The effects of clumping on derived abundances in Hii regions
The effects of clumping on derived abundances in Hii regions
复制标题
聚集对 Hii 地区衍生丰度的影响
DOI：
--
复制
发表时间：
2005
期刊：
影响因子：
--
通讯作者：
U. S. Andrews
中科院分区：
文献类型：
--
作者：
J. Mathis;Kenneth Wood The University of Wisconsin;U. S. Andrews
我们比较了具有均匀密度分布的HⅡ区的蒙特卡罗光电离化模型与具有相同中心恒星和化学成分但由团块内的团块组成的三维分级密度分布的模型，在四层方案上。目的是将标准分析得到的He、N、O、Ne和S的丰度(发射线强度和测量的平均温度)与我们模型中的丰度进行比较。我们考虑了恒星温度在37.5kK-45kK之间，电离光子光度在1048kK到1051kK之间，S−1。块状模型的离子丰度不同于均匀模型。对于热星，(He 0/H+)I S为2-3%，比均匀模型大得多。这一数量的He 0与金属丰度无关，因此影响了原始He丰度的测定。用通常的分析方法，用T([O III])表示高电离阶段，用T([N II])表示低电离阶段，得到的O、Ne和S的总丰度，对聚集模型的精度与对均匀模型的精度差不多，在∼内为真实值的20%。如果T([O III])被用来分析所有离子，则导出的(O/H)∼对于冷星来说太大了40%-60%，但对于热星来说是好的。统一模型有类似的误差，因此聚集不会改变丰度分析的准确性。离子丰度误差的物理原因存在于真实星云中。在块状模型中，来自高电离区(低He 0和低紫外线不透明度)的氦电离辐射可以穿透电离区边缘附近的区域。与均匀或径向对称的几何结构相比，这种效应允许He 0吸收更多的恒星光子。反过来，这些吸收会与O+等竞争那些高能的恒星光子。辐射传输-ISM：一般-H II区-宇宙学：杂项。
We have compared Monte Carlo photoionization models of H II regions with a uniform density distribution with models with the same central stars and chemical compositions but with a three-dimensional hierarchical density distribution consisting of clumps within clumps, on a four-tier scheme. The purpose is to compare the abundances of He, N, O, Ne and S obtained by standard analyses (emission-line strengths and measured mean temperatures from [O III] and [N II]) with the abundances in our models. We consider stellar temperatures in the range 37.5‐45 kK and ionizing photon luminosities from 10 48 to 10 51 s −1 . Clumped models have different ionic abundances than uniform. For hot stars, (He 0 /H + )i s 2‐3 per cent, much larger than with uniform models. This amount of He 0 is independent of metallicity and so impacts the determination of the primordial abundance of He. The total abundances of O, Ne and S obtained by the usual methods of analysis, using T([O III]) for high stages of ionization and T([N II]) for low, are about as accurate for clumped models as for uniform, and within ∼20 per cent of the true values. If T([O III]) is used for analysing all ions, the derived (O/H) is ∼40‐60 per cent too large for cool stars but is good for hot stars. Uniform models have similar errors, so the clumping does not change the accuracy of abundance analysis. The physical causes of the ionic abundance errors are present in real nebulae. In clumped models, helium ionizing radiation from zones of high ionization (low He 0 and low ultraviolet opacity) can penetrate nearby regions near the edge of the ionized zone. This effect allows He 0 to absorb more stellar photons than in uniform or radially symmetrical geometries. In turn, these absorptions compete with O + , etc., for those energetic stellar photons. Ke yw ords: radiative transfer ‐ ISM: general ‐ H II regions ‐ cosmology: miscellaneous.