Grid of theoretical NLTE equivalent widths of four Ba ii lines and barium abundance in cool stars

Abstract

We present a grid of computed non-local thermodynamic equilibrium (NLTE) equivalent widths (EW) and NLTE abundance corrections for four Ba II lines: 4554, 5853, 6141, and 6496 A. The grid can be useful in deriving the NLTE barium abundance in stars having parameters in the following ranges: effective temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5, microturbulent velocity 0 km s^-1 to 3 km s^-1, metallicity [Fe/H] from -2 to +0.5, and [Ba/Fe] from -0.4 to +0.6. The NLTE abundance can be either derived by EW interpolation (using the observed Ba II line EW) or by using the NLTE correction applied to a previously determined LTE abundance. Ba II line equivalent widths and the NLTE corrections were calculated using the updated MULTI code and the Ba II atomic model that was previously applied to determine the NLTE barium abundance in different types of stars. The grid is available on-line through the web, and we find that the grid Ba NLTE corrections are almost as accurate as direct NLTE profile fitting (to within 0.05-0.08 dex). For the weakest Ba II line (5853 A) the LTE abundances almost agree with the NLTE abundances, whereas the other three Ba II lines, 4554, 6141, and 6496 A, need NLTE corrections even at the highest metallicities tested here. The 4554 A line is extremely strong and should not be used for abundance analysis above [Fe/H]= -1. Furthermore, we tested the impact of different model atmospheres and spectrum synthesis codes and found average differences of 0.06 dex and 0.09 dex, respectively, for all four lines. At these metallicities we find an average Delta NLTE of +/-0.1 dex for the three useful Ba lines for subsolar cool dwarfs.