2.2. Observational constraints in the local ISM

Ideally, a viable interstellar dust model should fit all observational constraints arising primarily from the interactions of the dust with the incident radiation field or the ambient gas. These include:

1. the extinction, obscuration, and reddening of starlight;
2. the infrared emission from circumstellar shells and different phases of the ISM (diffuse H I, H II regions, photodissociation regions or PDRs, and molecular clouds);
3. the elemental depletion pattern and interstellar abundances constraints;
4. the extended red emission seen in various nebulae;
5. the presence of X-ray, UV, and visual halos around time-variable sources (X-ray binaries, novae, and supernovae);
6. the presence of fine structure in the X-ray absorption edges in the spectra of X-ray sources;
7. the reflection and polarization of starlight;
8. the microwave emission, presumably from spinning dust;
9. the presence of interstellar dust and isotopic anomalies in meteorites and the solar system;
10. the production of photoelectrons required to heat neutral photodissociation regions (PDRs); and
11. the infrared emission from X-ray emitting plasmas.

It is unreasonable to require that a single dust model simultaneously fit all these observational constraints since they vary in different astrophysical environment, reflecting the regional changes in dust properties.

However, a viable interstellar dust model should be derived by simultaneously fitting at least a basic set of observational constraints. Also, it must consist of particles with realistic optical, physical, and chemical properties, and require no more than the ISM abundance of any given element to be locked up in the dust. In practice, most interstellar dust models have been constructed by deriving the abundances and size distributions of some well studied solids, such as graphite or silicates, using select observations such as the average interstellar extinction, the polarization, or the diffuse infrared emission as constraints, and then checked the model for consistency with other observational constraints such as the wavelength dependent albedo and interstellar abundances.

Simultaneous fits to the average interstellar extinction curve and the infrared (IR) emission from the diffuse ISM have given rise to a standard interstellar dust model consisting of bare, spherical graphite and silicate particles and a population of polycyclic aromatic hydrocarbons (PAHs) [46]. This standard model can account for the observed 2175 Å bump in the UV extinction curve and the far-UV rise in extinction attributed to graphite and PAHs; for the mid-IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, most commonly associated with PAHs; and for the general continuous IR emission from the diffuse ISM, attributed to submicron size silicate and graphite grains. The choice of graphite and PAHs was mainly motivated by the UV extinction bump and the mid-IR emission features. Silicate particles were included to account for the presence of the 9.7 and 18 µm absorption features seen in a variety of astrophysical objects and Galactic lines of sights. Using this model as a benchmark, regional variations in the observational manifestations of the dust can then be attributed to local deviations from this standard model. For example: the lack of the mid-IR emission features from inside H II regions can be attributed to the depletion of PAHs in these regions; the flattening of the extinction curve at UV energies to a deficiency in PAHs and very small dust particles; and the presence of various absorption features in the spectra of some astronomical sources to the precipitation of ices onto the dust in these objects. Brief reviews on the history of the development of dust models were presented by [14, 13, 21].