Evidence for Cluster to Cluster Variations in Low-Mass Stellar Rotational Evolution

TL;DR

This study investigates variations in low-mass stellar rotational evolution across different clusters, testing existing models with new data and highlighting potential environmental influences on initial stellar rotation distributions.

Contribution

It provides evidence for cluster-to-cluster differences in stellar rotation evolution and discusses the limitations of current wind models in explaining rapid rotator evolution.

Findings

Slow rotators' evolution is consistent with star-disk interactions and core-envelope decoupling.

Wind models struggle to evolve rapid rotators from young to older clusters.

Environmental factors may influence initial stellar rotation distributions.

Abstract

A concordance model for angular momentum evolution has been developed by multiple investigators. This approach postulates that star forming regions and clusters are an evolutionary sequence which can be modeled with assumptions about the coupling between protostars and accretion disks, angular momentum loss from magnetized winds that saturates in a mass-dependent fashion at high rotation rates, and core-envelope decoupling for solar analogs. We test this approach by combining established data with the large h Per dataset from the MONITOR project and new low-mass Pleiades data. We confirm prior results that young low-mass stars can be used to test star-disk coupling and angular momentum loss independent of the treatment of internal angular momentum transport. For slow rotators, we confirm the need for star-disk interactions to evolve the ONC to older systems, using h Per (age 13~Myr) as our natural post-disk case. Further interactions are not required to evolve slow rotators from h Per to older systems, implying no justification for extremely long-lived disks as an alternative to core-envelope decoupling. However, our wind models cannot evolve rapid rotators from h Per to older systems consistently; this appears to be a general problem for any wind model that becomes ineffective in low-mass young stars. We outline two possible solutions: either there is cosmic variance in the distribution of stellar rotation rates in different clusters or there are substantially enhanced torques in low-mass rapid rotators. We favor the former explanation and discuss observational tests that could be used to distinguish them. If the distribution of initial conditions depends on environment, models which test parameters by assuming a universal underlying distribution of initial conditions will need to be re-evaluated.