Splinter Motivation
Observations of Sun-like stars starting 50 years ago suggested that their spin down follows a simple power law from the zero-age main sequence to the age of the Sun (e.g., Skumanich 1972). This led to the development of gyrochronology (Barnes 2003), whose underlying premise is that the rotation rates of low-mass stars can be used as a proxy for their ages.
Further studies showed that Sun-like stars do follow a Skumanich-type spin down at least over their first few billion years. The behavior of lower-mass stars, however, is not as easy to understand, nor does it follow from that of the more massive stars. A simple Skumanich-like power law for spin down cannot model the rotation rates of stars at all ages and masses (Meibom et al. 2009, 2011a, 2011b, 2015).
One possible explanation is that stars go through a period of stalling, where their rotation periods stay constant for some time before the stars resume spinning down. Evidence of this stalling has been observed in clusters (Agüeros et al. 2018, Douglas et al. 2019, Curtis et al. 2019, 2020), and may explain the period gap first observed in Kepler stars (McQuillan et al. 2013, 2014, Gordon et al. 2021).
In parallel, several theoretical frameworks have been developed to explain these observations. In one, stalling represents angular momentum transfer from the core to the envelope of low-mass stars. The envelope simulatneously loses angular momentum via stellar winds, leading to a constant surface rotation period. Due to evolutionary timescales and the thickness of the outer convective envelope, stars of different masses stall at different times, leading to the observed evolution in cluster rotation sequences (Spada & Lanzafame 2020). A competing framework focuses instead on the role of magnetic complexity, and suggests that stalling is due to a transition in magnetic field properties (Gossage et al. 2021).
Half of this splinter will focus on existing observational and theoretical evidence for stalling. The other half will dicuss future prospects for both emprical and theoretical studies. Our planned schedule, including both talks and panel discussions, will support dialogue between observers and theorists on this topic.