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.
Highlights
Mass loss is an important process that mainly occurs in the late stage of stellar evolution
It is expected that around a young planetary nebula there may be a significant amount of neutral material that was lost in the previous stage of stellar evolution
H I Raman spectroscopy provides an accurate determination of the expansion velocity of the H I region, for the measurement of which H I 21 cm radio observation has been the unique tool so far
Summary
Mass loss is an important process that mainly occurs in the late stage of stellar evolution. The proximity to resonance is responsible for a large scattering cross section requiring the existence of a neutral region with NHI ∼ 1020 cm−2 around a He II emission source Raman scattered He II features are reported in other symbiotic stars including He 2-106, HM Sagittae and V1016 Cygni (Lee, Kang & Byun 2001, Jung & Lee 2004b, Birriel 2004). Lee et al (2006) reported that the compact planetary nebula IC 5117 exhibits Raman scattered He II features blueward of Hα and Hβ In these objects, it appears that the central star is surrounded by a neutral region with a significant covering factor. We present our high resolution spectra of the young and compact planetary nebula NGC 6790 and report our finding of the Raman scattered He IIλ6545 feature in this object. We discuss briefly our observation and mass loss processes of NGC 6790
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