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15th of November 2023
ATNF Colloquium
Searches for Stellar Radio Activity in Circular Polarisation
by Joshua Pritchard (University of Sydney)
Abstract: Transient radio bursts produced in the coronae of stellar atmospheres and within planetary magnetospheres often feature a high degree of circular polarisation. These events reveal the presence of strong magnetic fields, and trace particle acceleration driven by magnetic reconnection, space weather, and auroral current systems. The detection of stellar radio emission has historically been challenged by the overwhelmingly higher surface density of extra-Galactic sources that dominate the radio sky, which produce a large number of false positive associations to optically identified stars. Our knowledge of the radio star population has therefore been primarily driven by targeted studies of the most active, nearby systems, impacting the inference of population statistics and discovery of new regimes of stellar radio activity.

In this talk I will present a body of work that exploits the circular polarisation of magnetically driven stellar radio emission to identify large samples of radio stars in widefield surveys. I will describe the application of this technique to two widefield surveys with the Australian Square Kilometre Array Pathfinder (ASKAP—the Rapid ASKAP Continuum Survey (RACS) and the Variables and Slow Transients (VAST) Pilot Survey—which resulted in 229 detections of a sample of 76 radio stars. Through repeat sampling of this group our observations place constraints on the statistical fraction of the M-dwarf population that produce radio bursts and provide a forecast of expected detection rates in future surveys with ASKAP and the Square Kilometre Array. Finally, I will discuss the utility of circular polarisation searches to uncover novel regimes of stellar radio activity. I will highlight the discovery of rotationally modulated auroral radio pulses from an old, slowly rotating M9.5 ultracool dwarf in a two year monitoring campaign with ASKAP and the Australia Telescope Compact Array. This object is the oldest and slowest rotator among known aurorally active ultracool dwarfs and displays a rich variety of radio burst features, forming an important probe of the dynamo mechanism responsible for strong magnetism in ageing cool stars.

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