Impulsive brightness enhancements are frequently observed in the light curves of other main sequence stars, and because these sudden brightenings resemble solar flares they are assumed to be equivalent. The likelihood of observing a flare on
another star is highly dependent on the spectral type of the star; a higher proportion of small, cool M-type red dwarf stars are observed to flare compared to larger, hotter F- and A-type stars. In addition the magnetically active cooler stars tend to produce flares more frequently than the hotter stars (e.g. Candelaresi et al. 2014). This may have much to do with the internal structures of main sequence stars, since cooler stars have larger convective zones than hotter stars and a sufficiently deep convective zone is necessary for a dynamo process to generate a magnetic field. Hence the hotter stars tend to not be active, but despite this there may be some flaring A-type stars (Pedersen et al. 2017; Van Doorsselaere et al. 2017). The rotation rate and age also relate to the flaring rate, since younger stars tend to rotate faster and faster rotation means a stronger dynamo (Pettersen 1989; Candelaresi et al. 2014). Even without observing any flares, magnetically active stars can be distinguished from inactive stars as the Hα spectral line will be in emission rather than absorption.
It is generally agreed that stellar flares are produced by magnetic activity, similar to solar flares. Observations show solar and stellar flares to have very simi- larly shaped time profiles, proxies of magnetic activity to be enhanced for stars that frequently produce large flares (Karoff et al. 2016), and evidence of the Neupert effect for stellar flares observed simultaneously in two different wavebands (Hawley et al. 1995). The possibility that at least some stellar flares may be produced by slightly different mechanisms should still be considered, however, especially since many stellar flares are orders of magnitude more energetic than any recorded solar flare (see Section 1.2.4 for more discussion of the implications of these ‘superflares’). So far the alternative mechanisms for stellar superflares include interactions of the magnetic field of the flaring star with that of a companion binary star (Simon et al. 1980), a close-in planet (Rubenstein & Schaefer 2000; Ip et al. 2004), or a disk (Hayashi et al. 1996). In addition, Cuntz et al. (2000) suggested that a tidal inter- action between a star and a close-in planet could enhance the magnetic activity of the star by altering flows and turbulence in the convection zone of the tidal bulge region, which in turn could lead to locally increased heating and dynamo action.
Although the majority of stellar flare studies focus on main sequence stars, there have been observations of flares or flare-like phenomena on post-main sequence stars and more exotic objects. Since flares are observed on stars across the main sequence, it is not surprising that as stars start to evolve off the main sequence they continue to flare. Some examples of early observations of flares on red giants in the white-light and radio wavebands were collected by Schaefer et al. (2000). In addition, Karovska et al. (2005) reported the observation of a SXR flare on the red giant Mira A. The recent observations fromKepler (see Section 2.6) have helped vastly expand the number of giants observed to flare (Van Doorsselaere et al. 2017). There is currently nothing to suggest that these red giant flares are fundamentally different
to solar flares and main sequence stellar flares, although a different mechanism cannot be ruled out.
The dramatic changes that a star undergoes as it transitions to a white dwarf, and the very different physics operating inside white dwarfs, mean that it is difficult to imagine how solar-like flares could continue into this phase of a star’s life. Although some white dwarfs have extremely strong magnetic fields (Hollands et al. 2015), there are no observations of typical stellar flares on white dwarf stars. There have been reports of ‘outbursts’ on white dwarfs by Hermes et al. (2015), however. These occur on much longer timescales than typical magnetic reconnection flares, with a rise phase that lasts several hours. The outbursts were found to be linked to asteroseismic g-mode pulsations of the star, with the pulsations having shorter periods and larger amplitudes during the outburst. Hermes et al. (2015) suggest that the mechanism behind the outbursts could be resonant mode coupling of the pulsations to daughter modes, which could then be rapidly damped by turbulence in the convective zone, thus depositing energy there.
In a more exotic case, flares are frequently detected from the supermassive black hole at the centre of the Milky Way, Sgr A* (e.g. Degenaar et al. 2013). It has been suggested that these could still result from some magnetic reconnection process in the accretion disk, although other mechanisms such as the infall of matter have also been proposed. Flares have also been detected on magnetars (neutron stars with extremely strong magnetic fields), which are attributed to large-scale reordering of the magnetic field (Woods et al. 2001). A particularly intriguing observed feature (with respect to this thesis) of some of these flares is the presence of quasi-periodic oscillations (e.g. Huppenkothen et al. 2013). Duncan (1998) suggested that these quasi-periodic oscillations might link to global seismic waves that could be initi- ated by the huge flares (referred to as ‘starquakes’ and synonymous to sunquakes; Matthews et al. 2015), and hence that they could be used for asteroseismology.