Research in the Interstellar Medium

Astrochemistry, the study of atoms and molecules in space, is one of the younger branches of astronomy. The atoms and molecules are detected spectroscopically though emission and/or absorption lines of electronic, rotational, and vibrational transitions. A wide variety of space born and ground based instruments are used to study these transitions. Most atoms are seen toward stars where the high temperatures excite their electrons. Some molecules, such as silicon monoxide [SiO] and water [H2O] are seen in the atmospheres of stars, while most molecules are seen in star forming regions (temperatures of a few hundred K) and cold dark clouds (temperatures of a few K). To date over 140 molecules have been detected in the interstellar medium. Many of these have also had one or more isotopologues detected.

By studying the chemical makeup of the interstellar medium we can learn about the different chemical processes that occur in low temperature (5-300 K) and very low density (<107 particles per square centimeter). Additionally, the physical conditions of the different regions where these molecules are found can be inferred by observing many transitions of each molecule.

Before any astronomical observations can take place, studies in chemistry labs must be conducted in order to characterize the molecules being searched for. In these studies, pure, gas-phase samples of the molecule are pumped into a chamber where their rotational and/or vibrational spectrum is recorded. From there, computer modeling is used to identify all of the spectral lines seen and to calculate the unique rotational, vibrational, and other molecular constants. Once this is completed one can observe astronomical sources for these same spectral lines.

While this may sound simple, in practice neither the research in the lab nor the astronomical search is trivial or simple. In many cases the molecule of interest is not stable on the earth (they may react very easily with other molecules) and needs to be created directly in the lab. Thus, the lab setups must continually be modified in order to create and hold these molecules for long enough for them to be studied. Also, as the complexity (number of constituent atoms) increases the number of internal interactions grows greatly, so once a spectrum is obtained it can be very challenging to identify and characterize all of the parameters. While the instrument setup for the astronomical searches rather straight forward, finding the transitions can be very challenging. The signals can be extremely weak and can be masked by transitions of other, more populous, molecules. So, one must observe numerous transitions in order to get even a tentative detection. See section 3 of Snyder et al. 2005 for a complete listing of the criteria for identifying interstellar molecules.

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© 2015, D. N. Friedel