Aims. Photodissociation by UV light is an important destruction mechanism for CO in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process.
Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C17O and 13C17O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks.
Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6e-10 s^-1, 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The photodissociation of C17O and 13C17O shows almost exactly the same depth dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally fractionated with respect to 16O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17O and 18O abundances in meteorites.
Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS
Chalmers University of Technology（查尔姆斯理工大学）；Uppsala University（乌普萨拉大学）；Joint ALMA Observatory（ALMA联合天文台）；European Southern Observatory（欧洲南方天文台）；Leiden University（莱顿大学）；Monash University（蒙纳士大学）；KU Leuven（荷语鲁汶大学）；Queen’s University Belfast（贝尔法斯特女王大学）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）；Gemini Observatory（双子座天文台）
C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud: An analysis of the precision achievable when modeling spectral line within the Local Thermodynamic Equilibrium approximation
通常在分子云中观察到CO同位素跃迁，以探测气体的柱密度，碳和氧的元素比，并追踪环境的运动学。我们的目标是估计丰度，激发温度，速度场和速度分散的三个主要的CO同位素对猎户座B分子云的子集。我们使用Cramer-Rao限（CRB）技术分析和估计的物理参数的精度在局部热力学平衡激发和辐射转移的框架内与加性高斯白噪声。我们提出了一个极大似然估计方法来推断CO同位素的1-0和2-1跃迁的物理条件。模拟结果表明，该估计器对常见的激发温度和柱密度范围（Tex > 6 K，N > 1 e 14 - 1 e 15 cm-2）是无偏的和有效的。与一般假设相反，不同的CO同位素体具有不同的激发温度，并且不同同位素体之间的线强度比不能准确地反映柱密度比。我们发现平均分数丰度与以前的决定对其他分子云是一致的。然而，显着的局部偏差推断，不仅在暴露于UV辐射场的区域，而且在屏蔽区域。这些偏差是由于选择性光解离、化学分馏和晶粒表面耗尽之间的竞争造成的。我们观察到C18 O发射的速度色散比13 CO的速度色散小10%。同时分析同一物种的两个不同的旋转跃迁所产生的大量增益被严格量化。CRB技术是一种很有前途的途径，用于分析从谱线拟合估计的物理参数。
Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. III. SiO and SiS abundances
Chalmers University of Technology（查尔姆斯理工大学）；Joint ALMA Observatory（ALMA联合 observatory）；European Southern Observatory（欧洲南方天文台）；Leiden University（莱顿大学）；Queen’s University Belfast（贝尔法斯特女王大学）
Exploring the Limits of Spectral Line Stacking in Spectral Line Data and Application Toward the Detection of Bulk $^{13}$C Enrichment of Aromatics in TMC-1
Massachusetts Institute of Technology（麻省理工学院）；NSF–Simons AI Institute for Cosmic Origins（美国国家科学基金会-西蒙斯宇宙起源人工智能研究所）；University of British Columbia（不列颠哥伦比亚大学）
