On the sun's absolute disk-center and mean disk intensities, its limb darkening, and its `limb temperature' (λλ330 to 1099 nm)

摘要

The coefficients A 0 of the limb-darkening functions I(μ)/I center=P 5(μ)=ΣA iμi (i=0,...5, μ=cos θ), published by Neckel and Labs (1994), and the corresponding disk-center intensities I center=I(μ=1), which were taken from the absolutely calibrated Kitt Peak FTS Atlas of the disk center (Brault et al., see Neckel and Labs, 1984, and Neckel 1999), are used to derive `limb intensities' I limb=I(μ=0)=A 0 I center. The corresponding `limb temperatures' T limb vary only slightly with wavelength; the mean value (≃4750 K) and the wavelength of maximum intensity (≃605 nm) conform to Wien's law (λmax T=0.288 cm K). Further, T limb agrees closely with that temperature, which follows from Avrett's (2000) model of the photosphere for τ500≃0.006; for this layer the optical thickness along the line of sight is close to 1 (`the limb'; compare Unsöld, 1968). The slight variation of T limb with wavelength is presumably due to systematic errors in the Neckel and Labs intensity data: it corresponds almost precisely to the differences between their data and the more recent ones provided by, e.g., Burlov-Vasiljev, Gurtovenko, and Matvejev (1995), and Burlov-Vasiljev, Matvejev, and Vasiljeva (1998). Two simple correction functions (for λ≦550 nm and λ≧550 nm) are proposed, which apply to all Neckel and Labs intensity data (disk center and full disk, line spectrum and (quasi) continuum), and to the absolutely calibrated Kitt-Peak FTS Atlas (spectra of disk center and full disk) as well.