Dr. Jean Chiar

Not everyone is aware that the space between the stars isn’t exactly empty. These vast interstellar tracts are filled with an evanescent fog of ice and dust particles, and understanding how this thin particle soup affects the formation of stars (and their accompanying planets) is the work of physicist Jean Chiar.

To study these small, dark particles, Jean uses infrared telescopes both on the ground and in orbit (including NASA’s Spitzer Space Telescope) to search for the absorption lines that are characteristic of ices, simple organic hydrocarbons, and silicates. While tiny shards of ice floating in space may seem like an esoteric field of study, Jean points out that it is exactly these small particles that can aggregate to form water-covered worlds such as Jupiter’s moon Europa. The main ingredient of life – water – eventually comes from the cold mists that waft between the stars.

Projects

"The Evolution of Astrophysical Ices: The Carbon Dioxide Diagnosis"

JPL 1266411

We have used the Infrared Spectrometer on board the Spitzer Space Telescope to carry out a comprehensive study of the carbon dioxide bending mode absorption feature centered near 15 micrometers in astrophysical ices. Previous observations with the Infrared Space Observatory, together with studies of laboratory analogs, have shown that this feature has strong diagnostic properties. Substructures within the feature are sensitive to the thermal history of the ices and to the formation of linked carbon dioxide/methanol complexes. Both of these molecules are important repositories for carbon in interstellar ices, and their roles in the chemical evolution of the ices and their sublimation products are intimately linked. The abundance of carbon dioxide relative to methanol is diagnostic of key reaction pathways, measuring the relative efficiencies of catalytic oxidation and hydrogenation reactions in cold dark clouds. In regions exposed to the interstellar radiation field, photolytic reactions may contribute to their formation. The distribution of carbon between these molecules may subsequently influence the production efficiencies of more complex organic molecules in regions of active star formation, where the ices are subject to heating, irradiation and shocks. By studying a range of absorbers, from pristine ices in dark clouds to processed ices in the vicinity of embedded stars, we will build a clear picture of the evolution of ices from the interstellar medium to protostellar envelopes and protoplanetary disks.

"Solid State Chemistry in Dense Clouds Along Quiescent Lines of Sight"

JPL 1267778

We will study the infrared spectra from 5.3 to 21.8 micrometers through dense interstellar clouds, with little or no star formation activity, to assess the early chemistry of molecular cloud dust. Dense clouds produce molecules and ices critical to star and planet formation. The formation of organic compounds in these ices is one of the first steps toward the complex molecular materials needed for life. Infrared spectroscopy provides a powerful tool for the study of the composition and evolution of interstellar ices. The most diagnostic features of solid-state materials occur in the mid-infrared. To date, mid-infrared absorption studies have primarily been toward embedded protostars where the ice may well have been processed either thermally or by far ultraviolet photons from the star. Such sightlines demonstrate a preponderance of simple molecules (water, methanol, carbon monoxide, carbon dioxide, and ammonia) and energetically processed species (nitriles and cyanates) in the surrounding ices, revealing that protostars strongly influence their circumstellar environments. Lines of sight to these objects are unlikely to be representative of dense cloud materials as a whole. A more complete understanding of the composition of dense clouds and their chemical dynamics requires that we also probe lines of sight through the general quiescent cloud medium. We have obtained low resolution spectra from the Spitzer Space Telescope's Infrared Spectrometer in the wavelength region that includes the absorption features of water, methanol, methane, and carbon dioxide, plus high resolution IRS spectra for selected sources, to study detailed band profiles. We will correlate band strengths with the amount of dust obscuration to determine the abundances and densities required for the ice components to appear, and study the chemical changes in molecular clouds as a function of temperature and density. These observations will provide a snapshot of the chemical state of a molecular cloud prior to the formation of stars, and a general baseline for studies of dust chemistry in regions of star formation.

"Unlocking the Mysteries of Interstellar Dust Composition and Icy Mantle Formation with Sensitive Infrared Spectroscopy"

NNA05CS35A

Interstellar dust is ubiquitous, yet its precise composition is still widely debated. The Spitzer Space Telescope's (SST) Infrared Spectrometer (IRS) provides the sensitivity to study previously unattainable lines of sight throughout the plane of the Milky Way Galaxy. We seek to study the hydrocarbon (carbon and hydrogen-containing molecules) and silicate (mineral) dust components of the interstellar medium for 56 lines of sight with varying amounts of dust obscuring the starlight. In addition, we will be probing the dust over a range of Galactic longitudes and latitudes. Specifically, we will measure the absorption features in infrared part of the electromagnetic spectrum. The hydrocarbon absorption features occur at 6.9 and 7.3 micrometers, and the silicate absorption features at 9.7 and 18.5 micrometers. The ratio of the depths of the hydrocarbon and silicate absorption features provides a direct handle on the hydrocarbon to silicate dust volume. It has been previously been noted that the ratio of the depth of the absorption to the amount of dust obscuration is distinct for locations within the Solar neighborhood (about 1000 lightyears from the Solar System) compared to the Galactic Center (which is 30,000 lightyears from our Solar System). Thus, these absorption features and their relative depths will also be related to the amount of dust obscuration and Galactic location. Finally, the silicate mineralogy can be assessed by studying the ratio of the two silicate absorption features, whose relative strengths have been shown to be indicative of olivine-rich or pyroxene-rich silicates.