The Interstellar Medium
Molecules and atoms in the ISM at high resolution
The interstellar medium contains a fingerprint of past chemical evolution, and provides the reservoir of gas for future star formation. Diffuse interstellar molecular clouds have been an invaluable tool to understand the nucleosynthesis history of the Milky Way, and HRMOS will allow us to efficiently extend this work to the Small and Large Magellanic Clouds. HRMOS will cover some of the most abundant molecules in the interstellar medium, including CN and CH+. The high resolution afforded by HRMOS will easily resolve the isotope shifts, allowing us to confidently measure the distribution of the 12CN/13CN, C15N/C14N, and 12CH+/13CH+ isotopologues in the interstellar medium of metal-poor galaxies. Furthermore, the relative population of the N’’=1 and N’’=0 levels of CN provide a reliable determination of the cosmic microwave background temperature.
Some interstellar sight-lines, such as those mentioned above, probe cold neutral gas, which is also well-suited to detect the absorption lines of Li I and thereby determine the abundance of lithium in the interstellar medium. Most of the lithium in the Universe was created a few minutes after the Big Bang, with later contributions from novae and spallation reactions. At present, measures of the Li/H abundance of stars disagree with the Big Bang production of lithium at more than 6sigma. By efficiently building a sample of 7Li/H and 7Li/6Li measures of the interstellar medium, we have an opportunity to study the build-up of a cosmologically important chemical element, and provide a new test of the cosmological model. Furthermore, given that lithium is produced both primordially and through novae/spallation, a galactic map of the lithium abundance will determine the relative importance of inflows (of primordial gas) and outflows (of enriched gas).