The Kraft Break Sharply Divides Low Mass and Intermediate Mass Stars

Main sequence stars are known to transition at mid-F spectral types from being slowly rotating (cooler stars) to being rapidly rotating (hotter stars), a transition known as the Kraft Break (Kraft1967) and attributed to the disappearance of the outer convective zone, causing magnetic braking to become ineffective. To investigate this Break more precisely, we assembled Gaia DR3 data and spectroscopic measurements of 405 F stars within 33.33 pc of the Sun. The Break boundaries occur at spectral types F4 and F5, effective temperatures of 6650 K and 6500 K, respectively. Using mass-temperature relations of eclipsing binaries, the Break boundaries correspond to stellar masses of 1.35 Msol and 1.28 Msol, Magnetic braking appears to become ineffective with only a small change in effective temperature and stellar mass. A study of F stars in the ~ 625 Myr Hyades cluster shows that the Break is nearly but not fully established by this age; we speculate that it should be established in populations older than 1 Gyr. We propose that the Kraft Break provides a less ambiguous division between low mass stars and intermediate mass stars. The Break is observationally well-defined and is physically linked to a change in stellar structure.