THE 6 μm FEATURE AS A TRACER OF ALIPHATIC COMPONENTS OF INTERSTELLAR CARBONACEOUS GRAINS (original) (raw)
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The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
The Astrophysical Journal, 2012
The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal-or kerogen-like organic nanoparticles with a mixed aromaticaliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 µm and 6.85 µm emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be < 15%. We conclude that the UIE emitters are predominantly aromatic, with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands.
Astronomy and Astrophysics, 2008
Context. The infrared emission features observed in the mid-infrared wavelength range in astronomical objects, often called the Aromatic Infrared Bands, exhibit differences in shape and position. Three astrophysical spectral classes have been proposed based on the spectral characteristics. The band positions in most sources are similar to those of aromatic materials, however, the exact nature of the emitters is still unknown. Aims. The spectral diversity of the bands provides a clue to the nature of the materials. An evolutionary scenario for the nature of the emitters can be inferred by comparison with laboratory analogues. Methods. The laboratory spectra of a wide range of soot material samples were recorded and a global analysis of the infrared absorption spectra performed. This spectral analysis, allied to the band shape and position variations, were then used to interpret the diversity and evolution of the features in the astronomical spectra. Results. We find correlations between the spectral regions characteristic of the CC and CH modes and use these to shed light on the origin of the infrared emission features. In particular, the observed shift in the position of the 6.2-6.3 µm band is shown to be a key tracer of the evolution of the aliphatic to aromatic component of carbonaceous dust.
Infrared Spectral Identification of Complex Organic Molecules in Interstellar Grains
The Search for Extraterrestrial Life: Recent Developments, 1985
The chemical evolution of interstellar grains leads ultimately to a composition consisting largely of complex organic molecules. Comparison of infrared absorption spectra of laboratory produced analogue materials with astronomical observations confirm the presence of similar molecules in interstellar space. The abundance of this complex organic matter derived from the strength of the absorption bands is of the order of ten million solar masses and is almost certainly as large or larger than all conceivable planets. 1.
2016
The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of < 9% was placed on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 µm and 3.3 µm emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A 3.4 /A 3.3 ≈ 0.68 derived from a small set of aliphatic and aromatic compounds, where A 3.4 and A 3.3 are respectively the band strengths of the 3.4 µm aliphatic and 3.3 µm aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources which exhibit both the 3.3 µm and 3.4 µm C-H features and determine I 3.4 /I 3.3 , the ratio of the power emitted from the 3.4 µm feature to that from the 3.3 µm feature. We derive the median ratio to be I 3.4 /I 3.3 ≈ 0.12. We employ density functional theory to compute A 3.4 /A 3.3 for a range of methyl-substituted PAHs. The resulting A 3.4 /A 3.3 ratio well exceeds ∼ 1.4, with an average ratio of A 3.4 /A 3.3 ≈ 1.76. By attributing the 3.4 µm feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form from I 3.4 /I 3.3 ≈ 0.12 and A 3.4 /A 3.3 ≈ 1.76 to be ∼ 2%. We therefore conclude that the UIE emitters are predominantly aromatic.
Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium
The Astrophysical Journal, 2001
Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 mid-infrared (5 to 12 µm) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina Nebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 µm. The relative band intensities (6.2/7.7 µm, 8.6/7.7 µm, and 11.2/7.7 µm) were derived from these spectra, and no systematic variation in these ratios was found in our observed regions, in spite of the fact that the incident radiation intensity differs by a factor of 1500. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carriers, PAHs in the DIM have no systematic variation in their size distribution, their degree of dehydrogenation is independent of the strength of UV radiation field, and they are mostly ionized. The finding that PAHs in the DIM with low UV radiation field strength are mostly ionized is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in this kind of environment. A plausible resolution of this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than those adopted in past theoretical studies. Because of the very low population of neutral state molecules, 1 Based on observations with ISO, an ESA project with instruments funded by ESA members states (especially the PI countries France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.
Astronomy and …, 1998
We present ISO-SWS spectroscopy of eight strong infrared sources with large extinction through the diffuse interstellar medium. These are five late-type Wolf-Rayet stars, the blue hypergiant Cyg OB2 #12 and the Galactic Center Sources 3 and 4. The spectra show a number of absorption features that can be ascribed to interstellar dust and gas. Features at 3.0, 3.4, 4.66 and 5.95 µm were already known from ground and airborne observations, while features at 4.27 and 6.21 µm are for the first time observed by ISO. We ascribe the 3.0, 4.27 and 4.66 µm features to H 2 O ice, CO 2 ice and gaseous CO, respectively, located in dense clouds along four lines-of-sight in our study. Based on an excellent correspondence with the well-known emission feature observed in many other objects, the 6.2 µm absorption band is identified with aromatic materials. The profile of the feature indicates that its carriers are PAHs or PAH clusters containing up to a few thousand carbon atoms. The lack of strong infrared features of hydroxy (OH), carbonyl (C=O) and aromatic CH groups towards the objects of the present study shows that the carbonaceous dust material in the diffuse ISM is poor in oxygen as well as hydrogen.
IRAS 08572+3915: constraining the aromatic versus aliphatic content of interstellar HACs
Astronomy and Astrophysics, 2007
We analyze dust features present in the mid-infrared (Spitzer) and recently published L-band (UKIRT) spectra of the infrared galaxy IRAS 08572+3915. The line of sight toward the AGN nucleus crosses a high column density of carbonaceous dust whose characteristic absorption features appear clearly. They provide a real insight into the chemical environment of the diffuse interstellar medium. Thanks to the moderate redshift of IRAS 08572+3915, the wavelength of the aromatic CH stretching mode is free of major telluric lines, and a strong observational constraint of H sp2 /H sp3 ≤ 0.08 has been determined. This limit clearly shows that the bonding of hydrogen atoms in interstellar hydrogenated amorphous carbon is highly aliphatic. The presence of a broad absorption feature centered at 6.2 µm, probably arising from olefinic/aromatic structures, corresponds to the backbone of this carbonaceous material, which is the major carbon-containing component of the interstellar medium along this line of sight.
Infrared Observations and Interstellar Molecules
Highlights of Astronomy
It has been common practice to separate the study of interstellar matter from that of stellar evolution. However, infrared astronomy deals mainly with observations of stars forming and stars dying. Interstellar matter represents a phase intermediate between these two stages, part of a cyclic process (Figure 1).We find molecules in interstellar space and want to know how they came to be there. Molecules form most easily at high densities and moderately high temperatures. These conditions prevail both in envelopes around forming stars and also around evolved red giants. However, the matter going into star formation is mainly leaving interstellar space, while that from the red giants is going into space. Therefore the evolved red giants are potentially the source of interstellar molecules.This paper will propose first that the chief interstellar solid molecules were formed in the atmospheres of red giant stars. Volatile solids that might condense in space do not seem to be a major cons...
Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life, 2004
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