By Christopher Duffy
Studies of the sun, our closest star, have shown that sun spot activity and solar eruptions, also known as solar flares, are strongly correlated – regions of the sun with many sun spots are found to be the typical point of origin for most solar flares. This leads one, perhaps intuitively, to expect that other stars would exhibit similar behaviour, however there is a growing amount of evidence to suggest that this is not the case. In her PhD thesis and latest publication, Lauren Doyle, a student at the Armagh Observatory and Planetarium has used the Transit Exoplanet Survey Satellite (TESS) telescope to study the flare and star spot activity of many low-mass M dwarfs, which are stars smaller and redder than our sun.
TESS (pictured above) is a space telescope launched in 2018 to primarily seek out new exoplanets by making observations of how the light seen from stars changes over time; drop in the amount of light seen from a star may indicate the presence of a planet. Likewise, a sudden brightening could signal a stellar flare. All of the stars studied by Lauren featured a large dominant star spot which, as it rotates in and out of view with the stellar rotation, results in an appreciable variation in the amount of light seen by TESS.
This mechanism, illustrated above, causes a periodic and gentle dimming of the observed brightness. Similarly, the sharp spikes in the light curve indicate the occurrence of a solar flare. If M-stars behaved like the Sun, we would expect more flares to be seen when the light was at a minimum. However, unexpectedly, this is not what Lauren found.
There are several possible explanations for this puzzling behaviour and why it differs from that seen in our sun. It is possible that the stars that Lauren studied show multiple spot locations, consequently it becomes far more difficult to establish the periodicity of the stellar rotation and thus star spot locations. Furthermore, stellar spots could be found at the stellar poles, something not seen in the sun, which depending on the viewing angle, may not be visible in the observations – thus tying stellar flares to stellar spots whilst providing no observational evidence for this.
Lauren’s paper is published in Monthly Notices of the Royal Astronomical Society. A short technical description of the work as well as a link to the full paper may be found here.