Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as Tracers of Photochemistry in Disks-猫眼课题宝

DOI：

10.3847/1538-4365/ac143a

发表时间：

2021-09

期刊：

The Astrophysical Journal Supplement Series

影响因子：

通讯作者：

中科院分区：

其他

文献类型：

作者：

J. Bergner;K. Öberg;V. Guzmán;C. Law;R. Loomis;G. Cataldi;A. Bosman;Y. Aikawa;S. Andrews;E. Bergin;A. Booth;L. Cleeves;I. Czekala;Jane Huang;J. Ilee;R. Le Gal;F. Long;H. Nomura;F. Ménard;C. Qi;K. Schwarz;R. Teague;T. Tsukagoshi;C. Walsh;D. Wilner;Yoshihide Yamato

原行星盘表层的紫外线光化学作用显著地改变了它们相对于恒星形成前阶段的组成。利用丰度比CN/HCN对各种天体的紫外场进行跟踪早已被提出；然而，到目前为止，CN、HCN和磁盘中UV场之间的关系仍然不明确。作为ALMA大程序MAPS（行星形成尺度上的ALMA分子）的一部分，我们提出了CN N = 1-0跃迁在0的观测结果。″3分辨率到5个磁盘系统。所有圆盘在~ 50 - 150au范围内显示明亮的CN发射，以及延伸至600 au的漫射发射架。在所有来源中，我们发现CN/HCN柱密度比随着圆盘半径从大约1增加到100，可能是由于紫外线穿透增加，增强了外圆盘中HCN的选择性光解。此外，多个毫米尘埃间隙和环分别与CN/HCN比值的波峰和波谷相吻合，这意味着一些毫米亚结构在更高的盘层中伴随着紫外线穿透的变化。CN/ hcn的比率普遍较高（>1），这表明光化学形成圆盘的化学成分很稳定，也意味着CN是大多数圆盘半径上具有益生元意义的腈基的主要载体。我们还发现，尽管从垂直分层的圆盘区域发射，但CN和HCN的局部柱密度呈正相关，表明不同的圆盘层之间存在化学联系。这篇论文是《天体物理学杂志增刊》MAPS特刊的一部分。

UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been proposed to trace the UV field in various astrophysical objects; however, to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN N = 1–0 transitions at 0.″3 resolution toward five disk systems. All disks show bright CN emission within ∼50–150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions and also means that CN is the dominant carrier of the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as Tracers of Photochemistry in Disks

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