Giant Molecular Cloud

Young stars like those of the Trapezium are forming out of a huge body of gas known as the Orion Molecular Cloud. The extent of this object is indicated by the shading in Figure 1.1. In its longest dimension, the cloud covers 15° in the sky, or 120 pc at the distance of 450 pc.1 Orion is but one of thousands of giant molecular clouds, or cloud complexes, found throughout the Milky Way. The gas here is predominantly molecular hydrogen, H2. With their total masses of

1 The reader unfamiliar with units commonly employed in astronomy, such as the parsec (pc), should consult Appendix A.

The Formation of Stars. Steven W. Stahler and Francesco Palla Copyright © 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-40559-3

Figure 1.1 A portion of the Northern sky. The Milky Way is depicted as light grey, while the darker patches indicate giant molecular clouds. Also shown, according to their relative brightness, are the more prominent stars, along with principle constellations.

order 105 Me, these structures are the largest cohesive entities in the Galaxy and are almost all producing new stars.

The fact that we know of molecular clouds at all is a triumph of radio astronomy. Gas in these regions is much too cold to radiate at visible wavelengths, but may be detected through its radio emission in trace molecules such as CO. Here observers most often rely on single dishes that can map extended areas of the sky. For more detailed studies of individual regions, one may utilize the fact that several dishes linked together effectively increase the detector area and hence the angular resolution. Such interferometers have been powerful research tools, especially for studying the distribution of matter around newly formed stars.

The left panel of Figure 1.2 is a high-resolution CO map of the entire Orion molecular cloud. In this case, the observations were made with a relatively large single-dish telescope. The spectral line being detected is the commonly used 2.6 mm transition of the main isotope, 12C16O. We have distinguished in the figure two major subunits, labeled Orion A and B. Both the elongated shape of the whole complex and its high degree of clumpiness are generic features of such structures.

Along with their gas, molecular clouds contain an admixture of small solid particles, the interstellar dust grains. These particles efficiently absorb light with wavelengths smaller than their diameters (about 0.1 pm) and reradiate this energy into the infrared. Regions where the dust effectively blocks the light from background stars are traditionally known as dark clouds. Generally, these represent higher-density subunits within a flocculent cloud complex, although they can also be found in isolation. Note that the extinction due to dust depends on its column density, i. e., the volume density integrated along the line of sight. Figure 1.3 depicts the major dark clouds in Orion, determined by tracing the regions of strong obscuration in optical photographs. A number of the most prominent structures, such as L1630 and L1641, are labeled by their designations in the Lynds cloud catalogue. The shaded areas, including those with NGC numbers, are chiefly reflection nebulae, i. e., dusty clouds that are scattering optical light from nearby stars into our direction.2

The mid- and far-infrared emission from warm dust particles provides yet another means to study the Orion region. The first instrument devoted exclusively to infrared mapping of the sky was IRAS (for Infrared Astronomical Satellite), launched in 1983. Figure 1.4, which spans the same angular scale as the previous two figures, shows the Orion Molecular Cloud as a composite of three monochromatic IRAS images taken at 12, 60, and 100 pm. Radiation in this spectral regime comes mainly from dust heated to roughly 100 K. The fact that even this modest temperature is maintained over such an extended region demands the presence of many stars of high intrinsic luminosity.

Returning to the 12 C16O map of Figure 1.2, we see that several areas associated with reflection nebulae have closed, nested contours, indicating a local buildup in radio intensity. The received intensity in 12C16O is correlated with the hydrogen column density, so that such buildups mark the presence of embedded clumps. The 2.6 mm transition is most readily excited by gas with number density near 103 cm-3 and is relatively weak at higher values. However, other tracers are available to explore denser regions. The inset in Figure 1.2 is a map of Orion B in the 3.1 mm line of CS, a transition excited near 104 cm-3. Here, most of the individual fragments have sizes of about 0.1 pc and inferred masses near 20 M0, while the few largest ones have masses ten times as great. Such localized peaks within the broad sea of molecular cloud gas are known as dense cores and are the actual sites of star formation.

The stars being born within dense cores shine copiously in optical light, but none of this short-wavelength radiation can penetrate the high column densities in dust. As before, however, the same dust can be heated to emit at wavelengths that can escape. The shaded portions of the Figure 1.2 inset show regions detected at 2.2 pm. What is being seen, in fact, are several

2 The NGC designation is historical and refers to the "New General Catalogue" of nebulous objects, dating from the

19th century.

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