i 1 , 1 i 1 , 1
Figure 4.6 Bolometric luminosity function for 55 p Ophiuchi stars, again in L1688. The distribution among three spectral classes is indicated.
Let us now turn to the morphology of actual clusters. A more complete picture is obtained by combining infrared studies of stars and their attendant dust with radio observations of the gas. One important example is the ensemble of clusters within the L1630 region of Orion B. Here, star formation is almost exclusively confined to four discrete regions: NGC 2071, 2068, 2023, and 2024 (recall Figures 1.2 and 1.3). Each of these clusters, roughly 1 pc in diameter, is associated with a previously known HII region or reflection nebula, signaling the presence of at least one O or B star. Stellar densities are of order 100 pc~3, similar to the nucleus of the L1688 cloud in p Ophiuchi. In both locations, molecular gas with nH > 104 cm~3 comprises from 50 to 90 percent of the total mass of several hundred MQ within the cluster borders.
The massive, bright stars spawned within many clusters allow these systems to be seen across the Galaxy. Subsequent mapping with near-infrared arrays has then revealed the lower-mass population. Plates 1 through 8 (see end of chapter) are a sequence of images showing groups in formation, at the distance of Orion and beyond. First is NGC 2024, richest of the L1630 clusters. On the left in Plate 1 is the optical view, which is notable for the broad, vertical dust lane obscuring most of the interior stars. Several hundred of these stars are revealed in the infrared image shown as the righthand part of Plate 1. Here it is apparent that the very brightest stars tend to lie in the most crowded portion of the cluster, a region that is invisible optically.
Most embedded systems like this one are not destined to form open clusters, but will become unbound after their gas is dispersed. The argument is a statistical one. Assuming that star formation in the Orion Molecular Cloud is representative of other complexes, then roughly 50 clusters like NGC 2024 should be forming now within 2 kpc of the Sun. We noted in Chapter 3 that much of the molecular gas associated with a cluster disappears by 5 x 106 yr, which we may take as a representative formation time. Thus, after 108 yr of steady cluster production, about 50 x 20 = 103 systems should be found in the same Galactic area. In fact, the total number of open clusters this age or younger is less than 100, so that their formation must be quite inefficient.
Plate 2 is a near-infrared view of S106, the nearest (at 600 pc) and best-studied example of a bipolar nebula illuminated by a massive star. Long known as an optical HII region, S106 again has a prominent dark lane in its central region. Here radio studies have uncovered a rich concentration of molecular gas. Situated within the obscuring slab is the infrared source IRS 4, a late-O or early-B star of 104 LQ that is driving high-velocity, ionized flows into each lobe. These latter two structures span a total length of 0.7 pc. Over 200 stars with mK < +14 are located within a 0.3 pc radius of IRS 4. The stellar density, which again peaks near the massive star, exceeds 103 pc~3, approaching that in the Trapezium cluster.
Not all massive stars coincide with low-luminosity clusters, at least according to present observations. Plate 3 is a red photograph showing three HII regions in the Gem OB1 association, 2.5 kpc distant. The left and center regions are denoted S255 and S257, respectively, while the more diffuse region to the right is S254. The bottom panel (Plate 4) is a near-infrared image covering a smaller scale. The two brightest objects on the extreme left and right are the isolated B0 stars exciting S255 and S257, respectively. These stars have no detected low-mass companions. On the other hand, the prominent cluster between them, revealed only in the infrared, contains about 70 members within a 0.5 pc radius. Near the cluster center is an embedded star, designated S255/IR, that radiates close to 105 Le, largely in the far-infrared. The cluster itself is sandwiched between two peaks of radio emission from a compact molecular cloud.
Intermediate-mass stars are also frequently located within clusters of less massive objects, but this retinue is less dense. Consider the Herbig Be star BD+40°4124, located 1 kpc away, in the direction of the Cygnus spiral arm. Near-infrared imaging (Plate 5) shows the star attended by several dozen embedded sources; these outnumber the nearby visible T Tauri stars by a factor of three. Observations in 12C18O and CS reveal two such visible stars, V1318 Cygni and V1686 Cygni, to be situated in a ridge of very dense gas, comprising several hundred Mq. The molecular outflow and maser activity also detected in the region stem from a bright infrared binary companion to V1318 Cygni, rather than the optical Be star. This companion has the largest infrared excess of any cluster member. Whether other visible Herbig stars actually drive molecular outflows remains an open question.
Occasionally, star forming regions of differing ages are found in proximity, as illustrated in Plate 6. This image shows the environment of NGC 7538, a previously known HII region in the Cas OB2 association. Diffuse radiation from the HII region itself, consisting of both reflected starlight and thermal gas emission, appears as the crescent-shaped nebulosity surrounding the hot, white OB stars. The two prominent red patches are even younger regions containing molecular gas and compact clusters of embedded stars.
One of the most spectacular HII regions is NGC 3603, shown in the central portion of Plate 7. This massive cluster, seen here in the near-infrared, is located in the Carina spiral arm, at a distance of 6 to 7 kpc. The O and B stars alone comprise some 2000 Mq and have 100 times the ionizing luminosity of the Trapezium Cluster. Indeed, the center of NGC 3603 is one of the densest concentrations of high-mass objects in the Galaxy. Many of the O and B stars are visible optically, despite the ambient dust. However, the far more numerous low-mass stars are only discernible in the infrared. Their placement in a color-magnitude diagram yields pre-main-sequence contraction ages of 3 x 105 to 1 x 106 yr.
The production of many thousands of stars in such a brief period makes NGC 3603 an impressive HII region but still not on the scale of a true starburst. To find these, we need to go outside the Milky Way. The neighboring Large Magellanic Cloud, for example, contains 30 Doradus (Plate 8). This giant HII region is morphologically similar to NGC 3603, but has a total luminosity ten times higher, i. e., of order 108 Lq. The central cluster is bright enough to be seen optically, even at a distance of 50 kpc. Note the great tendrils of gas around the compact, stellar group. These structures give the region its other name, The Tarantula Nebula. Even brighter starbursts lie within dwarf irregular and spiral galaxies, all located at greater distances.
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