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AGB - Asymptotic Giant Branch

AGB - Asymptotic 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/. „Asymptotic Giant Branch”. Harm Habing, Hans Olofsson (Eds.) A&A Library, 2004 Springer-Verlag.

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AGB - Asymptotic Giant Branch

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  1. AGB - Asymptotic Giant Branch RyszardSzczerba Centrum Astronomiczne im. M. Kopernika, Toruń szczerba@ncac.torun.pl (56) 62 19 249 ext. 27 http://www.ncac.torun.pl/~szczerba/

  2. „Asymptotic Giant Branch” Harm Habing, Hans Olofsson (Eds.) A&A Library, 2004 Springer-Verlag

  3. 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. What I will talk about?

  4. 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 giantwas 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) AGB Stars: History

  5. 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. AGB Stars: History

  6. 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. AGB Stars: History

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

  8. AGB Stars: History • CNO cycle (99.96 % up; 0.04% right).

  9. Stellar 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” AGB Stars: History

  10. 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. AGB Stars: History

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

  12. Stellar 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 b-decay (s-process). This process is important during TP-AGB. AGB Stars: History

  13. Merrill (1952) discovered lines of 99Tc, t~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-a process (formation of Father Secchi’s star has been explained). AGB Stars: History There are no stable isotopes with Atomic Mass 5 or 8 (i.e such that reactions like: 4He + 1H --> 5X or4He + 4He --> 8X may occur). The next stage is the triple-aprocess: 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-aprocess.

  14. New 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 mm. AGB Stars: History

  15. Transmission on Mauna Kea: 4.2 km. J:1.25, H:1.65, K:2.2 mm Water vapour: 1.6 mm

  16. Transmission on Mauna Kea: 4.2 km. L:3.5, M:4.7 mm Water vapour: 1.6 mm

  17. Transmission on Mauna Kea: 4.2 km. N:10.5mm

  18. Transmission on Mauna Kea: 4.2 km. Q:19.5mm

  19. 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..... Sir Frederick William Herschel

  20. Discovery of IR radiation.

  21. New results from new observing techniques – IR astronomy: • Neugebauer & Leighton (1969) – 2.2 mm survey (IRC). About 5000 sources were detected north of d=-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 mm, e.g. AFGL 2688 (Egg Nebula); AFGL 915 (Red Rectangle). • IRAS (1983) – photometry @ 12, 25, 60 and 100 mm (~250000) + LRS spectra (~10000) for the brightest sources. • ASTRO-F 2006!!! • ISO (1995-1998), SST (2003-...), HSO (2008-...), .... AGB Stars: History

  22. New 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. AGB Stars: History

  23. Mass loss on AGB: • Deutsch (1956) noticed that circumsttellar absorption lines in the MII component of the binary system a 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 mm at spectra of M-type giants as due to silicate dust. • Hachwell (1972) discovere 11.5 mm 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!). AGB Stars: History

  24. 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). AGB Stars: History

  25. AGB Stars: Overview

  26. 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). AGB Stars: observational characteristics

  27. 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, t~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. AGB Stars: observational characteristics

  28. 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: how to recognize them? 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.

  29. 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). AGB Stars: how to recognize them?

  30. 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 (DV > 2.5); • Semiregular variables of type a „SRa”: relatively regular with a smaller amplitude in the V-band (DV < 2.5); • Semiregular variables of type b „SRb”: poor regularity with a small amplitude in the V-band (DV < 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 AGB Stars: variability

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

  32. 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 l’s is a result of: • Strong variations of the TiO bands during pulsation cycle • A large change of flux in short l’s with Teff. AGB Stars: variability

  33. Thermal radiation l[mm] * T[K]=3000

  34. Useful relations 1 [Jy] = 10-23 [erg/cm2/s/Hz] Bn dn = Bl dl ; n l = c 1 [W] = 107 [erg/s]

  35. 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 (DV > 2.5); • Semiregular variables of type a „SRa”: relatively regular with a smaller amplitude in the V-band (DV < 2.5); • Semiregular variables of type b „SRb”: poor regularity with a small amplitude in the V-band (DV < 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) AGB Stars: variability

  36. MACHO red light curves (~0.7 mm) for some dust-enshrouded AGB stars found in the LMC by the MSX satellite. AGB Stars: variability

  37. 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. AGB Stars: masses

  38. 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” AGB Stars: HR diagram

  39. 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. AGB Stars: Period-Luminosity relations

  40. 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. AGB Stars: Period-Luminosity relations

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

  42. 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 AGB Stars: Period-Luminosity relations

  43. 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 AGB Stars: Period-Luminosity relations

  44. MACHO red light curves (~0.7 mm) for some dust-enshrouded AGB stars found in the LMC by the MSX satellite. AGB Stars: variability

  45. Schematic evolution of a star of 1 Mo mass: • 1-4 core H-burning • 5-8 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 Flash • 10-14 Core Helium burning • After 14 E-AGB Stellar evolution

  46. H mass profile during evolution of the MS

  47. H and He mass profiles

  48. H mass profile during shell H-burning

  49. T and density during shell H-burning

  50. CNO cycle: CN; ON • CNO cycle (99.96 % up; 0.04% right).

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