  It is known that the strength of neutral oxygen triplet lines at 7771-5 A
shows a luminosity effect in evolved A through G stars. However, its general
behavior across the HR diagram is not yet well understood, since the
applicability limit of the relations proposed by various previous work (tending
to be biased toward supergiants) still remains unclear. Besides, our
understanding on the nature of atmospheric micro-scale turbulence, which is
considered to play a significant role (along with the non-LTE line
intensification) for the cause of this effect, is still insufficient. Towards
clarifying these problems, we carried out an extensive non-LTE spectrum-fitting
analysis of O I 7771--5 lines for unbiased sample of 75 evolved A-, F,- and
G-type stars over wide luminosity classes (from subgiants through supergiants)
including rapid rotators, from which the total equivalent width (W_77) was
derived and the microturbulence (xi) was determined by two different (profile-
and abundance-based) methods for each star. While we confirmed that W_77 tends
to increase in the global sense as a star's absolute magnitude (M_V) becomes
more luminous, distinctly different trends were found between lower-gravity
(log g <~ 2.5) and higher-gravity (log g >~ 2.5) stars, in the sense that the
M_V vs. W_77 formulas proposed by past studies are applicable only to the
former supergiant group. In case of using W_77 for empirical M_V evaluation by
such simple formulas, it is recommended to confine only to supergiants of -5 >
M_V > -10. Regarding the microturbulence significantly controlling W_77, it
roughly shows an increasing tendency with a decrease in surface gravity.
However, the trend is not monotonic but rather intricate (e.g., hump,
stagnation, or discontinuously large increase) depending on the stellar type
and evolutionary stage.
