The kinematic behaviour of optical recombination lines and collisionally excited lines in Galactic planetary nebulae*†

Abstract

The kinematics of the plasma in 14 PNe is analysed by measuring the expansion velocities (V$_{exp}$) of different ions as derived from their collisionally excited lines (CELs) and optical recombination lines (ORLs). V$_{exp}$ are analysed as a function of the ionisation potential of ions that at first approximation represents the distance of the ion to the central star. In most cases the kinematics of ORLs is incompatible with the kinematics of CELs at the same ionisation potential, specially if CELs and ORLs of the same ion are considered. In general V$_{exp}$ from ORLs is lower than V$_{exp}$ from CELs indicating that, if the gas is in ionisation equilibrium, ORLs are emitted by a gas located closer to the central star. The velocity field derived from CELs shows a gradient accelerating outwards as predicted from hydrodynamic modelling of PNe ionisation structures. The velocity field derived from ORLs is different, in many cases the velocity gradient is flatter or nonexistent and high and low ionised species present nearly the same V$_{exp}$. In addition, the FWHM(ORLs) is usually smaller that the FWHM(CELs). Our interpretation is that ORLs are mainly emitted by a plasma that coexists with the plasma emitting CELs, but does not fit the ionisation structures predicted by models. Such a plasma should have been ejected in a different event that the plasma emitting CELs.