Searching for low-mass stars with magnetically-induced hyper-inflated radii

TL;DR

This paper investigates whether empirical data supports the existence of hyper-inflated low-mass stars caused by magnetic activity, using a magneto-convective model and Kepler binary data.

Contribution

It introduces a magneto-convective model to estimate magnetic field strengths needed for hyper-inflation in low-mass stars and analyzes Kepler binary data for evidence.

Findings

Empirical inflations can be explained with magnetic fields around 10 kG for stars >0.6 MSun.

Stars <0.4 MSun may require magnetic fields of 100-300 kG to explain hyper-inflation.

High-resolution spectroscopy can test the model's predictions.

Abstract

Precise empirical estimates of stellar radii have revealed that the radii of certain low-mass stars are inflated relative to stellar structure predictions: the largest inflations occur in magnetically active stars. Theoretically, the radii of magnetically active stars are in some cases found to be 'hyper-inflated' to roughly double the radius of a non-magnetic star with equal mass. Here we ask, do data exist which could allow us to search for empirical evidence in support of hyper-inflated stars? A photometric study of 44 eclipsing binaries in the Kepler field by Cruz et al. may help us in our search. The Cruz et al. study, although subject to large uncertainties, hints at the presence of hyper-inflation in some of the 88 stars in their sample. Their data enable us to set theoretical limits on the maximum strength Bc of magnetic fields inside their sample stars. According to our magneto-convective model, the average empirical inflations found from analysis of the Cruz et al. data can be replicated if Bc approx. 10 kG inside stars with masses greater than ~ 0.6 MSun. On the other hand, in stars with masses less than about 0.4 MSun, our model predicts that the average empirical inflations of the stars may approach hyper-inflated status. Such stars may require significantly stronger internal fields, i.e. Bc approx. 100-300 kG. High-resolution spectroscopy of the Kepler binaries could help to confirm or refute our conclusions.