7.2 Interstellar dust and extinction

Interstellar Dust Properties and Composition

Interstellar dust grains are tiny solid particles scattered throughout the space between stars. Though they make up only about 1% of the interstellar medium's mass, they have an outsized effect: they dim and redden starlight, serve as surfaces where molecules form, and supply raw material for building planets. Understanding dust is essential for correcting astronomical observations and for tracing the physical conditions inside galaxies.

Role of Interstellar Dust

Dust grains are concentrated in the galactic disk, especially within molecular clouds and star-forming regions, where they shape the structure of the interstellar medium. Their effects show up at every scale:

• Absorbing and scattering starlight, which changes the observed brightness and color of background stars. This is the primary reason astronomers need to account for dust when measuring distances.
• Catalyzing chemical reactions on grain surfaces. Many molecules that can't form efficiently in the gas phase, like $H_2O$ and $CH_3OH$ (methanol), assemble on dust grains instead.
• Providing building blocks for planetary systems. Dust grains in protoplanetary disks collide and stick together, eventually growing into planetesimals.
• Contributing to galactic mass and dynamics. Although dust is a small fraction of total mass, it dominates the opacity of the interstellar medium, controlling how radiation propagates through a galaxy.

Composition of Dust Grains

Dust grains aren't uniform. Their makeup depends on where they formed and what environment they currently sit in.

• Silicates form the cores of many grains. Common minerals include olivine ($(Mg,Fe)_2SiO_4$) and pyroxene ($(Mg,Fe)SiO_3$). These produce characteristic absorption features in the infrared.
• Carbonaceous materials make up another major component, appearing as graphite, amorphous carbon, or polycyclic aromatic hydrocarbons (PAHs). These are responsible for the prominent UV absorption bump at 2175 Å.
• Ice mantles coat grains in cold, dense regions (typically below ~20 K). Water ice dominates, but $CO_2$, $CO$, and $CH_4$ ices are also present. These mantles preserve volatile compounds until the grain is heated or disrupted.
• Metals and metal oxides such as iron and magnesium oxides add further diversity to grain composition.

Grain sizes follow a power-law distribution, roughly $n(a) \propto a^{-3.5}$ (the MRN distribution), spanning from a few nanometers up to about a micrometer. Most grains are smaller than visible-light wavelengths, which is why they interact more strongly with shorter-wavelength (bluer) light. Grains are also generally irregular and non-spherical, which matters for how they scatter light and polarize it.

Interstellar Extinction and Observation Techniques

Concept of Interstellar Extinction

Extinction is the total attenuation of starlight by dust through both absorption and scattering. It makes stars appear fainter than they truly are.

Extinction is wavelength-dependent: shorter wavelengths (blue light) are extinguished more than longer wavelengths (red light). This is why dust causes reddening, a systematic shift in a star's observed color toward the red. Reddening is not the same as redshift; it's a selective removal of blue photons, not a stretching of wavelengths.

The extinction at a given wavelength is written as $A_\lambda$, measured in magnitudes. Two key quantities tie everything together:

• Color excess: $E(B-V) = A_B - A_V$, which measures how much more extinction occurs in the B band than in the V band. This is a direct indicator of how much dust lies along the line of sight.
• Total-to-selective extinction ratio: $R_V = A_V / E(B-V)$. For the diffuse interstellar medium, $R_V \approx 3.1$ is typical. In dense clouds, $R_V$ can be larger (4–6), reflecting different grain size distributions.

If you don't correct for extinction, you'll systematically underestimate distances on the cosmic distance ladder, because dust-dimmed stars look farther away than they are.

Methods for Studying Dust

Astronomers use several complementary techniques to characterize dust:

1. Extinction curves. Plot $A_\lambda / A_V$ (or $A_\lambda / E(B-V)$) against $1/\lambda$. The shape of this curve reveals grain size distributions and compositions. A prominent feature is the 2175 Å bump, a broad absorption peak in the ultraviolet attributed to carbonaceous grains (likely graphite or PAHs).

2. Color excess measurements. By comparing a star's observed $(B-V)$ color to its expected intrinsic color (known from its spectral type), you get $E(B-V)$, which is proportional to the dust column density along that sightline.

3. Polarization measurements. Elongated dust grains tend to align with the local magnetic field. Starlight passing through these aligned grains becomes linearly polarized. Mapping this polarization across the sky traces the geometry of galactic magnetic fields.

4. Infrared emission. Dust grains absorb UV and optical photons and re-radiate thermally in the infrared. Observing this emission (with telescopes like Spitzer or Herschel) reveals dust temperatures, masses, and spatial distributions. Specific spectral features at ~10 μm and ~18 μm correspond to Si-O stretching and bending modes in silicate grains.

5. Reflection nebulae and diffuse galactic light. Starlight scattered (rather than absorbed) by dust illuminates nearby clouds, producing reflection nebulae. Studying the scattered light's color and intensity constrains grain albedo and scattering phase functions.