Barium in Galactic disc: nlte results S. M. Andrievsky, S. A. Korotin

tarix24.02.2018
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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

  • 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



Ba atomic model for NLTE calculations



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 between the 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



Ba atomic model for NLTE calculations (continued)



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



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



Distances

  • Very important parameter for the reliable gradient determination

  • RG=[R2G,Sun+(dcosb)2-2 R2G,Suncosbcosl]1/2

  • d=10-0.2(Mv--5+Av)

  • Av=[3.07+0.28(B-V)0+0.04E(B-V)]E)B-V)

  • RG,Sun = 7.9 kpc



Barium abundance distribution and abundance gradient



Longitudal dependence



2-D Ba abundance distrubution



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.



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

  • ?



SCOPES GRANT IS ACKNOWLEDGED Thank you!

:

ckf
ckf -> Henryk rymuza


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