Abstract
The sunspot models published so far do not reproduce the observed run of the umbral continuum intensities over the entire spectral range 0.5 < λ < 4 μm. Moreover, in several previous models is the temperature gradient smaller than both the adiabatic and the radiative equilibrium gradient.
Agreement between intensities computed from acceptable models and measured intensities can be obtained by introducing an additional opacity for λ ≲ 0.8 μm, which is probably caused by the crowding of atomic and molecular lines. We present a new umbral model atmosphere with a wavelength dependent opacity enhancement factor which explains the continuum intensities and also reproduces plausible center-to-limb variations and line profiles. This model is in radiative equilibrium down to τ0.5 = 1.5, with an effective temperature of 4000 ± 100K. For the deeper superadiabatic layers a small but probably significant departure from radiative equilibrium is indicated by the intensities in the range 1.5 < λ < 2.4 μm.
The uncertainties in the present model and the effect of the additional opacity on line profiles are briefly discussed.
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Zwaan, C. Structure of sunspots. Sol Phys 37, 99–111 (1974). https://doi.org/10.1007/BF00157847
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DOI: https://doi.org/10.1007/BF00157847