Cool Stellar Activity at Low Radio Frequencies

Abstract

Stellar magnetic fields drive a wide range of energetic phenomena, which have a crucial influence on their properties, evolution, and environment. In this thesis, we investigate the low-frequency radio emission from active low-mass stars. We harness this emission to constrain magnetospheric properties and processes arising from different magnetic activity paradigms. In Chapter 2, we present low-frequency observations of a sample of ultra-cool dwarfs. We show that while most ultra-cool dwarfs do not produce low-frequency emission, their magnetospheric properties can be constrained with measurements of the optically-thick side of the quiescent spectral energy distribution. In Chapter 3, we present Australian Square Kilometre Array Pathfinder (ASKAP) observations of the prototypical flare star, UV Ceti. Our observations reveal the presence of periodic, elliptically polarised radio pulses, confirming that large auroral current systems can be produced in the magneto- spheres of active M-dwarfs. In Chapter 4, we present results from one night of an 11-night multi-wavelength campaign targeting the nearest stellar neighbour, and magnetically-active planet host, Proxima Centauri. We detected a powerful flare with optical photometric and spectroscopic facilities, accompanied by a solar-like type IV radio burst. By analogy with the solar radio burst paradigm, we suggest that this event indicates a coronal mass ejection leaving the corona, and ongoing electron acceleration associated with a post-eruptive loop arcade. Together, the results presented in this thesis show the diverse forms of magnetic activity exhibited by low-mass stars. We have shown that low-frequency radio observations are sensitive to both auroral and solar-like activity. Future low-frequency radio surveys and multi-wavelength observations may reveal the role of stellar properties such as rotation, mass, and age, in driving these different forms of magnetic activity.