FOR RELEASE: Tuesday, March 23, 12:30

Spectroscopy of Interstellar PAHs: From Ground to Space - The New Frontiers

Farid Salama (650-604 3384, fsalama@mail.arc.nasa.gov)
NASA-Ames Research Center and SETI
Mountain View, California, USA

Author can be reached at (650) 604-3384 (office phone number) during week of meeting.

Popular Version of Paper JC18.05
Tuesday, March 23, 12:30
APS Centennial Meeting, Atlanta

Stars create carbon in their interiors and release it into space. This makes carbon an ideal probe and tracer of the birth and death of stars and planetary systems, and of the processes which occur in deep interstellar space. Without carbon, life as we know it would not exist. Because it is abundant and can form complex structures it plays key roles in the evolution of stars, planets, and human bodies. The finding that organic chemistry is abundant in the universe and that life's essential, pre-biotic, carbon-based building blocks are abundant in the ISM implies that carbon-based life could be possible throughout the universe.

Polycyclic aromatic hydrocarbons (PAHs), a family of carbon-containing compounds which are common on Earth in coal, soot, and automobile exhaust, may be the most abundant class of organic molecules in the Universe. PAHs are flat molecules of carbon and hydrogen in the form of hexagons - their skeleton looks like chicken wire. PAHs are very stable and are thought to be ubiquitous in the interstellar medium.

The existence of PAHs in the interstellar medium, present as a mixture of radicals, ions and neutral species, may explain the mysterious spectral signatures seen in both absorption and emission that are common throughout interstellar space.

The research presented here helps solve a problem scientists have struggled with for most of the century. They have detected more than 100 interstellar absorption lines in the spectra (range of frequencies or color) of starlight approaching the Earth. Absorption lines are discrete colors of light absorbed by intervening matter; this absorption leaves holes or "lines" in the spectral rainbow. The lines are called diffuse interstellar band (DIBs). Scientists believe a form of PAHs might be the long-sought matter producing the interstellar bands. PAHs are also good candidates to account for the infrared emission bands seen in the interstellar medium. These emission lines contribute to the cooling channels of interstellar clouds. It is important to understand how PAHs absorb stellar radiation and how they emit it back, because it contributes to the global energy balance in space.

The average temperature in the diffuse interstellar clouds is less than 100 K (less than about -170 C) and the density is very low (of the order of 50 particles per cubic cm as opposed to 10 e19 at the Earth's surface). The free interstellar PAH molecules and ions are bathed in stellar ultraviolet (UV) and visible radiation. Interstellar molecules and ions must be stable enough to survive in this harsh environment.

Simulating the conditions of space in the laboratory, we measured the spectra of large PAH molecules and ions in the ultraviolet and visible light bands and compared it to astronomical data from Kitt Peak and other observatories. We simulated the space environment in the sample chamber by creating very low temperatures (down to about -270 C) and a near-vacuum (pressures down to about 1 tenth of a billionth of the atmospheric pressure) with a liquid-helium cooler. The star light was simulated with artificial starlight. The PAH molecule was frozen in a cage of inert-gas (neon) atoms. This technique ensured that the trapped molecules were cold (down to about -270 C) and fully isolated (low density) in the condensed (solid) phase.

We have been able to make substantial progress by comparing the laboratory data measured in these solid inert matrices with an extensive set of high-resolution optical astronomical data. A list of promising PAH candidates as DIB carriers has been derived.

We also report recent breakthrough advances in gas phase supersonic jet experiments, where the PAH molecules and ions are free flying under conditions which closely mimic interstellar conditions. These gas-phase experiments provide for the first time the ultimate test for comparing laboratory spectra with interstellar spectra. It is now possible to definitively search for the unique signature of complex carbon-bearing, pre-biotic molecules in space.

This research is supported by NASA's Office of Space Science (Search for Origins and Structure and Evolution of the Universe Programs). The core research work presented here has been performed in collaboration with Thomas Halasinski, Bin Chen and Lou Allamandola from NASA-Ames for the laboratory studies and with Jacek Krelowski and Gazinur Galzutdinov from the University of Torun, Poland, for the astronomical observations.

The new gas-phase experiments were made in collaboration with Daniele Romanini, Ludovic Biennier and Frederic Stoeckel from the University of Grenoble, France, Jim Scherer and Anthony O'Keefe from Los Gatos Research, Mountain View and B. Provencal and Richard Saykally from the University of California, Berkeley.

About the author and list of related publications
Related NASA press release
Science Magazine: Carbon in the Universe
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