The Kraft Break Sharply Divides Low Mass and Intermediate Mass Stars

Main sequence stars transition at mid-F spectral types from slowly rotating (cooler stars) to rapidly rotating (hotter stars), a transition known as the Kraft Break (Kraft 1967) and attributed the disappearance of the outer convective envelope, causing magnetic braking to become ineffective. To define this Break more precisely, we assembled spectroscopic measurements of 405 F stars within 33.33 pc. Once young, evolved and candidate binary stars are removed, the distribution of projected rotational velocities shows the Break to be well-defined and relatively sharp. Nearly all stars redder than G_BP-G_RP = 0.60 mag are slowly rotating (vsini < 20 km/s), while only 4 of 40 stars bluer than G_BP-G_RP = 0.54 mag are slowly rotating, consistent with that expected for a random distribution of inclinations. The Break is centered at an effective temperature of 6550 K and has a width of about 200 K, corresponding to a mass range of 1.32 - 1.41 M_Sun. The Break is ~450 K hotter than the stellar temperature at which hot Jupiters show a change in their obliquity distribution, often attributed to tidal realignment. The Break, as defined above, is nearly but not fully established in the ~650 Myr Hyades cluster; it should be established in populations older than 1 Gyr. We propose that the Kraft Break provides a more useful division, for both professional and pedagogical purposes, between what are called low mass stars and intermediate mass stars; the Kraft Break is observationally well-defined and is linked to a change in stellar structure.