Statistical
Equilibrium of Sr II in the Atmospheres
of F and G Stars
E. V. Belyakova
and L. I. Mashonkina
Kazan State
University, ul. Lenina 18, Kazan, 420008 Tatarstan, Russia
Received September
27, 1996
Abstract—The
statistical equilibrium of Sr II in the atmospheres of stars with
Teff = 5500–6500 K, log g =
1.0–4.5, and [M/H] from 0 to –3 has been analyzed for
the first time. For Sr II, the pumping of excited states by
chains of radiative bound–bound (b–b)
transitions starting from the ground level and the subsequent
photoionization from these states underlie the mechanism of
departures from LTE. As a result, the upper excited levels and
the Sr III continuum are overpopulated relative to the
equilibrium state, the population of the ground 5s level
is essentially constant, and the 5p level is depopulated
when the b–b transitions are strong (in stars with [M/H] = 0
and –1) and is overpopulated when the b–b
transitions from it are unable to remove the pumping produced by
the resonance 5s–5p transition (stars with
[M/H] = –2 and –3). As a consequence, the subordinate
lines that arise during the transitions from 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 (
10400 Å) are small: the non-LTE corrections to
the strontium abundance do not exceed 0.05 dex. In
metal-deficient stars, the departures from LTE increase with
decreasing [M/H], with rising Teff, and with
decreasing log g. For the resonance lines, the correction
reverses its sign and becomes positive for [M/H]
–2; its value may reach 0.4 dex for
main-sequence stars and 0.8–1.2 dex for supergiants. This
may be the key to explaining the observed deficiency of s-process
elements relative to 
-process elements in stars of the Galactic halo.
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