

12C + 1H → 13N + γ +1,95 MeV13N → 13C + e+ + νe +1,37 MeV13C + 1H → 14N + γ +7,54 MeV14N + 1H → 15O + γ +7,35 MeV15O → 15N + e+ + νe +1,86 MeV15N + 1H → 12C + 4He +4,96 MeV

15N + 1H → 16O + γ

16O + 1H → 17F + γ

17F → 17O + e+ + νe

17O + 1H → 14N + 4He

• CNO cycle (99.96 % up; 0.04% right).

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AGB Stars: HistoryStellar models (Red Giants):• Progress possible because: development of observational techniques (photometry) and development of „electronic devices” – analytical solutions => the numerical ones. color-mag diagrams in globular clusters: Arp et al. (1953) „bifurcation of the red giant branch”

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AGB Stars: History•Sandage and Walker (1966) – were the first authors to use term AGB.• The term AGB originted as a description of the sequence of stars in the HR diagrams, the term AGB is now used to describe all stars with M < 8Mo that are on the second ascent (asymptotic) into the RG region of the HR-diagram.

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AGB Stars: History• Hoyle & Schwarzschild (1955) showed that evolution of stars through the RGB to max L and then down to HB can be understood.

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AGB Stars: HistoryStellar models (Thermal Pulses):• After a HB star has burned He in its core, He burns in a shell around the C/O core. When He-shell approaches the H-envelope => an instability „He-shell flash” (thermal pulse) develops (Schwarzschild and Harm 1965). • Schwarzschild and Harm (1967) showed that convective zone xtending from He-burning shell makes contact with the convective H-envelope an mixing results in: nucleosynthesis; mixing of new elements to the surface.

• Sanders (1967) argued that He-burning shell provides condition for the s-process.

Nucleosynthesis:•Burbidge et al. (1957) described nucleosynthesis due to absorption of free neutrons followed by -decay (s-process). This process is important during TP-AGB.

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AGB Stars: History• Merrill (1952) discovered lines of 99Tc, ~2 105 years!! (s-process element). The short half-life time showed that Tc has been recently dredge-up to the surface.• Iben (1975) showed models which produce C in He-burning shell by the triple- process (formation of Father Secchi’s star has been explained).There are no stable isotopes with Atomic Mass 5 or 8 (i.e such that reactions like:4He + 1H --> 5X or

4He + 4He --> 8X may occur). The next stage is the triple-process: 4He + 4He + 4He --> 12C This reaction requires both very high T (> 100 milion K) and very high densities which will occur only after the star has exhausted its H and has a core of nearly pure He. Only stars with masses > 0.4 Mo will can ignite 3-process.

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AGB Stars: HistoryNew results from new observing techniques - IR astronomy:•Infrared astronomy started in 1960’s due to strong intrest from military.• ~1970 observations were made in all telluric windows from 1-20 m.

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Transmission on Mauna Kea: 4.2 km.

J:.25, H:1.65, K:2.2 m

Water vapour: 1.6 mm

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Transmission on Mauna Kea: 4.2 km.

L:5, M:4.7 m

Water vapour: 1.6 mm

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Transmission on Mauna Kea: 4.2 km.

N: m

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Transmission on Mauna Kea: 4.2 km.

Q: m

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Sir Frederick William Herschel

• F.W. Herschel (1738 -1822) was born in Hanover.

• From 1757 he lived in England.

• A musician and an astronomer.

• In 1781 he discovered Uranus;

• He created catalogs of double stars and nebulae;

• In 1800 he discovered infrared radiation.....

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Discovery of IR radiation.

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AGB Stars: HistoryNew results from new observing techniques – IR astronomy:• Neugebauer & Leighton (1969) – 2.2 m survey (IRC). About 5000 sources were detected north of =-33o , e.g. IRC+10 216 (the nearest C-star), sources associated with Sgr A. Most of the sources were red giants.•Price & Walker (1976) – RAFGL – Revised Air Force ... ~2400 sources with photometry at 4 bands between 4 and 28 m, e.g. AFGL 2688 (Egg Nebula); AFGL 915 (Red Rectangle).•IRAS (1983) – photometry @ 12, 25, 60 and 100 m (~250000) + LRS spectra (~10000) for the brightest sources.• ASTRO-F 2006!!!•ISO (1995-1998), SST (2003-...), HSO (2008-...), ....

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AGB Stars: HistoryNew results from new observing techniques – radio astronomy:• Wilson & Barrett (1968) – OH maser line at 1612 MHz (18 cm) detected toward supergiant NML Cyg. • OH 1612 MHz line shows variations with period 300-1000 dyas (period like in LPV’s).• H2O and SiO masers were also detected from AGB stars (mostly! of spectral type M).• observations at milimeter wavelengths allowed to detetect thermal emission from many molecules. The first being CO (J=1->0) (Solomon et al. 1971) from IRC+10 216.

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AGB Stars: HistoryMass loss on AGB:•Deutsch (1956) noticed that circumsttellar absorption lines in the MII component of the binary system Her were seen also in the spectrum of the companion GI star => Renv ~ 2 105 Ro. With Vexp~10 km/s he estimated Mloss ~3 10-8 Mo/yr.•Auer and Wolf (1965) noted that Hyades cluster contains ~ 10 white dwarfs (WD) with M < 1.4 Mo. However, Hyades are young cluster and stars with mass ~ 2 Mo are still on MS!• Reimers (1975) collected data for many such systems and concluded that Mloss ~ L R / M (Reimer’s formula).• Gillet et al. (1968) identified emission band ~ 10 m at spectra of M-type giants as due to silicate dust.•Hachwell (1972) discovere 11.5 m band in C-stars (SiC) • Gilman (1969) explained the observed dust dichotomy as due to the high binding energy of CO molecule (like Russell 1934 for stellar spectra!).

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AGB Stars: History•At the begining of 1970’s it was clear that AGB stars produce dust (a question of dust origin was open from 1930’s when interstellar extinction was discovered).• Goldreich and Scoville (1976) developed a model of mass loss due to radiation pressure on dust and momentum exchange between dust and gas.

•In all calculations of stellar evolution before ~1980 the assumption was made that M* did not change!•Schoenberner (1979, 1980) was first who employed the Reimer’s formula for the stellar evolutionary calculations.• However, it was aalready then clear that Reimer’s formula predict too small mass loss rates for the AGB phase of stellar evolution (observations suggested Mloss up to ~10-4 Mo/yr).• The life of AGB stars is cut off by mass loss!!! (Iben & Renzini 1983).

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AGB Stars: Overview

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AGB Stars: observational characteristics•The most important spectral classes of AGB stars are M, S and C. MS –top: dominated by TiO (VO – in very cold stars); C- bottom: C2 and CN molecules dominate. S-stars have ZrO; Zr is s-process element.C-stars (N - on AGB) & (R - not on AGB).

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AGB Stars: observational characteristics•M-type stars: O > C; TiO, VO (very cold stars) MS-type

•S-type stars O ~ C; ZrO (Zr – is s-process element SC-type

•C-type stars O< C; C2, Cn....

•A particularly interesting s-process element found in the atmospheres of some AGB stars is 99Tc, ~2 105 years. Its presence means tht it has been brought to the stellar surface in the last few times 105 years. •This is direct observational evidence for the production of new elements inside stars.

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AGB Stars: how to recognize them?•The RGB stars has a maximum luminosity that is well explained theoretically. In addition the luminosity is well determined observationally for LMC Mbol=-3.9 (or L = 2900 Lo).•Stars more luminous than the tip of RGB are AGB or supergiants!

AGB stars in MC clusters. -3.6 < Mbol < -7.1. C-stars are shown as filled circles. The top axis shows the mass at the beginning of the AGB.

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AGB Stars: how to recognize them?•Other properties: TP – thermal pulse; presence of s-process elements (the efect of dredge-up after TP): Zr, V, ... and especially 99

Tc;S- and C-stars are AGB, but ... a care should be taken of a (possible) binarity;Mass loss > 10-7 Mo/yr is typical for AGB (supergiants, LBVs have also large mass loss rates – but they are rare);Long-period pulsations (AGB stars are Long Period Variables – LPVs).

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AGB Stars: variability•Classification of the light curves of LPVs (as defined in the General Catalogue of Variable Stars: GCVS): Mira-like „M”: regular variations with a large amplitude in the V-band (V > 2.5);Semiregular variables of type a „SRa”: relatively regular with a smaller amplitude in the V-band (V < 2.5);Semiregular variables of type b „SRb”: poor regularity with a small amplitude in the V-band (V < 2.5); Irregular „L”: irregular variations of low amplitude in the V-band.

•The high quality data are available now from microlensing surveys: MACHO (Alcock et al. 1995); EROS (Aubourg et al. 1993); OGLE (Udalski et al. 1993). See also: http://www.aavso.org/adata/curvegenerator.shtml or http://www.vsnet.kusastro.kyoto-u.ac.jp/vsnet/gcvs

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AGB Stars: variability

MACHO results: Miras (top four panels), semiregular variables (bottom six panels). All variables are from the sequence C.

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AGB Stars: variability

•The V-variations of Miras can reach 6 mag, but bolometric variability is smaller (most of the energy is emitted in the IR).

•The large amplitude in the shorter ’s is a result of:

Strong variations of the TiO bands during pulsation cycle

A large change of flux in short ’s with Teff.

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[m] [K]=3000

Thermal radiation

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Useful relations

ster Hz s cm

erg

1 exp

1222

3

kT

hvc

hvB v

ster cm s cm

erg

1

exp

1225

2

kT

hc

hcB

Bd= Bdc

1 [Jy] = 10-23 [erg/cm2/s/Hz]

1 [W] = 107 [erg/s]

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AGB Stars: variability•Classification of the light curves of LPVs (as defined in the General Catalogue of Variable Stars: GCVS): Mira-like „M”: regular variations with a large amplitude in the V-band (V > 2.5);Semiregular variables of type a „SRa”: relatively regular with a smaller amplitude in the V-band (V < 2.5);Semiregular variables of type b „SRb”: poor regularity with a small amplitude in the V-band (V < 2.5); Irregular „L”: irregular variations of low amplitude in the V-band.

•An important class of AGB variables not found in GCVS consists of the dust-enshrouded IR variables:OH/IR stars (P up to 2000 dyas Herman & Habing 1985)C-rich analogues of OH/IR stars.

•Examples of K-light curves: Whitelock et al. (1994), Wood et al. (1998) (see also Le Bertre 1993)

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AGB Stars: variability

MACHO red light curves (~0.7 m) for some dust-enshrouded AGB stars found in the LMC by the MSX satellite.

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AGB Stars: masses•Variable AGB stars occour over the whole mass range occupied (theoretically) by AGB stars:From about 0.85 Mo (the turn-off mass in some globular clusters – e.g. Menzies et al. 1985);Up to 6-8 Mo (identified in LMC – Wood et al. 1983)

•In the solar vicinity statistical studies have been performed (e.g. Feast & Whitelock 2000; Jura et al. 1993; Kerschbaum and Hron 1992). The results:Mira variables with P > (<) 300 dyas have M (<) > 1.1 Mo;The local semi-variables have masses similar to those of Mira variables with P > 300 dyas.

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AGB Stars: HR diagram•MACHO data for 0.5x0.5 degree region of the LMC bar (Wood et al. 1999).• Stars above dashed line have been searched for variability.•>90% of stars on TP-AGB are variables.•Stars below min of TP-AGB are probably binaries or „rotators with spots”

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AGB Stars: Period-Luminosity relations•Glass & Lloyd Evans (1981) discovered a linear relation between K-magnitude and log(P) in Mira variables. • Hipparcos distances have been used to look for P-L relations (e.g. Bedding and Zijlstra 1998; Whitelock & Feast 2000) •However, the most exciting results have been obtained from studies of AGB stars in the LMC, where the distance is known and the reddening is small.

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AGB Stars: Period-Luminosity relations•P-L relation for optically visible semiregular and Mira variables in the LMC.• 4 sequences are seen above Ko~12.9 (min L for TP-AGB of ~1Mo). • Miras: upper C; semiregulars: A, B and lower C. • The sequences A,B and C represent pulsations in different modes.

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AGB Stars: Period-Luminosity relations•Examples of light curves of stars on sequence A (left) and B (right column).

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AGB Stars: Period-Luminosity relations•Examples of light curves of stars on sequence D (left) and E (right column). •The origin of periods on the sequence D is unknown!•The sequence E stars: binaries or rotators

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AGB Stars: Period-Luminosity relations•P-L relation for optically visible and „dusty” (3x3.5o) variables from MSX.• Most of „dusty” variables evolved from the end of the Mira sequence (drop in K when optical depth becoming large)• Some MSX-selected sources lie above Mira sequence: they are more massive AGB stars

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AGB Stars: variability

MACHO red light curves (~0.7 m) for some dust-enshrouded AGB stars found in the LMC by the MSX satellite.

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Stellar evolution

•Schematic evolution of a star of 1 Mo mass:1-4 core H-burning5-8 shell H-burning (He core becomes electron degenerate)8 convection => the 1st dredge-up: 4He, 14N, 13C (CN + ON cycling) are mixed to the surface 9 Core He Flash10-14 Core Helium burningAfter 14 E-AGB

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H mass profile during evolution of the MS

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H and He mass profiles

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H mass profile during shell H-burning

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T and density during shell H-burning

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CNO cycle: CN; ON

12C + 1H → 13N + γ +1,95 MeV13N → 13C + e+ + νe +1,37 MeV13C + 1H → 14N + γ +7,54 MeV14N + 1H → 15O + γ +7,35 MeV15O → 15N + e+ + νe +1,86 MeV15N + 1H → 12C + 4He +4,96 MeV

15N + 1H → 16O + γ

16O + 1H → 17F + γ

17F → 17O + e+ + νe

17O + 1H → 14N + 4He

• CNO cycle (99.96 % up; 0.04% right).

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Convection and the 1st dredge-up

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Convection and the 1st dredge-up

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Core He-burning

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Stellar evolution

•Schematic evolution of a star of 5 Mo mass:

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E-AGB

•A schematic view of a 1Mo star. The structure is similar regardless of the stellar mass: CO degenerate core + He- and H-burning shells. Pulsations take place in the convective env.

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AGB: the unstable He-burning shell

•Physical reasons for the thermally unstable He-shell burning were recognized by Schwarzschild and Harm (1965). (The high temperature sensitivity of the 3- reaction and the thinness of the shell).

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AGB Stars: TP-phase•Computations of TP-AGB phase is difficult and time consuming (synthetic AGB calculations are involved). •The most extensive sets of full AGB calculations are those of Bloecker (1995) and Vassiliadis & Wood (1993).•L=59250(Mc-0.522) and tIP=3.05-4.5(Mc-1.0) the Paczynski’s relations. They are results of the presence of a radiative layer betwen H-burning shell and the convective envelope. • A typical liftime on the AGB is 106 years.

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Post-AGB Stars•When „superwind” reduces the mass of H-rich envelope below ~10-3 Mo the star begins to shrink.• In this phase of stellar evolution both: mass loss and nuclear reaction (~10-7 Mo/yr) lead to the reduction of the H-rich envelope.

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AGB Stars: TP-phase•The energy production by He-shell flash is very rapid (~106 Lo in this case).• When the He-shell flash energy escapes from the core, it leads to a peak in surface luminosity lasting several hundred years.!!

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AGB Stars: overview of the evolution

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AGB: the 3rd dredge-up and making C-stars • Iben (1975) and Sugimoto & Nomoto (1975) discovered how C-stars are produced during AGB evolution. • Iben identified four phases of a TP cycle: The „off” phase The „on” phase (inside intershell convective zone: 75% - 4He, 22% - 12C) The „power down” phase The „dredge-up” phase (energy released during shell flash escapes from the core => the convection extentds inward in mass).

•Dredge-up par: =Mdredge/Mc

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AGB Stars: TP-phase•Mass loss is crucial to study of AGB evolution => leads to the termination of evolution on the AGB. •Mloss is still unknon from the first principles! •Semi-empirical formulae adopt very strong dependence of Mloss on L.

•P~RM; ~1.5-2.5, ~0.5-1.0

The fundamental mode period grows rapidly during „superwind” phase.

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AGB Stars: linear pulsation models•Linear radial pulsation models for AGB stars (see e.g. Gautschy 1999 and referenes therein).•An attempt to model the dynamical and thermal coupling between pulsation and convection (Xiong et al. 1998).• The energy transport , which determines the stability and growth rates (fractional increase in amplitude per year) of pulsation, is dominated by convection!•Convection is usually treated in the simplifying approximation of mixing-length theory, so that derived growth rates need to be treated with caution!• Results of linear pulsation modelling which are discussed here are based on the code of Wood & Sebo (1996). this code computes a variation in convective flux through the pulsation cycle, but it does not include the effect of turbulent pressure.

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AGB Stars: linear pulsation models•Period for linear models in a 1 Mo AGB star as L increase up the AGB (mixing length l = 2.5 Hp pressure scale height) :the solid line – fundamental mode;the short dashed line – the first overtone;the long dashed line – the second overtone;the dotted line – the third overtone.

•P~RM; ~1.5-2.5, ~0.5-1.0 (increase with L)

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AGB Stars: linear pulsation models•solid – fundamental•short dashed – 1st overtone•long dashed – 2nd

overtone•dotted – 3rd overtone. Low L - 2nd overtone has the highest growth rateIntermediate L - 1st overtonehas the highest growthHigh L – the fundamental mode has the highest growth

The highest growth rate is a reasonable indicator that star will pulsate in this mode

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linear pulsation models vs. observations.•A good fit requires that at given L, the models have the observed T and P.•l/Hp =3.2 was assumed and M was changed to „move” model on the AGB.• Higher masses are required for pulsating AGB stars at higher L.• Having M, T and L => P was computed.•Po => sequence C; P1 –P3

sequences A & B.•Test from the period ratios.•Miras are fundamental mode pulsators!