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外文期刊>The Astrophysical journal
>Multiwavelength Imaging and Long-Slit Spectroscopy of the Planetary Nebula NGC 6884: The Discovery of a Fast Precessing, Bipolar Collimated Outflow
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Multiwavelength Imaging and Long-Slit Spectroscopy of the Planetary Nebula NGC 6884: The Discovery of a Fast Precessing, Bipolar Collimated Outflow
We present Hα, [N II] λ6583, [O III] λ5007 ground-based images, Hubble Space Telescope (HST) (V?band) archival images, VLA-A 3.6 cm continuum data, and high-resolution long-slit spectra of the planetary nebula NGC 6884. The VLA and HST images show an extremely knotty and filamentary nebula with an elliptical central structure also recognizable in Hα and [O III]. The [N II] image is different and is dominated by two bright arclike structures. The image ratio maps indicate a complex ionization structure within the nebula with low-excitation filaments surrounded by high-excitation regions. The spectra make it possible to distinguish three basic structural components in NGC 6884: (1) a compact (38 × 28, P.A. 32°), high-excitation ([N II] λ6583/Hα ≤ 0.15) ellipsoidal shell seen almost pole-on with polar and equatorial expansion velocities in the Hα line of 25 and 19 km s-1, respectively; (2) two point-symmetric, low-excitation ([N II]/Hα 0.6–1) spirals in which the distance to the central star and the radial velocity exhibit systematic variations; (3) two compact, apparently isolated, low-excitation bipolar knots. All data agree that the spirals represent precessing bipolar jetlike outflows. The observational properties of these precessing outflows can satisfactorily be explained with an approximately constant ejection velocity (55 km s-1) and a large precession angle (2 120°). The deduced precession period (500 × [D(kpc)/2] yr) is the shortest one measured so far in this kind of structure in planetary nebulae. The image ratio maps and the [N II]/Hα intensity ratio suggest shock excitation in the precessing jets. A general discussion of NGC 6884 is presented within the context of planetary nebulae with jetlike components.
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机译：我们展示了Hα，[N II]λ6583，[O III]λ5007地面图像，哈勃太空望远镜（HST）（V？band）档案图像，VLA-A 3.6厘米连续谱数据以及高分辨率长缝光谱行星状星云NGC 6884的图像。VLA和HST图像显示出一个极易打结的丝状星云，其椭圆中心结构在Hα和[O III]中也可以识别。 [N II]图像是不同的，并且由两个明亮的弧形结构主导。图像比例图指示星云内的复杂电离结构，其低激发丝被高激发区包围。光谱可以区分NGC 6884中的三个基本结构成分：（1）紧凑的（38×28，PA 32°），高激发度（[N II]λ6583/Hα≤0.15）椭圆形壳，几乎位于极点。在Hα线分别为25 km和19 km s-1时具有极地和赤道扩张速度; （2）两个点对称，低激发（[N II] /Hα0.6-1）螺旋，其中到中心星的距离和径向速度表现出系统的变化; （3）两个紧凑的，明显隔离的，低激励的双极结。所有数据都同意，螺旋线代表进动的双极射流状流出。这些进动流出的观测特性可以用大致恒定的喷射速度（55 km s-1）和大进动角（2 120°）令人满意地解释。推导的进动周期（500×[D（kpc）/ 2] yr）是迄今为止在行星状星云中这种结构中测得的最短的进动周期。图像比率图和[N II] /Hα强度比率建议在进动的射流中产生激振。 NGC 6884的一般性讨论是在具有喷射状成分的行星状星云的背景下进行的。
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