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

TL;DR

This study combines Kepler rotation data and stellar temperatures to reveal pileups at the edges of the period distribution in Sun-like stars, supporting theories of weakened magnetic braking and core-envelope coupling affecting stellar spin-down.

Contribution

It provides new observational evidence for the existence of period pileups and their explanation via Rossby number-related mechanisms in Sun-like stars.

Findings

Long-period pileup described by constant Rossby number, Ro_{crit}  2

Short-period pileup may be due to core-envelope coupling slowing spin-down

Period gap aligns with a constant Rossby number curve, supporting stellar physics explanations

Abstract

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 $\sim 5500$K. The long-period pileup is well-described by a curve of constant Rossby number, with a critical value of $\mathrm{Ro_{crit}} \lesssim 2$. The long-period pileup was predicted by van Saders et al. (2019) 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 ($\sim 2-6$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. (2020) 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. (2013a) in M-type stars. These observations provide further support for the hypothesis that the period gap is due to stellar astrophysics, rather than a non-uniform star-formation history in the Kepler field.