Hydrocarbons, Ices, and "XCN" in the Line of Sight toward the Galactic Center

SETI Institute, Mountain View, California, United States
The Astrophysical Journal (Impact Factor: 6.28). 05/2002; 570:198-209. DOI: 10.1086/339570

ABSTRACT We discuss 2.8-3.9 mum spectra from the United Kingdom Infrared Telescope of seven sight lines toward IR sources near Sagittarius A* in the Galactic center (GC). In all lines of sight, the 3.0 mum H2O ice feature is present with optical depths in the range 0.33-1.52. By constructing a simple ice model, we show that the ice pro le is not fully accounted for by pure H2O ice mantles. Residual absorption is present at 2.95 and 3.2-3.6 l m. Aliphatic hydrocarbon absorption at 3.4 mum is shown to vary by a factor of 1.7, indicating significant changes in the foreground extinction across the small field. By determining the true ice pro le for the GC line of sight, we reveal an additional broad absorption component around similar to3.3 mum, which partially underlies the similar to3.4 mum aliphatic hydrocarbon feature. Its carrier resides in the diffuse interstellar medium. The width of this absorption is deduced to be at least similar to100 cm(-1), much broader than individual polycyclic aromatic hydrocarbon molecules produced in the laboratory or unidentified infrared emission features observed in the interstellar medium. The 4.62 mum "XCN" feature is detected in the molecular clouds along the line of sight toward IRS 19. In the solar neighborhood, this feature is seen only toward some deeply embedded protostars. Toward the GC, it may indicate the serendipitous presence of such an object in the line of sight to IRS 19, or it might conceivably arise from the processing of ices in the circumnuclear ring of the GC itself.

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    ABSTRACT: Context. An absorption feature at 3.4 μm, widely observed in the diffuse interstellar medium (ISM), is produced by aliphatic hydrocarbon dust. In the local ISM there is in general a linear correlation between the 3.4 μm optical depth and the visual extinction AV. However, the 3.4 μm optical depth toward the young stellar object IRAS 18511+0146, the highest found in the local diffuse ISM apart from the Galactic centre lines of sight, is three times higher than predicted from the most likely value for the distance of the object, the extinction-distance relation for the Galactic diffuse ISM, and the value of AV / τ3.4 in the Galactic disk. Aims.IRAS 18511+0146 is associated with a small cluster of stars. We have obtained spectra of it as well as two other cluster members in order to verify the unusually high 3.4 μm optical depth toward IRAS 18511+0146 and test for patchiness in the foreground diffuse cloud material. Methods: Spectra covering 2.9-4.1 μm were recorded using the near-infrared imager/spectrograph (NIRI) at the Gemini North telescope. Results: After subtraction of the superimposed 3.47 μm band, produced in dense interstellar gas associated with the cluster, we have determined optical depths of 0.06-0.08 for the 3.4 μm absorption on the three lines of sight. We discuss the distance and extinction to the IRAS 18511+0146 cluster and possible interpretations of the high optical depth of the 3.4 μm absorption. Conclusions: The most probable distance to the IRAS 18511+0146 cluster is 3.9 kpc. Two different interpretations of the high observed τ3.4 are then possible. (i) The AV to distance ratio toward the cluster is approximately the one observed on average in the Galactic diffuse ISM, in which case AV ~ 7 and τ3.4 in this direction is three times higher than predicted by the usual AV / τ3.4 observed in local diffuse ISM. (ii) The visual extinction in the direction of the IRAS 18511+0146 cluster is higher (i.e. 15-20 mag), probably due to a diffuse cloud associated with the cluster, resulting in a high value of τ3.4. In either case, the high τ3.4 makes this cluster a unique and valuable line of sight for studying the Galactic hydrocarbon dust.
    Astronomy and Astrophysics 01/2012; 537:27-. DOI:10.1051/0004-6361/201117197 · 4.48 Impact Factor
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    ABSTRACT: Many materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains, we analyze the observed aromatic (3.28, 6.2 μm) and aliphatic (3.4 μm) hydrocarbon absorption features in the diffuse ISM along the line of sight toward the Galactic center Quintuplet Cluster. Observationally, sp 2 bonds can be measured in astronomical spectra using the 6.2 μm CC aromatic stretch feature, whereas the 3.4 μm aliphatic feature can be used to quantify the fraction of sp 3 bonds. The fractional abundance of these components allows us to place the Galactic diffuse ISM hydrocarbons on a ternary phase diagram. We conclude that the Galactic hydrocarbon dust has, on average, a low H/C ratio and sp 3 content and is highly aromatic. We have placed the results of our analysis within the context of the evolution of carbon dust in the ISM. We argue that interstellar carbon dust consists of a large core of aromatic carbon surrounded by a thin mantle of hydrogenated amorphous carbon (a-C:H), a structure that is a natural consequence of the processing of stardust grains in the ISM.
    The Astrophysical Journal 05/2013; 770(1):78. DOI:10.1088/0004-637X/770/1/78 · 6.28 Impact Factor
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    ABSTRACT: We present for the first time, an infrared data-cube of the central parsec of our Galaxy in the 2.8 to 4.2 micron range. This spectral band (the L-band) harbors important signatures of the interstellar and circumstellar medium, since the water ice absorption feature at 3 micron traces the dense medium and the hydrocarbon absorption at 3.4 micron is only observed in the diffuse gas.Thanks to a calibrator spectrum of the foreground extinction in the L-band derived in a previous paper, we corrected our data-cube for the line of sight extinction. Our observations performed with ISAAC spectrograph at the VLT-ESO telescope suggest that part of the absorption features takes place in the local environment of the Galactic center. This induces the presence of very low temperatures in the central parsec.
    Journal of Physics Conference Series 07/2012; 372(1). DOI:10.1088/1742-6596/372/1/012019

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