2009ApJ...695..542K

We present the high-resolution spectra of the young and compact planetary nebula NGC 6790 obtained with the Echelle Spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman-scattered He II λ6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He II λ1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profiles with a simple geometric model, in which the neutral region is in the form of a cylindrical shell that is expanding from the central star. From our line profile analysis, the expansion velocity of the H I region lies in the range v_exp= 15-19 km/s. Less stringent constraints are put on the H I column density N_HI and covering factor C, where the total flux of Raman He II λ6545 is consistent with their product CN_HI ∼ 0.5x10²⁰/cm2. The Monte Carlo profiles from stationary emission models exhibit deficit in the wing parts. A much better fit is obtained when the He II emission region is assumed to take the form of a ring that slowly rotates with a rotation speed of ∼18 km/s. Brief discussions are presented regarding the mass-loss processes and future observations.