1 introduction: agb - asymptotic giant branch ryszard szczerba centrum astronomiczne im. m....

1

Introduction: AGB - Introduction: AGB - Asymptotic Giant BranchAsymptotic Giant Branch

Ryszard Szczerba

Centrum Astronomiczne im. M. Kopernika, Toruń

[email protected]

2

„„Asymptotic Giant Branch”Asymptotic Giant Branch”

Harm Habing, Hans Olofsson (Eds.)

A&A Library, 2004 Springer-Verlag

3

Introduction

Tubingen, March 2009 4

NCACTORUN

POLAND


NCACTORUN

POLAND


NCACTORUN

TORUŃ

9

NCACTORUN

N. COPERNICUS (1473-1543)

10

NCACTORUN

TORUŃ

11

NCACTORUN

N. Copernicus Astronomical Center,Toruń


NCACTORUN

CAMK – opening ceremony: 24.05.1978 r.

13

What I will talk about?• Stellar evolution of low- and intermediate-mass stars;

• mass loss and the end of evolution during AGB;

• molecules and dust formation (circumstellar envelopes);

• dynamics and instabilities in dusty winds;

• post-AGB stellar evolution;

• observations and theory related to this phase of stellar evolution (results from IRAS, ISO, HST, SST, HSO)

• about future of the Sun.

14

AGB Stars: History•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)

15

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.

•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.

16

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.

17

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.

18

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”

19

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.

20

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.

21

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.

22

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.

23

Transmission on Mauna Kea: 4.2 km.

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

Water vapour: 1.6 mm

24


L:5, M:4.7 m

Water vapour: 1.6 mm

25


N: m

26


Q: m

27

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.....

28

Discovery of IR radiation.

29

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 (2009-2013), ....

30

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.

•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!).

31

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).

32

AGB Stars: Overview

33

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.

34

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.

35

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).

36

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

37

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.

38

[m] [K]=3000

Thermal radiation

39

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]

40

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.

41

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

42

H mass profile during evolution of the MS

43

H and He mass profiles

44

H mass profile during shell H-burning

45

T and density during shell H-burning

46

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).

47

Convection and the 1st dredge-up

48

Convection and the 1st dredge-up

49

Core He-burning

50

Stellar evolution

•Schematic evolution of a star of 5 Mo mass:

51

AGB stars: structure

•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.

52

AGB Stars: structure

•Comparison between structure of 1 and 5 Mo stars.

53

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).

54

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.

55

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.

56

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.!!

57

AGB Stars: characteristic „atmospheric” phenomena?

•low gravity – assumption of spherical symmetry less realistic.•instability against convection in deeper layers – due to H disociation and absorption (giant granular cells).•instability against pulsations – generation of shock fronts (complex dynamics) •Molecule and dust formation - complex radiation transfer & wind generation.

•Interaction between: 1.) convection, 2.) pulsations, 3.) radiation, 4.) molecular and dust formation and absorption, 5.) acceleration of the stellar wind.

1 introduction: agb - asymptotic giant branch ryszard szczerba centrum astronomiczne im. m....

Documents