' WOCyg Gould's Belt -C\ Lac1 OCys4


o 0792

Q Set 2

Ara 1

1 kpc

Ara 1

Sco 1

Figure 4.14 Galactic distribution of the closest OB associations. The Sun is indicated near the center of the frame. Sizes of the circles represent the physical dimension of each system and, in a few cases, the gross morphology. Note the ring of stellar and gaseous emission in Gould's Belt (shaded region), surrounding the solar symbol.

Before delving into the morphology of individual associations, let us first consider their spatial distribution. Figure 4.14 depicts all the systems within 1.5 kpc, as projected onto the Galactic plane. The reader may profitably compare Figure 3.2 for giant molecular clouds, which focuses on objects inside the Sun's galactocentric radius. Note that the nearest segment of the Perseus spiral arm, containing such associations as Cep OB1 and Per OB1, lies beyond the upper right border of Figure 4.14, between 2 and 3 kpc away. Note, too, we have already encountered many of these systems in other contexts. Thus, Mon OB1 is the relatively small association containing the cluster NGC 2264, and Mon OB2 encompasses NGC 2244 in the Rosette molecular cloud. Ori OB1 is the large association that includes the Trapezium, while Per OB2 surrounds the embedded cluster IC 348. These examples illustrate how expanding stellar systems may harbor smaller, interior clusters. They further remind us of the intimate relationship between OB associations and giant molecular clouds. Indeed, it is the rare association that is not in close proximity to some cloud complex.

Table 4.2 summarizes essential properties of the associations closer than about 600 pc, as measured by the Hipparcos satellite. Listed here are both a distance and an estimated physical diameter. The latter, indicated approximately in Figure 4.14, was obtained using the average angular extent of each system together with its distance. The table also gives separately the

Table 4.2 The Nearest OB Associations




O stars

B Stars

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