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

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!