Further Evidence of Modified Spin-down in Sun-like Stars: Pileups in the Temperature–Period Distribution

• Published in: The Astrophysical journal 2022-07, Vol.933 (1), p.114
• Main Authors: David, Trevor J., Angus, Ruth, Curtis, Jason L., van Saders, Jennifer L., Colman, Isabel L., Contardo, Gabriella, Lu, Yuxi, Zinn, Joel C.
• Format: Article
• Language: English
• Subjects: Angular momentum; ASTRONOMY AND ASTROPHYSICS; Astrophysics; Braking; Hypotheses; Kepler mission (NASA); M stars; Main sequence stars; Open clusters; Rossby number; Solar analogs; Star & galaxy formation; Star formation; Stellar evolution; Stellar magnetic fields; Stellar rotation; Stellar surfaces; Stellar winds; Sun; Surface temperature
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac6dd3

We combine stellar surface rotation periods determined from NASA’s Kepler mission with spectroscopic temperatures to demonstrate the existence of pileups at the long-period and short-period edges of the temperature–period distribution for main-sequence stars with temperatures exceeding ∼5500 K. The long-period pileup is well described by a curve of constant Rossby number, with a critical value of Ro crit ≲ Ro ⊙ . The long-period pileup was predicted by van Saders et al. as a consequence of weakened magnetic braking, in which wind-driven angular momentum losses cease once stars reach a critical Rossby number. Stars in the long-period pileup are found to have a wide range of ages (∼2–6 Gyr), meaning that, along the pileup, rotation period is strongly predictive of a star’s surface temperature but weakly predictive of its age. The short-period pileup, which is also well described by a curve of constant Rossby number, is not a prediction of the weakened magnetic braking hypothesis but may instead be related to a phase of slowed surface spin-down due to core-envelope coupling. The same mechanism was proposed by Curtis et al. to explain the overlapping rotation sequences of low-mass members of differently aged open clusters. The relative dearth of stars with intermediate rotation periods between the short- and long-period pileups is also well described by a curve of constant Rossby number, which aligns with the period gap initially discovered by McQuillan et al. in M-type stars. These observations provide further support for the hypothesis that the period gap is due to stellar astrophysics, rather than a nonuniform star formation history in the Kepler field.