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The Minimum Mass of Molecular Cloud Cores

We have found a physical reason why there exists an intrinsic scale in star-forming regions as, for example, in the Taurus region. This intrinsic scale is found in Larson's Figure 1 through the angular correlation function of the companions' surface density on the sky. In this figure, there is a knee in the curve at 0.04 pc which may be related to binary formation inside the molecular cloud cores. The existence of a knee in the correlation function figure also indicates that a physical process other than self-gravity performs during the evolution of molecular cloud cores. On the other hand, the internal motion of gas in a molecular cloud is turbulent. If molecular cloud cores are embedded in a molecular cloud, the cores suffer fluid dynamical instability, especially Kelvin-Helmholtz (KH) instability. This instability is fundamental if relative motion between blobs and ambient molecular gas exists. Through KH instability we can determine the minimum mass and size of molecular cloud cores, about 10 M☉ and about 0.1 pc, respectively. The mass is comparable to the Jeans mass of cores, and the growth rate of the KH instability is also comparable to that of the Jeans instability. Thus, we can conclude that the intrinsic scale in a star-forming region is also determined by the typical evolution of molecular cloud cores in a two-phase gas mixture in which cores suffer fluid dynamical instability. It is known that star formation occurs after the formation of a molecular cloud core. Then, if a star comes into existence as a binary in the core, the intrinsic scale may be lower than the 0.1 pc scale.