Empirical H - and K -band Limb Darkening for 31 CHARA Stars: A Near-infrared Benchmark for Stellar-Atmosphere Models

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Abstract
Limb darkening, the decrease in stellar intensity from the disk center to the limb, encodes the temperature structure and opacity of stellar atmospheres. Direct spatially resolved measurements of this center-to-limb variation remain scarce, especially in the near-infrared. We present interferometric limb-darkening measurements for 31 stars observed simultaneously in the H and K bands with the CHARA Array. The sample spans spectral types F–M and luminosity classes IV–I. The targets are well resolved in H and, for most targets, also in K . This coverage constrains the visibility curvature associated with limb darkening in joint H + K fits. We fit the combined H + K squared visibilities with four analytic limb-darkening laws and compare the resulting coefficients with bandpass-matched predictions from five stellar-atmosphere grids (Kurucz, MPS1, MPS2, Stagger, and spherical SATLAS, with reported coefficients placed on the Rosseland-radius convention) spanning one-dimensional plane-parallel, three-dimensional radiation–hydrodynamic, and spherical low-gravity models. The associated limb-darkened angular diameters are measured with median formal precisions of ≃0.2%–0.3%. The CHARA results show expected weaker limb darkening at longer wavelengths and also with increasing T eff . The clearest discrepancy with the atmosphere grids is in the wavelength dependence: the median fractional decrease in the power-law coefficient from H to K is ≃39% in the CHARA sample, compared with only ≃17%–22% with model predictions. Relative to the MPS2 comparison, the empirical coefficients are higher by ≃21% in H and lower by ≃5% in K . Synthetic recovery tests indicate that the excess scatter is unlikely to be explained solely by incomplete spatial-frequency coverage or numerical instability in the fitting pipeline. These results provide multiband interferometric limb darkening as a near-infrared benchmark for stellar-atmosphere models.
Funding Information
  • National Science Foundation (1506540)
  • National Science Foundation (1909165)
  • National Science Foundation (2034336)
  • National Science Foundation (2407956)
  • EC ∣ European Research Council (639889)
  • EC ∣ European Research Council (101003096)
  • National Science Foundation (2009489)
  • ANID ∣ Fondo Nacional de Desarrollo Científico y Tecnológico (1241073)

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