Frequency v (GHz)
Figure 5.2 Submillimeter spectrum of two regions in Orion. The KL Nebula (upper spectrum) shows many more molecular lines than the dense core in Orion 1.5' South (lower spectrum).
a general explanation. The observed gradients could also reflect the presence of still undetected young stars, whose luminosities would thermally alter the local chemistry.
Chemical gradients are also present in sites of high-mass star formation, such as the Orion BN-KL region. The fact that the luminous infrared sources IRc2 and BN have significantly affected their surroundings is evident from Figure 5.2. Here a portion of the spectrum near the 868 pm (v = 345 GHz) transition of H13CN is shown, both for the KL Nebula and for a starless dense core known as Orion 1.5' South, located 0.2 pc from IRc2. (Recall Figure 1.7.) Although the two cores have similar total column densities through their centers, the one in the KL Nebula has a far richer spectrum of molecular lines. At the same time, this region has a lower abundance of complex organics. In fact, Orion 1.5' South has a similar chemical makeup as TMC-1, although it has 5 times the temperature and 10 times the density of the latter. Clearly, the critical factor here is proximity to a massive star.
Several effects are at work to alter and enrich the molecular population near IRc2. The core material surrounding the star has a temperature of some 200 K, sufficient to vaporize grain mantles and thereby reinject molecules into the gas phase. Even closer to the star, shock waves driven by the massive outflow can heat the gas to the point of destroying the grain cores themselves. Such thermal processing explains why sulfur and silicon, both major grain constituents, are relatively abundant in this and other outflow regions. At the same time, such environments are unfavorable to long chains, which apparently need heavily shielded dense cores to form and survive.
Of the total list of known interstellar molecules, about half were first discovered in Sagittarius B2, a giant molecular cloud located within 200 pc of the Galactic center. With a visual extinction of some 30 mag, even near-infrared observations of the region are unable to detect any but the brightest sources. Figure 5.3 is a high-resolution map in 800 pm continuum emission from heated dust. The cloud complex Sgr B2 lies along the dust ridge that also contains Sgr A*, a compact radio source only a few parsecs from the Galaxy's true center. As in the case of Orion, the abundance of molecules is correlated with an elevated gas temperature created by the presence of many embedded stars. Most molecules are seen in Sgr B2/North, a compact clump with gas temperature 200 K and a density in H2 as high as 107 cm-3. Here, it is impossible to see the stellar cluster directly, but its presence is inferred from the 5 x 106 LQ in radiation emitted by heated dust. The pattern in molecular abundances broadly resembles Orion KL. It is intriguing that an equally luminous clump located just 2 parsecs away from Sgr B2/North is relatively sparse in molecular lines. The luminosity in this latter region, known as Sgr B2/Middle, stems from identifiable HII regions associated with several dozen O and B stars. The lesson here is that the ultraviolet radiation inflating an HII region also destroys the molecules that previously surrounded the star when it was more deeply embedded.
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