Flare Stars: A Short Review

Krstinja DzombetaDepartment of Astronomy and Astrophysics,University of TorontoToronto ONCanada M5S 3H4

and

John R. PercyDepartment of Astronomy and Astrophysics, andDunlap Institute of Astronomy and AstrophysicsUniversity of TorontoToronto ONCanada M5S 3H4

Abstract Flare stars, or UV Ceti stars, are a type of eruptive variable star, defined by theirflaring 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 researchdevelopments, 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 intensiveresearch, as a result of an abundance of new data, especially from the Kepler and TESS spacetelescopes. The large statistical samples of data have clarified the relation between flaring andstellar spectral type, luminosity, and rotation. They have allowed for the expansion of the range ofspectral types of flare stars, from K and M type dwarfs, to also F and G, and possibly even A. Theyhave 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 breakin the relationship has also been demonstrated. Current problems in flare-star research includeimproved modelling of the new observational results, using the dynamo theory which produces thestars’ magnetic field. What is the difference, if any, between the dynamo in completely-convectivestars 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 eruptivevariable stars which increase in brightness over the course of minutes, then return to their originalbrightness over the course of hours. Or, in the words of Balona (2015), “a flare star is a star inwhich at least one flare has been observed”. Although a large fraction of the stars in our galaxy areflare 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 ofthis Journal – skilled amateur astronomers, students, and others with a general interest in variablestars. This brief review is intended to close that gap. It emphasizes developments in the last fewyears. Percy (2007) and Templeton (2010) provided brief reviews a decade ago, and Benz andGudel (2010) provided a more technical review article. The proceedings of a recent internationalconference on Living Around Active Stars (Nandy et al. 2017) contains many interesting andrelevant technical papers.

Table 1: Examples of Flare StarsName Spectral Type Apparent Magnitude

UV Cet M6V 12.95-6.80Proxima Centauri M6V 11.13-10.43

AD Leo M4Vae 11.00-8.07Trappist 1 M8 18.8GJ 1243 M4V 12.83GJ 674 M3V 9.41LO Peg K3Ve 9.27-9.04

HD 219143 K3V 10.05-5.57λ 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 spectrumfrom gamma rays to radio waves, although it is more apparent at visual and UV wavelengths, asa result of the specific temperature of the material produced and emitted during the flare. Stellarflares 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 (Gershberg2014). 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 typeof star was named. Proxima Centauri is the nearest star to the sun; it is a flare star, and also hasan 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 (Davenportet 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 isthe 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 hadan energy of 5 x 1032 erg (Lingam and Loeb 2018), and was one of the most intense solar eventsever 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 singleflare on a F or G -type star is much smaller than that of the star, even if the energy of the flareis the same.

The flare mechanism was thought to be similar only on K and M dwarfs, although recentstudies show that these flares share certain qualitative similarities with those on hotter (F and Gtype) stars like the sun. K and M dwarfs are dimmer, cooler and less massive stars than F andG-types. Flares on the former may appear to brighten the entire star, while the latter does notappear to be as affected.

How are the data from flare stars analyzed? Light curves, plots of flux or brightness versustime, 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 ofthe star shown in blue. The small, smooth increases and decreases shown in blue reveal thestar’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 inblack and quiescent luminosity model plotted over it in red. The rotation period is also ∼ 8.5days. Flare candidates are shown as red points. Bottom: Selected flare candidates, showing theduration 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 regularincreases and decreases, indicating that the star also has variability due to spots. Spots, the cooldark regions where the magnetic field is strong enough to inhibit the passage of heat, occur onflare stars such as the M dwarf shown, as they do on the sun. See Percy and Rice (2017) for adiscussion 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 thanmagnitude 7. The brightest is ǫ Eri, V = 3.73. GAIA has recently discovered and measured therotation periods of almost 150,000 more (Lanzafame et al. 2019). The rotation period of the Mdwarf shown in Figure 1 is about ∼ 8.5 days, which is typical for these stars. The flare candidatesare 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 over30 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 panelshows the zoomed-in plots of the flares; it demonstrates their duration. For example, the leftplot of the K dwarf shows that the flare lasted about 0.5 day. The duration of a flare takes intoaccount the time required for the brightness to increase and then decrease to the initial level. Itis not possible to tell, from these flare light curves, whether there is any fine structure i.e. morerapid 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 havestrong global magnetic fields, while G-type stars like the sun have relatively weak versions – onlya few Gauss. How do magnetic fields produce such an energetic phenomenon? For a detaileddiscussion, see the excellent review, and simulations of stellar flares by Allred et al. (2015). Seealso Jouve and Kumar (2016) for a discussion of the connections between solar and stellar dynamomodels.

Flares are released when there is a reconnection of magnetic field lines on the star’s surfaceor atmosphere. This is the so-called impulsive phase, which causes the magnetic field to drop toa lower state of energy, thus releasing energy, and accelerating energetic particles into the stellaratmosphere. There is rapid heating and observable effects in the form of a flare. The acceleratedcharged particles travel along the magnetic field lines and collide with dense plasma, which isheated in reconnecting flux tubes at a temperature of at least 20MK. The charged particles includeelectrons, which are detected by so-called Bremsstrahlung radiation when they collide with theplasma. 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 difficultto 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 continuousconversion of kinetic to magnetic energy, which produces a magnetic field. The greater the rotationand convection, the stronger the magnetic field. This dynamo model also proposes that therotation and convection create differential rotation on the surface, which stretches the magneticfield 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 thepoles.)

While flares appear to be random, average flare occurrence frequency and energy are related tothe strength of the magnetic field (Davenport 2016). Young active M dwarfs have not had time tospin down their rotation; therefore they generate larger magnetic fields and hence produce largersingle 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 strongH-alpha emission lines in their spectra.

During the flaring episode, the spectrum changes throughout the various stages. It starts withflashes of Balmer emission lines of hydrogen, and helium lines, and at the maximum shows astrong continuum – white light emission with a temperature between 9000K and 10,000K – whichcan 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 thebase of magnetic field lines, because the amount of surface area covered depends on the strengthof 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 verycomplex and therefore interesting phenomena. They provide astrophysicists with another way tostudy 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 energeticradiation and particles, and coronal mass ejections, which eventually arrive at and interact with ourupper atmosphere and magnetic field, resulting in “space weather”. The general flaring principleand mechanism is the same on other types of flare stars as on the sun (though the dynamo on F andG stars, which are only partially convective may be somewhat different than that on K and M starswhich are completely convective). Studying flare stars may therefore help us to better understandflares on the sun, and how the sun compares with sun-like stars in general, revealing “the sunin space and time”. Whereas a solar flare is seen against a complex photosphere, photometry ofother 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 Mdwarfs, which have a smaller background intensity, have greater visibility, compared to those ofstars like the sun (Allred et al. 2015).

Understanding solar flares is of practical importance because of their potential negative effecton human activity and well-being. It’s also important for people to understand that global warmingcannot be blamed on solar activity. Solar flares can affect satellites, communication transmissionin 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 anestimated 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 solarflare, or “super-flare”, with energy greater than 1033 ergs, could have devastating effects on theearth’s environment, and on human lives (Shibata et al. 2013) – a good reason to increase ourunderstanding of such superflares.

3.2. Exoplanets

Thousands of exoplanets have been discovered in the last decade or so, mostly by the transitmethod, by the Kepler mission – almost all of them around cool stars. (The activity on thesestars actually makes exoplanets more difficult to detect by the transit method.) Many orbit in thehabitable zone, defined as the zone in which water on the planet would be in the liquid state. Manyorbit M stars, notably Proxima Centauri, the closest star to the sun. Given the low luminosity ofM 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 aboutthe effect of stellar flares on planetary atmospheres, and on the possible origin and development oflife on the planet – an exciting new interdisciplinary topic. High-energy stellar radiation (includingUV) 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 theplanetary atmosphere and magnetic field. There is also a possibility that stellar flares will affectthe protoplanetary disc of gas and dust from which the planets form.

Flares are more frequent and energetic on young, rapidly-rotating stars. While models predictthat the flares will not directly affect a habitable exoplanet, they do indicate that 94 percent ofthe planet’s ozone layer can be depleted in two years, if the planet does not have a magnetic fieldto 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 ofindividual stars, though over 1600 were known at that time. Flare star research has evolved andexpanded especially in the last decade as a result of the abundance of data made available byKepler. 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 thoseby 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 greatlyto expanding our understanding of flares.

NASA’s first Kepler mission operated from May 2, 2009 until May 8, 2013, primarily searchingfor exoplanets. In so doing, it constantly monitored over 200,000 stars including cool stars onwhich flares occur (NASA 2017; Johnson 2018). It rotated every 93 days to ensure that sunlightwould 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 morerapid, lower-energy flares, which are necessary for determining the number of flares as a function oftheir energy. This initial data allowed for greater insight into the properties of flares, particularlyon 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 starsexhibiting such a rare and randomly occurring phenomenon. They had focused mostly on activeM dwarfs because of their strong magnetic activity as indicated, for instance, by H alpha emissionlines. Kepler provided continuous monitoring of tens of thousands of stars, resulting in the datarequired to understand general properties of flare stars. The telescope had a high signal to noiseratio, 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 reactionwheels.

This led to K2, Kepler’s second mission or “new life”, which operated from May 2014 untilOctober 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 wellas continuing its predecessor’s search for planetary transits. Kepler found over 5,000 exoplanetcandidates and observed 200,000 stars, while K2 confirmed 300 exoplanets and identified 500 otherpossible candidates. NASA’s K2 mission came to an end in October 2018, as a result of lack offuel.

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 identifyflare candidates, such as requiring that a star have at least 100 flares in total and at least 10 withenergies above the local 68% completeness threshold (Davenport 2016). That particular searchprocess was therefore a conservative one.

Both Kepler and K2 revolutionized the study of flare stars, by providing an abundance ofdata, and prompting increased statistical analyses of general properties of flare stars. A resultof 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 (greypoints) versus effective temperature. The top axis gives the corresponding spectral type of thestars. Kepler data are shown in grey and TESS data shown in blue. Above and to the right of thesquare are histograms, showing the distribution of the points (Gunther et al. 2019). Note thatTESS observes primarily M type dwarfs.

May 2, 2009 until May 8, 2013, from data release 24 (Davenport 2016). The final version of thecatalog contains more than 850,000 flare events on 4041 stars, which is 1.9% of the Kepler stellardatabase. Their typical energy was 1034 to 1035 ergs – considerably greater than the maximumenergies observed in solar flares.

An updated flare catalog by Yang and Liu (2019) used observations from data release 25, andfound 3420 flare stars with more than 160,000 flare candidates. Davenport (2016) and Yang andLiu (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 flaresas a candidate, but showed that some of their missing data consisted of inactive flare stars, orthose 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, 209were 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 2shows the stars observed by Kepler in contrast to an initial data sample of stars observed byTESS, which focused more on early to late-type M dwarfs (Gunther et al. 2019). Figure 3 showsinformation 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, shownas 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. Fand G type stars have fewer high amplitude flares and follow the same decreasing trend – sameslope m of best fit. K and M types have a range of amplitudes, and a greater number of higheramplitude flares than F and G. Right: Top: Histogram (log scale) of energy distribution of flareson F types. There are few low and high energy flares; they are mostly concentrated in the middlewith a shallow distribution. Middle: Histogram (log scale) of energy distribution of flares on Gtypes, with most of the flares at lower energies, compared to F types. Bottom: Histogram (logscale) 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 accordingto spectral type and other properties, and perform analyses on a large statistical sample. Veryfortunately, the temperature and luminosity of each star in the Kepler input catalog had beendetermined prior to the mission. A significant amount of Kepler research was dedicated to Mdwarfs, as they experience the most flaring, followed by K dwarfs. This was confirmed in the initialKepler 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 moreenergetically, and lasting for a shorter duration. The same was confirmed by Van Doorsselaere etal (2017), also finding that K and M dwarfs show a decreasing trend in flare amplitude, as shown inFigure 4. Kepler data also showed that the energy distribution of flares follows a decreasing powerlaw fit for those with energy greater than 1031 ergs (Hawley et al. 2014). This was confirmed byVan Doorsselaere et al. (2017) for flares stars of type M, K and G. Davenport (2016) showed thatflare stars of spectral type G8 to M4 have decreasing flare luminosity with increasing rotationperiod. 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 4shows a histogram of the distribution of flare amplitudes.

Why do M dwarfs flare often? We have already noted that flares occur continuously along themain sequence, from G to M types. But the dynamo that causes the flare on M stars is believedto 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 initialdata, 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 astronger 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 consistsof cool, dim, small stars; hence during a flaring event, the flare can be significantly brighter thanthe star’s quiescent luminosity.

Kepler’s observations also provided data about F and G dwarfs. An analysis of these types ofstars showed that their flares are similar with regard to the dynamo driving the phenomenon (VanDoorsselaere et al. 2017). The number of flaring F and G type stars decreases with increasingamplitude, 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. Forinstance: flares of a given energy will be more difficult to detect in the more luminous F and Gstars 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 abroad 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 themechanism generating flares on F and G is the same on the two types, but differs from that whichcauses 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 andM were similar, but F had a shallower distribution, thus contradicting the possible categorizationof flare stars as groups F and G versus K and M. It also appears that G type stars have the leastnumber of high energy flares, although Van Doorsselaere et al. (2017) noted that there was a biasfor 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 havespun down as a result of their evolution and expansion. Nevertheless, Balona (2015) found a fewflaring giants, and Van Doorsselaere et al. (2017) found 653 more – the incidence being similarto 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 ofrotational variability due to spots. One wonders whether the flares on these two different samplescaused by the same process.

What about rotation period? There were several observations that early G-type flare stars donot have a correlation with their rotation period, thus increasing the debate on the mechanismcausing flares (Davenport 2016). However, it is expected that rotation would be linked to thestrength of the stellar magnetic field, and other measures of activity in cool stars indicate adecrease with increasing rotation period. Davenport (2016) finds that, within each spectral typerange from G8 to M4, total flare luminosity decreases with increasing rotation period. Results areless 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 samemagnetic phenomena causing these energetic events (Van Doorsselaere et al. 2017). The energydistribution is quite different for both of these types, therefore there is no definite conclusion thatthe 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 canbe detected through the rotational variability that they produce. Kepler measured the rotationperiods 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 flaresand starspots. They found that the strongest flares in the sample did not seem to be correlated tothe largest starspot group, but the weaker versions occurred more frequently close to the starspotgroup. 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 willbe interesting to follow some of the Kepler and TESS stars to see whether there are equivalentcycles on those stars.

An analysis of the Rossby number, a dimensionless number which indicates the significanceof rotation, for all flare stars (F, G, K, and M) showed that the rotation period should be verysignificant for all flares (Davenport 2016, Van Doorsselaere et al. 2017). It confirmed that fasterrotating stars have a greater probability of flaring, as well as more frequently and producing highlyenergetic flares. This leads to the question, what about the sun? Before we can fully understandsolar 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 arereferred 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). Theyare therefore potentially relevant to our well-being. But are they just the high-energy tail of theflare distribution? Can the standard flare mechanism produce flares which are this energetic? Orare they a separate phenomenon, requiring a separate mechanism?

Kepler has detected superflares with energies of 1036 erg (Wichmann et al. 2014, Katsovaand Nizamov 2018), or up to 1038 ergs according to Notsu et al. (2019). Superflares usually lasta 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 indynamo activity for hotter stars (Candelarisi et al. 2014). An analysis of superflares and rotationfound that the number of stars with superflares decreases as rotation period increases, matchingthe results discussed above in regards to the Rossby number (Maehara et al. 2017). They alsofound, however, that superflares may occur, albeit not very frequently, on slow rotating G-typestars (periods 20 to 30 days), as well as fast-rotating ones. G-type stars have weak magneticfields (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 nophysical mechanism could be used to explain the storage of such a large magnetic flux before theoccurrence of a superflare.

Notsu et al. (2019) recently studied superflares on solar-type stars from Kepler data, newspectroscopic observations, and results from Gaia data release 2, to determine whether Keplersuperflare stars include slow-rotating Sun-like stars. They found that some of the superflarestars were actually subgiants, rather than dwarfs. The subgiants were identified through theclassification of Kepler G-type stars using the recent Gaia data release 2 stellar radius data. Somewere in binary systems, but not all, so the superflare phenomenon could not always be ascribed tointeractions 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 thehypothesis that quasi-periodic variations in Kepler sun-like superflare stars is caused by rotationwith 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 energystored around large starspots.

Superflares can possibly occur on stars like the sun, so there are interesting studies going onto search for evidence of these in the distant past, using ice cores, tree rings, and even historicalrecords.

4.3. Do Flares Occur on A-type Stars?

There have been occasional reports of flares on hot stars, but these reports are rare andunconfirmed. Solar-type flares would not be expected on hot stars, since hot stars do not havethe 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 Keplerlight curves, detected flare-like events in 33 A-type stars. Van Doorsselaere et al. (2017) found28 A-type flare star candidates. There are a number of reasons, however, why the flares might bespurious, including instrumental effects, and the possibility that the flares were occurring on coolbinary companions to the A stars (though, if so, the flare would tend to be overwhelmed by themuch greater luminosity of the A star). Pedersen et al. (2017) therefore undertook a very carefulre-examination of Balona’s 33 stars, using Kepler and other data. They concluded that “we findpossible alternative explanations for the observed flares for at least 19 of the 33 A stars, but findno 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 aremore difficult to observe, because of the much greater background luminosity of an A star, ascompared 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 typeF 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” torotation rates several times faster than normal. It is additionally possible that the magnetic fieldsof the two stars could interact though, in three eclipsing binaries studied by Balona (2015), therewas no correlation between flare frequency and orbital phase. The eclipsing binary KIC 12418816shows an exceptionally high level of magnetic activity (Dal and Ozdarcan 2018). RS CVn starsand 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 additionaltestbed for stellar dynamo theories (Hill 2016).

The spot distribution on RS CVn stars, and their consequent rotational variability changesfrom year to year, and they became a popular target for amateur astronomers in the 1980’s asphotometric photometry became more accessable and organized. Several are on the AAVSO visualand/or photoelectric program, including λ And, HK Lac, and SZ Psc. Unlike most single flarestars, many RS CVn stars are relatively bright.

A related, and increasingly-interesting topic, is what effect a massive exoplanet might have onthe activity of its parent star, especially if it orbited very close to that star (Lanza 2018). We willnot 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 starsin 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 UVCet, 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 mayshow detectable superflares; Kepler studies show that superflares can brighten a star by up to 30percent. Furthermore: superflare stars tend to have larger spots, and therefore larger rotationalvariability, which could be detected photometrically.

Much early research on flare stars was done by photographing star clusters. By imaging dozensof 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 minuteor 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). Sincethese exoplanets’ host stars are cool stars, there’s the possibility that you will observe a stellarflare. So be observant!

The sun is a flare star! The AAVSO has had an active and varied solar observing program formany decades. There are lots of instructions and resources on the AAVSO website.

Professional astronomers often organize observing “campaigns” on variable stars, in whichskilled 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 Commissionand consisting of various European institutions (F-Chroma 2015), organized two campaigns toobserve solar flares. They were able to obtain significant data from amateur astronomers, foruse in scientific analysis. The campaigns are over, but their website provides excellent, detailedinformation 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 reasonsoutlined above. There is every reason to think that will continue:

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

• Data from the Kepler archive, from ongoing missions like TESS and GAIA, and futuremissions will provide even more information about the relationship between the frequencyand 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 occuror have occurred on the sun.

• This new information will require theoretical explanation, yet the theory of stellar convectionand 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, andespecially 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 hasrevised and edited it for dissemination. The Dunlap Institute is funded through an endowmentestablished by the David Dunlap Family and the University of Toronto.

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