In Bell et al. 2012 it was shown that M-dwarf stars cooler than about 4000K exhibit half as much flux at optical wavelengths than is suggested by their equivalent stellar models. This can cause the ages of stars in young clusters, (less than 10Myr in age), to be underestimated by a factor of 2-3 depending on choice of colour. Not only does this in turn cause large discrepancies with planet formation timescales, but the excess flux in the models can drastically affect studies of exoplanet climatology.

By observing broadband visible spectra of stars in the Pleiades and Praesepe, the two clusters for which we have robust memberships, ages and metallicities, and deriving robust flux calibrations for those spectra, we can directly compare them to the suspect stellar atmospheres. We have performed synthetic photometry on the spectra and compared them to photometric values to ensure accurate fluxes. In order to confirm the comparisons to models are also reliable we transform them to the observational plane and pass them through a resolution model of the instrument on which the spectra were observed.

Initial findings suggest that the discrepancy is caused by both grey opacity, such as that caused by an overabundance of ionised hydrogen in the stellar photosphere, as well as excess opacity localised around molecular absorption features, such as TiO and VO.