부산대학교 도서관

We derived position-velocity density distribution diagrams along the major (PA = 77$^{\circ}$) axis of the elliptical planetary nebula NGC 7009 with the Keck HIRES [SII] 6716/6731 \A, doublet spectral images. The average densities of the main shell and knots of NGC 7009 derived from the [SII] 6716/673 \A, fluxes integrated over the line of sight indicate a density range of $N_{\rm e}$ = $10^{3.4}$ to 10$^{3.9}$ $cm^{-3}$, while the local densities from the volume fraction resolved in radial velocities along the line of sight show a considerably large range of about 10$^{2.8}$ -- 10$^{4.7}$ $cm^{-3}$. The derived projection angle of the major axis of the main shell is about $\psi \sim$18.3($\pm$2)$^{\circ}$. Assuming that the main shell is an ellipsoidal shell with $a \simeq$16$''$ and $b \simeq 6''$, we found the range of expansion velocity, radius, and latitude of four knots and a hot bubble. The four knots at the points in symmetrical positions are roughly aligned with the same axis of expansion of latitudes $\phi \sim \pm 34.5(\pm 0.6)^{\circ}$: One pair expands at about 35 $kms^{-1}$ close to the main ellipsoidal shell, and the other expands rapidly at about 60 $kms^{-1}$ at a distance of $r \sim 16''$. In the latitude range $\phi = $65 -- 75$^{\circ}$, the hot bubble of a relatively large structure expands rapidly with a velocity of 130 -- 150 $kms^{-1}$. Four knots and hot bubble points that expand faster than the main shell appear to have been formed by two to three eruptions at a different epoch than the primary structure formation.
Comment: 18 pages, 7 figures Accepted for publication in ApJS