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Barium in Galactic disc: nlte results S. M. Andrievsky, S. A. Korotin
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tarix | 24.02.2018 | ölçüsü | 445 b. | | #28025 |
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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 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 (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 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 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
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). characteristic time of mixing, any gradient will be 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!
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