Stars form through the gravitational collapse of molecular cloud cores. Before collapsing, the cores are supported by thermal pressure and turbulent motions. A question of critical importance for the understanding of star formation is how to observationally discern whether a core has already initiated gravitational collapse or is still in hydrostatic balance. The canonical method to identify gravitational collapse is based on the observed radial density profile, which would change from Bonnor-Ebert type toward power laws as the core collapses. In practice, due to the projection effect, the resolution limit and other caveats, it has been difficult to directly reveal the dynamical status of cores, particularly in massive star forming regions. We here propose a novel, straightforward diagnostic, namely, the collapsing index (CI), which can be modeled and calculated based on the radial profile of the line width of dense gas. A meaningful measurement of CI requires spatially and spectrally resolved images of optically thin and chemically stable dense gas tracers. ALMA observations are making such data sets increasingly available for massive star forming regions. Applying our method to one of the deepest dense-gas spectral images ever taken toward such a region, namely, the Orion molecular cloud, we detect the dynamical status of selected cores. We observationally distinguished a collapsing core in a massive star forming region from a hydrostatical one. Our approach would help significantly improve our understanding of the interaction between gravity and turbulence within molecular cloud cores in the process of star formation.

恒星是通过分子云核心的引力坍缩形成的。在坍塌之前，核心由热压和湍流运动支撑。对于理解恒星形成至关重要的一个问题是如何通过观测辨别核心是否已经开始引力坍缩或仍处于流体静力平衡中。识别引力坍塌的典型方法是基于观察到的径向密度剖面，当核心坍塌时，它会从 Bonnor-Ebert 型变为幂律。在实践中，由于投影效应、分辨率限制和其他警告，很难直接揭示核心的动力学状态，尤其是在大质量恒星形成区。我们在这里提出了一种新颖、直接的诊断方法，即崩溃指数 (CI)，可以根据稠密气体线宽的径向剖面进行建模和计算。有意义的 CI 测量需要光学薄且化学稳定的致密气体示踪剂的空间和光谱分辨图像。ALMA 观测使此类数据集越来越多地可用于大质量恒星形成区域。将我们的方法应用于有史以来最深的密集气体光谱图像之一，即猎户座分子云，我们检测了选定核心的动态状态。我们通过观察将大质量恒星形成区域中的坍缩核心与静水核心区分开来。我们的方法将有助于显着提高我们对恒星形成过程中分子云核心内重力和湍流之间相互作用的理解。