The Eagle Nebula

Credit: T.A. Rector (University of Alaska, NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A. Wolpa (NOAO/AURA/NSF)

Jeffrey Linsky investigates clouds of warm gas in the interstellar medium close to the Sun, i.e., within 50 light years. There are 15 of these turbulent clouds, which were formed several million years ago by the winds from young, massive stars and supernova explosions in the Scorpio-Centaurus Association. Two of the nearest clouds, the Local Interstellar Cloud (LIC) and the G cloud, cover 70% of the sky as seen from Earth. Linsky recently determined that the solar system has just left the LIC and is now on a collision course with the G cloud, which is a little cooler than Earth’s current space environment.

In his studies of the motions and physical properties of the 15 nearby clouds, Linsky has been able to explain the phenomenon of scintillating quasars. Quasars twinkle relatively slowly, changing in intensity over several hours. They also twinkle at different rates, depending on the time of year. These twinkling patterns result from the interaction of Earth’s speed (30 km/s) around the Sun and its position with respect to the irregular shape of the turbulence in nearby interstellar clouds. If Earth didn’t move through space, quasars wouldn’t twinkle. In fact, the twinkling of quasars at radio wavelengths and the twinkling of stars at visible wavelengths in the night sky are essentially the same process. The only difference is that for the quasars, it’s the Earth that moves. For the stars, it’s the Earth’s atmosphere that moves around.

Linsky has solved other galactic puzzles. Together with his research group, known as the "Cool Star Mafia," he figured out why concentrations of deuterium, a heavy form of hydrogen, vary in our galaxy. Most of the Universe's deuterium was created during the Big Bang 14 billion years ago. Scientists have known for some time that the region of space near our Sun has higher concentrations of deuterium than other parts of the Milky Way. In 2004, Linsky's group explained why: Supernova explosions have blasted deuterium atoms and molecules out of carbon-rich dust grains, i.e., soot, that tightly bind them in other, less active regions of the galaxy. Once freed from the dust grains, deuterium mixes with clouds of regular hydrogen. Because our solar system has experienced several nearby supernovae in recent history, concentrations of deuterium are relatively high near us. Scientists believe that studies like Linsky's will help them develop a detailed picture of the chemical evolution of our galaxy.

Rosalba Perna has developed a new technique for gathering information about the composition and size of dust grains in the interstellar medium of galaxies beyond the Milky Way. Information about galactic dust can be extracted from the afterglow spectra of gamma-ray bursts, particularly the long bursts resulting from the collapse of a massive star. Perna and her colleagues analyze absorption spectra in the infrared, visible, near-ultraviolet, and soft X-ray regions to reconstruct the dust composition in the galaxy where a gamma-ray burst recently occurred. In one such study, they found that the dust profile of a host galaxy was very different from that of the Milky Way. Rather, it was comparable to that of the Large Magellanic Cloud, an irregular galaxy located about 160,000 light years from our galaxy. Such dust profiles are shedding light on the star formation rate in a particular galaxy and the environment in which the stars formed. Perna’s goal is to learn more about how dust and stars evolve in the Universe.

Chris Greene investigates important chemical reactions that occur in interstellar clouds and analyzes the dissociative recombination of simple polyatomic molecules such as HCO⁺, HeH⁺, LiH₂⁺, and H₃⁺. Recent calculations of the dissociative recombination rate of HCO⁺ resulted in much better agreement between theory and experiment. His analysis of the dissociative recombination rate for HeH+ also confirmed previous theoretical work and agreed with experiment. This work included the development of a new theoretical tool kit for this type of analysis. With LiH₂⁺, Greene extended and improved upon methods originally developed for predicting dissociative recombination rate of H₃⁺, which is abundant in the hydrogen-rich interstellar environment.

Infrared light absorption by H₃⁺

Greene's theoretical analysis of H₃⁺ showed that free low-energy electrons can blow apart H₃⁺ with surprising efficiency, forming either three hydrogen atoms or a hydrogen molecule (H₂) and a hydrogen atom. Experiments performed at Sweden's CRYRING storage ring and Germany's Test Storage Ring confirmed Greene's prediction that the rate of this chemical reaction is quite fast under interstellar conditions. This was good news because there were early, less well-controlled measurements of the recombination rate suggesting that it was at least 1000 times faster than the standard models of this process suggested. The refinements of the original experiments, in combination with new theoretical work, eliminated all remaining doubt. Since H₃⁺ disappears so rapidly, astrophysicists must now explain its large observed abundance in interstellar gas clouds. A likely explanation is an enhanced rate of ionization by cosmic rays. Greene's study is an example of the increasing connection between the research efforts of JILA's atomic physicists and astrophysicists.

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