| Description |
Infrared Dark Clouds (IRDCs) are thought to host the earliest stages of high-mass star formation, an epoch that is still poorly understood. Gound-based millimeter observations have found that many IRDCs have embedded cores, and Herschel PACS and SPIRE photometry has made it possible to image many of these very cold cores via their dust emission. We have used the Submillimeter Array (230GHz) to observed a sample of cold IRDCs that are dark even at PACS70, but which contain bright cores seen in emission in the SPIRE bands. The SMA spectral maps and the molecular chemistry suggest at least three early stages of star formation that we have tentatively modeled as reflecting three progressive temperature and evolutionary stages. Unfortunately in the youngest, coldest sources most diagnostic molecules are either frozen out onto grains or have not yet assembled. Nitrogen hydrides, however, do not suffer from depletion effects in cold, dense regions, and so offer an invaluable tool to probe the early stages. Herschel HIFI is uniquely suited to observe the key ground-state absorption lines of four nitrogen hydride species: NH, NH2, o-NH3, and p-NH3. Our chemical models show that the line ratios are sensitive measure of density, temperature, and inferred evolutionary stage. We therefore propose observations of four very cold cores in IRDCs that we have already studied with the SMA; we supplement them with two progressively warmer IRDCs for reference with only a modest increase in time. Our goal is to quantify the early evolutionary stages of IRDC cores before they form stars, and the associated chemical activity in the cloud. We will also use the results to refine our chemical models of the nitrogen hydrides under these conditions. Our team is expert in chemical modeling of dark clouds, IRDCs, millimeter and submillimeter astronomy, and Herschel HIFI data reduction and analysis. |
