Testing Disk-Locking in NGC 2264

TL;DR

This study tests magnetospheric accretion models in T Tauri stars of NGC 2264, finding support for non-dipolar field geometries and the X-wind model, while challenging the pure dipole and stellar wind models, and observing evolutionary changes in magnetic interactions.

Contribution

It provides empirical support for the modified X-wind model with non-dipolar fields and highlights the evolution of magnetic interactions in young stars.

Findings

Support for non-dipolar magnetic field geometries.

Evidence for the X-wind model's trapped flux concept.

No support found for accretion-powered stellar winds.

Abstract

We test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model (Shu et al. 1994) which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age~1-2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess, finding good agreement. In addition, we use our accretion parameters to determine the relationship between accretion and H-beta luminosity, again finding good agreement with previously published results; however, we also find that care must be used when applying this relationship due to strong chromospheric emission in young stars which can lead to erroneous results in some cases.