2003ApJ...594..340S

Extensive and sensitive strip-scan observations of the southeastern envelope of Heiles Cloud 2 in Taurus were carried out in ¹²CO J=1-0 emission with an angular resolution of 15" or 0.010 pc at 140 pc. We discovered in the cloud envelope a number of small-scale structures with large line widths, while in the main body of the cloud, relatively narrow line width and small velocity gradient was observed. The typical size and line width of the small scale features in the cloud envelope is 0.1 pc and 2 km.s^–1, respectively. This line width is among the largest of this size observed in nonstar forming regions and even larger than high-mass cores of comparable size. Most of them have low gas density (≲10² cm^–3) and sizes significantly smaller than their corresponding Jeans lengths and seem stable under external pressure at a level of 10³-10⁴ K.cm^–3. These features may be evidence for structure formation through thermal instability. In the cloud envelope, there exist small features with large peak-to-peak separation of velocity centroid (∼7 km.s^–1), but they suddenly disappear in regions with A_V≳1. This sudden change of velocity structure may be due to the gas phase transition from two-phase (WNM+CNM) to single CNM phase.