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tokens: of ∼30 km s −1 . The same grid-based Monte Carlo code “STaRS 2 2 http://github.com/csj607/STaRS ” is used to investigate the distribution and kinematics of the neutral hydrogen regions in NGC 6886 and NGC 6881. Figure 2 shows a schematic illustration of the scattering geometry adopted in this work. The scattering region is a partial spherical shell surrounding the central He ii region. The covering factor CF is defined as , where θ is the opening angle of the partial spherical shell. The H i region is assumed to be of uniform density expanding with a single speed . Figure 2. Schematic illustration of the simulation. The neutral region is assumed to be a partial spherical shell with the inner and outer radii R in and R out , respectively, and the opening angle θ . The scattering region is divided into a large number of small cubical grids to trace each photon until escape. Far-UV He ii λ 1025 photons generated in the central He ii region are incident on the H i region. The neutral region is optically thick to Rayleigh-scattered UV photons, and optically thin to Raman-scattered optical photons. Each far-UV photon is traced until escape from the neutral region through multiple Rayleigh scattering, or after Raman scattering. The incident far-UV He ii λ 1025 is assumed to be described by a single Gaussian function determined in Section 3.2 . A simulation model is set with the following free parameters: the covering factor CF of the scattering region, H i column density N HI , and expanding speed . The incident He ii λ 1025 flux is deduced from the observed He ii λ 6560 line flux and using case B recombination theory (Storey Hummer 1995 ). We refer to Hyung et al. ( 1995 ) and Pottasch Surendiranath ( 2005 ) for NGC 6886 and also Kaler et al. ( 1987 ) for NGC 6881 for the values of T e = 1.25 × 10 4 K and n e = 10 4 cm −3 . In Figure 3 , we present our best-fit results for NGC 6886 and NGC 6881 in the middle panels. The gray lines represent the observational data and the black lines show our best-fit simulation results. Despite severe blending of the Raman-scattered He ii features with strong [N ii ] λ 6548 lines, the broad features with a red-tail structure are conspicuous in both PNe. These red-tailed features are attributed to the expansion of H i region. Figure 3. Comparison of the observational data (gray) and our simulation data (black) for NGC 6886 (left) and NGC 6881 (right). The best-fit models are found in the middle panels. In the middle-left panel, the model parameters are CF = 0.3, N HI = 5 × 10 20 cm −2 , and (CF30-N5E20-V25). We also present models with a lower expansion speed of (CF30-N5E20-V20, top-left) and a higher value of 30 km s −1 (CF30-N520-V30, bottom-left) for comparison. In the middle-right panel, the model parameters are CF = 0.6, N HI = 2 × 10 20 cm −2 , and (CF60-N3E20-V30). The top-right and bottom-right panels show the models with (CF60-N3E20-V20) and 40 km s −1 (CF60-N3E20-V40), respectively. In the case of NGC