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.
The so-called diffuse interstellar bands (DIBs) are a group of broad and shallow absorption features superimposed on the spectra of stars whose light passes through clouds of interstellar gas and dust. Though long believed to be produced by the largest reservoir of organic material in the Milky Way, a definitive chemical identification of these DIB compounds has remained elusive for 80 years. This collaborative investigation was part of a 15-year effort to capture high-quality spectra of approximately 200 sightlines to investigate the mutual dependences of DIB features and the degree to which they DIB carrier compounds are influenced by their local conditions. A primary challenge to such an effort is a data challenge: converting the raw pictures of stellar spectra to a set of reduced, calibrated spectra, to measurements of DIB features are the first steps. The next challenges relate to gleaning useful patterns in thousands of measurements and spectra and designing appropriate analysis methods and tools. The present investigation concentrated primarily on these foundational tasks as the necessary prelude to solving this long-standing mystery in astrophysics, involving seven undergraduate students in summer research project to learn more about basic spectroscopy and transferable data skills. The most significant scientific finding of this project was the serendipitous detection of peculiarly high temperature in molecular gas towards Herschel 36, the central star of the Lagoon Nebula. Simple, well-studied interstellar molecules CH+ and CH show population of excited rotational levels which had never been observed before in the diffuse interstellar medium. While these observations are well explained by a very nearby, veiled star acting as an infrared heat source in Herschel 36’s close proximity, more intriguing still is the apparently similar broadening effect on some of the narrower DIBs. This observation suggests that many of the DIB carriers are located close enough to Herschel 36 to be affected by excess infrared radiation in much the same was as CH+ and CH. Until this time, it had not been possible to determine where DIB carriers were located within a sightline or to easily detect differences in the shapes of their absorption features. DIB absorption line structures in the Herschel 36 sightline possess extended absorption tails towards red light (ETR), which offer a valuable clue about both the overall structure of the carrier molecules as well as the number of atoms they likely contain. From this study, we can infer that the DIBs affected by ETR of contain only about 5-7 carbon (or nitrogen or oxygen) atoms and that they must have a distinct linear configuration in order to be so affected by the nearby infrared source. These findings represent an unexpected and unpredicted result that may completely alter scientists’ expectations about what DIB carriers are and how they come to exist in interstellar space. Evidence suggests that DIB carriers do not form from simple-to-complex forms but are instead by-products of material breaking down from macroscopic material consistently exposed to the harsh ultraviolet light of interstellar space. Further study is needed to test these new ideas and to more completely analyze the full dataset collected in this project.
