On the Absorption and Emission Properties of Interstellar Grains

Our current understanding of the absorption and emission properties of interstellar grains are reviewed. The constraints placed by the Kramers-Kronig relation on the wavelength-dependence and the maximum allowable quantity of the dust absorption are discussed. Comparisons of the opacities (mass absorption coefficients) derived from interstellar dust models with those directly estimated from observations are presented.Comment: invited talk for "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data" (Heidelberg, Germany; 4--8 October 2004), edited by C.C. Popescu & R.J. Tuffs, AIP Conf. Ser., in press; (11 pages, no figures, no tables

Nanodust in the Interstellar Medium in Comparison to the Solar System

Nanodust, which undergoes stochastic heating by single starlight photons in the interstellar medium, ranges from angstrom-sized large molecules containing tens to thousands of atoms (e.g. polycyclic aromatic hydrocarbon molecules) to grains of a couple tens of nanometers. The presence of nanograins in astrophysical environments has been revealed by a variety of interstellar phenomena: the optical luminescence, the near- and mid-infrared emission, the Galactic foreground microwave emission, and the ultraviolet extinction which are ubiquitously seen in the interstellar medium of the Milky Way and beyond. Nanograins (e.g. nanodiamonds) have also been identified as presolar in primitive meteorites based on their isotopically anomalous composition. Considering the very processes that lead to the detection of nanodust in the ISM for the nanodust in the solar system shows that the observation of solar system nanodust by these processes is less likely.Comment: 28 pages, 12 figures; invited book chapter for "Nanodust in the Solar System: Discoveries and Interpretations" (2012

Interstellar Silicon Depletion and the Ultraviolet Extinction

Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Angstrom interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [Si/H]_{dust} and the 2175 Angstrom bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance [Si/H]_{gas} is known. We derive [Si/H]_{dust} either from {[Si/H]_{ISM} - [Si/H]_{gas}} or from the Kramers-Kronig relation which relates the wavelength-integrated extinction to the total dust volume, where [Si/H]_{ISM} is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive [Si/H]_{dust} from fitting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We find that in all three cases [Si/H]_{dust} shows no correlation with the 2175 Angstrom bump, while the carbon depletion [C/H]_{dust} tends to correlate with the 2175 Angstrom bump. This supports carbon grains instead of silicates as the possible carrier of the 2175 Angstrom bump. We also find that neither [Si/H]_{dust} nor [C/H]_{dust} alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.Comment: 38 pages, 15 figures, 2 tables; accepted for publication in The Astrophysical Journal (2017). arXiv admin note: text overlap with arXiv:1507.0659

The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic 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 and 6.85 micrometer 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.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter