The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons

Abstract

Infrared (IR) absorption spectra of individual polycyclic aromatic hydrocarbons (PAHs) containing methyl (-CH3), methylene (>CH2), or diamond-like *CH groups and IR spectra of mixtures of methylated and hydrogenated PAHs prepared by gas phase condensation were measured at room temperature (as grains in pellets) and at low temperature (isolated in Ne matrices). In addition, the PAH blends were subjected to an in-depth molecular structure analysis by means of high-performance liquid chromatography, nuclear magnetic resonance spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Supported by calculations at the density functional theory level, the laboratory results were applied to analyze in detail the aliphatic absorption complex of the diffuse interstellar medium at 3.4 mu-m and to determine the abundances of hydrocarbon functional groups. Assuming that the PAHs are mainly locked in grains, aliphatic CHx groups (x = 1,2,3) would contribute approximately in equal quantities to the 3.4 mu-m feature (N_{CHx} / N_{H} approx 10^{-5} - 2 * 10^{-5}). The abundances, however, may be two to four times lower if a major contribution to the 3.4 mu-m feature comes from molecules in the gas phase. Aromatic =CH groups seem to be almost absent from some lines of sight, but can be nearly as abundant as each of the aliphatic components in other directions (N_{=CH} / N_{H} < 2 * 10^{-5}; upper value for grains). Due to comparatively low binding energies, astronomical IR emission sources do not display such heavy excess hydrogenation. At best, especially in proto-planetary nebulae, >CH2 groups bound to aromatic molecules, i.e., excess hydrogens on the molecular periphery only, can survive the presence of a nearby star.

34 pages, 19 figures, ApJS, 208, 26

References in corpus (6)

• The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
• Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump
• The role of the charge state of PAHs in ultraviolet extinction
• Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
• The smoothness of the interstellar extinction curve in the UV. Comparison with recent laboratory measurements of PAH mixtures
• Experimental and theoretical study on the infrared spectroscopy of astrophysically relevant PAH derivatives 2- and 9-vinylanthracene

Cited by in corpus (18)

• Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 $\mu$m ratio
• High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 micron region: role of hydrogenation and alkylation
• The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared
• Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions
• Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
• On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
• VUV spectroscopy of carbon dust analogs: contribution to interstellar extinction
• Aliphatic hydrocarbon content of the interstellar dust
• PDRs4All VI: Probing the Photochemical Evolution of PAHs in the Orion Bar Using Machine Learning Techniques
• Machine-learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons
• The 2175 Angstrom Interstellar Extinction Bump: Is the Wavelength Variable?
• Visible absorptions of potential diffuse ISM hydrocarbons: C${}_9$H${}_9$ and C${}_9$H${}_5$ radicals
• Spitzer's perspective of polycyclic aromatic hydrocarbons in galaxies
• CO desorption from interstellar icy grains induced by IR excitation of superhydrogenated PAHs
• Formation of methane and cyclohexane through the hydrogenation of toluene
• The Influence of Aliphatic Components on the Aromatic Emission Characteristics of Polycyclic Aromatic Hydrocarbons