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Basic Definitions

Basic Definitions. Terms we will need to discuss radiative transfer. r. n. s. $. dA. Specific Intensity I. I ν.

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Basic Definitions

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  1. Basic Definitions Terms we will need to discuss radiative transfer.

  2. r n s $ dA Specific Intensity I Iν • Iν(r,n,t) ≡ the amount of energy at position r (vector), traveling in direction n (vector) at time t per unit frequency interval, passing through a unit area oriented normal to the beam, into a unit solid angle in a second (unit time). • If θ is the angle between the normal s (unit vector) of the reference surface dA and the direction n then the energy passing through dA is • dEν = Iν(r,n,t) dA cos (θ) dν dω dt • Iν is measured in erg Hz-1 s-1 cm-2 steradian-1 Basic Definitions

  3. Specific Intensity II • We shall only consider time independent properties of radiation transfer • Drop the t • We shall restrict ourselves to the case of plane- parallel geometry. • Why: the point of interest is d/D where d = depth of atmosphere and D = radius of the star. • The formal requirement for plane-parallel geometry is that d/R ~ 0 • For the Sun: d ~ 500 km, D ~ 7(105) km • d/D ~ 7(10-4) for the Sun • The above ratio is typical for dwarfs, supergiants can be of order 0.3. Basic Definitions

  4. Z Specific Intensity III • Go to the geometric description (z,θ,φ) - polar coordinates • θ is the polar angle • φ is the azimuthal angle • Z is with respect to the stellar boundary (an arbitrary idea if there ever was one). • Z is measured positive upwards • + above “surface” • - below “surface” φ Iν θ Basic Definitions

  5. Mean Intensity: Jν(z) • Simple Average of I over all solid angles • Iν is generally considered to be independent of φ so • Where µ = cosθ ==> dµ = -sinθdθ • The lack of azimuthal dependence implies homogeneity in the atmosphere. Basic Definitions

  6. Physical Flux • Flux ≡ Net rate of Energy Flow across a unit area. It is a vector quantity. • Go to (z, θ, φ) and ask for the flux through one of the plane surfaces: • Note that the azimuthal dependence has been dropped! Basic Definitions

  7. R r To Observer Astrophysical Flux θ θ • FνP is related to the observed flux! • R = Radius of star • D = Distance to Star • D>>R  All rays are parallel at the observer • Flux received by an observer is dfν = Iν dω where • dω = solid angle subtended by a differential area on stellar surface • Iν = Specific intensity of that area. Basic Definitions

  8. R r To Observer Astrophysical Flux II For this Geometry: dA = 2πrdr but R sinθ = r dr = R cosθ dθ dA = 2R sinθ Rcosθ dθ dA = 2R2 sinθ cosθ dθ Now μ = cosθ dA = 2R2μdμ By definition dω = dA/D2 dω = 2(R/D)2μ dμ θ θ Basic Definitions

  9. Astrophysical Flux III Now from the annulus the radiation emerges at angle θ so the appropriate value of Iν is Iν(0,μ). Now integrate over the star: Remember dfν=Iνdω NB: We have assumed I(0,-μ) = 0 ==> No incident radiation. We observe fν for stars: not FνP Inward μ: -1 < μ < 0 Outward μ: 0 < μ < -1 Basic Definitions

  10. Moments of the Radiation Field Basic Definitions

  11. s s′ P P′ dA′ dA Invariance of the Specific Intensity • The definition of the specific intensity leads to invariance. • Consider the ray packet which goes through dA at P and dA′ at P′ • dEν = IνdAcosθdωdνdt = Iν′dA′cosθ′dω′dνdt • dω = solid angle subtended by dA′ at P = dA′cosθ′/r2 • dω′ = solid angle subtended by dA at P′ = dAcosθ/r2 • dEν = IνdAcosθ dA′cosθ′/r2 dνdt = Iν′dA′cosθ′dAcosθ/r2 dνdt • This means Iν = Iν′ • Note that dEν contains the inverse square law. θ θ′ Basic Definitions

  12. Energy Density I • Consider an infinitesimal volume V into which energy flows from all solid angles. From a specific solid angle dω the energy flow through an element of area dA is • δE = IνdAcosθdωdνdt • Consider only those photons in flight across V. If their path length while in V is ℓ, then the time in V is dt = ℓ/c. The volume they sweep is dV = ℓdAcosθ. Put these into δEν: • δEν = Iν (dV/ℓcosθ) cosθ dω dν ℓ/c • δEν = (1/c) Iν dωdνdV Basic Definitions

  13. Energy Density II • Now integrate over volume • Eνdν = [(1/c)∫VdVIνdω]dν • Let V → 0: then Iν can be assumed to be independent of position in V. • Define Energy Density Uν ≡ Eν/V • Eν = (V/c)Iνdω • Uν = (1/c) Iνdω = (4π/c) Jν • Jν = (1/4π)Iνdω by definition Basic Definitions

  14. Photon Momentum Transfer • Momentum Per Photon = mc = mc2/c = hν/c • Mass of a Photon = hν/c2 • Momentum of a pencil of radiation with energy dEν = dEν/c • dpr(ν) = (1/dA) (dEνcosθ/c) = ((IνdAcosθdω)cosθ) / cdA = Iνcos2θdω/c • Now integrate: pr(ν) = (1/c)  Iνcos2θdω = (4π/c)Kν Basic Definitions

  15. Radiation Pressure Photon Momentum Transport Redux • It is the momentum rate per unit and per unit solid angle = Photon Flux * (”m”v per photon) * Projection Factor • dpr(ν) = (dEν/(hνdtdA)) * (hν/c) * cosθ where dEν = IνdνdAcosθdωdt so dpr(ν) = (1/c) Iνcos2θdωdν • Integrate dpr(ν) over frequency and solid angle: Basic Definitions

  16. Isotropic Radiation Field I(μ) ≠ f(μ) Basic Definitions

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