The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
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NGC 7009（土星星云）的 3D 电离结构和演化
DOI：
10.1051/0004-6361:20031729
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发表时间：
2003
影响因子：
6.5
通讯作者：
R. Ragazzoni
中科院分区：
文献类型：
--
作者：
F. Sabbadin;M. Turatto;E. Cappellaro;S. Benetti;R. Ragazzoni
对扩展发射线物体的断层扫描和 3D 分析应用于有趣的行星状星云 NGC 7009 的长缝 ESO NTT + EMMI 高分辨率光谱，涵盖十二个位置角。我们推导出气体膨胀定律、诊断和离子径向剖面、距离和中心恒星参数、星云光电离模型以及等离子体结构和演化的空间恢复。土星星云（${\rm distance}\simeq1.4$ kpc、${\rm Age}\simeq6000$ yr、电离${\rm Mass}\simeq0.18$ $M_\odot$）由几个相互关联的组件组成，其特征在于不同的形态、物理条件、激发和运动学。我们确定了四个“大规模”、平均到高激发子系统（内壳、主壳、外壳和晕），以及许多“小规模”子系统：帽（外壳内的低激发结串）、ansae（极地、低激发、可能受到冲击的层）、流（连接主壳与ansae的高激发极地区域）和 外壳内赤道、中低激发赝环。内壳、主壳、溪流和安塞在 $V{\rm_{exp}}\simeq4.0 \times R$´´ km s -1 处膨胀，外壳、盖和赤道伪环在 $V{\rm_{exp}}\simeq3.15 \times R$´´ km s -1 处膨胀，晕在 $V{\rm_{exp}}\simeq10$ km s -1 。我们将八个子系统中观测到的物理条件和线通量的径向分布与来自光电离代码 CLOUDY 的理论剖面进行比较，推断 NGC 7009 的所有光谱特征都可以用中心恒星的光电离来解释，即热星 ($\log\, T_* \simeq4.95$) 和发光星 ($\log\, L_*/L_\odot\simeq3.70$) 0.60–0.61 $M_\odot$ 处于氢壳核燃烧阶段的 AGB 后恒星。土星星云的 3D 形状在以光电离为主并由快速恒星风支持的演化场景中进行了讨论：它从超级风喷射开始（首先是各向同性，然后是极性不足），经过中性过渡阶段（持续 $\simeq$3000 年），电离开始（发生在 $\simeq$2000 年前），以及完全 主壳层的电离（$\simeq$1000年前），最终达到了今天：整个星云对于紫外恒星通量来说在光学上是薄的，除了帽（外壳中的平均纬度凝聚，被主壳遮蔽）和ansae（沿主轴的超音速电离前沿）。
Tomographic and 3-D analyses for extended, emission-line objects are applied to long-slit ESO NTT + EMMI high-resolution spectra of the intriguing planetary nebula NGC 7009, covered at twelve position angles. We derive the gas expansion law, the diagnostics and ionic radial profiles, the distance and the central star parameters, the nebular photo-ionization model and the spatial recovery of the plasma structure and evolution. The Saturn Nebula (${\rm distance}\simeq1.4$ kpc, ${\rm age}\simeq6000$ yr, ionized ${\rm mass}\simeq0.18$ $M_\odot$) consists of several interconnected components, characterized by different morphology, physical conditions, excitation and kinematics. We identify four “large-scale”, mean-to-high excitation sub–systems (the internal shell, the main shell, the outer shell and the halo), and as many “small-scale” ones: the caps (strings of low-excitation knots within the outer shell), the ansae (polar, low-excitation, likely shocked layers), the streams (high-excitation polar regions connecting the main shell with the ansae), and an equatorial, medium-to-low excitation pseudo-ring within the outer shell. The internal shell, the main shell, the streams and the ansae expand at $V{\rm_{exp}}\simeq4.0 \times R$´´ km s -1 , the outer shell, the caps and the equatorial pseudo-ring at $V{\rm_{exp}}\simeq3.15 \times R$´´ km s -1 , and the halo at $V{\rm_{exp}}\simeq10$ km s -1 . We compare the radial distribution of the physical conditions and the line fluxes observed in the eight sub-systems with the theoretical profiles coming from the photo-ionization code CLOUDY, inferring that all the spectral characteristics of NGC 7009 are explainable in terms of photo-ionization by the central star, a hot ($\log\, T_* \simeq4.95$) and luminous ($\log\, L_*/L_\odot\simeq3.70$) 0.60–0.61 $M_\odot$ post–AGB star in the hydrogen-shell nuclear burning phase. The 3–D shaping of the Saturn Nebula is discussed within an evolutionary scenario dominated by photo-ionization and supported by the fast stellar wind: it begins with the superwind ejection (first isotropic, then polar deficient), passes through the neutral, transition phase (lasting $\simeq$3000 yr), the ionization start (occurred $\simeq$2000 yr ago), and the full ionization of the main shell ($\simeq$1000 yr ago), at last reaching the present days: the whole nebula is optically thin to the UV stellar flux, except the caps (mean latitude condensations in the outer shell, shadowed by the main shell) and the ansae (supersonic ionization fronts along the major axis).