We present results of long-slit and panoramic spectroscopy of extended gaseous disks in 18 nearby S0 galaxies, mostly in groups. The gas in our S0s is found to be often accreted from outside that is implied by its decoupled kinematics: at least 5 galaxies demonstrate strongly inclined large-scale ionized-gas disks smoothly coupled with their outer HI disks, 7 galaxies reveal circumnuclear polar ionized-gas disks, and in NGC 2551 the ionized gas though confined to the main galactic plane however counterrotates the stellar component. The ionized-gas excitation analysis reveals the gas ionization by young stars in 12 of 18 S0 galaxies studied here; the current star formation in these galaxies is confined to the ring-like zones coinciding with the UV-rings. The gas oxygen abundance estimates in the rings are closely concentrated around the value of 0.7 $Z_odot$ and do not correlate either with the ring radius nor with the metallicity of the underlying stellar population. By applying the tilted-ring analysis to the 2D velocity fields of the ionized gas, we have traced the orientation of the gas rotation-plane lines of nodes along the radius. We have found that current star formation proceeds usually just where the gas lies strictly in the stellar disk planes and rotates there circularly; the sense of the gas rotation does not matter (the counterrotating gas in NGC 2551 form stars currently). In the galaxies without signs of current star formation the extended gaseous disks are either in steady-state quasi-polar orientation (NGC 2655, NGC 2787, NGC 3414, UGC 9519), or are acquired recently through the highly inclined external filaments provoking probably shock-like excitation (NGC 4026, NGC 7280). Our data imply crucial difference of the external-gas accretion regime in S0s with respect to spiral galaxies: the geometry of the gas accretion in S0s is typically off-plane.
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