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My post-doc at NASA Ames focused on the emission from organic molecules, which can be seen in star formation regions and around dying stars. These molecules exist deep within dust clouds where they are protected from the harsh radiation in interstellar space. When exposed to energetic photons, they produce a strong set of infrared emission bands. If the photons are strong enough, they break the PAHs apart.
Generally, these organic molecules are described as Polycyclic Aromatic Hydrocarbons (PAHs for short). PAH emission can be seen from a wide variety of astronomical sources, whereever you have energetic photons and dust containing carbon. Sources of PAH emission include carbon-rich planetary nebulae, reflection nebulae, the edges of ionized regions around young stars, and even starburst galaxies like M82. PAH molecules must be a very common component of the interstellar medium. Lou Allamandola estimates they may represent more than 15% of all interstellar carbon. So, a proper underrstanding of how PAHs produce the spectrum we see, allows us to use them as probes of the interstellar medium in a wide range of physical conditions.
During my three years at NASA Ames, I worked with Jesse Bregman on this problem. Using a combination of narrow-band imaging and long-slit spectroscopy at several telescopes (including the 4 m UKIRT reflector and the 5 m Hale telescope at Palomar), we observed a number of extended sources of PAH emission in the near infrared (3-4 µm) and the mid-infrared (8-13 µm). We chose to concentrate on spatially extended sources because this is an excellent way to see how the PAH emission varies with changing physical conditions. Long-slit spectroscopy in extended regions of emission has proven to be a powerful diagnostic, constraining what molecular bonds are responsible for previously known bands. We have even discovered some new bands. The proximity of the Astrochemistry Group at NASA Ames allowed a close collaboration, enabling us to interpret our findings in light of the latest laboratory results.
Our near-infrared studies of the Orion Bar led to the discovery that the emission features at 3.40 µm can be separated into two spectrally and spatially distinct components. We believe both arise from different aliphatic C-H bonds associated with PAH material. Aliphatic bonds require some explanation. They are not as strong as aromatic bonds, and appear in hydrocarbons like methane (CH4), ethane (C2H6), and longer chains.
In our mid-infrared studies of NGC 1333 we made the first identification of ionized PAHs in the interstellar medium. The ionized PAHs can only be seen in the immediate vicinity of the central star, which is nicely consistent with what we would expect.
Now that I'm working with infrared spectra from the Spitzer Space Telescope, I have changed my approach. Instead of studying how the characteristics of the PAH spectra can vary within an extended source, I'm now looking at how PAHs can vary from one source to the next. Spitzer has built a sample of spectra from much cooler objects than one usually expects to produce PAH spectra. My collaborators on this project include Kathleen Kraemer at the Air Force Research Lab. and Jeronimo Bernard-Salas here at Cornell, who have been working with me to study PAHs in nearby galaxies in the Local Group, and Mike Jura at UCLA and Luke Keller at Ithaca College. Mike and Luke are members of the IRS Disks Team.
Mike used Spitzer to study red giants with disks around them. At least two of his sample show PAHs, which is a surprise since we have long thought that red giants are too cool to excite PAH emission. Luke focused on intermediate-mass pre-main-sequence stars. They're known as Herbig Ae/Be stars, and many show PAH emission. These stars are warmer than red giants, but still cooler than the usual sources of PAH emission.These collaborations led to an unusual collection of PAH spectra, and our analysis has revealed tell-tale shifts in the wavelengths of the PAH emission features as the temperature of the star exciting the emission changes. Our first clue in a sample of Herbig Ae/Be stars Luke and I worked on (Sloan et al. 2005). We noticed that the PAHs had shifted slightly to longer wavelengths in these cool stars, with the largest shift in the coolest star. Then, Mike Jura led a paper on HD 233517, a cool red giant with PAH emission. The star was even cooler and the PAHs shifted to even longer wavelengths. More cool PAH sources with shifted features appeared, including MSX SMC 029, a dying star in the Small Magellanic Cloud which has nearly evolved into a planetary nebulae in the (Kraemer et al. 2006). Jeronimo Bernard-Salas provided another clue with a spectrum of SMP LMC 11, an object like MSX SMC 029, but showing a host of absorption lines from simple aliphatic molecules which could only survive in such a cool radiation field (Bernard-Salas et al. 2006).
The clincher was HD 100764, another of Mike Jura's strange red giants with unusual shifts in the PAH features and a cool radiation field. Mike let me use this spectrum as centerpiece of a paper discussing all of these unusual PAH spectra (Sloan et al. 2007) and showing an excellent correlation between the wavelengths of the PAH features and the temperature of the exciting star. These shifts in wavelength are consistent with a shift in the chemistry of the carbon-rich material producing the PAH features. Walt Duley at the University of Waterloo has long argued that the PAHs are a special case of a more general class of hydrocarbons called HAC, or hydrogenated amorphous carbon. Unlike PAHs, HACs are not purely aromatic. Instead, they are a mixture of hydrocarbons with aromatic and aliphatic bonds.
A year later, Luke Keller published a larger sample that verified our observations. It appears that the weak aliphatic bonds that are usually obliterated in hot radiation fields are able to survive in the unusually cool radiation fields we are able to study with Spitzer. This result is exciting, because it is another clue, along with our Orion Bar studies, of the true nature of insterstellar carbon grains. When we see PAH emission, we are actually seeing the hydrocarbons as they are being destroyed by the radiation field. Our rare glimpses of PAHs in cooler environments suggest that PAHs are just the sturdy survivors of larger carbon grains with contributions from aliphatic hydrocarbons.
More recently, Mikako Matsuura and I published two studies with spectra from highly evolved carbon stars that are almost planetary nebulae (Matsuura et al. 2014, Sloan et al. 2014). These spectra show hydrocarbons in a variety of forms, including PAHs, aliphatic species, and even fullerenes. We continue to discover new forms for hydrocarbons and relate those forms to the physical condistions in their environments, and we are steadily building a picture of how these hydrocarbons form and evolve in space.
Bernard-Salas, J., et al. 2006, “The Spitzer-IRS spectrum of SMP LMC 11” ApJ Letters, 652, L29.
Jura, M., et al. 2006, “Polycyclic aromatic hydrocarbons orbiting HD 233517, an evolved oxygen-rich giant,” ApJ Letters, 637, L45.
Keller, L.D., et al. 2008, “PAH emission from Herbig Ae/Be stars,” ApJ, 684, 411.
Kraemer, K.E., et al. 2006, “A post-AGB star in the Small Magellanic Cloud observed with the Spitzer Infrared Spectrograph,” ApJ Letters, 652, L25.
Matsuura, M., et al. 2014, “Spitzer Space Telescope spectra of post-AGB stars in the Large Magellanic Cloud - polycyclic aromatic hydrocarbons at low metallicities,” MNRAS, 439, 1472.
Sloan, G.C., et al. 1997, “Variations in the 3 µm spectrum across the Orion Bar: PAHs and related molecules,” ApJ, 474, 735.
Sloan, G.C., et al. 1999, “Direct spectroscopic evidence for ionized PAHs in the interstellar medium,” ApJ Letters, 513, L65.
Sloan, G.C., et al. 2007, “The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics,” ApJ, 664, 1114.
Sloan, G.C., et al. 2014, “Carbon-rich dust past the asymptotic giant branch: Aliphatics, aromatics, and fullerenes in the Magellanic Clouds,” ApJ, 791, 28.
Last modified 13 December, 2014. © Gregory C. Sloan.