星际介质PDR和XDR结构的微波谱线诊断

Diagnosis of PDRs and XDRs of the ISM Using Radio-waveband Spectral Lines

星际介质(ISM)的物理性质和化学平衡受多种加热和冷却过程的影响。光子主导区(PDR)和X射线主导区(XDR)是恒星形成过程中星际介质的两个主要结构。PDR和XDR区的主导能源机制不同,造成星际介质的物理性质和化学成分上有很大不同,反映在不同种类分子、同种分子不同跃迁能级之间以及分子同位素之间的谱线强度比有显著差异,因此可以利用分子谱线强度比作为区分PDR和XDR的示踪工具。PDR和XDR模型的数值模拟表明,分子谱线中HNC/HCN、HCN/HCO+以及HCN/CO谱线强度比可以有效地区分PDR区和XDR区。此外,CN/HCN、NO/CO以及HOC+/HCO+的柱密度比值也可以指示PDR和XDR区。PDR和XDR的微波分子谱线诊断技术已经应用在河外星系的物理和化学演化研究中,不仅证实了该技术的有效性,而且在PDR和XDR模型中引入机械能反馈机制后,使得模型能够更贴切地符合观测结果,为理解河外星系的演化打开了一扇新的窗口。

The physical properties and chemical balance of the interstellar medium （ISM） are affected by a variety of heating and cooling mechanisms. Photon dominated region （PDR） and X-ray dominated region （XDR） are two structures in the process of star formation. The dominant energy sources in PDRs and XDRs are different, resulting in variations of physical properties and chemical composition of the ISM. The emission line intensity changes with different molecule species, different transitions of a same molecule, and isotopes. Therefore the ratios beween different molecular lines can be used as indicators of the PDRs and XDRs. The modeling of the PDR and XDR shows the intensity ratios of HNC/HCN, HCN/HCO＋, HCN/CO and CO（16-15）/CO（1-0） are good tracers to diagnose PDRs and XDRs. In addition, the column density ratios of CN/HCN, NO/CO and HOC＋/HCO＋ may also be used as tracers. The diagnosis tool using molecular line ratios has been successfully applied to extraalaxies study, opening a new window to learn the molecular gas properties in the centers of galactic nuclei. The inclusion of mechanical feedback to the models makes the fitting more consistent with the observational data.