The Chemistry of Star-Forming Regions

Abstract

During the last quarter century, chemists have responded magnificently to the challenges raised by astronomers in their attempts to understand the variety of molecules detected in interstellar clouds. Observations have shown the chemistry of these regions to be surprisingly complex, and now more than 100 molecular species have been identified in interstellar and circumstellar regions of the Galaxy. The chemistry of interstellar clouds that gives rise to these molecules is now believed to be reasonably well understood (see section 2) in terms of a network of some thousands of binary reactions between several hundred species.1,2 Astronomers are now applying the techniques of astrochemistry to interpret observations of star-forming regions.3 These regions are much more complex in physical terms than quiescent interstellar clouds. In starforming regions, interstellar gas is being compressed, the force of gravity overcoming the resistance provided by gas pressure, magnetohydro-dynamic (MHD) turbulence, magnetic pressure, and rotation. The chemistry is not in steady state during this collapse, and can therefore be used as a tracer of the evolution of the collapse. In addition, the chemistry modifies and controls the collapse through the provision of molecular coolants of the gas, and by determining the fractional ionization in the gas. It is this ionization that affects the level of magnetic and turbulent support available to the cloud. Molecular rotational emissions at millimeter and submillimeter wavelengths are both the main cooling processes and the most effective probes of these regions. Interstellar gas in the Galaxy is observed to be distributed in an irregular fashion, in clouds of a range of sizes. Much of the mass is encompassed in so-called giant molecular clouds (GMCs) which range in mass from about 104 to about 106 solar masses (the solar mass is about 2 × 1030 kg), and have linear extents of several hundred light years (a light year, ly, is about 1 × 1016 m). The gas in GMCs is largely H2, but because that molecule has no dipole moment, the material is most effectively traced in the 1-0 rotational emission of CO, the next most abundant molecule (CO/H2 = 10-4 by number). Isotopomers of CO are also used. The CO emission identifies cold gas of number density ∼103 H2 molecules cm-3. A detailed study4 of one particular GMC, the Rosette molecular cloud (RMC), shows that it contains almost 2 × 105 solar masses of gas, extending over 100 ly. The gas in the RMC is fragmented into about 70 clumps with masses ranging from a few tens to a few thousands of solar masses. The clumps are embedded in a more tenuous medium, typically contain 102-103 H2 molecules cm-3, and are cool (j30 K). Observations show that clumps with larger column densities of CO (J1016 CO molecules cm-2) are more likely to contain embedded stars. Therefore, clumps satisfying this criterion are likely to be the sites of star formation in the RMC. Collapse of a clump leads to fragmentation and the formation of a cluster of dense cores. Carbon monoxide (12C16O) is not an effective tracer of gas in dense cores because the CO lines are optically thick and CO level populations are thermalized at lower densities. However, species of lower abundance than 12C16O can trace the dense gas in cores (J104 H2 molecules cm-3), and they include NH3, CN, H2CO, and CS. A typical core cluster5 is illustrated in Figure 1. It is a contour map in intensity of 1-0 rotational emission from the minor isotopic species 12C18O. This core cluster contains cores which may evolve to form new stars. Several stars have already formed and are detected as infrared sources (IRS 1-4). A primary goal of astrophysics is the detailed study of the collapse of a dense core to form a star. It is also important to gain an understanding of how gravity overcomes the various resistances to collapse, and how a young star interacts with its environment through the stellar winds and jets that develop at the earliest stages of the star’s existence. The answers to these questions are certainly contained in the emissions from the molecules and dust present in the collapsing core. Identifying a collapsing core is observationally difficult. The indicators should be molecular lines that are broadened by the infalling velocities. In this Account, we describe how the search for the infall signature has led to a recognition that the interaction of gas and dust in the infalling gas produces profound changes to the chemistry and physics of star-forming regions. This interaction is poorly understood, and the nature of the star-forming process will