[Paper Review] In Dust We Trust: An Overview of Observations and Theories of Interstellar Dust

Aigen Li, J. Mayo Greenberg|arXiv (Cornell University)|Apr 23, 2002

Astrophysics and Star Formation Studies31 references3 citations

TL;DR

This comprehensive review synthesizes observational and theoretical advances in interstellar dust over the past century, establishing dust as a dynamic, chemically active agent in galactic evolution. It details how dust forms in stellar outflows, evolves through interstellar and molecular clouds via accretion and processing, and critically enables star and planet formation through shielding, cooling, and catalysis—highlighting unresolved questions in dust composition, carriers of key spectral features, and its role in astrochemistry and life's origins.

ABSTRACT

The past century of interstellar dust has brought us from first ignoring it to finding that it is an important component of the interstellar medium and plays an important role in the evolution of galaxies, the formation of stars and planetary systems, and possibly, the origins of life. Current observational results in our galaxy provide a complex physical and chemical evolutionary picture of interstellar dust starting with the formation of small refractory particles in stellar atmospheres to their modification in diffuse and molecular clouds and ultimately to their contribution to star forming regions. In this review, a brief history of the studies of interstellar dust is presented. Our current understanding of the physical and chemical properties of interstellar dust are summarized, based on observational evidences from interstellar extinction, absorption, scattering, polarization, emission (luminescence, infrared vibrational emission, and microwave rotational emission), interstellar depletions, and theoretical modelling. Some unsolved outstanding problems are listed.

Motivation & Objective

• To synthesize the historical and current understanding of interstellar dust as a key component of the interstellar medium.
• To examine the physical and chemical evolution of dust from stellar formation to molecular cloud processing and star formation.
• To identify unresolved problems in dust composition, carrier species, and evolutionary pathways.
• To assess the role of dust in catalyzing molecular formation, shielding molecular regions, and contributing to IR luminosity.
• To outline future observational and theoretical challenges in understanding dust's role in galaxy evolution and the origins of life.

Proposed method

• Synthesis of observational data from interstellar extinction, polarization, infrared and microwave emission, and depletions.
• Integration of theoretical models of grain formation, growth, destruction, and processing in various interstellar environments.
• Use of laboratory astrophysics and radiative transfer models to interpret dust emission and absorption features.
• Analysis of dust evolution through cycles between diffuse and molecular clouds, including accretion of ice mantles and UV processing.
• Application of models to explain the 2175 Å hump, diffuse interstellar bands (DIBs), and unidentified infrared bands (UIRs).
• Evaluation of dust's role in star formation via energy dissipation (IR emission), shielding from UV radiation, and ambipolar diffusion modulation.

Experimental results

Research questions

• RQ1 What are the carriers of the 2175 Å extinction hump and the diffuse interstellar bands (DIBs)?
• RQ2 What is the origin and carrier of the unidentified infrared bands (UIRs), and are they primarily from PAHs formed in situ or in carbon-rich outflows?
• RQ3 What causes the extended red emission (ERE), and what is its carrier?
• RQ4 Where and how are interstellar dust grains formed—primarily in the cold ISM or as stardust condensing in evolved stars?
• RQ5 What is the true composition, morphology, and size distribution of interstellar dust grains, and why are crystalline silicates rare in the ISM despite being present in comets and stardust?

Key findings

• Interstellar dust contributes ~30% of the total Galactic luminosity through infrared emission and plays a central role in energy dissipation during star formation.
• Dust grains form in the outflows of evolved stars, Wolf-Rayet stars, and supernovae, and are later processed in diffuse and molecular clouds through accretion, coagulation, and UV irradiation.
• The 2175 Å extinction hump is likely due to small silicate or carbonaceous grains, though its exact carrier remains unidentified.
• Unidentified infrared bands (UIRs) are strongly associated with polycyclic aromatic hydrocarbons (PAHs), but their formation mechanism—whether in situ in molecular clouds or from carbon-rich outflows—remains uncertain.
• Extended red emission (ERE) is linked to dust with specific optical properties, possibly involving silicon carbide or amorphous carbon, but its carrier is still unknown.
• Dust destruction occurs via sputtering, vaporization, and shattering in shocks, while grain growth and mantle formation occur in dense molecular clouds, with significant implications for chemical evolution and star formation efficiency.