Equivalent width and spectral-line saturation
An absorption line's wavelength equivalent width is W_λ=∫[1−F_λ/F_{λ,c}]dλ, using the local continuum.
Key knowledge
An absorption line's wavelength equivalent width is W_λ=∫[1−F_λ/F_{λ,c}]dλ, using the local continuum. In a purely absorbing foreground slab, W_λ=∫[1−e^(−τ_λ)]dλ. For small optical depth this is approximately ∫τ_λdλ and increases linearly with absorber column density at fixed level populations and line properties. Saturation and broad wings break that linear inference.
Values and equations
W_λ=∫[1−F_λ/F_{λ,c}]dλ; pure slab: W_λ=∫[1−e^(−τ_λ)]dλ
- W_λ,λ
- wavelength equivalent width and wavelength m
- F_λ,F_{λ,c}
- measured spectral flux density and local continuum spectral flux density W m⁻² m⁻¹
- τ_λ
- line-of-sight wavelength optical depth 1
Conditions and scope
- Specify the continuum, integration range and wavelength units; the slab calculation neglects its own emission and scattered light entering the beam.
- The curve of growth depends on the opacity profile, Doppler broadening and damping. Thermal and unresolved turbulent motion can contribute to the same width; a single unresolved line does not uniquely determine abundance or temperature.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Jeremy B. Tatum — University of Victoria, Stellar Atmospheres2 sources
Evidence and review
Read the equivalent-width definition, pure-slab exponential and optically thin expansion, saturation/wing mechanism and observational degeneracy discussion. Expressed the same normalized measurement in unresolved source flux; did not extend the foreground-slab solution to a general emitting stellar atmosphere.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
Entry details
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- uni-cosmos-equivalent-width-growth
- Revision
- core.1
- Structured reference record
- University core expansion