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Figure 4.17 Main-sequence turnoff in Upper Scorpius. The dashed curve represents the ZAMS, while the solid curve is the 5 x 106 yr isochrone.

Figure 4.17 Main-sequence turnoff in Upper Scorpius. The dashed curve represents the ZAMS, while the solid curve is the 5 x 106 yr isochrone.

of lower mass. Within Upper Scorpius, X-ray observations by the Einstein satellite turned up dozens of previously unknown members. Subsequent photometry and spectroscopy showed that most of these are weak-lined T Tauri stars. Deeper surveys by the ROSAT and Chandra X-ray satellites have uncovered even more sources, which by now outnumber the massive stars shown in Figure 4.17.

4.3.4 The Orion Association

Let us apply these ideas to the best known of all OB associations, that in Orion. Figure 4.18 shows the familiar outline of CO emission from the giant molecular cloud, along with the approximate boundaries of the four identified subgroups. That labeled 1c largely coincides with the Ori R1 association, while the small 1d subgroup is the region of radius 2.5 pc that includes the even more compact Trapezium cluster.

The spatial pattern of all the subgroups again suggests vividly the progression of massive star formation and further demonstrates how this process serves to clear out molecular gas. Thus, the oldest and largest 1a subgroup lies in an area currently free of CO emission. With a little imagination, one can picture how the Orion A cloud once extended northward into this region. The somewhat younger 1b system still partially encompasses dense gas, while the smallest 1c and 1d groups are wholly embedded within Orion A. This temporal ordering is confirmed by the stellar distribution. For example, the most luminous star in 1b is the supergiant Ç Ori, with a

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Figure 4.18 Subgroups in the Orion OB association. The outline of CO emission is also shown.

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Right Ascension a

Figure 4.18 Subgroups in the Orion OB association. The outline of CO emission is also shown.

mass of 49 Mq and a main-sequence lifetime of 4 x 106 yr. The corresponding member of 1a is n Ori, a B1 main-sequence star of 16 Mq with a lifetime of 1.4 x 107 yr.

We saw in Chapter 1 how near-infrared surveys have revealed many young, low-mass stars in the Orion B cloud. Similarly, in the 1d subgroup, a large, low-mass population is manifest both in the optical and infrared. A similar distribution surely holds in Orion 1a, where a turnon should exist somewhat below 1.5 Mq, the mass with a pre-main-sequence contraction time of 1.4 x 107 yr. Picking out F and G stars from the plethora of background sources over such a wide area is a challenging task.

Returning, then, to Figure 4.18, we can now appreciate how the OB association represents but one aspect (albeit the most conspicuous one) of much more extensive formation activity. Failure to recognize this fact has misled some into positing a causal link in subgroup formation. The idea is that the creation of O and B stars in one locale somehow induces collapse in a neighboring region, leading to a kind of OB chain reaction. However, while there is ample evidence that massive stars can terminate formation activity over a substantial volume, there is little to suggest that they also initiate it (except on a restricted spatial scale; see Chapter 15.) The relative ages and locations of OB subgroups are certainly of interest, but the true global pattern of stellar birth can only be discerned through multiple observations. A case in point is the L1641 region of Orion A, also depicted in Figure 4.18. Here, X-ray and infrared studies have uncovered a distributed population consisting of hundreds of low-mass stars. The oldest are weak-lined T Tauri's, with ages that rival high-mass members of the 1a subgroup, while the youngest are embedded infrared sources or classical T Tauri stars near the birthline.

The physical picture emerging is that the Orion Molecular Cloud has been gravitationally settling over some period exceeding 107 yr. This contraction has proceeded locally at different rates and with diverse outcomes. It apparently began within the present 1a subgroup, at an epoch which can best be measured once a turnon in low-mass stars is observed. Eventually, enough massive stars formed here to disperse the surrounding gas. Sometime later, the L1641 region also condensed to the point of star formation, but never attained the compactness necessary for massive stars. The 1c and 1d regions followed suit, and intense formation activity continues today in both Orion A and B. The details in this highly incomplete picture will undoubtedly change as future studies focus increasingly on the low-mass stellar component. What seems secure, both in Orion and elsewhere, is that star formation in any particular region can occur without an external trigger, purely through the gravitational contraction of a large cloud region. We shall elaborate this key idea in subsequent chapters.

4.3.5 Embedded and Runaway Stars

We have been focusing on massive stars that are optically visible. These have either moved away from, or else destroyed, the gas and dust in their immediate vicinity within the last few million years. The HII regions they excite are extended structures, with typical diameters of 1018 cm (i. e., 0.3 pc). Even younger O and early-B stars exist, for which the enshrouding matter completely absorbs all ultraviolet and visible photons. Constituting some 10 percent of the massive star population in the solar neighborhood, these objects are detectable through their reemission of stellar photons at radio and infrared wavelengths. Such ultracompact HII regions, roughly 1017 cm in size, are among the most luminous Galactic objects in the far infrared. The powerful radio source W49 in Aquila contains at least seven of these regions crowded into an area only 0.8 pc in diameter. Dense systems like this one could represent the ancestors of Trapezium-like clusters within visible OB associations and probably contain numerous low-mass members that are currently beyond detection.

Figure 4.19 shows the distribution of ultracompact HII regions in the Galactic plane, as revealed through IRAS observations in the far infrared. There is nearly a perfect match with the corresponding distribution of giant molecular clouds, delineated by the CO contours in the same figure. This agreement underscores the extreme youth of the deeply embedded stars.

At the other extreme are massive objects with little or no associated interstellar matter. As we mentioned, about 25 percent of O stars do not appear to be members of clusters or associations. These field objects tend to be farther from the Galactic midplane than their counterparts within groups. Their radial velocities also exhibit more dispersion about the local value expected from Galactic rotation alone. Statistical analysis of the velocities reveals that most objects are leaving the plane, rather than entering it from above and below. Thus, the stars were likely born in ordinary associations, but with speeds that were higher than average.

A large fraction of the field objects are runaway OB stars. These have exceptionally high spatial velocities, typically from 50 to 150 km s_1, and are sometimes located high above the Galactic plane. In one sense, the origin of runaways is not a mystery, since their proper motions can often be traced back to a known OB association. The real problem is their velocities, which indicate that the objects were once subject to strong forces. Thus, each runaway might originally

naturally has a lower speed than would a single, runaway object. The reader should keep these possibilities in mind as we periodically return to explore the special problems connected with massive star formation.

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