The recent observations show that comet C/2016 R2 (Pan-Starrs) has a unique and peculiar composition when compared with several other comets observed at 2.8 au heliocentric distance. Assuming solar resonance fluorescence is the only excitation source, the observed ionic emission intensity ratios are used to constrain the corresponding neutral abundances in this comet. We developed a physico-chemical model to study the ion density distribution in the inner coma of this comet by accounting for photon and electron impact ionization of neutrals, charge exchange and proton transfer reactions between ions and neutrals, and electron-ion thermal recombination reactions. Our calculations show that CO2+ and CO+ are the major ions in the inner coma, and close to the surface of nucleus CH3OH+, CH3OH2+ and O2+ are also important ions. By considering various excitation sources, we also studied the emission mechanisms of different excited states of CO+, CO2+, N2+, and H2O+. We found that the photon and electron impact ionization and excitation of corresponding neutrals significantly contribute to the observed ionic emissions for radial distances smaller than 300 km and at larger distances, solar resonance fluorescence is the major excitation source. Our modelled ion emission intensity ratios are consistent with the ground-based observations. Based on the modelled emission processes, we suggest that the observed ion emission intensity ratios can be used to derive the neutral composition in the cometary coma only when the ion densities are significantly controlled by photon and photoelectron impact ionization of neutrals rather than by the ion-neutral chemistry.

中文翻译：

---

用于研究彗星 C/2016 R2 (Pan-STARRS) 内彗发中离子密度分布的物理化学模型

最近的观测表明，与在 2.8 au 日心距离观察到的其他几颗彗星相比，彗星 C/2016 R2（泛星）具有独特而奇特的成分。假设太阳共振荧光是唯一的激发源，观察到的离子发射强度比用于限制这颗彗星中相应的中性丰度。我们开发了一个物理化学模型，通过考虑中性物质的光子和电子碰撞电离、离子与中性物质之间的电荷交换和质子转移反应以及电子-离子热复合反应来研究这颗彗星内部彗发中的离子密度分布。我们的计算表明，CO2+和CO+是彗发内部的主要离子，靠近原子核表面的CH3OH+、CH3OH2+和O2+也是重要的离子。通过考虑各种激发源，我们还研究了 CO+、CO2+、N2+ 和 H2O+ 不同激发态的发射机制。我们发现，相应中性粒子的光子和电子碰撞电离和激发对观察到的径向距离小于 300 公里的离子发射有显着贡献，而在较大距离处，太阳共振荧光是主要的激发源。我们模拟的离子发射强度比与地面观测结果一致。基于模拟的发射过程，我们建议观察到的离子发射强度比可用于推导出彗星彗发中的中性成分，只有当离子密度显着受中性粒子的光子和光电子碰撞电离控制而不是由离子-中性化学。我们还研究了 CO+、CO2+、N2+ 和 H2O+ 不同激发态的发射机制。我们发现，相应中性粒子的光子和电子碰撞电离和激发对观察到的径向距离小于 300 公里的离子发射有显着贡献，而在较大距离处，太阳共振荧光是主要的激发源。我们模拟的离子发射强度比与地面观测结果一致。基于模拟的发射过程，我们建议观察到的离子发射强度比可用于推导出彗星彗发中的中性成分，只有当离子密度显着受中性粒子的光子和光电子碰撞电离控制而不是由离子-中性化学。我们还研究了 CO+、CO2+、N2+ 和 H2O+ 不同激发态的发射机制。我们发现，相应中性粒子的光子和电子碰撞电离和激发对观察到的径向距离小于 300 公里的离子发射有显着贡献，而在较大距离处，太阳共振荧光是主要的激发源。我们模拟的离子发射强度比与地面观测结果一致。基于模拟的发射过程，我们建议观察到的离子发射强度比可用于推导出彗星彗发中的中性成分，只有当离子密度显着受中性粒子的光子和光电子碰撞电离控制而不是由离子-中性化学。