arXiv subjects

Timothy W. H. Yiu

Publications and source records attributed to Timothy W. H. Yiu.

3 recordsLinked to original sources

arXiv2026

The magnetic field strength of a methane dwarf measured from its radio spectral cutoff

Brown dwarfs and gas-giant planets are expected to emit coherent radio emission via the electron cyclotron maser instability (ECMI) mechanism powered by non-thermal electrons in their magnetospheres. Detection of a characteristic spectral cutoff in their cyclotron emission is likely to be the only feasible route to measure their magnetic field strengths. Previous radio measurements of a subset of cold brown dwarfs (spectral type T or methane dwarfs) have not shown such a cutoff at frequencies as high as $\gtrsim 10\,{\rm GHz}$, implying field strengths far higher than canonical expectations from dynamo scaling laws. However, it is also possible that these high-frequency emissions did not originate in the large-scale magnetic field but rather from small high-field-strength loops at the surface, which the scaling laws are not designed to predict. Here we present radio observations of a methane dwarf binary (WISEP J101905.63+652954.2) whose radio emission was first detected in an untargeted low-frequency survey, making it more likely that the emission traces the object's large-scale magnetic field. The object's spectrum shows a clear radio spectral cutoff whose location yields a polar surface field strength of around 126 gauss. Canonical dynamo scaling laws based on energy balance now over-predict the field strength for this object, but the scaling law based on Lorentz-Coriolis force balance (Elsasser number rule) is consistent with the measurement. Our results suggest that multi-frequency (MHz to GHz) monitoring of brown-dwarf electron cyclotron maser emission is essential to separately measure the `mean field' on large scales and `fluctuating' fields on smaller spatial scales in order to determine the dynamo scaling law that most accurately predicts field generation in the metallic hydrogen layer of gas giants and cold brown dwarfs.

astro-ph.SR↗

arXiv2025

Unusual periodic modulation in the radio emission of the methane dwarf binary WISEP J101905.63+652954.2

Brown dwarfs display Jupiter-like auroral phenomena, such as rotationally modulated electron cyclotron maser radio emission. Radio observations of cyclotron maser emission can be used to measure their magnetic field strength, topology, and to deduce the presence of magnetically interacting exoplanets. Observations of the coldest brown dwarfs (spectral types T and Y) are especially intriguing, as their magnetospheric phenomena could closely resemble those of gas-giant exoplanets. Here we report observations made over ten epochs, amounting to 44 hours, of WISEP J101905.63+652954.2 (J1019+65 hereinafter) using the LOFAR telescope between 120 and 168 MHz. J1019+65 is a methane dwarf binary (T5.5+T7) whose radio emission was originally detected in a single-epoch LOFAR observation to be highly circular polarised and rotationally modulated at $\approx 3$h. Unexpectedly, our long-term monitoring reveals an additional periodic signature at $\approx 0.787$h. We consider several explanations for the second period and suggest that it could be the rotationally modulated emission of the second brown dwarf in the binary, although follow-up infrared observations are necessary to confirm this hypothesis. In addition, the data also allow us to statistically estimate the duty cycle and observed radio-loud fraction of the 120-168\,MHz cyclotron emission from methane dwarfs to be $\langle D \rangle = 0.030^{+0.034}_{-0.030}$ and $F^{'}_{\rm radio} = 0.088^{+0.168}_{-0.088}$ respectively.

astro-ph.SR↗

arXiv2023

Radio emission as a stellar activity indicator

Radio observations of stars trace the plasma conditions and magnetic field properties of stellar magnetospheres and coronae. Depending on the plasma conditions at the emitter site, radio emission in the metre- and decimetre-wave bands is generated via different mechanisms such as gyrosynchrotron, electron cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most sensitive wide-field radio sky surveys ever conducted. Because these surveys are untargeted, they provide an opportunity to study the statistical properties of the radio-emitting stellar population in an unbiased manner. Here, we perform an untargeted search for stellar radio sources down to sub-mJy level using these radio surveys. We find that the population of radio-emitting stellar systems is mainly composed of two distinct categories: chromospherically active stellar (CAS) systems and M dwarfs. We also seek to identify signatures of a gradual transition within the M-dwarf population from chromospheric/coronal acceleration close to the stellar surface similar to that observed on the Sun, to magnetospheric acceleration occurring far from the stellar surface similar to that observed on Jupiter. We determine that radio detectability evolves with spectral type, and we identify a transition in radio detectability around spectral type M4, where stars become fully convective. Furthermore, we compare the radio detectability vs spectra type with X-ray and optical flare (observed by TESS) incidence statistics. We find that the radio efficiency of X-ray/optical flares, which is the fraction of flare energy channelled into radio-emitting charges, increases with spectral type. These results motivate us to conjecture that the emergence of large-scale magnetic fields in CAS systems and later M dwarfs leads to an increase in radio efficiency.

astro-ph.SR↗