AGB Stars as Tracers of Stellar Populations From the Milky Way to the Magellanic Clouds 

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The structure of AGB stars. The dredge-up: stellar surface enriched in nuclear burning products, especially carbon. The atmosphere: molecules formation; shocks develop; grain condensation. The circumstellar envelop: mass loss & super wind; escaping gas & dust particles. 

Introduction: Why can we use AGB stars as tracers? • The most luminous red stars in any galaxy • At their brightest in the near-infrared (extinction unimportant) • Rarity (short-lived) For convenience: study those brighter than tRGB 

The (I,V-I) diagram for stars in the LMC. tRGB: Mbol = -3.9 

• Defining feature of OH/IR stars: powerful OH maser action at 1612 MHz; ΔV/2 (Vexp) ~ gas expansion velocity of the CSE; ΔV: indicator of initial mass of the star. • Age: Mbol: upper limit of a single star’s age; Cluster: highest luminosity ~ AGB star with the age of the cluster (M-type) LPVs: longer period, younger age. ΔV: rough age indicator. P < 255d , > 5 Gyr; 255 < P < 450d , 1 ~ 3 Gyr; P > 450d , < 1 Gyr. ∆V > 38km/s, < 1Gyr ∆V < 36km/s, > 4Gyr 

Wood et al. (1999) The PL relations for optically visible red variables in a O.5xO.5 degree area of the LMC. Luminosity of tRGB and TPAGB min for a star of M ≈ 1⨀ indicated by arrows. Data from MACHO. 

The MWG • Solar neighborhood Z: distance from the Galactic plane for Miras. Conclusion: Shorter period, greater heights; so Shorter period, lower main-sequence mass. (because they’re older?) The kinematics of LPVs depend on the period. Conclusion: Miras with period longer than 200d are disk stars. Shorter period, lower main-sequence mass. (quick rotation — lower mass?) 

• The Galactic Nucleus: the center & its surroundings (l, V) diagram of OH/IR stars near the Galactic center. Conclusion: Vrad = A*l, A = 30km/(s*pc) ➡ Uniform mass density ~ 1.3*10^5⨀ /(pc^3) ∆V < 36km/s, older ∆V > 38km/s, younger Distribution of OH/IR stars within a few hundred pc from the center Same A = 2.5km/(s*pc) ➡ Uniform mass density ~ 1.1*10^4⨀/(pc^3) Younger group suggests different origin. 

Period distribution of OH/IR stars (shaded) Vexp distribution of different samples. P < 255d , > 5 Gyr; 255 < P < 450d , 1 ~ 3 Gyr; P > 450d , < 1 Gyr. ∆V > 38km/s, < 1Gyr ∆V < 36km/s, > 4Gyr 

The Inner MWG: -30˚ < l < 30˚; the Bar and the Bulge (l, V) diagram of interstellar CO and OH/IR stars Near-IR maps of the MWG made by the COBE satellite. Evidence of the Bar from AGB stars. 

The distribution of IRAS Miras in distance modulus. Open circles: negative longitude. Closed triangles: positive longtitude. 

Ishihara, 2011 The Galactic Disk 

Marigo, 2008 Evolutionary tracks of AGB stars Orange: M-type stars; Green: Carbon stars 

• PL relation (Miras in metal-rich clusters obey sequence C) All brighter than tRGB are Miras Matsunaga et al. (2006) NIR survey for red variables in GCs Filled circles: variables in metal-rich clusters ([Fe=H] > 1) Star symbols: those in metal-poor ones Gray points: data of the LMC Globular clusters 

Yoshifusa Ita et al. (2003) Upper: LMC; Bottom: SMC Data from OGLE-II & SIRIUS θ<0.55: ‘regularly pulsating variables’; θ>0.55: ‘less regularly pulsating variables’. J-K > 1.4: mostly C-rich AGB stars; J-K < 1.4: mostly O-rich AGB stars. The Magellanic Clouds