Molecular Clouds in W49 and W51

Abstract

Radio observations of six molecular lines have been obtained in the W49 and W51 H ii region sources as part of an investigation of the physical conditions in molecular clouds and the relationship of these clouds to the H ii regions. The principal observations are maps with 4' spacing of the 6-cm formaldehyde (H_2CO) absorption and strip maps with 1' spacing of carbon monoxide (CO, J = 1 → 0) emission at 2.6 mm. A few selected positions were also observed in ^(13)CO and C^(18)O as well as the carbon monosulfide lines (CS, 3 → 2 and 2 → 1) at 2 and 3 mm. Seven distinct clouds are found, and five of these are associated with or near H ii regions. The molecular clouds are all much larger than the H ii regions; one near W51 (at 65 km s^(-1)) extends over an area containing at least three H ii regions and appears as a self-absorption feature in CO. The mass of three of these molecular clouds is estimated to be at least 10^4 - 10^5 M_⊙, 10 times as great as the ionized regions. Average hydrogen molecule densities, over the entire clouds, are about 200 - 1000 cm^(-3); but near the H ii regions there are probably compact, high-density (≳ 10^6 cm^(-3)) molecular regions which are responsible for the high-excitation lines of CS. The excitation temperature of the 6-cm H2_CO transition in two clouds in the direction of W49A is estimated to be 1.76 ± 1.2° K. The large velocity difference between these clouds and the H ii region and the narrowness of the lines suggest that the clouds are dark nebulae and unrelated to W49A; however, there is H_2O maser emission at the velocities of the dark cloud lines suggesting that some of the maser emission may be from foreground objects.