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

¹ Dublin Institute for Advanced Studies, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland
e-mail: antonfj@cp.dias.ie
² School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo 7991 PD, The Netherlands
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 72, 97200 AB Groningen, The Netherlands
⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁸ GEPI & USN, Observatoire de Paris, CNRS Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁹ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

Received: 22 March 2021
Accepted: 27 June 2021

Abstract

In recent years, thanks to new facilities such as LOFAR that are capable of sensitive observations, much work has been done on the detection of stellar radio emission at low frequencies. Such emission has commonly been shown to be coherent emission, generally attributed to electron-cyclotron maser (ECM) emission, and has usually been detected from main-sequence M dwarfs. Here we report the first detection of coherent emission at low frequencies from T Tauri stars, which are known to be associated with high levels of stellar activity. Using LOFAR, we detect several bright radio bursts at 150 MHz from two weak-line T Tauri stars: KPNO-Tau 14 and LkCa 4. All of the bursts have high brightness temperatures (10¹³ − 10¹⁴ K) and high circular polarisation fractions (60–90%), indicating that they must be due to a coherent emission mechanism. This could be either plasma emission or ECM emission. Due to the exceptionally high brightness temperatures seen in at least one of the bursts (≥10¹⁴ K), as well as the high circular polarisation levels, it seems unlikely that plasma emission could be the source; as such, ECM is favoured as the most likely emission mechanism. Assuming this is the case, the required magnetic field in the emission regions would be 40–70 G. We determine that the most likely method of generating ECM emission is plasma co-rotation breakdown in the stellar magnetosphere. There remains the possibility, however, that it could be due to an interaction with an orbiting exoplanet.