The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust
The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust
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行星状星云 SwSt 1 的 FUSE 光谱：气体和尘埃不均匀性的证据
DOI：
10.1086/429613
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
S. Redfield
中科院分区：
文献类型：
--
作者：
N. Sterling;H. Dinerstein;C. W. Bowers;S. Redfield
我们展示了远紫外光谱探测器（FUSE）对年轻的致密行星状星云（PN） SwSt 1沿着其中心恒星HD 167362的视线进行的观测。我们在中心星的连续体上发现了几个物种的星周吸收线。从FUSE数据中得到的星云的物理参数与从发射谱线中得到的有很大的不同。我们从激发的S III精细结构能级的柱密度比中得到电子密度ne = 8800 cm-3，这比所有先前的估计至少低3倍。从紫外谱线得到的气态铁丰度相当高（[Fe/S] = -0.35±0.12），这意味着铁没有明显地耗尽到尘埃中。相比之下，光学和近红外发射谱线表明，Fe的损耗更为强烈：[Fe/H] = -1.64±0.24,[Fe/S] = -1.15±0.33。我们没有探测到星云H2的吸收，极限N(H2) < 7 × 1014 cm-2，比红外H2发射谱线估计的柱密度至少低4个数量级。综上所述，缺乏H2吸收，低ne，以及从FUSE光谱中得到的高气态Fe丰度提供了强有力的证据，表明致密结构（可以保护分子和尘埃免受高能恒星光子的破坏性影响）并不存在于通往中心恒星的视线中。另一方面，有大量证据表明，在SwSt 1的其他地方存在尘埃、分子物质和致密气体。因此，我们得出结论，星云一定具有非均匀结构。我们在1040.94和1041.69 Å从两个激发的精细结构水平的中性氧探测到星云吸收。这些能级产生了63 μm和145 μm的远红外发射线，通常用于推断气体性质，特别是温度，假设它们是碰撞激发的。我们发现SwSt 1的O - I精细结构水平具有倒总体比。这需要一种非热激发机制，我们将其确定为恒星连续体的荧光激发。由于荧光对水平种群的影响，远红外[O - I]线强度不能直接用作密度和温度的诊断。
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the young, compact planetary nebula (PN) SwSt 1 along the line of sight to its central star HD 167362. We detect circumstellar absorption lines from several species against the continuum of the central star. The physical parameters of the nebula derived from the FUSE data differ significantly from those found from emission lines. We derive an electron density ne = 8800 cm-3 from the column density ratio of the excited S III fine-structure levels, which is at least a factor of 3 lower than all prior estimates. The gaseous iron abundance derived from the UV lines is quite high ([Fe/S] = -0.35 ± 0.12), which implies that iron is not significantly depleted into dust. In contrast, optical and near-infrared emission lines indicate that Fe is more strongly depleted: [Fe/H] = -1.64 ± 0.24 and [Fe/S] = -1.15 ± 0.33. We do not detect nebular H2 absorption, to a limit N(H2) < 7 × 1014 cm-2, at least 4 orders of magnitude lower than the column density estimated from infrared H2 emission lines. Taken together, the lack of H2 absorption, low ne, and high gaseous Fe abundance derived from the FUSE spectrum provide strong evidence that dense structures (which can shield molecules and dust from the destructive effects of energetic stellar photons) are not present along the line of sight to the central star. On the other hand, there is substantial evidence for dust, molecular material, and dense gas elsewhere in SwSt 1. Therefore, we conclude that the nebula must have an inhomogeneous structure. We detect nebular absorption at 1040.94 and 1041.69 Å from the two excited fine-structure levels of neutral oxygen. These levels give rise to far-infrared emission lines at 63 and 145 μm, which are often used to infer gas properties, particularly temperature, under the assumption that they are collisionally excited. We find that the O I fine-structure levels in SwSt 1 have an inverted population ratio. This requires a nonthermal excitation mechanism, which we identify as fluorescent excitation by the stellar continuum. To the extent that fluorescence affects the level populations, the far-infrared [O I] line strengths cannot be directly used as diagnostics of density and temperature.