STATISTICAL EQUILIBRIUM OF SR-II IN THE ATMOSPHERES OF F-STARS AND G-STARS

The statistical equilibrium of Sr II in the atmospheres of stars with T-eff = 5500-6500 K, logg = 1.0-4.5, and [M/H] from 0 to -3 has been a nalyzed for the first time. For Sr II, the pumping of excited states b y chains of radiative bound-bound (b-b) transitions starting from the ground level and the subsequent photoionization from these states unde rlie the mechanism of departures from LTE. As a result, the upper exci ted levels and the Sr III continuum are overpopulated relative to the equilibrium state, the population of the ground 5s level is essentiall y constant, and the 5p level is depopulated when the b-b transitions a re strong (in stars with [M/H] = 0 and -1) and is overpopulated when t he b-b transitions from it are unable to remove the pumping produced b y the resonance 5s-5p transition (stars with [M/H] = -2 and -3). As a consequence, the subordinate lines that arise during the transitions f rom the 5p level weaken compared to the LTE case over the entire range of the parameters, while the resonance lines strengthen for [M/H] = 0 and -1 and weakened for [M/H] = -2 and -3. In stars with normal heavy -element abundances the departures from LTE for all Sr II lines except for the infrared triplet in the 4d-5p transition (lambda 10400 Angstr om) are small: the non-LTE corrections to the strontium abundance do n ot exceed 0.05 dex. In metal-deficient stars, the departures from LTE increase with decreasing [M/H], with rising T-eff, and with decreasing logg. For the resonance Lines, the correction reverses its sign and b ecomes positive for [M/H] less than or equal to -2; its value may reac h 0.4 dex for main-sequence stars and 0.8-1.2 dex for supergiants. Thi s may be the key to explaining the observed deficiency of s-process el ements relative to a-process elements in stars of the Galactic halo.