7.2 Interstellar dust and extinction

Interstellar dust plays a crucial role in shaping our universe. These tiny particles absorb and scatter starlight, catalyze chemical reactions, and provide building blocks for planetary systems. They're found throughout galaxies, influencing their dynamics and evolution.

Dust grains are made of silicates, carbonaceous materials, and ices. Their size and composition affect how they interact with light, causing extinction and reddening of starlight. Scientists study dust through various methods, including extinction curves and infrared observations.

Interstellar Dust Properties and Composition

Role of interstellar dust

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• Microscopic solid particles in space between stars composed of silicates, carbonaceous materials, and ices
• Absorbs and scatters starlight altering observed brightness and color of celestial objects
• Catalyzes chemical reactions in space facilitating formation of complex molecules (water, methanol)
• Provides formation sites for molecules acting as building blocks for planetary systems
• Contributes to overall mass of galaxy affecting galactic dynamics and evolution
• Concentrated in molecular clouds and star-forming regions shapes structure of interstellar medium
• Present throughout galactic disk influences large-scale galactic properties

Composition of dust grains

• Silicates form core of many dust grains (olivine, pyroxene)
• Carbonaceous materials contribute to grain structure (graphite, amorphous carbon)
• Ices coat grains in cold regions preserving volatile compounds (water, carbon dioxide, methane)
• Metals and metal oxides add to grain diversity (iron, magnesium oxides)
• Size distribution ranges from nanometers to micrometers following power-law distribution
• Majority of grains smaller than visible light wavelengths affects interaction with electromagnetic radiation
• Irregular and non-spherical shapes influence scattering properties and polarization of light

Interstellar Extinction and Observation Techniques

Concept of interstellar extinction

• Attenuation of starlight by dust reduces apparent brightness of stars
• Wavelength-dependent process causes color changes (reddening) of starlight
• Alters spectral features complicating spectroscopic analysis
• Extinction law describes wavelength dependence often parameterized by $R_V = A_V / E(B-V)$
• Impacts distance measurements leading to underestimation if not accounted for
• Requires correction for accurate cosmic distance ladder calculations

Methods for studying dust

• Extinction curves plot extinction vs wavelength revealing dust grain properties
• Prominent 2175 Å bump indicates presence of carbonaceous materials
• Color excess measurements $E(B-V)$ indicate dust column density
• Comparison of observed vs intrinsic stellar colors determines reddening
• Linear polarization measurements of starlight by aligned dust grains reveal magnetic field structure
• Infrared observations detect thermal emission from dust grains
• Spectral features in IR indicate dust composition (silicate features at 10 and 18 μm)
• Scattering observations of reflection nebulae and diffuse galactic light provide insights into dust distribution