APSPM2015 Nov 2 Seoul Univ 14:00-14:25 invited. Stellar Superflares. Kazunari Shibata. Kwasan and Hida Observatories, Kyoto University, Kyoto, Japan

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1 APSPM2015 Nov 2 Seoul Univ 14:00-14:25 invited Stellar Superflares Kazunari Shibata Kwasan and Hida Observatories, Kyoto University, Kyoto, Japan

2 Carrington flare (1859, Sep 1, am 11:18 ) The first flare that human beings observed by Richard Carrington (England) white flare for 5 minutes very bright aurora appeared next day morning at many places on Earth, e.g. Cuba, the Bahamas, Jamaica, El Salvador, and Hawaii. Largest magnetic storm (> 1000 nt) in recent 200 yrs. Telegraph systems all over Europe and North America failed. Telegraph pylons threw sparks and telegraph paper spontaneously caught Fire (Loomis 1861)

3 Will the Carrington-class flare occur again? If the Carrington-class flare occur now, what will happen? According to a study by the National Academy of Sciences (2008), the total economic impact could exceed $2 trillion

4 Will the Carrington-class flare occur again? Can much bigger flares, superflares (>10^33 erg), occur on the Sun at present? If yes, what is frequency of superflares? Why and how can superflares occur on the Sun? To answer these questions is the subject of my talk.

5 statistics of occurrence frequency of solar flares, microflares, nanoflares nanoflare microflare 1000 in 1 year 100 in 1 year solar flare 10 in 1 year 1 in 1 year Largest solar flare 1 in 10 year superflares 1 in 100 year? 1 in 1000 year C M X X10 X100 1 in year

6 How can we observe superflares on the Sun? If empirical statistics rule of solar flares is applied to much larger flares (superflares), then the frequency of superflares with energy 1000 times larger than the largest solar flares might occur once in years. However, the period of modern observations of the Sun with telescope is only 400 years. How can we observe the Sun for years? If we observe solar type stars (similar to our Sun) for 1 year, we can get the data similar to the data obtained from years observtions of the Sun! Prof Sekiguchi kindly told me that the Kepler satellite is taking such data!

7 Kepler satellite (NASA) Space mission to detect exoplanets by observing transit of exoplanets 0.95 m telescope Observing 160,000 stars continuously (from 2009 to 2013). Among them, are solar type stars. ~30 min time cadence (public data)

8 Superflares on Solar Type Stars : Our study (Maehara et al. 2012) Hence we searched for superflares on solar type stars using Kepler satellite data, which include data of solar type stars Since the data are so large, we asked 1 st year undergraduate students to help analyzing these stars, because students have a lot of free time (2010 fall) Surprisingly, we (they) found 365 superflares on 148 solar type stars (G-type main sequence stars)

9 Published in Nature (2012, May) Undergraduate students Superflares on Solar type stars H. Maehara, T. Shibayama, S. Notsu, Y. Notsu, T. Nagao, S. Kusaba, S. Honda, D. Nogami, K. Shibata

10 typical superflare observed by Kepler Brightness of a star and a flare Total energy ~ 10^35 erg Time (day) Maehara et al. (2012)

11 typical superflare observed by Kepler Brightness of a star and a flare Total energy ~ 10^36 erg Time (day) Maehara et al. (2012)

12 What is the cause of typical stellar superflare brightness observed variation by? Kepler Brightness of a star and a flare Total energy ~ 10^36 erg It is likely due to rotation of a star Time (day) with a big star spot Maehara et al. (2012)

13 Model calculation of stellar brightness variation KIC model(green) inclination = 45 Stellar brightness Starspot radius 0.16 R* 2 % 平均基準 ( ) time 5 days Notsu et al. 2013

14 Model calculation of stellar brightness variation KIC model(green) inclination = 45 Stellar brightness Starspot radius 0.16 R* 2 % 平均基準 )5 days ( time Notsu et al. 2013

15 Flare energy vs rotational period Fast rotation (young) Solar Rotation Slow rotation (old) Stars with period longer than 10 days cf solar rot period ~ 25days Maehara+(2012), Notsu+ (2013)

16 Flare energy vs rotational period Fast rotation (young) Solar Rotation Slow rotation (old) Stars with period longer than 10 days There is no hot Jupiter in these superflare stars against previous prediction (Schaefer+ 2000) cf solar rot period ~ 25days Maehara+(2012), Notsu+ (2013)

17 Number of superflares per year per star What is the frequency of superflares? (NotsuY+ 2013) Solar Rotation Fast rotation (young) Slow rotation (old) Rotational Period (days)

18 Once in 1000 years Flare energy vs sunspot area Flare Energy 10^35 erg X10000 Superflares on solar type stars Once in 100 years 10^34 erg X1000 Once in 10 years 10^33 erg X100 Once in 1 year 10^32 erg X in 1 year 10^31 erg X Sunspot area su (in unit of solar surface area) 100 in 1 year 10^30 erg M 1000 in 1 year 10^29 erg C Solar flare Sammis et al. 2000

19 Once in 1000 years Once in 100 years Once in 10 years Flare energy vs sunspot area 10^35 erg X ^34 erg X ^33 erg X100 Flare Energy Superflares on solar type stars Once in 1 year 10^32 erg X in 1 year 10^31 erg X Sunspot area (in unit suof solar Surface area) 100 in 1 year 10^30 erg M 1000 in 1 year 10^29 erg C Solar flare Sammis et al. 2000

20 Flare energy vs sunspot area (magnetic flux) 2 3/ / ] [ cm A G B f erg A B f L B f fe E spot spot mag flare Shibata et al. (2013)?

21 Flare energy vs. area of starspots Spectroscopic rotational velocity (Subaru obs) low inclination angle Photometric rotational velocity Notsu, Y. et al. (2015)

22 Comparison of statistics between solar flares/microflares and superflares Shibata et al nanoflare microflare solar flare Largest solar flare superflare? C M X X10 X100

23 Comparison of statistics between solar flares/microflares and superflares 1000 in 1 year 100 in 1 year 10 in 1 year 1 in 1 year 1 in 10 year 1 in 100 year 1 in 1000 year 1 in year X10000 flare Superflares of 1000 times more Energetic than the largest solar flares occur once in 5000 years! nanoflare solar flare microflare C M X X10 X100 Largest solar flare superflare Shibata et al. 2013

24 Fundamental Question Why and how can superflares occur on Sun-like stars (i.e., present Sun)? Superflares occur because of the presence of large spots. => Why and how can large spots be generated on Sun-like stars (i.e., present Sun)?

25 Spot Area (in unit of total area of a star) Amplitude Many stars without superflares What is the spot area statistics? Show evidence of large spots! T eff = K Solar Rotation Period (day) Maehara+ (2015)

26 Statistics of Spot Area on the Sun Courtesy of Ishii and Maehara+ (2015)

27 Statistics of Spot Area on the Sun and Sun-like Stars Large spots exist in many Sun-like stars though frequency Is small Courtesy of Ishii and Maehara+ (2015)

28 d dt t Necessary time to generate magnetic flux producing superflares (Shibata et al. 2013) Why and how can large spots be generated on the present Sun? (Shibata et al. 2013) t 40 2 p B 2 2R t p Mx 10 Mx p p 1 r Hz 1 years The necessary time to generate magnetic flux of Mx that can produce superflares of erg are 40 years (<< 5000 years) (but > 11 years) only 8 years (< 11 years) to generate 2x10 23 Mx producing superflares of erg Is it possible to store such huge magnetic flux below the base of convection zone? => big challenge to dynamo theorist! => easily occur!?

29 Stellar Brightness variation Most active Sun-like star 4 superflares in 500 days! Shibayama et al 2013 day KIC (rot period 15 days)

30 What is Solar/Stellar Cycle dependence of Flare frequency? Shibayama et al 2013 nanoflare Solar maximum 2001 solar flare Solar minimum 2008 microflare Most active Sun-like stars Largest solar flare superflare

31 Spectroscopic Observations of Solar type stars causing superflares will be extremely important Okayama 3.8m New Technology Telescope of Kyoto Univ (under construction) New Technology 1. Making Mirrors with Grinding 2. Segmented mirror 3. Ultra Light mounting Budget for operation Is still lacking. Please support us! Will be completed ~ 2017 High speed photometric and spectroscopic observation of Transient objects Gamma ray bursts Exoplanets Stellar flares (superflares) courtesy of Prof. Nagata (Department of Astronomy, Kyoto University)

32 Summary Using Kepler data, we found 365 superflares (10^33-10^36 erg) on 148 solar type stars (among stars) during 120 days (Maehara+ 2012). => 1547 superflares from 279 solar type stars during 500 days (Shibayama+ 2013). Superflares occur on Sun-like stars ( K and slow rotation) with frequency such that superflares with energy 10^34-10^35 erg ( times of the largest solar flare) occur once in years There is no hot Jupiter around these superflare stars. These stars have big star spot (Notsu+ 2013). Rotational velocity and big star spot of 50 superflare stars has been confirmed by spectroscopic observations (Notsu+ 2015) Hence we cannot reject the possibility that superflares of 10^34 10^35 erg would occur once in years on the present Sun (Shibata+ 2013, Nogami+ 2014) Thank you for your attention