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

Astronomy and Astrophysics – Astronomy

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Ism: Clouds, Hii Regions, Ism: Individual: W51B, Ism: Molecules, Radio Lines: Ism

Scientific paper

We present ^13CO J=1-0 line observations of the Hii 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 kms^-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 ~48x17 pc^2 and ~2.4x10^5 Msolar respectively. In a position-velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v_LSR = +60 to +73 kms^-1. The velocity gradient of this ring is ~0.5 kms^-1 pc^-1, and the mass is ~6.2 x 10^4 Msolar. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ~7 kms^-1, ~13 pc and ~3.0 x10^49 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 Hii 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.

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