The shaping of planetary nebula Sh 2-188 through interaction with the interstellar medium

Abstract

Sh 2-188 is an example of strong interaction between a planetary nebula (PN) and the interstellar medium (ISM). Its structure is postulated to be the result of motion through the ISM. We present new H$α$ images from the Isaac Newton Telescope Photometric H$α$ Survey of the Northern Galactic Plane which reveal new structure. The nebula extends 15 arcmin on the sky in total. We have developed a `triple-wind' hydrodynamical model, comprising of the initial `slow' asymptotic giant branch (AGB) wind and the later `fast' stellar wind plus a third wind reflecting the motion through the ISM. Simulations at various velocities of the central star relative to the ISM indicate that a high velocity of 125 kms is required to reproduce the observed structure. We find most of the structure already forms during the AGB phase. The closure of the ring arises from the slow--fast wind interaction. Most of the mass lost on the AGB has been swept downstream, providing a potential explanation of the missing mass problem in PNe. We report a proper motion for the central star of 30 mas yr^-1 in the direction of the bright limb, implying a distance to the nebula of 850^{+500}_{-420} pc, consistent with a spectroscopic distances. Expansion velocities measured from spectroscopic data are consistent with velocities measured from the simulation. The model shows that the size of the PN was already set during the AGB phase.