2023ApJ...949..106C

The young planetary nebula NGC 6302 is known to exhibit Raman-scattered He II features at 6545 Å and 4851 Å. These features are formed through inelastic scattering of He IIλλ 1025 and 972 with hydrogen atoms in the ground state, for which the cross sections are 1.2 × 10^–21 cm² and 1.4 × 10^–22 cm², respectively. We investigate the spectrum of NGC 6302 archived in the ESO Science Portal. Our Gaussian line-fitting analysis shows that the Raman-scattered He II features are broader and more redshifted than the hypothetical model Raman features that would be formed in a cold static H I medium. We adopt a simple scattering geometry consisting of a compact He II emission region surrounded by an H I medium to perform Monte Carlo simulations using the radiative transfer code STaRS. Our simulations show that the H I region is characterized by the H I column density N_H I_ = 3 × 10²¹ cm^–2, with the random speed component v_ran = 10 km s^–1 expanding with a speed ${v}_{\exp }=13\,\mathrm{km}\ {{\rm{s}}}^{-1}$ from the He II emission region. Based on our best-fit parameters, we estimate the H I mass of the neutral medium M_H I_ ≃ 1.0 × 10^–2M_⊙, pointing out the usefulness of Raman He II spectroscopy as a tool to trace H I components.