Velocity Vr (km s 1)

Figure 3.7 Profiles of HI emission in the dark cloud complex p Ophiuchi (a) directly toward the complex, and (b) adjacent to the molecular gas.

ble total masses. They appear as an excess of 21 cm emission associated in both position and velocity with giant molecular clouds. The temperature of the envelope gas lies in the range of 50 to 150 K, i. e., similar to other HI clouds found throughout the Galaxy. Because of the much poorer spatial resolution available with centimeter radiation, these structures have not been mapped with nearly the detail of their molecular interiors, but there is evidence for filaments, arcs, and other inhomogeneities. The dashed contour in Figure 3.3 traces, for the Rosette system, the locus of the HI intensity at half the peak value. Similar envelopes, but on a reduced scale, are also detected around isolated dark clouds.

3.1.4 Origin and Demise

We have already seen one indication that giant molecular clouds are formed through the accumulation of many individual clumps. The clustering of the complexes along Galactic spiral arms suggests that this buildup occurs as gas flows into the potential wells associated with the arms (recall Figure 1.19). Here, the original gas is presumably atomic. Molecular clumps could form inside the condensing medium through their self-shielding from ultraviolet radiation (see Chapter 8), leaving behind the present HI envelopes. Note further that the observed drop in the H2 surface density between the arms implies that a typical giant cloud cannot survive as long as the interarm travel time of the Galactic gas, i. e., about 108 yr at the solar position.

What destroys the complexes? There is strong empirical evidence that the powerful winds and radiative heating associated with massive, embedded stars are the primary agents. In the Rosette cloud, the O stars within the Nebula have already dispersed much of the adjacent atomic and molecular gas, and are driving a thick HI shell into the remainder. The shell radius of 18 pc, in combination with the expansion velocity of 5 km s_1 obtained from the 21 cm line, implies that the expansion has proceeded for 4 x 106 yr. This time matches the age of the partially embedded NGC 2244 cluster, as determined by the main-sequence turnoff method (see Chapter 4).

For other systems, it is also possible to witness the progressive alteration in cloud properties as a function of the age of an associated stellar cluster. Older complexes generally contain o> o

Figure 3.8 Molecular mass near open stellar clusters as a function of the cluster age. The vertical arrows signify upper bounds.

Figure 3.8 Molecular mass near open stellar clusters as a function of the cluster age. The vertical arrows signify upper bounds.

more clumps of smaller diameter and show evidence for streams of ionized gas. In addition, the cloud fragments are receding from the stars, with a typical speed of 10 km s^1. Figure 3.8 displays the total molecular mass, as gauged by CO emission, lying within 25 pc of known stellar clusters. This mass is plotted as a function of the cluster age. (We will see how the second quantity is obtained in Chapter 4.) Note the marked decline in the mass by an age of 5 x 106 yr (log t = 6.7), with essentially complete disappearance after 5 x 107 yr. The first O stars must appear relatively soon after the formation of a complex, since the majority of giant clouds seen today contain a massive association. Thus, the disappearance time also provides an estimate of the maximum duration of complexes.

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