Raman Scattered HeII 6545 in the Young and Compact Planetary Nebula NGC 6790

TL;DR

This paper reports the discovery of Raman scattered He II 6545 in the young planetary nebula NGC 6790, using high-resolution spectra and Monte Carlo modeling to analyze the neutral hydrogen region and its dynamics.

Contribution

It is the first to identify Raman scattered He II 6545 in NGC 6790 and models the neutral region with a rotating ring, providing new insights into the nebula's structure and kinematics.

Findings

Raman He II 6545 line detected in NGC 6790.

Neutral hydrogen shell expanding at 15-19 km/s.

Rotating He II emission region with ~18 km/s rotation speed.

Abstract

We present the high resolution spectra of the youn and compact planerary nebula NGC 6790 obtained with the echelle spectrograph at Bohyunsan Optical Astronomy Observatory and report the discovery of Raman scattered He II 6545 in this object. This line feature is formed in a thick neutral region surrounding the hot central star, where He II 1025 line photons are scattered inelastically by hydrogen atoms. A Monte Carlo technique is adopted to compute the line profile 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 HI region lies in the range V_exp = 15- 19 Km/s. Less stringent constraints are put on the HI column density N_HI and covering factor C, where the total flux of Raman He II 6545 is consistent with the product of N_HI and C being CN_HI ~ 0.5 X 10^20 cm^-2. 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 ~ 18Km/s. Brief discussions are presented regarding the mass loss processes and future observations.