标题： [C I] 和 [C II] 辐射在 IRC +10216 的星际包层中的研究 I. 观测数据和 [C I] 辐射的非局部热平衡建模 显示英文标题

标题： [C I] and [C II] emission in the circumstellar envelope of IRC +10216 I. Observational data and NLTE modeling of the [C I] emission

摘要： 目标：本研究旨在描述原子碳C0在包层中的分布，以支持对其光化学过程的更深入理解。此外，我们还简要讨论了朝向恒星的[CII]发射观测。方法：我们在距离恒星最大78"的投影距离处获得了[CI]$\mathrm{^3P_1} \rightarrow \mathrm{^3P_0}$亚能级线的光谱。线轮廓通过直接拟合高斯组分以及通过建模[CI]三重态的观测线来表征。我们还报告了在中心位置以及距离恒星32"处从C+亚能级单重态检测到的$\mathrm{^2P_{3/2}} \rightarrow \mathrm{^2P_{1/2}}$线。结果：IRC +10216的[CI]发射的整体情况与之前更为有限的研究一致。观测和建模的线轮廓之间令人满意的一致性表明，在系统速度处的发射出现在距离恒星一个光束以外，排除了C0位于薄壳中的可能性。鉴于CO的键能仅比C0的电离阈值低0.1 eV，因此在投影距离超过$\sim 10^{17}$cm的视线方向上未观察到可辨识的[CII]发射（采用130 pc的距离）并不矛盾于一种情景，即大部分C0位于CO和C+之间，这符合外部远紫外辐射场的预期。这一假设也得到了一个模型的支持，该模型中C0壳层位于更外侧，无法再现距离恒星中间天空平面距离的[CI]线轮廓。将文献中采用的光化学模型与假设C0丰度相对于$\mathrm{H}_2$密度为常数的简化条件进行比较，我们限定了[CI]发射壳层的内边界，该边界位于距离恒星$\sim 10^{16}$cm处。 显示英文摘要

摘要： Aims: The study at hand aims to describe the distribution of atomic carbon, C0, throughout the envelope, in support of an improved understanding of its photo-chemistry. Additionally, we also briefly discuss the observation of [CII] emission towards the star. Methods: We obtain spectra of the [CI] $\mathrm{^3P_1} \rightarrow \mathrm{^3P_0}$ fine structure line at projected distances of up to 78" from the star. The line profiles are characterized by both direct fitting of Gaussian components, and by modeling the observed line of the [CI] triplet. We also report the detection of the $\mathrm{^2P_{3/2}} \rightarrow \mathrm{^2P_{1/2}}$ line from the C+ fine structure singlet at the central position and at 32" from the star. Results: The overall picture of the [CI] emission from IRC +10216 agrees with more limited previous studies. The satisfying agreement between the observed and modeled line profiles, with emission at the systemic velocity appearing beyond one beam from the star, rules out that the C0 is located in a thin shell. Given that the bond energy of CO falls only 0.1 eV below the ionization threshold of C0, the absence of observable [CII] emission from sightlines beyond a projected distance of $\sim 10^{17}$ cm from the star (adopting a distance of 130 pc) does not contradict a scenario where the bulk of C0 is located between that of CO and C+, as expected for an external FUV radiation field. This conjecture is also corroborated by a model in which the C0 shell is located farther outside, failing to reproduce the [CI] line profiles at intermediate sky-plane distances from the star. Comparing a photo-chemical model adopted from literature with the simplifying assumption of a constant C0 abundance with respect to the $\mathrm{H}_2$ density, we constrain the inner boundary of the [CI] emitting shell, located at $\sim 10^{16}$ cm from the star.