Low-velocity streams inside the planetary nebula H2-18

Abstract

Aims. Numerous planetary nebulae show complicated inner structures that are not obvious to explain. For one such object, we undertook a detailed 3D photoionisation and kinematical model analysis to gain a better understanding of the underlying shaping processes. Methods. We obtained 2D ARGUS/IFU spectroscopy covering the whole nebula in selected representative emission lines. We used 3D photoionisation modelling to compute images and line profiles, and a comparison of the observations with the models was performed to fine-tune the model details. This procedure predicts the approximate nebular 3D structure and kinematics. Results. We find that within a cylindrical outer nebula, there is a hidden, very dense bar-like or cylindrical inner structure. Both structures are co-axial and are inclined to the sky by 40 deg. We propose that the wide, asymmetric, one-sided plume is a flat structure attached to one end of the bar. All nebular components share the same kinematics, with an isotropic velocity field that monotonically increases with distance from the star before reaching a plateau. The relatively low velocities indicate that the observed shapes do not require particularly energetic processes, and there is no indication for the current presence of a jet. The 3D model reproduces the observed line ratios and the detailed structure of the object significantly better than previous models.