Molecular clouds are irregularly shaped objects that are spread over tens of parsecs (1 parsec ~ 3.1 x 10 cm or 2 x 10 miles) and contain enough material to build over a million solar systems such as our own. Within a single cloud the structure is often complex, with each cloud containing cores that are characterized by interstellar standards as having extremely high densities, n > 10 cm, surrounded by the more extended low density molecular cloud. This structure is illustrated in Figure 5, where many small dense cores are shown embedded within a larger cloud.

Since the density inside a star is significantly greater than the average density of a molecular cloud, astronomers theorize that, for unknown reasons, perhaps the passage of a shock-wave due to an expanding region of highly ionized gas surrounding a newly formed star (H II region) or a nearby supernova, portions of the low density molecular cloud collapse to form cores, which then provide the building blocks for star formation.

It is expected that the O and HO emission will be strongest in regions that are warm, i.e., temperatures > 15 K (-433 degrees Fahrenheit), and dense, i.e., densities of the primary constituent molecular hydrogen (H), > 10 cm, (ten thousand H molecules per cubic centimeter). For this reason the prime candidates for SWAS observations are giant and dark cloud cores.

I. GIANT CLOUD CORES

Giant molecular cloud (GMC) cores are regions of high density ( ~ 10 cm) and warm temperature (T ~ 35 K), located within a much larger lower density extended cloud. These cores have diameters of ~ 1 parsec and are generally associated with sites of massive star formation. The closest and most studied giant cloud core is the OMC-1 core associated with the Orion A (M42) H II region. Inside the Orion A H II region thousands of stars of the Trapezium cluster have been born from the parent cloud less than a million years ago.

With the proximity of numerous star forming sites, the conditions in OMC - 1 are favorable enough to enable observations of a large number of molecular transitions. In fact this core has proven to be one of the best laboratories to study chemical interactions in molecular clouds. The central portion of the core close to the embedded stars has provided for the first observation of CO (and many other species) in interstellar space. The entire core has been studied in many molecular transitions and shows considerable chemical complexity. Figure 6 shows the emission from the densest part of the core in transitions of several molecules, isotopic CO (CO), methanol (CHOH), and
diazenylium (NH).

The emission can, in general, be characterized as the number of photons received by the telescope, with stronger emission therefore tracing more photons. The differences seen in the spatial distribution of molecular emission within the Orion cloud are quite striking, with the CHOH emission strongest near the center of the map at the position of an embedded cluster of young stars, while the NH emission has a minimum at the position of the embedded stars, reaching a peak further to the north. These differences have been demonstrated by SWAS team members Ted Bergin, Paul Goldsmith, and Ronald Snell to be the result of variations in the chemical abundances of these species. The OMC-1 core is one of many GMC cores,in our galaxy and with such a high degree of chemical complexity sources like OMC-1 will be prime targets for the SWAS satellite.

In addition, models of the chemistry in OMC-1 region and in other cores have predicted that the atomic oxygen abundance is depleted onto grains from the cosmic value. Since the abundance of water and molecular oxygen are also dependent on the abundance of atomic oxygen SWAS observations will provide the only direct method of verifying these claims. The large scale mapping observations of neutral carbon and CO planned by SWAS will also be valuable in testing current theoretical models of chemistry and structure of dense star forming molecular cores.

II. DARK CLOUD CORES

Dark cloud cores are also sites of high density situated within a larger extended cloud. However, these cores are, in general, smaller (diameter 0.3 ~ pc), colder (T ~ 10 K), and less dense (n ~ 10 cm) than their giant cloud counterparts. In addition to being smaller, dark cloud cores do not form as many massive stars as giant cloud cores and therefore are thought to contain more pristine molecular material, material which has not been affected by the formation of stars. As such, these cores offer excellent targets for SWAS because an understanding of the conditions of molecular clouds prior to the formation of stars is required in order to understand the formation process itself.

Figure 7 presents a picture of one quiescent dark cloud core, L134N (Lynds 134N) in a transition of the oxygen bearing molecule sulfur monoxide (SO).

L134N will be a key SWAS target because this source is known to contain high amounts of oxygen bearing molecules. In fact, this is the only source in this galaxy, or any galaxy, where molecular oxygen has been observed. In 1993 astronomers using the POM-2 telescope in Meudon, France announced the tentative detection of isotopic oxygen OO in L134N after many days of integration. Isotopic oxygen can be observed from the Earth only under certain conditions. Using SWAS, astronomers should be able to verify this detection within a few orbits because SWAS will be observing the most abundant form of molecular oxygen, O, which is believed to be 500 times more abundant than OO.