Radiative transfer simulation of water rotational excitation in comets. Comparison of the Monte Carl
Radiative transfer simulation of water rotational excitation in comets. Comparison of the Monte Carl
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彗星中水旋转激发的辐射传输模拟。
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发表时间：
2007
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通讯作者：
A. Lecacheux
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作者：
V. Zakharov;D. Bockelée;N. Biver;J. Crovisier;A. Lecacheux
上下文。最近天基远红外和亚毫米波长探测器的出现，为观测彗星水的旋转跃迁提供了可能。亚毫米波天文卫星和奥丁卫星在几颗彗星上探测到了557 GHz的110−101正射电基线。这条线以及其他的水和h18o线将由欧空局赫歇尔空间天文台的三个仪器进行观测。为了准备或解释这些观测结果，需要包括H2O自吸收在内的激发模型。目标。Bockelee-Morvan （1987, A&A, 181,169）采用局域近似法（EP）处理彗星中水的辐射转移。Bensch & Bergin （2004, ApJ, 615,531）使用蒙特卡罗方法（MC），从物理角度来看，该方法更精确，但对CPU时间的要求更高。本研究的目的是比较两种方法的结果，并调查EP方法在多大程度上提供了可接受的结果来合成线剖面和分析观测。方法。我们开发了两个一维数字代码。MC代码基于Hogerheijde & van der Tak （2000, A&A, 362, 697）提出的加速蒙特卡罗算法。EP码基于Bockelee-Morvan（1987）提出的算法。它们包括七个最低旋转水平的正水，这是旋转冷彗发中主要的人口水平。假设密度分布为球对称且膨胀速度恒定。与水和电子的碰撞，以及红外泵浦，都被考虑在内。利用远红外外差仪计算与Odin和未来赫歇尔观测有关的合成线剖面，计算出产水速率范围为10 28至10 30 s−1。结果。结果表明，EP方法在预测旋转激励、线强度和线形状方面具有足够的精度。能级居群的差异不超过20%，除非在电子温度和密度存在强梯度的狭窄区域。线条形状非常一致，线条面积相差不到7%。
Context. The recent advent of space-based detectors at far-infrared and submillimetre wavelengths opened up the possibility of observing cometary water from its rotational transitions. The 110−101 fundamental line of ortho H2O at 557 GHz was detected in several comets by the Submillimeter Wave Astronomical Satellite and Odin. This line as well as other water and H 18 O lines will be observed by the three instruments of the ESA Herschel Space Observatory. In order to prepare or interpret these observations, excitation models including H2O self-absorption are required. Aims. For treating radiation transfer of water in comets, Bockelee-Morvan (1987, A&A, 181, 169) used the local approximation with the escape probability method (EP). Bensch & Bergin (2004, ApJ, 615, 531) used the Monte-Carlo method (MC), which is more exact from a physical point of view, but is much more CPU time-demanding. The aim of this study is to compare the results of the two methods and to investigate the extent to which the EP method provides acceptable results for synthesizing line profiles and analysing observations. Methods. We developed two 1D numerical codes. The MC code is based on the accelerated Monte Carlo algorithm proposed by Hogerheijde & van der Tak (2000, A&A, 362, 697). The EP code is based on the algorithm proposed by Bockelee-Morvan (1987). They include the seven lowest rotational levels of ortho-water, which are the primarily populated levels in the rotationally cold coma. A spherically symmetric density distribution with constant expansion velocity is assumed. Collisions with water and electrons, and infrared pumping, are taken into account. Synthetic line profiles pertaining to Odin and future Herschel observations with the Heterodyne Instrument for Far-Infrared are computed for water production rates ranging from 10 28 to 10 30 s −1 . Results. We show that the EP method has sufficient accuracy to predict rotational excitation, line intensities and line shapes. Differences in level populations do not exceed 20%, except in the narrow region where a strong gradient in electron temperature and density is present. Line shapes are in excellent agreement and line areas differ by less than 7%.