Solution

ID: past-exam-of-the-short-options-paper-of-the-physics-prelims-of-the-university-of-oxford/2022/s02/1/a/solution

Assume spherical stars, central eclipses, uniform disc surface brightness, and negligible third light. Let denote the smaller star and the larger one, with and . Normalize the out-of-eclipse flux to one, and denote the deep and shallow fractional losses by and . The graph gives approximately , , , so , ; the last digits are graphical estimates.
At total occultation of the smaller star, the lost flux is . At its central transit across the larger star, the lost flux is . There are two identifications:
  • Smaller star hidden at the deep eclipse. Then and , giving and . Thus . The smaller star has the higher surface brightness; the deep eclipse hides it and the shallow eclipse is its transit.
  • Smaller star hidden at the shallow eclipse. Then and , giving and . Thus . The smaller star has the lower surface brightness; the deep eclipse is its transit across the hotter, larger star.
These are ratios of luminosities in the observed band. Treating them as bolometric luminosities requires similar band corrections or an explicitly bolometric light curve. Limb darkening and finite exposure times limit the precision of these elementary estimates.

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