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

Abstract

Sh 2-188 is an example of strong interaction between a planetary nebula (PN) and the interstellar medium (ISM). It shows a single arc-like structure, consisting of several filaments, which is postulated to be the result of motion through the ISM. We present new Hα images from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic Plane which reveal structure behind the filamentary limb. A faint, thin arc is seen opposite the bright limb, in combination forming a closed ring. Behind the faint arc a long wide tail is detected, doubling the size of the nebula. 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 (the interacting stellar winds model), 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 km s-1 is required to reproduce the observed structure. We find that the bright limb and the tail already formed during the AGB phase, prior to the formation of the PN. 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 ± 10 mas yr -1 in the direction of the bright limb. Assuming the central star is moving at 125 ± 25 km s-1, the distance to the nebula is estimated to be 850+500-420 pc, consistent with a spectroscopic distance to the star. Expansion velocities measured from spectroscopic data of the bright filaments are consistent with velocities measured from the simulation. Sh 2-188 is one of the largest PNe known, with an extent of 2.8 pc. The model shows that this size was already set during the AGB phase. © 2005 RAS.