Year: 2026
Type: article
Abstract: Context. Central flashes may occur during stellar occultations by objects in the Solar System. Aims. Catalog diffraction effects on the flash with point-like stars, monochromatic waves, and different cases of spherical transparent atmosphere; describe the corrections due to stellar diameters. Methods. To describe diffraction, we used the Huygens principle, the Sommerfeld lemma, and the stationary phase method, and we treated the effects of finite stellar diameter using Clausius’ theorem. Results. For point-like stars, the central flash shape is that of the classical Poisson spot, but with a greater height. For tenuous atmospheres that cannot focus the stellar rays at the shadow center, the flash is amplified by the factor (R0/r0)2 compared to the Poisson spot, where R0 and r0 are the object and the shadow radii, respectively. For denser atmospheres that can focus the rays at the shadow center, the flash peaks at 2π2(RCF/λF)2ϕ⊥(0), where RCF is the central flash layer radius, λF is the Fresnel scale, and φ⊥(0) is the flux that would be observed at the shadow center without focusing. For isothermal atmospheres with scale height H, the height is 2π2(RCFH)/λF2. Fringes surrounding the central flash are separated by λP = λF2/RCF, which is related to the separation between the primary and secondary stellar images. For a projected stellar diameter D* ≫ λP, the flash is described by complete elliptic integrals, and has a full width at half maximum of 1.14D* and a peak value of 8H/D*. Conclusions. For Earth-based occultations by Pluto and Triton observed in the visible with point-like stars, diffraction causes flashes with very large heights of ∼104−105, spread over a very small meter-sized region in the shadow plane. In practice, the flash is usually smoothed by the stellar diameter, but still reaches high values of ∼50 and ∼200 during Pluto and Triton occultations, respectively. Diffraction dominates when using millimeter wavelengths or longer. We discuss the effects of departure from sphericity, atmospheric waves, and stellar limb darkening.
Authors: B. Sicardy, L. Dettwiller
Language: English
Topic: Astro and Planetary Science
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7130543441
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