This work investigates which atoms, ions, small and large molecules cause certain interstellar absorption features known as "Diffuse Interstellar Bands" (DIBs). About 600 absorption lines associated with these families of interstellar bands are known since the 1920s but their association which specific molecules is still unknown. Here researchers with different expertise in atomic and molecular spectroscopy, interstellar gas and dust, and laboratory chemistry, come together to tackle this long-standing open question.
The approach is to compile and organize existing observations in databases and to add new measurements of DIBs seen toward about 220 stars to assess and to search for empirical relations between the different DIB absorption features as well as other interstellar parameters such as density and ionization state. The existing list of DIBs of all spectral widths to uniform limit is expanded with new observations to include broad and narrow lines that only appear under certain physical conditions. For this purpose, a new atlas of stars illuminating such interstellar regions will be assembled. Another goal is to observe certain diffuse interstellar clouds to obtain the types and absorption strengths of DIBs as a function of increasing density and ionization state, which relates to the spatial distribution of particular compounds and on-going reactions within the cloud. The team members have guaranteed access to observing facilities (e.g., Apache Point Observatory) to do this part of work.
As long as the types of compounds (molecules with more than 15 atoms are suspected) responsible for the DIB absorptions remain elusive, it is not possible to fully account for the chemical inventory and processes of the interstellar medium. The identification of the carriers of the DIBs will be a major step forward in understanding the chemical inventory in molecular clouds from which stars and planets form.
The interdisciplinary nature of this collaborative project is very strong. It provides undergraduate- and graduate students with opportunities to work on problems related to more than one discipline.
This multi-institution collaborative research program involves investigating clouds of gas and dust that represent the "galactic recycling" process whereby material ejected from dying stars can eventually be turned into new generations of stars and planets. Our research group has studied various aspects of the so-called "Diffuse Interstellar Bands" (DIBs), which are part of the "fingerprint" of these clouds that is imprinted on starlight that passes through the cloud. The exact chemical substances that cause any particular DIBs are unknown, but are most likely molecules that represent a significant fraction of the carbon - a key element for life as we know it - that exists in these clouds. This particular portion of the research involved measuring the effect that light from recently formed stars has on hydrogen molecules in the vicinity. Detailed observations of ultraviolet light coming from the stars reveal the fingerprint of the hydrogen gas in front of the stars. In this particular region, the hydrogen molecules are unusually energetic due to the effects of the starlight. Our group has found that certain DIBs show evidence that the relevant molecules are also highly energized, setting constraints on the types of molecules that cause these particular DIBs. In this vein, since exploring relationships between the DIBs and other characteristics of the gas and dust in these cosmic clouds can constrain the identification of the molecules that cause the DIBs, our group has been building a database of such information. A portion of this project thus involved compiling information on hydrogen gas as well as dust in the dozens of clouds investigated by our collaborative group, and this database of information will be made public. Four undergraduate students participated in this project, including students from traditionally underrepresented groups in the physical sciences. These students gained experience that will enhance their future careers in science-related fields and one graduating student currently works at a NASA facility.
