This page hosts an interactive calculator that estimates the critical value for coevality of two binary stars.

If you use the calculator, please acknowledge the following paper:

Valle G., Dell'Omodarme M., Prada Moroni P.G., Degl'Innocenti S. (2015). A statistical test on the expected difference on grid-based binary stars age estimates. In preparation.


The following information are required:
  • M1: the mass of the primary star in solar unit (range: 0.80 - 1.60 Msun).
  • M2: the mass of the secondary star in solar unit (range: 0.80 - 1.60 Msun).
  • Metallicity: the metallicity [Fe/H] of the primary star.
  • r: the relative age of the primary star, defined as the ratio of the age af the star and the age at the TAMS.
  • Delta Teff: error on the effective temperature (in K).
  • Delta [Fe/H]: error on the metallicity (in dex).
  • Delta M: relative error in the mass determination (range: [0.0; 1.0]).
  • Delta R: relative error in the radius determination (range: [0.0; 1.0]).


The following information are computed:
  • W0.95: the critical value at level 0.05.
  • beta W0.95: the approximated critical values from the beta distribution.


For the computations, the following correlations ara assumed:

  • rho = 0.95 between the two effective temperatures
  • rho = 0.95 between the two [Fe/H]
  • rho = 0.8 between the two masses
  • rho = 0.0 between the two radii.


Please provide the required input, press the button "Compute...", and wait a few seconds.

M1 (Msun) M2 (Msun) [Fe/H] r Δ Teff Δ [Fe/H] Δ M Δ R