Two Molecular Clouds near M17

Abstract

We present fully sampled images in the C 18 O J = 2-1 line extending over 13' × 23', made with the Heinrich Hertz Telescope (HHT) on Mount Graham, AZ. The HHT has a resolution of 35'' at the line frequency. This region includes two molecular clouds. Cloud A, to the north, is more compact, while cloud B is to the west of the H II region M17. Cloud B contains the well-known source M17SW. In C 18 O we find 13 maxima in cloud A and 39 in cloud B. Sixteen sources in cloud B are in M17SW, mapped previously with higher resolution. In cloud B, sources outside M17SW have line widths comparable to those in M17SW. In comparison, cloud A has lower C 18 O line intensities and smaller line widths but comparable densities and sizes. Maps of the cores of these clouds were also obtained in the J = 5-4 line of CS, which traces higher H 2 densities. Our images of the cores of clouds A and B show that for V LSR ≤ 20 km s -1 , the peaks of the CS emission are shifted closer to the H II region than the C 18 O maxima, so higher densities are found toward the H II region. Our CS data give additional support to the already strong evidence that M17SW and nearby regions are heated and compressed by the H II region. Our data show that cloud A has a smaller interaction with the H II region. We surmise that M17SW was an initially denser region, and the turn-on of the H II region will make this the next region of massive star formation. Outside of M17SW, the only other obvious star formation region may be in cloud A, since there is an intense millimeter dust continuum peak found by Henning et al. (1998) but no corresponding C 18 O maximum. If the CO/H 2 ratio is constant, the dust must have a temperature of ~100 K or the H 2 density is greater than 10 6 cm -3 or both to reconcile the C 18 O and dust data. Alternatively, if the CO/H 2 ratio is low, perhaps much of the CO is depleted.