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arXiv 2005.06063astro-ph.SRastro-ph.GA
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AGB星外流中的CO和HCN同位素比值

CO and HCN isotopologue ratios in the outflows of AGB stars

M. Saberi, H. Olofsson, W. H. T. Vlemmings, E. De Beck, T. Khouri, S. Ramstedt

备注：
14 pages, 11 figures
发表信息：
A&A 638, A99 (2020)
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摘要：

星周分子的同位素谱线强度比已被广泛用于追踪演化恒星的光球元素同位素比。然而，根据分子种类和环境的物理条件，星周同位素比值可能会与恒星大气值有很大的偏差。在本文中，我们的目的是研究如何的CO和HCN丰度比径向变化，由于在流出的AGB星的化学反应和激发和光学深度的影响，从而产生的线强度比。我们发现，星周12 CO/13 CO可以偏离其大气值高达25-94%和6-60%，分别为C-和O-型CSE。我们发现，ISRF的强度和气体动力学温度的变化可以显着影响CO同位素比在外部CSE。相反，H12 CN/H13 CN比率对于所有测试的质量损失速率是稳定的。RT模拟表明，不同的旋转跃迁的CO的综合线强度比变化显着的恒星与中间的质量损失率，由于化学和激发效应相结合。相反，两种HCN同位素物的激发条件是相同的。我们证明了使用的同位素丰度配置文件从化学模型作为输入RT模型中的同位素观测的解释的重要性。以前的研究CO同位素比值是基于多个过渡数据的个别来源，这是很难估计的误差，由于假设是不完全正确的，根据这项研究报告的值。如果有的话，以前的研究可能高估了拱星12 CO/13 CO丰度比。使用HCN作为示踪剂的C同位素比的影响较少复杂的问题，提供一个帐户的高光学深度的校正。

英文摘要：

Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the circumstellar isotopologue ratio may deviate considerably from the stellar atmospheric value. In this paper, we aim to examine how the CO and HCN abundance ratios vary radially due to chemical reactions in the outflows of AGB stars and the effect of excitation and optical depth on the resulting line intensity ratios. We find that the circumstellar 12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for C- and O-type CSEs, respectively. We show that variations of the intensity of the ISRF and the gas kinetic temperature can significantly influence the CO isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is stable for all tested mass-loss rates. The RT modeling shows that the integrated line intensity ratio of CO of different rotational transitions varies significantly for stars with intermediate mass-loss rates due to combined chemical and excitation effects. In contrast, the excitation conditions for the both HCN isotopologues are the same. We demonstrate the importance of using the isotopologue abundance profiles from chemical models as inputs to RT models in the interpretation of isotopologue observations. Previous studies of CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C isotope ratios is affected by fewer complicating problems, provided one accounts corrections for high optical depths.

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