Direct Radio Discovery of a Cold Brown Dwarf

Vedantham, H. K.; Callingham, J. R.; Shimwell, T. W.; Dupuy, Trent J.; Best, William M. J.; Liu, Mengmeng; Zhang, Zhoujian; De, Kishalay; Lamy, Laurent; Zarka, Philippe; Röttgering, H. J. A.; Shulevski, A.

doi:10.3847/2041-8213/abc256

Cited by 34 publications

(37 citation statements)

References 71 publications

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“…The proposed acceleration mechanisms that could produce the observed coherent low-frequency emission from most of the LoTSS detected M dwarfs are similar to the magnetospheric processes observed on the Solar System's gas giant planets and ultracool dwarfs (Hallinan et al 2008;Vedantham et al 2020a;Callingham et al 2021). For example, the star itself can produce Jovian-like emission through electric field-aligned currents resulting from a breakdown in co-rotation between a rotating plasma disk and its magnetosphere (Schrijver 2009;Nichols et al 2012;Pineda et al 2017).…”

Section: Introductionsupporting

confidence: 53%

Low-frequency monitoring of flare star binary CR Draconis: long-term electron-cyclotron maser emission

Callingham

Pope

Feinstein

et al. 2021

A&A

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Recently detected coherent low-frequency radio emission from M dwarf systems shares phenomenological similarities with emission produced by magnetospheric processes from the gas giant planets of our Solar System. Such beamed electron-cyclotron maser emission can be driven by a star-planet interaction or a breakdown in co-rotation between a rotating plasma disk and a stellar magnetosphere. Both models suggest that the radio emission could be periodic. Here we present the longest low-frequency interferometric monitoring campaign of an M dwarf system, composed of twenty-one ≈8 h epochs taken in two series of observing blocks separated by a year. We achieved a total on-source time of 6.5 days. We show that the M dwarf binary CR Draconis has a low-frequency 3σ detection rate of 90−8+5% when a noise floor of ≈0.1 mJy is reached, with a median flux density of 0.92 mJy, consistent circularly polarised handedness, and a median circularly polarised fraction of 66%. We resolve three bright radio bursts in dynamic spectra, revealing the brightest is elliptically polarised, confined to 4 MHz of bandwidth centred on 170 MHz, and reaches a flux density of 205 mJy. The burst structure is mottled, indicating it consists of unresolved sub-bursts. Such a structure shares a striking resemblance with the low-frequency emission from Jupiter. We suggest the near-constant detection of high brightness temperature, highly-circularly-polarised radiation that has a consistent circular polarisation handedness implies the emission is produced via the electron-cyclotron maser instability. Optical photometric data reveal the system has a rotation period of 1.984 ± 0.003 days. We observe no periodicity in the radio data, but the sampling of our radio observations produces a window function that would hide the near two-day signal.

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Section: Introductionsupporting

confidence: 53%

Low-frequency monitoring of flare star binary CR Draconis: long-term electron-cyclotron maser emission

Callingham

Pope

Feinstein

et al. 2021

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“…These discoveries herald an unprecedented opportunity to constrain magnetic activity in main-sequence stars other than the Sun as well as the impact of the ensuing space-weather on exoplanets, as exemplified by the 19 other detections presented by Callingham et al (under review). Additionally, the recent direct discovery of a cold brown dwarf using LoTSS data (Vedantham et al 2020a) also demonstrates the new potential of deep lowfrequency surveys in helping us to understand the properties of planetary-scale magnetic fields outside of the Solar System.…”

Section: Stars and Exoplanetsmentioning

confidence: 96%

The LOFAR LBA Sky Survey

Gasperin

Williams

Best

et al. 2021

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Context. The LOw Frequency ARray (LOFAR) is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. < 1 mJy beam −1 and < 15) observations at ultra-low frequencies (< 100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. Aims. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range between 42 − 66 MHz, at a resolution of 15 and at a sensitivity of 1 mJy beam −1 (1σ). In this work, we outline the survey strategy, the observational status, and the calibration techniques. We also briefly describe several of our scientific motivations and present the preliminary public data release. Methods. The preliminary images were produced using a fully automated pipeline aimed at correcting all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, have not yet been corrected, the images presented in this work are still ten times more sensitive than previous available surveys at these low frequencies. Results. The preliminary data release covers 740 deg 2 around the HETDEX spring field region at an angular resolution of 47 with a median noise level of 5 mJy beam −1. The images and the catalogue of 25,247 sources have been publicly released. We demonstrate that the system is capable of reaching a root mean square (rms) noise of 1 mJy beam −1 and an angular resolution of 15 once directiondependent effects are accounted for. Conclusions. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique data set that can be used for a wide range of science topics, such as the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, as well as those produced by active galactic nuclei (AGN).

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“…The vast majority of stellar radio emission has only been studied at frequencies greater than 1 GHz (Güdel 2002), mainly owing to sensitivity constraints. The advent of sensitive radio telescopes at metre wavelengths has opened up the lowfrequency window to stars and brown dwarfs (Lynch et al 2017;Villadsen & Hallinan 2019;Vedantham et al 2020c;Callingham et al 2021a;Vedantham et al 2020a). As part of our ongoing effort to constrain coronal parameters using low-frequency emission, here we focus on WX UMa, a system blindly detected (Callingham et al 2021b) in the ongoing Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS) (Shimwell et al 2017(Shimwell et al , 2019 with the LOFAR telescope (van Haarlem et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Large closed-field corona of WX Ursae Majoris evidenced from radio observations

Davis

Vedantham

Callingham

et al. 2021

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The space-weather conditions that result from stellar winds significantly impact the habitability of exoplanets. The conditions can be calculated from first principles if the necessary boundary conditions are specified, namely the plasma density in the outer corona and the radial distance at which the plasma forces the closed magnetic field into an open geometry. Low frequency radio observations (ν 200 MHz) of plasma and cyclotron emission from stars probe these magneto-ionic conditions. Here we report the detection of low-frequency (120 − 167 MHz) radio emission associated with the dMe6 star WX UMa. If the emission originates in WX UMa's corona, we show that the closed field region extends to at least ≈ 10 stellar radii, which is about a factor of a few larger than the solar value, and possibly to 20 stellar radii. Our results suggest that the magnetic-field structure of M dwarfs is in between Sunlike and planet-like configurations, where compact over-dense coronal loops with X-ray emitting plasma co-exist with a large-scale magnetosphere with a lower plasma density and closed magnetic geometry.

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Direct Radio Discovery of a Cold Brown Dwarf

Cited by 34 publications

References 71 publications

Low-frequency monitoring of flare star binary CR Draconis: long-term electron-cyclotron maser emission

Low-frequency monitoring of flare star binary CR Draconis: long-term electron-cyclotron maser emission

The LOFAR LBA Sky Survey

Large closed-field corona of WX Ursae Majoris evidenced from radio observations

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