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

Abstract

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.

Detection of coherent low-frequency radio bursts from weak-line T Tauri stars

“…However, this does not rule out the possibility of linear polarisation as we may be unable to detect it due to lack of sensitivity. For example, Callingham et al (2021) detect linear polarisation from similar emission in CR Draconis with a linear polarisation fraction of <10%, which would be too faint to detect in the emission we observe.…”

“…Effectively this is a scaled-up version of the mechanism driving auroral radio emission in the case of Jupiter (Zarka 1998) and ultra-cool dwarfs (Nichols et al 2012). More recently, this mechanism has been proposed to explain the LOFAR detected main-sequence M dwarf CR Draconis (Callingham et al 2021).…”

“…Assuming we are dealing with coherent radiation, the options are either plasma emission, as seen for example in the Sun from Type II radio bursts (Hariharan et al 2014), or ECM emission, which is associated with auroral emitters such as planets, brown dwarfs and main sequence M dwarfs (Lynch et al 2017;Pineda et al 2017;Villadsen & Hallinan 2019;Zic et al 2019;Vedantham et al 2020;Callingham et al 2021) and possibly Type IV solar radio bursts (Treumann et al 2011).…”

“…Recently, coherent radio emission was detected from the flare star CR Draconis by Callingham et al (2021) using the Low-Frequency Array (LOFAR; Van Haarlem et al 2013). When observed for a total of 6.5 days over one year, three bright radio bursts were detected in addition to an almost steady radio flux.…”

The population of M dwarfs observed at low radio frequencies

“…It is also important to note that a high coronal temperature does not immediately imply that plasma emission is producing the low-frequency emission. For example, all 20 detections of CR Dra have an identical circularly polarised handedness, implying that the emission site is situated in a stable magnetic field arrangement 31 . Plasma emission would be expected to flip in circularly polarised handedness as it emerges from areas of complex magnetic geometry 2 .…”