The high-velocity molecular stream in W51B: a ring structure with compact H ii regions

We present ¹³ CO J  = 1 ? 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ～ + 65 km s^?1 of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ～ 48 × 17 pc² and ～ 2.4 × 10⁵ M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v _LSR=+ 60 to +73 km s^?1. The velocity gradient of this ring is ～ 0.5 km s^?1 pc^?1, and the mass is ～ 6.2 × 10⁴ M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ～ 7 km s^?1, ～ 13 pc and ～ 3.0 × 10 ⁴⁹ erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0?0.3 and G48.9?0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.