The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula
The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula
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NGC 6720（环形星云）的三维电离结构和演化
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发表时间：
2007
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通讯作者：
W. Henney
中科院分区：
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作者：
C. O’Dell;F. Sabbadin;W. Henney
我们利用断层扫描技术和长缝高分辨率光谱三维恢复技术，确定了NGC 6720（环状星云）的气体运动学、诊断和离子径向分布、空间结构和演化阶段。环状星云的主壳是一个三轴椭球体（半径为0.10,0.13和0.20 pc），接近极点，并以近似弹道的方式膨胀（Vexp = 0.65 km s-1 arcsec-1）。中心星的特征[log(L*/L⊙)≃2.3,T*≃120000 K]，结合星云年龄为7000 yr的特征，表明这颗M*≃0.61-0.62 M⊙的agb后恒星正接近白矮星冷却序列。环状星云的赤道在光学上是厚的，比光学上薄的两极密度大得多。NGC 6720周围的内晕是直接被中心恒星电离的AGB风在高纬度（环极）的极上投影，而外围较暗的圆形晕是AGB风在中低纬度（被主星云遮蔽）的复合投影。环形星云的空间运动学特性和在晚期行星状星云中常见的致密结的起源与辐射-流体动力学和风相互作用模型的预测进行了严格的比较。
We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L⊙) ≃ 2.3, T* ≃ 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* ≃ 0.61-0.62 M⊙ post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models.