Computes the two quadratic limb-darkening coefficients (u1, u2) for exoplanet transit light-curve fits, by trilinear interpolation of published Claret tables in Teff, log(g), and [Fe/H].
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I(μ) / I(1) = 1 − u1·(1−μ) − u2·(1−μ)²
Quadratic limb-darkening coefficients are obtained by trilinear interpolation of published tables in Teff, log(g), and [Fe/H]. For Johnson–Cousins, Sloan/SDSS, Stromgren, Kepler, CoRoT, and Spitzer filters, coefficients are from Claret & Bloemen (2011) using ATLAS9 plane-parallel models with the least-squares fitting method (100 equally-spaced μ points). For TESS, coefficients are from Claret (2018) using PHOENIX-COND spherical models at solar metallicity. For the CBB (Blue Blocking Exoplanet) filter, coefficients are from Claret, Mullen & Gary (2022) RNAAS, which were derived specifically for this filter using ATLAS9 atmosphere models. For the CHEOPS filter, coefficients are from Claret (2021, RNAAS 5, 13), using ATLAS plane-parallel models across all tabulated metallicities and PHOENIX-COND spherical models at solar metallicity, with the least-squares fitting method.
The quadratic limb-darkening law is I(μ)/I(1) = 1 − u1(1 − μ) − u2(1 − μ)2, where μ = cos θ is the cosine of the angle between the line of sight and the stellar surface normal. Microturbulent velocity is fixed at ξ = 2.0 km/s, the standard value for transit work. The LSM (least-squares) fitting method is used throughout. No extrapolation is performed beyond the grid boundaries. A REST API is available and is documented here.