Massive star formation in W51 A triggered by cloud–cloud collisions

Abstract

WA is one of the most active star-forming regions in the Milky Way, and includes copious amounts of molecular gas with a total mass of . The molecular gas has multiple velocity components over similar to 20kms(-1), and interactions between these components have been discussed as the mechanism that triggered the massive star formation in WA. In this paper, we report on an observational study of the molecular gas in WA using the new (CO)-C-12, (CO)-C-13, and (CO)-O-18 (J = 1-0) data covering a 14 x 10 area of WA obtained with the Nobeyama 45m telescope at 20" resolution. Our CO data resolved four discrete velocity clouds with sizes and masses of similar to 30pc and 1.0-1.9 x 10(5) M-circle dot around radial velocities of 50, 56, 60, and 68kms(-1). Toward the central part of the H II region complex G49.5-0.4 in WA, in which the bright stellar clusters IRS1 and IRS2 are located, we identified four (CO)-O-18 clumps having sizes of similar to 1pc and column densities of higher than 10(23)cm(-2), which are each embedded within the four velocity clouds. These four clumps are concentrated within a small area of 5pc, but show a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each other by bridge features having weak intensities. The high intensity ratios of (CO)-C-13 (J = 3-2)(J = 1-0) also indicate that these four clouds are associated with the H II regions, including IRS1 and IRS2. We also reveal that, in the other bright H II region complex G49.4-0.3, the 50, 60, and 68kms(-1) clouds show a complementary distribution, with two bridge features connecting between the 50 and 60kms(-1) clouds and the 60 and 68kms(-1) clouds. An isolated compact H II region G49.57-0.27 located similar to 15pc north of G49.5-0.4 also shows a complementary distribution and a bridge feature. The complementary distribution on the sky and the broad bridge feature in the position-velocity diagram suggest collisional interactions among the four velocity clouds in WA. The timescales of the collisions can be estimated to be several 0.1Myr as crossing times of the collisions, which are consistent with the ages of the H II regions measured from the sizes of the H II regions with the 21cm continuum data. We discuss a scenario of cloud-cloud collisions and massive star formation in WA by comparing these with recent observational and theoretical studies of cloud-cloud collision.