Unification of the nearby and photometric stellar luminosity functions

We introduce a model Galactic field low-mass stellar population that has a proportion of binary systems as observed, with a mass ratio distribution consistent with observational constraints. The model single-star and system luminosity function agrees with the nearby and the Malmquist-corrected photometric luminosity function, respectively. We tabulate the model luminosity functions in the photometric V, I, and K bands, and in bolometric magnitudes. Unresolved binary systems are thus a natural explanation for the difference between the nearby and photometric luminosity functions. A local overdensity of faint stars need not be postulated to account for the difference and is very unlikely. We stress that the nearby luminosity function can only be used to weakly constrain the stellar mass function below 0.5 M_sun because of the small sample size. The photometric luminosity function can only be used to put lower limits on the stellar mass function because most binary systems are not resolved. Taken together, however, the nearby and photometric stellar luminosity function data do not imply a mass function with a peak at a mass of 0.2-0.3 M_sun. Instead, the data are consistent with a power-law mass function between 0.08 M_sun and 0.5 M_sun. We urge researchers to use only star count data that are properly corrected for all contributions to cosmic scatter, photometric uncertainties, and unresolved binaries, and to be aware of the severe limitations of theoretical mass-luminosity relations for low-mass stars, when drawing conclusions about structure in the stellar mass function.