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Barium in Galactic disc: NLTE results

Barium in Galactic disc: NLTE results. S.M. Andrievsky, S.A. Korotin, (Odessa National University, Ukraine) R.E. Luck (Case Western Reserve University, USA). General problem. Up to now nobody measured barium abundance at large distances in the disc

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Barium in Galactic disc: NLTE results

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  1. Barium inGalactic disc: NLTE results S.M. Andrievsky, S.A. Korotin, (Odessa National University, Ukraine) R.E. Luck (Case Western Reserve University, USA)

  2. General problem • Up to now nobody measured barium abundance at large distances in the disc • For this one needs to use supergiant spectra • Ba II lines available for measurement are very strong in supergiant spectra • LTE methods are not applicable in this case

  3. Ba atomic model for NLTE calculations

  4. Ba atomic model for NLTE calculations (continued) • 31 levels of Ba I,101 levels of Ba II, the ground level of Ba III • 91 b-b transitions betweenthe first 28 levels of Ba II with n < 12 and l < 5 • the fine structure 5d2D and 6p2P0 • compilation of log g data • photoionization cross-sections (Thomas-Fermi method, Hofsaess, 1979), and H-like approximation • b-b experimental and theoretical collisional data • inelastic collisions of Ba II with H I: formula with S = 0.1 • four Ba II lines: 4554, 5853, 6141 and 6496 A • even-to-odd abundance ratio for 4554 A isotopic components: 82:18 • logε(Ba)O = 2.17

  5. Ba atomic model for NLTE calculations (continued)

  6. Sample of the stars • Pulsating supergiants (Cepheids) • Why Cepheids? • They are luminous stars • Many lines in their spectra (Ba lines in particular) • Almost all elements are not altered (Ba in particular) • Distances are very reliable (P-L relation) • Hobby-Eberly telescope (9.2 m mirror) • 210 stars, 301 spectra • R = 30000, S/N is abot 100

  7. Reliability of the results • For typical Cepheid parameters: Teff = 5000-6300 K, logg=1.5-2.5 • Vt:+/- 0.2-0.3 km/s: Ba abundance +/-0.12 • Teff:+/-150 K: Ba abundance +/-0.09 • logg:+/-0.2: Ba abundance +/-0.06 • Standard error of the Ba abundance determination is about 0.15 (from 3-4 lines) • The total error is about 0.22 dex

  8. Distances • Very important parameter for the reliable gradient determination • RG=[R2G,Sun+(dcosb)2-2 R2G,Suncosbcosl]1/2 • d=10-0.2(Mv-<V>-5+Av) • Av=[3.07+0.28(B-V)0+0.04E(B-V)]E)B-V) • RG,Sun = 7.9 kpc

  9. Barium abundance distribution and abundance gradient

  10. Longitudal dependence

  11. 2-D Ba abundance distrubution

  12. Simple(st) interpretation • Why Ba abundance distribution is flat? 1. Random and systematic gas velocities in the disc of about 4-17 km/s (Brand & Blits,1993) 2. Characteristic time mixing over a baseline of 10 kpc is about 1 Gyr (corresponds to the life-time of the star with 3 M0). 3. Ba nuclei are mainly produced by the low-mass AGB stars (1-2 M0). 4. Since life-time for such stars is large than characteristic time of mixing, any gradient will be erased (but not for O, Fe etc produced in the short living SNe.

  13. Why Ba abundance gradient is about zero, while that of La, Ce, Nd etc is about -0.03 (Luck & Lambert, 2011)? ?

  14. SCOPES GRANT IS ACKNOWLEDGEDThank you!

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