恒星形成于主要成分为分子的星际介质中。因此，分子的形成是恒星形成中不可或缺的步骤。用传统方法，比如HI发射和CO发射，难以追踪从原子到分子的转变。我们发展了一种观测技术，中性氢窄线自吸收（HINSA），这种技术提供了对分子云内的原子氢丰度进行直接测量的目前唯一的方法， 受到相关领域综述性文章的肯定（例如， McKee & Ostriker 2007, ARA&A). .为了构建更加完整的星际介质原子向分子演化的图像，我们计划在HINSA选源的基础上，观测和分析碳的演化。在HI转变为H2的同时，ISM中的碳由CII演化为CI并且最终被固定在CO中。本项目建议利用现存的Herschel，SOFIA，德令哈和CSO数据，争取CSO及APEX新观测时间，研究CII->CI->CO在银河系暗云中的演化。本项目的成果有可能对约束分子演化时标，确定恒星的形成初始条件产生重大影响。

Stars form in dense, largely molecular interstellar medium. The formation of molecules is thus an essential step in star formation. The transitional phase of ISM (HI to H2) cannot be directly traced by tranditional methods, such as HI emission and CO emission. We have developed an obserational technique, HI Narrow Self-Absorption (HINSA), which provides the only direct measurement of HI abundance INSIDE molecular clouds availabe so far. HINSA was refered to as a "chemical clock" by several review articles, e.g., "Theory of Star Formation" (McKee & Ostriker 2007, Annual Review of Astronomy and Astrophysics). .Based on HINSA, we can select targets showing signs of on-going HI-H2 Transition. As HI is turned into H2 in the ISM, Carbon changes from ionized carbon (CII) to atomic carbon (CI), and will finally be locked into CO. This proposal will focus on existing Herschel, SOFIA, Delingha, and CSO data of CII, CI, and CO. We also plan to obtain new data from either CSO or APEX. The analysis of these data sets will help build a better picture of the evolution of carbon in Milky way dark clouds. The understand of carbon evolution and the concurrent HI-H2 transition should shed significant lights into critical questions of star formation.