On posterior probability and significance level: application to the power spectrum of HD 49933 observed by CoRoT

Aims. We emphasize that the mention of the significance level when rejecting the null hypothesis (H0 which assumes that what is observed is pure noise) can mislead one to think that the H0 hypothesis is unlikely to occur with that significance level. We show that the significance level has nothing to do with the posterior probability of H0 given the observed data set, and that this posterior probability is very much higher than what the significance level naively provides. Methods. We use Bayes theorem for deriving the posterior probability of H0 being true assuming an alternative hypothesis H1 that assumes that a mode is present, taking some prior for the mode height, for the mode amplitude and linewidth. Results. We report the posterior probability of H0 for the p modes detected on HD49933 by CoRoT. Conclusions. We conclude that the posterior probability of H0 provide a much more conservative quantification of the mode detection than the significance level. This framework can be applied to any stellar power spectra similar to those obtained for asteroseismology.