Kraft break

In astronomy, the Kraft break refers to the abrupt decrease in stars' average rotation rates at surface temperatures below about 6,500 Kelvin. This temperature corresponds to mid-F type stars. This feature bears the name of astronomer Robert Kraft, though its existence was recognized prior to his publications on the topic.

The break is understood to arise from the presence of a magnetic dynamo in cooler stars, which generates a magnetic field that transfers the star's rotational angular momentum to the stellar wind, thus slowing down stellar rotation through magnetic braking. A convective envelope thick enough for convection timescales to exceed rotation period is required to sustain this solar-type dynamo. Hotter stars, lacking deep convective envelopes, are unable to sustain the dynamo, and so magnetic braking is much less efficient. As a result, they continue to rotate quickly.

The Kraft break coincides with a transition in hot Jupiters' spin-orbit alignment. Hot Jupiters orbiting cooler stars tend to orbit in the same plane as stellar rotation, while those orbiting hotter stars are more likely to be misaligned.

References

  1. 1 2 Wang 汪, Xian-Yu 宪钰; Wang 王, Songhu 松虎; Ong 王, J. M. Joel 加冕 (2026-01-01). "Unified Kraft Break at ∼6500 K: A Newly Identified Single-star Obliquity Transition Matches the Classical Rotation Break". The Astrophysical Journal Letters. 996 (1): L7. arXiv:2511.15610. Bibcode:2026ApJ...996L...7W. doi:10.3847/2041-8213/ae21c5. ISSN 2041-8205.
  2. 1 2 Durney, B. R.; Latour, J. (1977). "On the angular momentum loss of late-type stars". Geophysical & Astrophysical Fluid Dynamics. 9 (1): 241–255. Bibcode:1977GApFD...9..241D. doi:10.1080/03091927708242330. ISSN 0309-1929. Retrieved 2026-09-23.
  3. 1 2 Beyer, Alexa C.; White, Russel J. (2024-09-01). "The Kraft Break Sharply Divides Low-mass and Intermediate-mass Stars". The Astrophysical Journal. 973 (1): 28. arXiv:2408.02638. Bibcode:2024ApJ...973...28B. doi:10.3847/1538-4357/ad6b0d. ISSN 0004-637X.
  4. ↑ Kraft, Robert P. (1967). "Studies of Stellar Rotation. V. The Dependence of Rotation on Age among Solar-Type Stars". The Astrophysical Journal. 150: 551. Bibcode:1967ApJ...150..551K. doi:10.1086/149359. ISSN 0004-637X. Retrieved 2026-08-19.
  5. ↑ Schatzman, E. (1962). "A theory of the role of magnetic activity during star formation". Annales d'Astrophysique. 25: 18. Bibcode:1962AnAp...25...18S.
  6. ↑ Charbonneau, Paul (2014-08-18). "Solar Dynamo Theory". Annual Review of Astronomy and Astrophysics. 52 (1): 251–290. Bibcode:2014ARA&A..52..251C. doi:10.1146/annurev-astro-081913-040012. ISSN 0066-4146. Retrieved 2026-09-23.
  7. ↑ Maeder, A. (2009), Physics, Formation and Evolution of Rotating Stars (PDF), Astronomy and Astrophysics Library, Bibcode:2009pfer.book.....M, doi:10.1007/978-3-540-76949-1, ISBN 978-3-540-76948-4