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arXiv 1012.5076astro-ph.SR
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IRC +10216中的水:内风中激波诱导化学作用的真实形成过程

Water in IRC+10216: a genuine formation process by shock-induced chemistry in the inner wind

Isabelle Cherchneff

摘要:

在极端碳星IRC+10216的风中存在水已经被赫歇尔望远镜证实。高J H2O谱线被探测到的区域靠近恒星半径r \geq 15 R \ast。目的:我们研究了IRC+10216内部层中的水和相关分子的形成,在那里灰尘也会形成并加速风。方法:我们描述的分子形成的化学动力学网络,涉及碳和氧为基础的分子。然后,我们将这个网络的物理条件有关的灰尘形成区的经验之间的1和5 R \ast的脉动驱动的冲击通过。我们解决了一个系统的刚性,耦合,普通和微分方程。结果:成尘区存在非平衡化学。H2O在光球层上方快速形成,这是由热激波后气体中CO的热碎裂引起的羟基OH的合成。导出的5 R \ast处H2的丰度为1.4 \times 10-7,这与Herschel观测值非常吻合。无论恒星的C/O比如何,水的非平衡形成过程都是活跃的,因此,在渐近巨星分支上的所有恒星的风加速区都应该存在H2O。

英文摘要:

Context: The presence of water in the wind of the extreme carbon star IRC+10216 has been confirmed by the Herschel telescope. The regions where the high-J H2O lines have been detected are close to the star at radii r \geq 15 R\ast. Aims: We investigate the formation of water and related molecules in the periodically-shocked inner layers of IRC+10216 where dust also forms and accelerates the wind. Methods: We describe the molecular formation by a chemical kinetic network involving carbon-and oxygen-based molecules. We then apply this network to the physical conditions pertaining to the dust-formation zone which experiences the passage of pulsation- driven shocks between 1 and 5 R\ast. We solve for a system of stiff, coupled, ordinary, and differential equations. Results: Non-equilibrium chemistry prevails in the dust-formation zone. H2O forms quickly above the photosphere from the synthesis of hydroxyl OH induced by the thermal fragmentation of CO in the hot post-shock gas. The derived abundance with respect to H2 at 5 R\ast is 1.4\times10-7, which excellently agrees the values derived from Herschel observations. The non-equilibrium formation process of water will be active whatever the stellar C/O ratio, and H2O should then be present in the wind acceleration zone of all stars on the Asymptotic Giant Branch.

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