Spectroscopy of Hydrocarbon Grains toward the Galactic Center and Quintuplet Cluster

Jean E. Chiar1, A. J. Adamson2, D. C. B. Whittet3, Y.J. Pendleton4

1 NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 and SETI Institute
2 Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, Hawaii 96720
3 Rensselaer Polytechnic Institute, Department of Physics, Troy, NY 12180
4 NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035

AUTHORS

E-mail contact: chiar@misty.arc.nasa.gov

See ApJ, 570, 198 (2002), ApJS, 138, 75 (2002), and ApJ, 537, 749 (2000) for discussion of some of this work.

Investigations of objects located near the Galactic center require an understanding of the physical properties of the intervening dust through which all observations are made. Some of the dust is local to the Galactic center, while much of it lies along the line-of-sight. The enormous extinction seen towards the center of our Galaxy ( magnitudes) provides an excellent opportunity to study the properties of interstellar dust using infrared spectroscopy. Here we present new ground based observations of the 2.8-3.9 um region made towards several Galactic center infrared sources, including those of the central cluster and the Quintuplet cluster.

The Galactic center line-of-sight suffers extinction from both diffuse and dense cloud dust; spectroscopy reveals significant differences among the dust grains residing in these two regimes. The observations presented here show differences exist in the spectra of relatively nearby objects in the Galactic center: the Quintuplet and Galactic center cluster sources. Using UKIRT's CGS4, we have carried out spectroscopy of individual lines-of-sight toward a substantial number of sources, and have obtained high S/N so that the diffuse and dense interstellar cloud components can be deconvolved and assessed in the resulting spectra. The depth of the 3.0 um water-ice feature loosely corresponds to the amount of dense molecular cloud material along the line-of-sight. This is found to vary by a factor of almost 5 across a 2 parsec region, perhaps reflecting the clumpy nature of the clouds. One of the primary spectral signatures of the organic component of interstellar dust in the diffuse interstellar medium is the aliphatic carbon component observed toward the Galactic center in absorption at 3.4 um. Our observations provide additional data and reveal that the 3.4 um hydrocarbon feature varies in depth across the areas studied here. This is likely a reflection of the distribution of extinction from the foreground diffuse ISM.

Our group has also explored the nature of the hydrocarbon dust in the diffuse ISM along the line of sight toward the Galactic center. While many laboratory analog materials have provided insight into the carrier of the interstellar band based on absorption signatures at 3.4 um, longer wavelength data obtained from space-based spectroscopy using ISO's Short Wavelength Spectrometer revealed vital information regarding the corresponding deformation modes at 6.85 and 7.25 um toward the Galactic center. The relative strengths of these three features, along with a detailed analysis of laboratory data produced via competing processes, have revealed that hydrogenated amorphous carbon produced through plasma processing, closely matches the interstellar data.

The results presented here show significant differences between the hydrocarbon spectra between the two sets of Galactic center objects. For instance, a sharp, narrow 3.28 um absorption feature, attributed to polycyclic aromatic hydrocarbons (PAHs), is present in the Quintuplet cluster spectra, but absent in the spectra of the Galactic center sources. In addition, both regions show evidence for broad absorption in the region where PAHs are expected to absorb, but the width of the feature is too great to be simply reconciled with PAHs in solid grain material. Variations of the ice and hydrocarbon bands among spectra of objects located relatively close to each other, as in the case of the sources presented here, requires a deeper investigation into the morphology of the regions and of the evolution of dust in the diffuse and dense ISM. Here we present additional steps in the direction of that understanding.