Distribution and Kinematics of H I through Raman He II Spectroscopy of NGC 6302

Abstract: The young planetary nebula NGC 6302 is known to exhibit Raman-scattered He II features at 6545 and 4851 Angstrom. These features are formed through inelastic scattering of He II$\lambda\lambda$ 1025 and 972 with hydrogen atoms in the ground state, for which the cross sections are $1.2 \times 10^{-21}$ and $1.4\times 10^{-22} {\rm\ cm^2}$, 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 a H I medium to perform Monte Carlo simulations using the radiative transfer code ${\it STaRS}$. Our simulations show that the H I region is characterized by the H I column density $N_{\rm HI}=3\times 10^{21}{\rm\ cm^{-2}}$ with the random speed component $v_{\rm ran}=10{\rm\ km\ s^{-1}}$ expanding with a speed $v_{\rm exp}= 13{\rm\ km\ s^{-1}}$ from the He II emission region. Based on our best fit parameters, we estimate the H I mass of the neutral medium $M_{\rm HI} \simeq 1.0\times 10^{-2}\ {\rm M_\odot}$, pointing out the usefulness of Raman He II spectroscopy as a tool to trace H I components.