The NLTE formation of neutral-boron lines in cool stars

We study the formation of B i lines in a grid of cool stellar model atmospheres without the assumption of local thermodynamic equilibrium (lte). The non-lte modelling includes the effect of other lines blending with the B i resonance lines. Except for the cases where the B i lines are very strong, the departures from lte relevant for the resonance lines can be described as an overionisation effect and an optical-pumping effect. This causes the lines to be weaker than in lte so that an abundance analysis assuming lte will underestimate stellar boron abundances. We present non-lte abundance corrections useful to improve on abundances derived from the B i 250nm and 209 nm lines under the lte assumption. Application of the results on literature data indicates that the B/Fe ratio in metal-poor stars is constant.