Flare Stars: A Short Review

Krstinja Dzombeta Department of Astronomy and Astrophysics, University of Toronto Toronto ON Canada M5S 3H4 and John R. Percy Department of Astronomy and Astrophysics, and Dunlap Institute of Astronomy and Astrophysics University of Toronto Toronto ON Canada M5S 3H4

Abstract Flare stars, or UV Ceti stars, are a type of eruptive variable star, defined by their flaring behavior – a rapid (minutes) increase in brightness, followed by a slower (hours) decrease. This short review outlines current knowledge about flare stars, their importance, recent research developments, future research directions, and some practical activities for skilled amateur as- tronomers and students. Over the past decade, flare stars have been the subject of intensive research, as a result of an abundance of new data, especially from the Kepler and TESS space telescopes. The large statistical samples of data have clarified the relation between flaring and stellar spectral type, luminosity, and rotation. They have allowed for the expansion of the range of spectral types of flare stars, from K and M type dwarfs, to also F and G, and possibly even A. They have confirmed the greater frequency of flares on M dwarfs, compared to K, and that flare stars’ energies follow a decreasing power law fit for the number of high-energy flares, although a break in the relationship has also been demonstrated. Current problems in flare-star research include improved modelling of the new observational results, using the dynamo theory which produces the stars’ magnetic field. What is the difference, if any, between the dynamo in completely-convective stars such as M dwarfs, and in stars such as the sun with only partial convective zones? AAVSO keywords = flare stars ADS keywords = flare stars 1. Introduction We live around an active flare star – the sun. Flare stars, or UV Ceti stars, are eruptive variable stars which increase in brightness over the course of minutes, then return to their original brightness over the course of hours. Or, in the words of Balona (2015), “a flare star is a star in which at least one flare has been observed”. Although a large fraction of the stars in our galaxy are flare stars, the flares are rare, brief, and unpredictable. Most readers will have never observed one – except perhaps on the sun. There has not been a focussed review of them for the readership of this Journal – skilled amateur astronomers, students, and others with a general interest in variable stars. This brief review is intended to close that gap. It emphasizes developments in the last few years. Percy (2007) and Templeton (2010) provided brief reviews a decade ago, and Benz and G¨udel (2010) provided a more technical review article. The proceedings of a recent international conference on Living Around Active Stars (Nandy et al. 2017) contains many interesting and relevant technical papers. Table 1: Examples of Flare Stars Name Spectral Type Apparent Magnitude UV Cet M6V 12.95-6.80 Proxima Centauri M6V 11.13-10.43 ADLeo M4Vae 11.00-8.07 Trappist 1 M8 18.8 GJ1243 M4V 12.83 GJ674 M3V 9.41 LOPeg K3Ve 9.27-9.04 HD219143 K3V 10.05-5.57 λ And G8III-IV 4.05-3.65 V374 Peg M3.5 16.0-3.5

The flare-star phenomenon can be observed at wavelengths across the electromagnetic spectrum from gamma rays to radio waves, although it is more apparent at visual and UV wavelengths, as a result of the specific temperature of the material produced and emitted during the flare. Stellar flares can be observed visually, or by using other optical techniques, such as photometry (CCDs) or spectroscopy. The total energies of flares range approximately from 1027 to 1035 erg (Gershberg 2014). The energies of solar flares are typically 1027 ergs, and generally do not exceed 1032 ergs. Table 1 provides some examples of flare stars. UV Cet is the flare star after which this type of star was named. Proxima Centauri is the nearest star to the sun; it is a flare star, and also has an Earth-sized exoplanet in close orbit (Anglada et al. 2017). AD Leo is the most magnetically- active M dwarf (Segura et al 2010). Trappist 1 has three exoplanets within 1 AU (Mullan et al. 2018). GJ 1243 has a relatively small amplitude of differential rotation for a cool star (Davenport et al. 2015). GJ 674 is optically inactive but exhibits frequent far ultraviolet flares (Froning et al. 2019). LO Peg shows X-ray flares, similar to low-mass fast-rotating stars (Lalitha et al. 2017). HD 219143 has a 12-year activity cycle, similar to the solar cycle (Johnson et al. 2016). λ And is the brightest RS CVn (flaring binary) star. V374 Peg reached magnitude 3.5 at maximum. The study of stellar flares stems from the first observation of a solar flare in 1859 by R. C. Carrington and R. Hodgson. This flare, known as the Carrington event, is calculated to have had an energy of 5 x 1032 erg (Lingam and Loeb 2018), and was one of the most intense solar events ever recorded (Russell et al. 2016). Occasionally in the analysis of flare stars, there is an artificial categorization created by dif- ferentiating those whose luminosity is much greater than that of the star, such as on low-mass, low-luminosity K and M type stars (Walkowicz et al. 2011). The relative luminosity of a single flare on a F or G -type star is much smaller than that of the star, even if the energy of the flare is the same. The flare mechanism was thought to be similar only on K and M dwarfs, although recent studies show that these flares share certain qualitative similarities with those on hotter (F and G type) stars like the sun. K and M dwarfs are dimmer, cooler and less massive stars than F and G-types. Flares on the former may appear to brighten the entire star, while the latter does not appear to be as affected. How are the data from flare stars analyzed? Light curves, plots of flux or brightness versus time, are used to analyze flares, as they show the changing luminosity of the star, including the Figure 1: Top: Light curve (flux versus time) of an M dwarf (KIC 6224062) (Davenport 2016). The flares recovered from the data are shown in red, with the modelled quiescent luminosity of the star shown in blue. The small, smooth increases and decreases shown in blue reveal the star’s spotted variability; the flare star has a rotation period of ∼ 8.5 days. Middle: Light curve (fractional flux change versus time) of a K dwarf with (Teff = 4546) with original data shown in black and quiescent luminosity model plotted over it in red. The rotation period is also ∼ 8.5 days. Flare candidates are shown as red points. Bottom: Selected flare candidates, showing the duration of the flares. Middle and bottom from Walkowicz et al. (2011).

flare (Gershberg 2014). The top panel in Figure 1 shows a light curve of an M dwarf, GJ 1243 (Davenport 2016). The quiescent luminosity model, shown in blue, demonstrates slow regular increases and decreases, indicating that the star also has variability due to spots. Spots, the cool dark regions where the magnetic field is strong enough to inhibit the passage of heat, occur on flare stars such as the M dwarf shown, as they do on the sun. See Percy and Rice (2017) for a discussion of the rotational variability of spotted stars, and some “maps” of spotted photospheres. See also Balona (2015) for many more examples of flare-star light curves. Cool spotted stars which are rotational variables, but have not (yet) been observed to flare, are called BY Dra stars. Almost a thousand were previously known, and 33 are brighter than magnitude 7. The brightest is ǫ Eri, V = 3.73. GAIA has recently discovered and measured the rotation periods of almost 150,000 more (Lanzafame et al. 2019). The rotation period of the M dwarf shown in Figure 1 is about ∼ 8.5 days, which is typical for these stars. The flare candidates are shown in red, although the light curve does not clearly show the duration of these events. Another example of a light curve from a flare star (a K dwarf) is provided on the middle and bottom panels in Figure 1. The middle panel shows the full light curve of the star for a little over 30 days, with the quiescent luminosity model plotted in red over the original data (black points), and flare candidates shown as red points. The rotation period is about 9 days. The bottom panel shows the zoomed-in plots of the flares; it demonstrates their duration. For example, the left plot of the K dwarf shows that the flare lasted about 0.5 day. The duration of a flare takes into account the time required for the brightness to increase and then decrease to the initial level. It is not possible to tell, from these flare light curves, whether there is any fine structure i.e. more rapid variability within the flare. 2. What Causes a Flare? Flaring is a type of stellar activity caused by magnetic phenomena in the star. M dwarfs have strong global magnetic fields, while G-type stars like the sun have relatively weak versions – only a few Gauss. How do magnetic fields produce such an energetic phenomenon? For a detailed discussion, see the excellent review, and simulations of stellar flares by Allred et al. (2015). See also Jouve and Kumar (2016) for a discussion of the connections between solar and stellar dynamo models. Flares are released when there is a reconnection of magnetic field lines on the star’s surface or atmosphere. This is the so-called impulsive phase, which causes the magnetic field to drop to a lower state of energy, thus releasing energy, and accelerating energetic particles into the stellar atmosphere. There is rapid heating and observable effects in the form of a flare. The accelerated charged particles travel along the magnetic field lines and collide with dense plasma, which is heated in reconnecting flux tubes at a temperature of at least 20MK. The charged particles include electrons, which are detected by so-called Bremsstrahlung radiation when they collide with the plasma. The electrons are significant in the heating during the flaring as they transport energy. Besides electrons, the reconnection of magnetic field lines accelerates ions, which are more difficult to detect, but are important and therefore must be used in computational models of stellar flares. For the sun, the reconnection occurs around sunspot pairs. More specifically, the magnetic field lines are stressed by convection in the star’s outer layers. Convection and rotation generate a magnetohydrodynamic dynamo in the star, a continuous conversion of kinetic to magnetic energy, which produces a magnetic field. The greater the rotation and convection, the stronger the magnetic field. This dynamo model also proposes that the rotation and convection create differential rotation on the surface, which stretches the magnetic field lines. (In differential rotation, the rotation period is a function of the latitude on the star. For instance: the sun rotates with a shorter period at the equator, and a longer period toward the poles.) While flares appear to be random, average flare occurrence frequency and energy are related to the strength of the magnetic field (Davenport 2016). Young active M dwarfs have not had time to spin down their rotation; therefore they generate larger magnetic fields and hence produce larger single flares, even 1000 times more energetic than solar flares. The flare causes high-speed shocks, with speeds of greater than 600 km/s, as well as increased density and radiation in the atmosphere (Allred et al. 2015). The strong magnetic activity on M dwarfs has also been indicated by strong H-alpha emission lines in their spectra. During the flaring episode, the spectrum changes throughout the various stages. It starts with flashes of Balmer emission lines of hydrogen, and helium lines, and at the maximum shows a strong continuum – white light emission with a temperature between 9000K and 10,000K – which can be fit by a blackbody (Gershberg 2014, Walkowicz et al. 2011). For a stellar flare, the white light emission covers most of the surface while, in solar flares it only covers small areas near the base of magnetic field lines, because the amount of surface area covered depends on the strength of the magnetic field. 3. Why are Flare Stars Important? Besides being one of the most common and energetic events in stars, stellar flares are also very complex and therefore interesting phenomena. They provide astrophysicists with another way to study magnetic fields in stars – a topic which is still poorly understood, but increasingly relevant. 3.1. Solar Flares Solar flares are rapid local brightenings on the sun. They result in the emission of energetic radiation and particles, and coronal mass ejections, which eventually arrive at and interact with our upper atmosphere and magnetic field, resulting in “space weather”. The general flaring principle and mechanism is the same on other types of flare stars as on the sun (though the dynamo on F and G stars, which are only partially convective may be somewhat different than that on K and M stars which are completely convective). Studying flare stars may therefore help us to better understand flares on the sun, and how the sun compares with sun-like stars in general, revealing “the sun in space and time”. Whereas a solar flare is seen against a complex photosphere, photometry of other flare stars enables us to obtain a full light curve of a single flaring event, as in Figure 1, and study it in detail, such as calculating the total energy (Gershberg 2014). Flares on K and M dwarfs, which have a smaller background intensity, have greater visibility, compared to those of stars like the sun (Allred et al. 2015). Understanding solar flares is of practical importance because of their potential negative effect on human activity and well-being. It’s also important for people to understand that global warming cannot be blamed on solar activity. Solar flares can affect satellites, communication transmission in space, astronauts participating in spacewalks, and the stability of the electrical power grid. The most powerful solar flare since the Carrington event occurred on March 13, 1989, with an estimated energy of 1032 ergs, and caused a geomagnetic storm and blackout in and around Quebec, Canada (Shibata et al. 2013, Choudhuri 2017). There is a conjecture that an even stronger solar flare, or “super-flare”, with energy greater than 1033 ergs, could have devastating effects on the earth’s environment, and on human lives (Shibata et al. 2013) – a good reason to increase our understanding of such superflares. 3.2. Exoplanets Thousands of exoplanets have been discovered in the last decade or so, mostly by the transit method, by the Kepler mission – almost all of them around cool stars. (The activity on these stars actually makes exoplanets more difficult to detect by the transit method.) Many orbit in the habitable zone, defined as the zone in which water on the planet would be in the liquid state. Many orbit M stars, notably Proxima Centauri, the closest star to the sun. Given the low luminosity of M stars, the planet would have to orbit very close to the star to be habitable. With this increasing interest in exoplanets in habitable zones, there is increased concern about the effect of stellar flares on planetary atmospheres, and on the possible origin and development of life on the planet – an exciting new interdisciplinary topic. High-energy stellar radiation (including UV) and particles can result in the heating and escape of the gas in the planetary atmosphere. But the effects will depend on both the evolution of the stars’ activity, and the evolution of the planetary atmosphere and magnetic field. There is also a possibility that stellar flares will affect the protoplanetary disc of gas and dust from which the planets form. Flares are more frequent and energetic on young, rapidly-rotating stars. While models predict that the flares will not directly affect a habitable exoplanet, they do indicate that 94 percent of the planet’s ozone layer can be depleted in two years, if the planet does not have a magnetic field to protect it from energetic charged particles (Segura et al. 2010). 4. New Developments The study of stellar flares, prior to a decade ago. relied on ground-based observations of individual stars, though over 1600 were known at that time. Flare star research has evolved and expanded especially in the last decade as a result of the abundance of data made available by Kepler. Several groups have searched the Kepler database for flares (e.g. Walkowicz et al. 2011, Balona 2015, Davenport 2016, Van Doorsselaere et al. 2017). Other observations, such as those by the Microvariability and Oscillations of STars telescope (MOST: Hunt-Walker et al. 2012), K2 (Kepler’s second mission: Howell et al. 2014) and the Transiting Exoplanet Survey Satellite (TESS: Gunther et al. 2019), as well as studies of individual stars, have also contributed greatly to expanding our understanding of flares. NASA’s first Kepler mission operated from May 2, 2009 until May 8, 2013, primarily searching for exoplanets. In so doing, it constantly monitored over 200,000 stars including cool stars on which flares occur (NASA 2017; Johnson 2018). It rotated every 93 days to ensure that sunlight would not enter the telescope. It operated in both a long-cadence (LC: 30 minute observations) and short-cadence (SC: one minute observations) mode. The latter were critical for studying more rapid, lower-energy flares, which are necessary for determining the number of flares as a function of their energy. This initial data allowed for greater insight into the properties of flares, particularly on M dwarfs. Up until Kepler, flare star research had primarily focused on pre-selected objects (active stars, known for flaring) because of limited telescope time to observe such faint stars exhibiting such a rare and randomly occurring phenomenon. They had focused mostly on active M dwarfs because of their strong magnetic activity as indicated, for instance, by H alpha emission lines. Kepler provided continuous monitoring of tens of thousands of stars, resulting in the data required to understand general properties of flare stars. The telescope had a high signal to noise ratio, allowing the detection of low energy, short-lasting flares, even with energies of only 1030 erg (Ramsay et al. 2013). Kepler’s mission ended as a consequence of the failure of its reaction wheels. This led to K2, Kepler’s second mission or “new life”, which operated from May 2014 until October 2018. The second mission used the repurposed Kepler telescope, rotating every 80 days. K2 focused on targets proposed by the community through a Guest Observer program, as well as continuing its predecessor’s search for planetary transits. Kepler found over 5,000 exoplanet candidates and observed 200,000 stars, while K2 confirmed 300 exoplanets and identified 500 other possible candidates. NASA’s K2 mission came to an end in October 2018, as a result of lack of fuel. The large number of observations also encouraged the development of carefully-designed au- tomated search processes of both the LC and SC data, with specific parameters used to identify flare candidates, such as requiring that a star have at least 100 flares in total and at least 10 with energies above the local 68% completeness threshold (Davenport 2016). That particular search process was therefore a conservative one. Both Kepler and K2 revolutionized the study of flare stars, by providing an abundance of data, and prompting increased statistical analyses of general properties of flare stars. A result of this was the creation of the Kepler Catalog of Stellar Flares, based on the observations from Figure 2: A graph of the TESS magnitude of flare stars from TESS (blue points) and Kepler (grey points) versus effective temperature. The top axis gives the corresponding spectral type of the stars. Kepler data are shown in grey and TESS data shown in blue. Above and to the right of the square are histograms, showing the distribution of the points (Gunther et al. 2019). Note that TESS observes primarily M type dwarfs.

May 2, 2009 until May 8, 2013, from data release 24 (Davenport 2016). The final version of the catalog contains more than 850,000 flare events on 4041 stars, which is 1.9% of the Kepler stellar database. Their typical energy was 1034 to 1035 ergs – considerably greater than the maximum energies observed in solar flares. An updated flare catalog by Yang and Liu (2019) used observations from data release 25, and found 3420 flare stars with more than 160,000 flare candidates. Davenport (2016) and Yang and Liu (2019) had only 396 flares stars in common in their respective catalogs. Yang and Liu (2019) attributed this difference to Davenport (2016) identifying any star exhibiting more than 100 flares as a candidate, but showed that some of their missing data consisted of inactive flare stars, or those with small flares. Balona (2015) also searched the Kepler light curves – both long-cadence (LC) and short-cadence (SC). Of 20,810 stars in the LC, 743 were identified as flare stars; of 4758 stars in the SC, 209 were flare stars. The SC data enabled him to study the flare shapes, durations, and fine structure. NASA’s current search for and study of exoplanets is being conducted by TESS. Figure 2 shows the stars observed by Kepler in contrast to an initial data sample of stars observed by TESS, which focused more on early to late-type M dwarfs (Gunther et al. 2019). Figure 3 shows information about flare stars obtained from TESS SC observations. 4.1. Flares on F, G, K, M type stars Figure 3: Histograms of the number (top, log scale) and fraction (bottom) of flaring stars (blue), compared with the total number of stars (grey) in the TESS short-cadence observations, shown as a function of the stellar effective temperature. M dwarfs dominate the sample of flaring stars, while F, G and K stars rarely have detectable flares (Gunther et al. 2019). Figure 4: Behavior of flare stars from Kepler data, from Doorsselaere et al. (2017). Left: His- togram (log scale) showing the distribution of flare amplitudes in F, G, and K-M type stars. F and G type stars have fewer high amplitude flares and follow the same decreasing trend – same slope m of best fit. K and M types have a range of amplitudes, and a greater number of higher amplitude flares than F and G. Right: Top: Histogram (log scale) of energy distribution of flares on F types. There are few low and high energy flares; they are mostly concentrated in the middle with a shallow distribution. Middle: Histogram (log scale) of energy distribution of flares on G types, with most of the flares at lower energies, compared to F types. Bottom: Histogram (log scale) of energy distribution of flares on K and M types. They can exhibit higher energy flares, with a shallower line of best fit than G types.

Kepler’s observations presented an opportunity to survey and compare flare stars according to spectral type and other properties, and perform analyses on a large statistical sample. Very fortunately, the temperature and luminosity of each star in the Kepler input catalog had been determined prior to the mission. A significant amount of Kepler research was dedicated to M dwarfs, as they experience the most flaring, followed by K dwarfs. This was confirmed in the initial Kepler data (quarter 1 - observations between May 13, 2009 and June 25, 2009) by Walkowicz et al (2011). Their analysis showed that M dwarfs flare more frequently than K dwarfs, and do so more energetically, and lasting for a shorter duration. The same was confirmed by Van Doorsselaere et al (2017), also finding that K and M dwarfs show a decreasing trend in flare amplitude, as shown in Figure 4. Kepler data also showed that the energy distribution of flares follows a decreasing power law fit for those with energy greater than 1031 ergs (Hawley et al. 2014). This was confirmed by Van Doorsselaere et al. (2017) for flares stars of type M, K and G. Davenport (2016) showed that flare stars of spectral type G8 to M4 have decreasing flare luminosity with increasing rotation period. This was consistent with previous findings and shows that, as stars age and spin-down, their magnetic activity decreases and hence they produce fewer flares. The left panel in Figure 4 shows a histogram of the distribution of flare amplitudes. Why do M dwarfs flare often? We have already noted that flares occur continuously along the main sequence, from G to M types. But the dynamo that causes the flare on M stars is believed to be different than that of other flare stars. The dynamos differ due to the role of convection; stars of spectral type M are nearly or fully convective. Figure 3, a histogram from TESS’s initial data, shows that more M dwarfs experience flares than other types of stars (Davenport et al. 2019). The fraction peaks at mid-M (M4-M6) type, and decreases for earlier or later M types. A possible explanation for this could be that the dynamo of these convective stars generates a stronger magnetic field, and hence the reconnection of its lines leads to greater release of energy. The large flare amplitude in K and M dwarfs is attributed to the fact that this spectral type consists of cool, dim, small stars; hence during a flaring event, the flare can be significantly brighter than the star’s quiescent luminosity. Kepler’s observations also provided data about F and G dwarfs. An analysis of these types of stars showed that their flares are similar with regard to the dynamo driving the phenomenon (Van Doorsselaere et al. 2017). The number of flaring F and G type stars decreases with increasing amplitude, with Van Doorsselaere et al. (2017) showing that they have the same rate of decrease, as shown in Figure 4. It’s important to keep in mind the selection effects and other limitations of these surveys. For instance: flares of a given energy will be more difficult to detect in the more luminous F and G stars than in K and M types. Balona (2015) suggests that, when these are taken into account, the incidence of flares may not change much from the cool dwarfs to the hotter ones. F and G type stars do not have the same flare duration. Specifically, F-type stars showed a broad distribution of flare duration, while G was more concentrated on shorter durations, appear- ing to be more similar to K and M dwarfs. This contradicted postulates which suggested that the mechanism generating flares on F and G is the same on the two types, but differs from that which causes the flares on both K and M (Van Doorsselaere et al. 2017). As shown in the right panel of Figure 4, the energy distribution of flare stars of type G, K and M were similar, but F had a shallower distribution, thus contradicting the possible categorization of flare stars as groups F and G versus K and M. It also appears that G type stars have the least number of high energy flares, although Van Doorsselaere et al. (2017) noted that there was a bias for high energy flares and the distribution for F types was shifted toward higher energies. Cool giant stars would perhaps not be expected to flare, because their rotation would have spun down as a result of their evolution and expansion. Nevertheless, Balona (2015) found a few flaring giants, and Van Doorsselaere et al. (2017) found 653 more – the incidence being similar to that in dwarf F and G stars. Their average durations were longer than those of GKM dwarfs, and more similar to the average duration of F dwarfs. None of these giants showed evidence of rotational variability due to spots. One wonders whether the flares on these two different samples caused by the same process. What about rotation period? There were several observations that early G-type flare stars do not have a correlation with their rotation period, thus increasing the debate on the mechanism causing flares (Davenport 2016). However, it is expected that rotation would be linked to the strength of the stellar magnetic field, and other measures of activity in cool stars indicate a decrease with increasing rotation period. Davenport (2016) finds that, within each spectral type range from G8 to M4, total flare luminosity decreases with increasing rotation period. Results are less clear for G0-G8 stars, which includes the sun’s spectral type. Van Doorsselaere et al. (2017) also find that the flare occurrence rate (and flare energy) is greatest in rapidly-rotating stars. The amplitudes of flares in F and G stars exhibit similar behaviour, possibly due to the same magnetic phenomena causing these energetic events (Van Doorsselaere et al. 2017). The energy distribution is quite different for both of these types, therefore there is no definite conclusion that the mechanism is the same for F and G type stars. Another type of stellar activity whose relation to flares can be studied is starspots, which can be detected through the rotational variability that they produce. Kepler measured the rotation periods of thousands of stars (e.g. Figure 1). Rottenbacher and Vida (2018) studied flares on late- F to mid-M types from the Kepler data in an attempt to find a possible correlation between flares and starspots. They found that the strongest flares in the sample did not seem to be correlated to the largest starspot group, but the weaker versions occurred more frequently close to the starspot group. It seems that the correlation between flares and starspots is inconclusive. On the sun, of course, spots and other forms of activity take place in an 11-year cycle. It will be interesting to follow some of the Kepler and TESS stars to see whether there are equivalent cycles on those stars. An analysis of the Rossby number, a dimensionless number which indicates the significance of rotation, for all flare stars (F, G, K, and M) showed that the rotation period should be very significant for all flares (Davenport 2016, Van Doorsselaere et al. 2017). It confirmed that faster rotating stars have a greater probability of flaring, as well as more frequently and producing highly energetic flares. This leads to the question, what about the sun? Before we can fully understand solar flares, it is important to analyze the highly energetic variants, known as superflares. 4.2. Superflares Flares can be classified according to the spectral type and other properties of the host star, but they can also be classified and analyzed according to energy. The most powerful ones are referred to as “superflares”, defined as white light flares with energies from 1033 to 1035 erg – thousands of times more powerful than the most powerful solar flare (Shibata et al. 2013). They are therefore potentially relevant to our well-being. But are they just the high-energy tail of the flare distribution? Can the standard flare mechanism produce flares which are this energetic? Or are they a separate phenomenon, requiring a separate mechanism? Kepler has detected superflares with energies of 1036 erg (Wichmann et al. 2014, Katsova and Nizamov 2018), or up to 1038 ergs according to Notsu et al. (2019). Superflares usually last a few hours and can account for 0.1-1% of the total stellar luminosity, depending on the star. Superflares occur on dwarfs from G to K to M but, as stellar effective temperature increases, the rate of superflares decreases. This matched previous studies that had indicated a decrease in dynamo activity for hotter stars (Candelarisi et al. 2014). An analysis of superflares and rotation found that the number of stars with superflares decreases as rotation period increases, matching the results discussed above in regards to the Rossby number (Maehara et al. 2017). They also found, however, that superflares may occur, albeit not very frequently, on slow rotating G-type stars (periods 20 to 30 days), as well as fast-rotating ones. G-type stars have weak magnetic fields (only a few Gauss), but can experience strong magnetic activity, like flares (Notsu et al. 2019). How? It has been suggested that the sun generates its own large magnetic flux, but no physical mechanism could be used to explain the storage of such a large magnetic flux before the occurrence of a superflare. Notsu et al. (2019) recently studied superflares on solar-type stars from Kepler data, new spectroscopic observations, and results from Gaia data release 2, to determine whether Kepler superflare stars include slow-rotating Sun-like stars. They found that some of the superflare stars were actually subgiants, rather than dwarfs. The subgiants were identified through the classification of Kepler G-type stars using the recent Gaia data release 2 stellar radius data. Some were in binary systems, but not all, so the superflare phenomenon could not always be ascribed to interactions in close binary systems. Notsu et al. (2019) also found from their own spectroscopic observations of 18 sun-like stars which were also observed by Kepler, that there is support for the hypothesis that quasi-periodic variations in Kepler sun-like superflare stars is caused by rotation with large starspots, contrary to the results of Roettenbacher and Vida (2018). Notsu et al. (2019) also conclude that the energy released by superflares is not inconsistent with the magnetic energy stored around large starspots. Superflares can possibly occur on stars like the sun, so there are interesting studies going on to search for evidence of these in the distant past, using ice cores, tree rings, and even historical records. 4.3. Do Flares Occur on A-type Stars? There have been occasional reports of flares on hot stars, but these reports are rare and unconfirmed. Solar-type flares would not be expected on hot stars, since hot stars do not have the large outer convection zones required to generate a magnetic field. It was therefore of special interest when Balona (2012, 2013), from a careful analysis of Kepler light curves, detected flare-like events in 33 A-type stars. Van Doorsselaere et al. (2017) found 28 A-type flare star candidates. There are a number of reasons, however, why the flares might be spurious, including instrumental effects, and the possibility that the flares were occurring on cool binary companions to the A stars (though, if so, the flare would tend to be overwhelmed by the much greater luminosity of the A star). Pedersen et al. (2017) therefore undertook a very careful re-examination of Balona’s 33 stars, using Kepler and other data. They concluded that “we find possible alternative explanations for the observed flares for at least 19 of the 33 A stars, but find no truly convincing (evidence) to support the hypothesis of flaring A-type stars”. The question therefore remains open. We note that flares of a given energy on A stars are more difficult to observe, because of the much greater background luminosity of an A star, as compared with an M star. 4.4. Flares on RS CVn (and Other) Binary Stars RS CVn stars are binary stars with orbital periods of 1 to 14 days, and components of type F or cooler. For half a century, they have been known to exhibit unusual stellar activity – spots, hot coronae, strong and variable emission lines, and flares (e.g. Frasca et al. 2008). This activity, which may occur on one or both of the components is attributed to their unusually rapid rotation, which has resulted from tidal interactions with their companion, which has “spun them up” to rotation rates several times faster than normal. It is additionally possible that the magnetic fields of the two stars could interact though, in three eclipsing binaries studied by Balona (2015), there was no correlation between flare frequency and orbital phase. The eclipsing binary KIC 12418816 shows an exceptionally high level of magnetic activity (Dal and Ozdarcan¨ 2018). RS CVn stars and other close binaries (and also cataclysmic variables with a cool, main sequence component) provide observations of spun-up, differentially-rotating stars, and therefore provide an additional testbed for stellar dynamo theories (Hill 2016). The spot distribution on RS CVn stars, and their consequent rotational variability changes from year to year, and they became a popular target for amateur astronomers in the 1980’s as photometric photometry became more accessable and organized. Several are on the AAVSO visual and/or photoelectric program, including λ And, HK Lac, and SZ Psc. Unlike most single flare stars, many RS CVn stars are relatively bright. A related, and increasingly-interesting topic, is what effect a massive exoplanet might have on the activity of its parent star, especially if it orbited very close to that star (Lanza 2018). We will not discuss that topic here. 5. Practical Activities Readers of this review might wish to observe a stellar flare, but they are rare, brief, and random, and they occur on faint stars, so they require planning and great patience. Observing variable stars in general requires guidance and experience. Start with the AAVSO webpage (www.aavso.org), including the various observing manuals. Read Templeton (2010); his instructions still apply. Choose a target, such as UV Ceti itself. Or search the General Catalogue of Variable Stars (GCVS) (Samus et al. 2017), which can also be accessed through SIMBAD (simbad.u-strasbg.fr/simbad/) and Vizier. To give you a sense of what you might observe, there are many observations of UV Cet, HM CMa, and AD Leo in the AAVSO International Database (Kafka 2019). Wikipedia (2019) lists a few stars, including the fifth-magnitude o Aql and 5 Ser, which may show detectable superflares; Kepler studies show that superflares can brighten a star by up to 30 percent. Furthermore: superflare stars tend to have larger spots, and therefore larger rotational variability, which could be detected photometrically. Much early research on flare stars was done by photographing star clusters. By imaging dozens of stars at once, the chance of recording a flare was greatly increased – and still is. In the GCVS, you will find dozens of UV Cet stars in the Pleiades and Praesepe, for instance. Once you are observing, record the star’s magnitude and the time of observation every minute or two. Of course, if you have an automated telescope and CCD camera, that helps! Some AAVSO observers observe exoplanet transits (www.aavso.org/exoplanet-section). Since these exoplanets’ host stars are cool stars, there’s the possibility that you will observe a stellar flare. So be observant! The sun is a flare star! The AAVSO has had an active and varied solar observing program for many decades. There are lots of instructions and resources on the AAVSO website. Professional astronomers often organize observing “campaigns” on variable stars, in which skilled amateur astronomers can help. This rarely happens with flare stars – other than the sun. In 2015-16, F-HUNTERS, part of the F-CHROMA project funded by the European Commission and consisting of various European institutions (F-Chroma 2015), organized two campaigns to observe solar flares. They were able to obtain significant data from amateur astronomers, for use in scientific analysis. The campaigns are over, but their website provides excellent, detailed information on observing solar flares, as well as more information on these energetic phenomena. 5. Future Directions, and Conclusions There has been a revival of interest and accomplishment in flare star research, for reasons outlined above. There is every reason to think that will continue:

• The automated flare-finding routines used so far are rather conservative. It may be possible to refine these, to recover even more flares.

• Data from the Kepler archive, from ongoing missions like TESS and GAIA, and future missions will provide even more information about the relationship between the frequency and energy of flares, and the stars’ spectral type, luminosity class, rotation and binarity.

• There would be special interest in further studies of superflares, and whether they can occur or have occurred on the sun.

• This new information will require theoretical explanation, yet the theory of stellar convection and magnetic fields is still far from complete. • Interest in exoplanets, both among scientists and the public, will continue to expand, espe- cially with regard to the planets’ habitability and possible habitation. How will flares, and especially superflares affect the atmospheres of these planets – especially close-in planets? How did they affect the young earth? How might they affect the earth in the future?

Acknowledgements This review was prepared by co-author Krstinja Dzombeta as a senior thesis project in the Astron- omy Major Program at the University of Toronto, supervised by co-author John Percy, who has revised and edited it for dissemination. The Dunlap Institute is funded through an endowment established by the David Dunlap Family and the University of Toronto.

References Allred, J.C. et al 2015, Astrophys. J., 809, 104. Anglada, G. et al. 2017, Astrophys. J., 850, L6. Balona, L.A. 2012, Mon. Not. Roy. Astron. Soc., 423, 3420. Balona, L.A. 2013, Mon. Not. Roy. Astron. Soc., 431, 2240. Balona, L.A. 2015, Mon. Not. Roy. Astron. Soc., 447, 2714. Benz, A.O. and G¨udel, M. 2010, Ann. Rev. Astron. Astrophys., 48, 241. Candelarisi, S. et. al. 2014, Astrophys. J., 792, 67. Choudhuri, A.R. 2017, Sci. China Phys., Mech., Astron., 60, 19601. Dal, H.A. and Ozdarcan,¨ O. 2018, Mon. Not. Roy. Astron. Soc., 474, 326. Davenport, J.R.A. 2016, Astrophys. J., 829, 23. Davenport, J.R.A. et al. 2019, Astrophys. J., 871, 241. Davenport, J.R.A., Hebb, L., and Hawley, S.L. 2015, Astrophys. J., 806, 212. Davenport, J.R.A. et al. 2016, Astrophys. J. Lett., 829, L31. F-Chroma, 2015, http://fchroma.astro.uni.wroc.pl/index.php/observing-guide.html Frasca, A. et al. 2008, Astron. Astrophys., 479, 557. Froning, C.S. et al. 2019, arXiv:1901.08647 Gershberg, R.E. 2014, Bull. Crimean Astrophys. Obs., 110, 50. G¨unther, M.N. et al. 2019, arXiv:1901.00443 Hawley, S.L. et al. 2014, Astrophys. J., 797, 121. Hill, C.A. 2016, in Living Around Active Stars, ed. D. Nandy et al., 54. Hunt-Walker, N.M. et al. 2012, Publ. Astron. Soc. Pacific, 124, 545. Johnson, M. 2018, https://www.nasa.gov/mission pages/kepler/overview/index.htm Johnson, M.C. et al. 2016, Astrophys. J., 821, 74. Jouve, L. and Kumar, R. 2016, in Living Around Active Stars, ed. D. Nandy et al., Cambridge University Press, 12. Kafka, S. 2019, variable star observations from the AAVSO International Database (https://www.aavso.org/aa international-database Katsova, M.M. and Nizamov, B.A. 2018, Geomagnetism and Aeronomy, 58, 890. Lalitha, S., Schmitt, J.H.M.M., and Singh, K.P. 2017, Astron. Astrophys., 602, A26. Lanza, A.F. 2018, Astron. Astrophys., 610, A81. Lanzafame, A.C. et al. 2019, Astrophys. J., in press. Lingam, M. and Loeb, A. 2018, arXiv:1810.002007 Maehara, H. et al. 2017, Publ. Astron. Soc. Japan, 69, 41. Mullan, D.J. et al. 2018, Astrophys.. J., 869, 149. Nandy, D., Valio, A., and Petit, P. (editors) 2017, Living Around Active Stars, Cambridge Uni- versity Press. NASA 2017, https://keplerscience.arc.nasa.gov/objectives.html Notsu, Y. et al. 2019, arXiv:1904.00142 Notsu, Y. et al. 2016, in 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, 119. Pedersen, M.G. et al. 2017, Mon. Not. Roy. Astron. Soc., 466, 3060. Percy, J.R. 2007, Understanding Variable Stars, Cambridge University Press. Percy, J.R. and Rice, J.B. 2017, www.aavso.org/rotating-variables-mapping-surfaces-stars Ramsay, G. et al. 2013, Mon. Not. Roy. Astron. Soc., 434, 2451. Roettenbacher, R.M. and Vida, K. 2018, Astrophys. J., 868, 3. Russell, C.T., Luhmann, J.G., and Riley, P. 2016, in Living Around Active Stars, ed. D. Nandy et al., 204. Samus, N.N. et al. 2017, General Catalogue of Variable Stars, www.sai.msu.su/gcvs/gcvs/ Segura, A. et al. 2010, Astrobiology, 10, 751. Shibata, K. et al. 2013, Publ. Astron. Soc. Japan, 65, 49. Templeton, M.R. 2010, https://www.aavso.org/vsots uvcet Van Doorsselaere, T., Shariati, H., and Debosscher, J. 2017, Astrophys. J. Suppl., 232, 26. Walkowicz, L.M. et al. 2011, Astron. J., 141, 50. Wichmann, R. et al. 2014, Astron. Astrophys., 567, A36. Wright, N.J. et al. 2018, Mon. Not. Roy. Astron. Soc., 479, 2351. Yang, H. and Liu, J. 2019, arXiv:1903.01056