Expansion velocities of the planetary nebula NGC 7009—the high dispersion spectra line profiles

Abstract

We investigated the line profiles of the high dispersion spectra of He I, He II, N III, [N II], [O III], [Ar III], [Ar IV], [S II], [S III ], and [Cl IV], secured at the center of the planetary nebula NGC 7009 with the fiber-fed Bohyunsan Echelle Spectrograph (BOES). The expansion velocities for the main shell and the faint outer thin shell were derived based on stronger double Gaussian profiles and the fast blue wing component, respectively. With the Keck 2D spectral images, we set the main shell boundaries as R∼4″ and 6″ and the faint outer thin shell boundaries as R∼11″ and 13″. The radial variation of expansion velocities of both shells does not follow a linear acceleration. The acceleration of the main shell gas seems to be in a retarded mode, while that of the outer faint shell gas seems to be affected by an additional force. The fast wing component appears also in the He I, He II, and N III line profiles which are likely to have been formed in a compact region, located inside the main shell. The additional deceleration factor, such as the rotation of the central star or the dust gas in the gas shell might cause the non-linear acceleration. Based on the line profiles responsible for the three zones, we conclude that there were at least three major changes occurred in the central star temperature since the first outflow from the central star.