An absence of gaps in the main-sequence population of field stars

Using high-precision parallaxes from the Hipparcos catalog, we construct H-R diagrams for two samples of bright stars. The first is a magnitude-limited sample that is over 90% complete and uses uniform photometry from the Catalog of WBVR Magnitudes of Northern Sky Bright Stars (delta > -14 degrees). This sample shows a smooth distribution of stars along the main sequence, with no detectable gaps. The second contains all of the stars closer than 100 pc in the Hipparcos catalog with delta < -12 degrees. Uniform spectroscopy from the Michigan Spectral Survey shows that some stars that appear on the main sequence in the H-R diagram, particularly those in the 0.2 < B - V < 0.3 region that has been labeled the Bohm-Vitense gap, are classified as giants by the MK system of spectral classification. Other gaps that have been identified in the main sequence are also affected by such classification criteria. This analysis casts doubt on the existence of the Bohm-Vitense gap, which is thought to result from the sudden onset of,convection in stars. The standard identification of main-sequence stars with luminosity class V and giants with luminosity class III must be reconsidered for some spectral types. The true nature of the stars that lie on the main sequence in the H-R diagram but do not have luminosity class V designations remains to be investigated.