Search

We implement a new observational method for mapping the aliphatic hydrocarbon content in the solid phase in our Galaxy, based on spectrophotometric imaging of the 3.4  |$\mu$| m absorption feature from interstellar dust. We previously demonstrated this method in a field including the Galactic Centre cluster. We applied the method to a new field in the Galactic Centre where the 3.4  |$\mu$| m absorption feature has not been previously measured and we extended the measurements to a field in the Galactic plane to sample the diffuse local interstellar medium, where the 3.4  |$\mu$| m absorption feature has been previously measured. We have analysed 3.4  |$\mu$| m optical depth and aliphatic hydrocarbon column density maps for these fields. Optical depths are found to be reasonably uniform in each field, without large source-to-source variations. There is, however, a weak trend towards increasing optical depth in a direction towards b  = 0° in the Galactic Centre. The mean value of column densities and abundances for aliphatic hydrocarbon were found to be about several |$\rm \times 10^{18} \, cm^{-2}$| and several tens × 10−6, respectively for the new sightlines in the Galactic plane. We conclude that at least 10–20 per cent of the carbon in the Galactic plane lies in aliphatic form. [ABSTRACT FROM AUTHOR]

In the interstellar medium, the cosmic elemental carbon abundance includes the total carbon in both gas and solid phases. The aim of the study was to trial a new method for measuring the amount and distribution of aliphatic carbon within interstellar dust over wide fields of view of our Galaxy. This method is based on the measurement of the 3.4- |$\mu$| m absorption feature from aliphatic carbonaceous matter. This can readily be achieved for single sources using infrared (IR) spectrometers. However, making such measurements over wide fields requires an imaging IR camera, equipped with narrow-band filters that are able to sample the spectrum. While this cannot produce as good a determination of the spectra, the technique can be applied to potentially tens to hundreds of sources simultaneously, over the field of view of the camera. We examined this method for a field in the centre of the Galaxy, and produced a map showing the variation of 3.4- |$\mu$| m optical depth across it. [ABSTRACT FROM AUTHOR]

The mid-IR spectrum of the interstellar medium contains both aromatic and aliphatic hydrocarbon features. These are generally attributed to carbonaceous dust. The aliphatic component is of particular interest because it produces a significant 3.4 μm absorption feature. The optical depth of this feature is related to the number and type of aliphatic carbon C–H bonds in the line of sight. It is possible to estimate the column density of aliphatic carbon from quantitative analysis of the 3.4 μm interstellar feature, providing that the absorption coefficient of interstellar aliphatic hydrocarbon is known. We produced interstellar dust analogues with spectra closely matching astronomical observations. Using a combination of FTIR and 13C NMR spectroscopy, we determined an integrated absorption coefficient of the aliphatic component. The results thus obtained permit direct calibration of astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the ISM. [ABSTRACT FROM AUTHOR]

There is considerable uncertainty as to the amount of carbon incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 μm absorption feature when viewed against a background radiation source. The optical depth of the 3.4 μm absorption feature is related to the number of aliphatic carbon C–H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 μm interstellar absorption feature provided that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We report laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. Measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure combining FTIR and 13C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement [4.76(8) × 10−18 cm group−1 and 4.69(14) × 10−18 cm group−1], less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium. [ABSTRACT FROM AUTHOR]