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Register a new userTheoretical study of deuteronated PAHs as carriers for IR emission features in the ISM
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Mridusmita Buragohain
Publication date
2015
fields
Physics
and research's language is
English
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This work proposes deuteronated PAH (DPAH+ ) molecules as a potential carrier of the 4.4 and 4.65 {mu}m mid infrared emission bands that have been observationally detected towards the Orion and M17 regions. Density Functional Theory calculations have been carried out on DPAH+ molecules to see the variations in the spectral behaviour from that of a pure PAH. DPAH+ molecules show features that arise due to the stretching of the aliphatic C-D bond. Deuterated PAHs have been previously reported as carriers for such features. However, preferred conditions of ionization of PAHs in the interstellar medium (ISM) indicates the possibility of the formation of DPAH+ molecules. Comparison of band positions of DPAH+ s shows reasonable agreement with the observations. We report the effect of size of the DPAH+ molecules on band positions and intensities. This study also reports a D/H ratio ([D/H]sc ; the ratio of C-D stretch and C-H stretch bands per [D/H]num ) that is decreasing with the increasing size of DPAH+ s. It is noted that large DPAH+ molecules (no. of C atoms ~ 50) match the D/H ratio that has been estimated from observations. This ratio offers prospects to study the deuterium abundance and depletion in the ISM.
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We present recent UV laboratory spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. From a detailed comparison of gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we infer upper limits in the abundance of these PAHs in the interstellar medium. Upper limits in the column densities of anthracene of $0.8 - 2.8 times 10^{12}$ cm$^{-2}$ and of pyrene and 2,3-benzofluorene ranging from $2 - 8 times 10^{12}$ cm$^{-2}$ are inferred. Upper limits in the column densities of benzo[ghi]perylene are $0.9 - 2.4 times 10^{13}$ and $10^{14}$ cm$^{-2}$ for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times $10^{-10}$. Upper limits in the fractional abundance of benzo[ghi]perylene of a few times $10^{-9}$ and of phenanthrene of few times $10^{-8}$ are inferred. {Toward CPD $-32^circ 1734$, we found near 3584 {AA} an absorption line of OH$^+$, which was discovered in the interstellar medium only very recently. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds, or that a PAH population with a large variety of molecules is present.
We employ an all-sky map of the anomalous microwave emission (AME) produced by component separation of the microwave sky to study correlations between the AME and Galactic dust properties. We find that while the AME is highly correlated with all tracers of dust emission, the best predictor of the AME strength is the dust radiance. Fluctuations in the AME intensity per dust radiance are uncorrelated with fluctuations in the emission from polycyclic aromatic hydrocarbons (PAHs), casting doubt on the association between AME and PAHs. The PAH abundance is strongly correlated with the dust optical depth and dust radiance, consistent with PAH destruction in low density regions. We find that the AME intensity increases with increasing radiation field strength, at variance with predictions from the spinning dust hypothesis. Finally, the temperature-dependence of the AME per dust radiance disfavors the interpretation of the AME as thermal emission. A reconsideration of other AME carriers, such as ultrasmall silicates, and other emission mechanisms, such as magnetic dipole emission, is warranted.
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Blue Luminescence (BL) was first discovered in a proto-planetary nebula, the Red Rectangle (RR) surrounding the post-AGB star HD 44179. BL has been attributed to fluorescence by small, 3-4 ringed neutral polycyclic aromatic hydrocarbon (PAH) molecules, and was thought to be unique to the RR environment where such small molecules are actively being produced and shielded from the harsh interstellar radiation by a dense circumstellar disk. In this paper we present the BL spectrum detected in several ordinary reflection nebulae illuminated by stars having temperatures between 10,000 -- 23,000 K. All these nebulae are known to also exhibit the infrared emission features called aromatic emission features (AEFs) attributed to large PAHs. We present the spatial distribution of the BL in these nebulae. In the case of Ced~112, the BL is spatially correlated with mid-IR emission structures attributed to AEFs. These observations provide evidence for grain processing and possibly for in-situ formation of small grains and large molecules from larger aggregates. Most importantly, the detection of BL in these ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR.
The high interstellar abundances of polycyclic aromatic hydrocarbons (PAHs) and their size distribution are the result of complex chemical processes implying dust, UV radiation, and the main gaseous components (H, C+, and O). These processes must explain the high abundance of relatively small PAHs in the diffuse interstellar medium (ISM) and imply the continuous formation of some PAHs that are small enough (number of carbon atoms NC <~ 35-50) to be completely dehydrogenated by interstellar UV radiation. The carbon clusters Cn thus formed are constantly exposed to the absorption of ~10-13.6 eV UV photons, allowing isomerization and favoring the formation of the most stable isomers. They might tend to form irregular carbon cages. The frequent accretion of interstellar C+ ions could favor further cage isomerization, as is known in the laboratory for C60, possibly yielding most stable fullerenes, such as C40, C44, and C50. These fullerenes are expected to be very stable in the diffuse ISM because C2 ejection is not possible by single UV photon absorption, but could need rare two-photon absorption. It is possible that at least one of these fullerenes or its cation is as abundant as C60 or C60+ in the diffuse ISM, although this abundance is limited by the lack of observed matching features in observed mid-infrared spectra. B3LYP calculations of the visible spectrum for a number of fullerene isomers with 40 <~ NC <~ 50 show that they generally have a few spectral bands in the visible range, with f-values in the range of a few 10-2. This could make such fullerenes interesting candidates for the carriers of some diffuse interstellar bands.
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