Physical properties of a very diffuse HI structure at high Galactic latitude
Physical properties of a very diffuse HI structure at high Galactic latitude
复制标题
高银河纬度处非常弥散的 HI 结构的物理特性
DOI：
10.1051/0004-6361:20065523
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发表时间：
2007
影响因子：
6.5
通讯作者：
M. Martin
中科院分区：
文献类型：
--
作者：
M. Martin
目标。分析的主要目的是提出一种新的方法来估计在银河系高纬度观测到的氢原子扩散云的物理性质。方法.该方法基于与分数布朗运动模拟的观测结果的比较，使用积分发射、质心速度和线宽的统计特性来约束3D密度和速度场的物理特性，以及Hi的平均温度。结果我们应用这种方法来解释21厘米的观测获得的绿色银行望远镜的一个非常弥漫的HI云在银河系高纬度位于Firback北1。我们首先表明，观察不能再现单独由高度湍流的CNM型气体，并有显着的贡献的热加宽观察到的线宽。为了再现这些曲线，需要调用具有不同平均温度和填充因子的两个分量。我们确定，在ISM的这个非常扩散的部分中，2/3的柱密度由WNM和1/3的热不稳定气体（T = 2600 K）组成。WNM气体是温和的超音速（� M �1）和不稳定的阶段是肯定的亚音速（� M �0.3）。密度对比度（即，相对于密度分布平均值的标准偏差）接近0.8。WNM的填充因子是不稳定气体的10倍，其密度结构更接近于CNM气体的预期。该气田含有一个CNM型天然气的特征，其浓度很低（NH = 3 × 10 19），可能是由WNM型天然气的汇聚流形成的。
Aims. The main goal of this analysis is to present a new method to estimate the physical properties of diffuse cloud of atomic hydrogen observed at high Galactic latitude. Methods. This method, based on a comparison of the observations with fractional Brownian motion simulations, uses the statistical properties of the integrated emission, centroid velocity and line width to constrain the physical properties of the 3D density and velocity fields, as well as the average temperature of Hi. Results. We applied this method to interpret 21 cm observations obtained with the Green Bank Telescope of a very diffuse HI cloud at high Galactic latitude located in Firback North 1. We first show that the observations cannot be reproduced solely by highly-turbulent CNM type gas and that there is a significant contribution of thermal broadening to the line width observed. To reproduce the profiles one needs to invoke two components with different average temperature and filling factor. We established that, in this very diffuse part of the ISM, 2/3 of the column density is made of WNM and 1/3 of thermally unstable gas (T ∼ 2600 K). The WNM gas is mildly supersonic (� M �∼ 1) and the unstable phase is definitely sub-sonic (� M �∼ 0.3). The density contrast (i.e., the standard deviation relative to the mean of density distribution) of both components is close to 0.8. The filling factor of the WNM is 10 times higher that of the unstable gas, which has a density structure closer to what would be expected for CNM gas. This field contains a signature of CNM type gas at a very low level (NH ∼ 3 × 10 19 ) which could have been formed by a convergent flow of WNM gas.