ISO LWS observations of planetary nebula fine‐structure lines
ISO LWS observations of planetary nebula fine‐structure lines
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行星状星云精细结构线的 ISO LWS 观测
DOI：
10.1046/j.1365-8711.2001.04180.x
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发表时间：
2001
影响因子：
4.8
通讯作者：
D. Péquignot
中科院分区：
文献类型：
--
作者：
Xiaowei Liu;M. Barlow;M. Cohen;I. J. Danziger;S. Luo;J. Baluteau;P. Cox;R. Emery;T. Lim;D. Péquignot
我们用红外空间天文台（ISO）上的长波光谱仪（LWS）获得了51个银河系行星状星云（PN）和原行星状星云（PPN）的43-198 μm远红外光谱。还获得了前PN候选物Lo 14的光谱。光谱产生了电离区发射的[N ii] 122 μm、[N iii] 57 μm和[O iii] 52和88 μm精细结构谱线以及光解离区（PDR）发射的[O i] 63和146 μm以及[C ii] 158 μm精细结构谱线的通量，这些谱线已被用来确定电离区的电子密度和离子丰度以及密度，PDR的温度和气体质量。在以Lo 14为中心的LWS光谱中检测到的强[N iii]和[O iii]发射线可能与附近的强射电和红外源G 331.5−0.1有关。
我们发现，由[O iii] 88 μm/52 μm的二重态比得到的电子密度系统地低于由光学[Ar iv] λ4740/λ4711和[Cl iii] λ5537/λ5517的二重态比得到的电子密度，后者的临界密度比52和88 μm的线高得多，这表明星云中存在密度不均匀性。离子丰度，N+/H+，N2+/H+和O2+/H+，以及N2+/O2+丰度比，它提供了一个很好的近似N/O元素丰度比，推导。虽然从远红外精细结构谱线推导出的相对于H+的离子丰度对所采用的电子密度和密度不均匀性的存在很敏感，但由于所涉及的谱线的临界密度相似，当N2+/O2+由57 μm/（52 μm+88 μm）通量比计算时，对星云物理条件的强烈依赖在很大程度上被抵消了。
从观测到的[O i]和[C ii]线强度比确定了24 PN附近PDR的温度和密度。除了少数天体外，推断出的温度在200到500 K之间，峰值在250 K左右。PDR的密度在104-105 cm−3之间，在一些年轻的致密PN中达到3×105 cm−3。NGC 7027的PDR的温度为1600 K，密度为105 cm−3，是所研究的星云中最热的星云之一，同时也是密度最大的星云之一。对于大多数研究的PN，[C ii]发射区只包含少量的材料，气体质量为0.1 M Ω。在少数星云中发现了异常大的PDR质量，包括NGC 7027，双极星云M2-9和NGC 6302，年轻的致密行星状星云BD+30°3639，IC 418和NGC 5315，以及老的，可能是重组的星云IC 4406和NGC 6072。
We have obtained 43–198 μm far-infrared (IR) spectra for a sample of 51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN), using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). Spectra were also obtained of the former PN candidate Lo 14. The spectra yield fluxes for the fine-structure lines [N ii] 122 μm, [N iii] 57 μm and [O iii] 52 and 88 μm emitted in the ionized regions and the [O i] 63- and 146-μm and [C ii] 158-μm lines from the photodissociation regions (PDRs), which have been used to determine electron densities and ionic abundances for the ionized regions and densities, temperatures and gas masses for the PDRs. The strong [N iii] and [O iii] emission lines detected in the LWS spectrum taken centred on Lo 14 could be associated with the nearby strong radio and infrared source G 331.5−0.1.
We find that the electron densities yielded by the [O iii] 88 μm/52 μm doublet ratio are systematically lower than those derived from the optical [Ar iv] λ4740/λ4711 and [Cl iii] λ5537/λ5517 doublet ratios, which have much higher critical densities than the 52- and 88-μm lines, suggesting the presence of density inhomogeneities in the nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as well as the N2+/O2+ abundance ratio, which provides a good approximation to the N/O elemental abundance ratio, are derived. Although ionic abundances relative to H+ deduced from the far-IR fine-structure lines are sensitive to the adopted electron density and the presence of density inhomogeneities, the strong dependence on the nebular physical conditions is largely cancelled out when N2+/O2+ is calculated from the 57 μm/(52 μm+88 μm) flux ratio, owing to the similarity of the critical densities of the lines involved.
The temperatures and densities of the PDRs around 24 PN have been determined from the observed [O i] and [C ii] line intensity ratios. Except for a few objects, the deduced temperatures fall between 200 and 500 K, peaking around 250 K. The densities of the PDRs vary from 104–105 cm−3, reaching 3×105 cm−3 in some young compact PN. With a derived temperature of 1600 K and a density of 105 cm−3, the PDR of NGC 7027 is one of the warmest and at the same time one of the densest amongst the nebulae studied. For most of the PN studied, the [C ii]-emitting regions contain only modest amounts of material, with gas masses ≲0.1 M⊙. Exceptional large PDR masses are found for a few nebulae, including NGC 7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetary nebulae BD+30°3639, IC 418 and NGC 5315, and the old, probably recombining, nebulae IC 4406 and NGC 6072.