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AGB - AGB - Asymptotic Giant Branch Asymptotic Giant Branch

wykład VII: wykład VII: AGB stars as tracers of AGB stars as tracers of stellar populationsstellar populations

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” Part 8: Part 8:

Harm Habing & Patricia WhitelockHarm Habing & Patricia Whitelock

Harm Habing, Hans Olofsson (Eds.)

A&A Library, 2004 Springer-Verlag

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AGB Stars

• AGB stars may be used to trace the history of galaxy:

The brightest ( ~30 000 Lo) are ~100 Myr old The weakest ( ~3000 Lo) are ~ 10 Gyr old.

Paczyński’s relation: L= 59250 (Mc-0.522) Lo

• The advantages of AGB stars: They are (except of supergiants) the most luminous stars Maximum of their SED ~ NIR (low extinction).

• However, AGB stars are short-living (rare!). For AGB stars more luminous than the tip of RGB, we have:

1 Mo =>1 AGB is predicted for 12 000 MS stars; 5 Mo => 240 MS stars.

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Overlap between E-AGB and RGB

•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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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: overview of the evolution

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Stellar populations

• The word „population” was introduced in 1944 by Baade:

He recognized that stars in the inner part of M31 are fainter that similar stars in the outer parts of this galaxy.

• Population: a sample of stars of same a (age) and Z (metallicity), but different MMS – like in clusters.• The phase-sphace distribution function f(x,v,t) - (number of stars as a function of their location in the galaxy and of their velocity) can be decomposed into „stellar populations”• The Color – magnitude diagrams are useful tool to identify and characterize „stellar populations”

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AGB Stars in galaxies of the Local Group

• Milky Way: nucleus; buldge; thin & thick disk; halo.

• Magellanic Clouds;• M31 (Andromeda), M32, M33, NGC 205• other galaxies of the LG.

• Search for realation between different components seen in MWG => template for studying other galaxies.

• Large scale surveys: ISOGAL, MSX, DENIS, 2MASS, radio surveys at 1612 MHz

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„Satelites” of Milky Way

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Local Group of Galaxies

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AGB Stars: History

Habing et al. (1985): IRAS sources with colors of mass losing AGB stars

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AGB Stars in the Milky Way Galaxy

• 3-D distribution of AGB stars is not axially symmetric, because:

the existence of spiral arms; the existence of bar; at distance of the GC only the brightest AGB stars are seen.

•NOTE: A complete description requires not only 3-D distribution of AGB stars, but also distribution of their velocities.

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the Milky Way Galaxy

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the Milky Way Galaxy

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AGB Stars in the Milky Way Galaxy

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Solar neighborhood (d<1 kpc)

• The solar neighborhood – is a site where several different stellar populations coexist.• At distance < 1 kpc all Miras are bright in the NIR, so we can assume that ALL local Miras are known.• The total number of AGB stars with „significant” envelopes (F25 > F12) is ~ 200 000! The mass of the MWG is (1.8-3.7) x 1011

Mo.

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LMC

MWG: distance can be estimated from the P-L correlation

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Solar neighborhood

Whitelock et al. (1994)

open circles – C filled circles – O Miras

Stars with longer periods have larger MMS than stars of shorter periods.

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Galactic Center (d<8 pc)

• Galactic Center – coincides with radio so urce SgrA* (its coordinates are (l,b)=(-0.056 deg, -0.046 deg) – about 10 pc from the origin of the galactic coordinate system adopted in 1970s).• Distance to the GC is ~ 8 kpc: 1’-> 2.3 pc• In spite of high extinction (AV up to 30 , but AK only about 3!), such surveys as 2MASS and DENIS have revealed most of the AGB stars close to the GC except of those with thich CSE’s.• OH and SiO maser surveys are independent of extinction.

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Galactic Center (d<8 pc)Lindqvist et al. (1992)

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Galactic Center

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A few hundered pc from GC

Lindqvist et al. (1992): 134 OH/IR sources + 21 sources from deeper survey

Top – all sources

Middle – V/2 < 18 km/s

Bottom – V/2 > 18 km/s

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A few hundered pc from GC

Wood et al. (1998): Monitoring of 75 sources at K + OH/IR sources

Baade’s window: „Sgr I window” (l,b)=(1.37o,-2.63o); „NGC6522 window” (l,b)=(1.03o,-3.83o);

AV~2

There is no C-stars there!

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A few hundered pc from GC

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Bars in galaxies

M100: Non-barred spirals

NGC 2442: Barred spirals

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Evidence of bar in the MWG

COBE: NIR data

Dwek et al. (1995)

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Evidence of bar in the MWG

• An analysis of the distribution of AGB stars (Weinberg1992, 1993) shows an assymetry consistent with the presence of a bar:

Miras at positive longitudes are closer to the Sun than those at negative longitudes.

•In the survey for OH/IR stars at 1612 MHz more stars have been detected at positive longitudes than at the corresponding negative longitudes (Sevenster 1999). • There is a region of intense star formation between l=20o and 30o, where the bar joins a spiral arms (Sevenster 1999, Lopez-Corredoira et al. 2001). SiO maser sources have velocities differing strongly from the galactic rotation (Deguchi et al 1999). • OGLE and MACHO contributed as well ....

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The outer galactic disk (30o<l<360o)

• At longitudes 30o<l<360o the dominant stellar component is disk• There is much smaller number of AGB stars in the anticenter direction;• The probability to detect OH or SiO masers in the anticenter direction is small (lower metallicity?); •The ratio of C-stars to O-stars is larger in the anticenter direction;• From the COBE measurements the disk has an outer edge at RGC~ 12.1 kpc;

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Globular cluster & halo

• Globular cluster are among the oldest objects in the MWG;• Low metallicity clusters [Fe/H]<-1 appear to be part of the halo;• Clusters with [Fe/H]>-1 are found at low latitudes only and are (probably) related to the bulge;• There is no C-stars (intrinsic) in the globular clusters! (presently low mass stars are seen on the AGB).• There are a few M-stars with luminosities above tRGB (only for more metal rich clusters)

• In the halo there is some number of C-stars originally belonged to the surrounding dwarf Sagitarius galaxy (tidal stripping)

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Globular cluster

Frogel & Elias (1988)

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AGB stars in LMC and SMC

Cioni et al. (2000).

Based on the DENIS data

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AGB stars in LMC and SMC

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LPV in LMC from MACHO

Wood (2000)

OGLE and MACHO data are and will be very useful in search for LPVs!!

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AGB stars in LMC

Van Loon et al. (1999)

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• In LMC a total of 11 000 C-stars is expected, of which < 10% have been detected (Azzopardi 2000);• In SMC a total of 3100 C-stars is expected, of which 1700 have been identified!• Intrinsic C-stars occur more frequently in the SMC then in the LMC (effect of metallicity, but also stellar mass).• The total mass of the LMC estimated from the radial velocities measurements for C-stars (so the LMC rotation curve has been determined) is 5.3+-1 x 109 Mo.

C-stars in LMC and SMC

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AGB stars in M31 (Andromeda Galaxy)

M32 (left) and M100 (bottom right)

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• Mass of M31 is estimated to be (2-4) x 1011 Mo;• dMWG~760 kpc => 1”=3.7 pc;• tRGB has K=17.8• There are many red stars above the tRGB => AGB stars•.....

AGB stars in M31 (Andromeda Galaxy)