Radio-loud ultracool dwarfs allow analysis of magnetic fields
8 February 2016
The group of lowest mass stars and brown dwarfs are collectively called ultracool dwarfs. A number of these objects are sources of both burst and quiescent radio emission. The radio bursts are sometimes found to occur periodically on the timescale of the rotation of the ultracool dwarf or as isolated events. They are highly circularly polarised and occur over a timescale of a few minutes. Alternatively, the quiescent emission is observed to have very little variability and low circular polarisation. Both radio emission components are thought to be the result of magnetic processes and imply that ultracool dwarfs are able to generate and sustain strong magnetic fields. This is unexpected though, given their non-solar-like interior and the observed decline in the strength of magnetic activity tracers at other wavelengths.
Radio surveys of ultracool dwarfs have found that about 10% of these systems are radio luminous, with 21 currently known to have radio emission. Correlations between the presence of radio emission and other dwarf properties such as rotation are not well established. Furthermore, little is known about the magnetic environment responsible for the radio emission observed in ultracool dwarfs. To address these issues, CAASTRO members Dr Christene Lynch and Dr Tara Murphy (University of Sydney), together with colleagues from Australia and overseas, carried out a survey of 15 ultracool dwarfs located in the Southern hemisphere using the Australia Telescope Compact Array. ATCA is able to simultaneously observe over a wide range of frequencies, providing detailed information on the time-frequency structure of radio bursts and quiescent emission. Such a characterisation is required if we want to constrain the magnetic properties of ultracool dwarfs.
The researchers detected radio emission from three of the 15 observed sources, including the detection of a new source, 2MASSW J0004348–404405. The emission from these three sources showed no variability or burst emission and was consistent with emission from a gyrosynchrotron mechanism. To characterise the magnetic conditions responsible for the observed radio emission, the team constructed a simple stellar magnetospheric model consisting of mildly-relativistic electrons that spiralled in a uniform magnetic field. They found the observed quiescent emission to be consistent with radio emission expected from a magnetic environment with a field strength 10 – 230 G and electron density 104 – 108 cm-3. Additionally, they analysed the general trends of the radio emission for this sample of 15 sources and found that the radio activity increased for later spectral types and more rapidly rotating objects.