Stellar flares

The dominant sources of radio emission in the solar system are the Sun and Jupiter. Solar radio studies are important for investigating fundamental properties of plasma and magnetohydrodynamics. The Sun is the brightest radio object in our sky, with radio flares up to (and possibly over) 109 Jy. Solar phenomena and events can occur in the frequency range from 50 MHz to 3 GHz and higher. Solar flares can be seen on timescales of less than a minute, to tens of minutes.


Almost any type of magnetically active star has the possibility of flaring. Types of stars typically known for flaring are dwarf stars of type M and later, also known as Ultracool Dwarf (UCD) stars, and UV-Ceti type stars. There are two types of radio stellar flare, incoherent and coherent. Incoherent flares occur on timescales of tens of minutes to days, and as such are not considered fast radio transients. Coherent radio flares are highly circularly polarised and vary on timescales of milliseconds to hours with both frequency and time structure. Given the current observations of flare stars there are already constraints on properties of the stars, such as a minimum brightness temperature of 1012 K. However, it is still not clear what mechanism is responsible for stellar flares and whether this mechanism is the same for coherent flares from different stellar types. The two main competing mechanisms are plasma radiation or an electron cyclotron maser. Observing radio flares with high time resolution and large frequency bandwidth can help to differentiate between these models, for example Osten & Bastian (2006) find the plasma radiation mechanism more likely for AD Leo after observing flares with the Arecibo telescope with high time resolution and wide bandwidth.


Investigating coherent stellar flares is essential for probing stellar atmospheric properties and magnetic activity. Observing flare stars with high time resolution and large bandwidth can help answer key open questions about stellar flares such as whether the same mechanism is at play in different stellar systems and for different stellar types, whether radio flares correlate with emission at different wavelengths, and what radio flares can tell us about the atmosphere and magnetic field structuring of stars. However, few stars have been observed to flare in the radio.


In the late 1960s and 1970s telescopes such as the Lovell Telescope at the Jodrell Bank Observatory and the Arecibo radio telescope were used to observe flares on UV Ceti, YZ Canis Minoris, EV Lacertae, V371 Orionis, and Wolf 424. Since then, individual flare stars have been observed in more detail and high time resolution observations of flares have revealed fine time structure; for example, structure on the order of 20–30 ms for AD Leo. Part of the reason for the lack of radio observations is the difficulty of differentiating between a stellar flare and radio frequency interference (RFI) in a single dish radio observation. In radio observations there are three main methods for distinguishing between RFI and stellar flares: observing with two widely separated instruments (RFI does not have the same characteristics for different telescopes), interferometry (gives a high resolution to determine position), and high spectral resolution.


MeerKAT has the required time and frequency properties to observe and detect stellar flares from flare type stars, and MeerTRAP will be observing the same field over long periods of time when commensal with for example LADUMA and Fornax. There at are least 147 flare type stars in the field of view of MeerKAT large survey project pointings, and there may be flares from flare type stars that are not yet known or from other systems, such as systems similar to Jupiter and Io, that produce flares that MeerTRAP detects.

Examples of two flares from AD Leo observed with Arecibo by Osten & Bastian (2006). These observations have 10 ms time resolution.


The Jodrell Bank Centre for Astrophysics

The University of Manchester

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement number 694745).