Abstract
Infrared observations of the secondary minimum of the eclipsing system of Algol, secured recently by Nadeauet al. (1978) with the 200 in and 60 in reflectors of Mount Wilson and Palomar Observatories at the effective wavelength of 10 μm, show its light curve to be distinctlydish-shaped-i.e., the light diminishes relatively fast in the early stages of the eclipse, and its rate of decline slows down in advanced partial phases. This fact indicates convincingly that the light distribution over the apparent disc of Algol's late-type (contact) component is akin to that produced by the phenomenon of ‘gravity-darkening’ to a very pronounced degree.
Past observational evidence on close binary systems has indicated that gravity-darkening consistent with the equilibrium structure of the stars was adequate to account for the light variations of such systems exhibited between minima, as long as their components remain on (or do not depart much from) the Main Sequence (Kopal, 1968b). However, for components close to (or at) their Roche limits, the photometric effects of gravity-darkening appear to be greatly exaggerated (Kopal, 1968a)—a result confirmed by subsequent investigators (cf. Parthasarathy, 1972; or Niarchos, 1978).
An analysis of Algol's infrared light curve during the secondary minimum (when its contact component undergoes eclipse by its nearly spherical mate) observed at an effective wavelength of 10 μm, discloses now that the (monochromatic) coefficient of the linear law of gravity-darkening, characterizing the distribution of brightness over the apparent disc of the contact star comes out again at least twice as large as one which would correspond to a purely radiative energy transfer of total light in the far interior of this star. No physical theory can be advanced to explain this fact—except, possibly, a hypothesis that the observed enhancement of the monochromatic coefficient τ of gravity-darkening over that appropriate for total radiation may be caused by a very wide departure of the outer layers of the respective stars from thermodynamic equilibrium.
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Kopal, Z. Grativy-darkening in the Algol system. Astrophys Space Sci 60, 441–453 (1979). https://doi.org/10.1007/BF00644344
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DOI: https://doi.org/10.1007/BF00644344