Chemistry and rotational excitation of O_2_ in interstellar clouds. I. Predicted emissivities of lines for the ODIN, SWAS, PRONAOS-SMH and PIROG 8 submillimeter receivers.
Chemistry and rotational excitation of O_2_ in interstellar clouds. I. Predicted emissivities of lines for the ODIN, SWAS, PRONAOS-SMH and PIROG 8 submillimeter receivers.
复制标题
星际云中 O_2_ 的化学和旋转激发。
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
J. Benayoun
中科院分区：
文献类型：
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作者：
P. Maréchal;Y. Viala;J. Benayoun
由于地球大气层的不透明性，分子氧还没有被发现存在于星际云中。由于O2在星际化学中的潜在重要性，几个试图探测O2旋转谱线的项目正在使用安装在卫星或平流层气球上的毫米和亚毫米接收器进行研究：ODIN用于119和487 GHz谱线，SWAS用于487 GHz谱线，PRONAOS-SMH用于368 GHz谱线，PIROG 8用于425 GHz谱线。作为这些项目的理论准备，并利用星际化学的最新发展以及O2碰撞激发的最新截面计算，我们使用星际云模型进行了O2转动布居的非LTE计算。本文预测了在不同条件下，在漫射、高密度和稠密的暗云中，O2柱密度及其在（亚）毫米波段的转动线发射率，覆盖了从1到30的可见消光范围。研究了密度、温度、外紫外辐射场和气相元素丰度对O2丰度和转动激发的影响。我们的结果与Black & Smith（1984）的结果是对立的，后者首先提出了星际O2的可探测性问题。如果密度和温度对O_2丰度的影响很小，那么当紫外辐射场相对于当地标准值增加1000倍时，它就不会有效地破坏O_2，从而阻止在大多数云中检测到O_2。影响氧气丰度最剧烈的参数|以及OH和H2O|是气相C/O丰度比：当C/O比大于0.7时，除119 GHz谱线外，所有即将进行的飞行任务都无法观测到O2，该谱线在C/O = 1以下的非常不透明的云（AV 20）中仍可观测到。O+OH！产生分子氧的O2+H对云层中O2丰度的影响很小，云层足够不透明以允许检测;然而，它控制OH丰度，因为它是该分子的主要破坏过程。P. Marechal（Priscilla. obspm.fr）预测O2旋转能级的辐射去激发可能是冷分子云中的重要冷却剂，其效率与CO相当。
Molecularoxygenhasnotyetbeenobservedininter- stellar clouds because of the opacity of the Earth's atmosphere. Due to its potential importance in interstellar chemistry, several projets attempting to detect rotational lines of O2 are being de- velopedusingmillimeterandsubmillimeterreceiversembarked on satellites or stratospheric balloons: ODIN for the 119 and 487 GHz lines, SWAS for the 487 GHz line, PRONAOS-SMH for the 368 GHz line and PIROG 8 for the 425 GHz line. As a theoretical preparation to these projects and taking advantage of recent developments in interstellar chemistry as well as re- cent cross-section calculations of collisional excitation of O2, we have used an interstellar cloud model to perform a non-LTE calculation of O2 rotational population. O2 column densities and emissivities of its rotational lines at (sub)millimeter wave- lengths are predicted for various conditions in diffuse, translu- centanddensedarkclouds,coveringarangeofvisualextinction from 1 to 30. The effects of density, temperature, external ul- traviolet radiation eld and gas phase elemental abundances on the O2 abundance and rotational excitation have been investi- gated. Our results are confronted to the ones obtained by Black & Smith (1984) who adressed initially the problem of the de- tectability of interstellar O2. If density and temperature have little influence on the O2 abundance, it is not the case for the UV radiation eld which efciently destroys O2 so as to pre- vent its detection in most clouds as soon as it is enhanced by a factor of 1000 with respect to the local standard value. The most drastic parameter that influences the abundance of O2 | as well as that of OH and H2O | is the gas-phase C/O abun- danceratio:forC/Oratiolargerthan0.7,O2 becomesunobserv- able by all forthcoming missions except for the 119 GHz line which remains observable in very opaque clouds (AV 20) up to C/O = 1. The rate coefcient of the reaction O+OH!O2+H which produces molecular oxygen has little influence on the O2 abundance in clouds sufciently opaque to allow detection; it however controls the OH abundance since it is the main de- struction process of this molecule. Our model calculations also Send offprint requests to: P. Marechal (Priscilla.Marechal@obspm.fr) predict that the radiative de-excitation of O2 rotational levels could be an important cooling agent in cold molecular clouds with an efciency comparable to that of CO.