Superflares on Solar Type Stars

Kazunari Shibata

Astronomical Observatory,

Kyoto University, Kyoto, Japan

Symposium "Environments of Terrestrial Planets Under the Young Sun: Seeds of Biomolecules",NASA Goddard Space Flight Center, Greenbelt, MD, USAApril 9-13, 2018, 10:00-10:30 invited

Solar Coronaseen in X-rays

(Yohkoh)

Soft X-rays（1 keV)2MK – a few 10MK

2

Corona is full of

Flares !

Simultaneous Halpha and X-ray view of a flare Magnetic

reconnecitonHα X-ray

Plasmoid ejections are ubiquitous

impulsive flares~ 10^9 cm

LDE(Long DurationEvent) flares~ 10^10 cm

CMEs(Coronal Mass Ejections) from Giant arcades~ 10^11 cm

Unifiedmodel

(Shibata+1995)

Plasmoid-Induced-Reconnection(see review by Shibata+Magara 2011)

X-ray Observations of Stellar FlaresSolar Flare

X-rayIntensity(3-24keV)

time

10 hours

Stellar Flare of Prox Cen (Haisch et al. 1983)

Protostellar Flare of YLW15 (Monmerle,Tsuboi + 2000)

time

X-rayIntensity(~ 1 keV)

time

X-rayIntensity(2-10 keV)

Koyama+ 1996 discovered protostellar flares

Can stellar flares be explained by magnetic reconnection ?

• Yes !

• Indirect evidence has been found in empirical correlation between

Emission Measure ( )

and Temperature from soft X-ray obs

(Shibata and Yokoyama 1999, 2002)

32LnEM

Emission Measure （ＥＭ＝ｎ２Ｖ） of Solar and Stellar Flares increases with Temperature （Ｔ）

（n：electron density、Ｖ： volume）(Feldman et al. 1995)

soft X-ray observations

ＥＭ－Ｔ relation of Solar and Stellar Flares

Log-log plot

of Feldman

etal (1995)’s

figure

Shibata and Yokoyama, 1999, 2002

ＥＭ－Ｔ relation of Solar and Stellar Flares

microflare

(Shimizu 1995)

Shibata and Yokoyama, 1999, 2002

young-star and protostellar flares

Tsuboi (1998)Pallavicini(2001)

Koyama(1996)

Class I protostar

Shibata and Yokoyama (1999) ApJ 526, L49

Flare Temperature Scaling Law

• Reconnection heating＝conduction cooling（Yokoyama and Shibata 1998)

LTVB A 2/4/ 2/72

7/27/6 LBT L

• Emission Measure

• Dynamical equilibrium (evaporated plasma must be confined in a loop)

• Using Flare Temperature scaling law, we have

Flare Emission Measure(Shibata and Yokoyama 1999)

32LnEM

2/175TBEM

8/2 2BnkT

EM-T correlation for solar/stellar flares

Shibata and Yokoyama (1999, 2002)

Magnetic field strength（Ｂ）＝constant

Magnetic field strengths of solar and stellar flaresare comparable ~ 50-100 G

2/175TBEM

Shibata and Yokoyama (1999, 2002)

Total energy of stellar flares

Superflares

Their energy= 10-10^6times thatofthe largestsolar flares

Solar flares

Solar microflares

Stellar flares

Their host starsare young starsand binary starswith fast rotation

Q: What determines flare total energy ? A: loop length (because magnetic field strength is

roughly constant)

The reason why stellar flares are hot => loop lengths of stellar flares are large

Shibata and Yokoyama (2002)

Cf Isobe et al. 2003, Kawamiti et al. 2008

Why young stars produce superflares ?

• Answer：young star’s rotation is fast

（so dynamo is active and total magnetic flux is large =>

loop is large）

Young stars

Stellar X-rayLuminosity

Stellar rotational velocity

Young stars

Aged stars (Sun)）

Aged stars (Sun)

(Pallavicini et al. 1981, ApJ 248, 279)

Young stars

Aged stars (Sun)

Can Superflares Occur on the Sun ?

Standard view on the possibility of superflares on the Sun before 2012

• The Sun is old, and is slowly rotating.

• Hence large spots will not be formed, and thus superflares will not occur.

？superflare

nanoflare

microflare

solar flare

Comparison of statistics between solar flares/microflares and superflares

1000 in 1 year100 in 1 year10 in 1 year1 in 1 year1 in 10 year1 in 100 year1 in 1000 year1 in 10000 year

C M X X10 X1000 X100000

Largest solar flare

Shibata et al. 2013

C M X X10 X100

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 superflareswith energy 1000 times larger than the largest solar flares might occur once in 10000 years.

• However, the period of modern observations of the Sun with telescope is only 400 years.

• How can we observe the Sun for 10000 years ?

• If we observe 10000 solar type stars (similar to our Sun) for 1 year, we can get the data similar to the data obtained from 10000 years observtions of the Sun !

Prof Sekiguchi kindly told me that the Kepler telescope is taking such data !

Kepler telescope (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,83000 are solar type stars.

• ~30 min time cadence (public data)

• Hence we searched for superflares on solar type stars using Kepler satellite data, which include data of 83000 solar type stars

• Since the data are so large, we asked 1st 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 148solar type stars (G-type main sequence stars)

Superflares on Solar Type Stars :Our study (Maehara et al. 2012)

Superflares on Solar type stars

H. Maehara, T. Shibayama, S. Notsu, Y. Notsu, T. Nagao, S.

Kusaba, S. Honda, D. Nogami, K. Shibata

Published in Nature (2012, May)

Undergraduate students

typical superflare observed by Kepler

Brightnessof a starand a flare

Time (day)

Total energy~ 10^35 erg

Maehara et al. (2012)

typical superflare observed by Kepler

Brightnessof a starand a flare

Time (day)

Total energy~ 10^36 erg

Maehara et al. (2012)

typical superflare observed by Kepler

Brightnessof a starand a flare

Time (day)

Total energy~ 10^36 erg

Maehara et al. (2012)

What is the cause of stellar brightness variation ?

It is likely due to rotation of a star with a big star spot

Model calculation of stellar brightness variationKIC6034120

2％(

平均基準)

model(green)inclination = 45°

Starspot radius0.16 R*

5 daysNotsu et al. 2013time

Stellar brightness

KIC6034120

2％(

平均基準) 5 days

Notsu et al. 2013

Model calculation of stellar brightness variation

model(green)inclination = 45°

Starspot radius0.16 R*

time

Stellar brightness

Flare energy vs rotational period

Stars with period longerthan 10 days

cf solar rot period ~ 25days

Maehara+(2012),Notsu+ (2013)

Fast rotation(young)

Slow rotation(old)

0.5Ｇyr <= Tu+ 20151Ｇyr 4.6Ｇyr0.1Ｇyr

Flare energy vs rotational period

Stars with period longerthan 10 days

cf solar rot period ~ 25days

Maehara+(2012),Notsu+ (2013)

Fast rotation(young)

Slow rotation(old)

Solar Rotation

0.5Ｇyr <= Tu+ 20151Ｇyr 4.6Ｇyr0.1Ｇyr

Flare energy vs rotational period

Stars with period longerthan 10 days

cf solar rot period ~ 25days

Maehara+(2012),Notsu+ (2013)

Fast rotation(young)

Slow rotation(old)

Solar Rotation

0.5Ｇyr <= Tu+ 20151Ｇyr 4.6Ｇyr0.1Ｇyr

There is no hot Jupiter in these superflare stars against previous prediction (Schaefer+ 2000)

What is the frequency of superflares ?(NotsuY+ 2013)

Rotational Period (days)

Nu

mb

er o

f s

up

erfl

ares

per

yea

rp

er s

tar

Solar Rotation

Fast rotation(young)

Slow rotation(old)

0.5Ｇyr<= Tu+ 20151Ｇyr 4.6Ｇyr0.1Ｇyr

Flare frequency vs. rotation period

• The frequency of superflares decreases as the rotation period increases (P>2-3days).

– The frequency of superflares shows the “saturation” for a period range < 2-3 days.

G-dwarfs, Eflare > 5×1034 erg

Notsu et al. (2013)

(Ro = P_rot/t_conv) Rotational Period (days)

superflare

nanoflare

microflare

solar flare

Comparison of statistics between solar flares/microflares and superflares

？

Largest solar flare

Shibata et al. 2013

C M X X10 X100

superflare

nanoflare

microflare

solar flare

Comparison of statistics between solar flares/microflares and superflares

1000 in 1 year100 in 1 year10 in 1 year1 in 1 year1 in 10 year1 in 100 year1 in 1000 year1 in 10000 year

C M X X10 X1000 X100000

Largest solar flare

Superflares of 1000 times more Energetic than the largest solar flares occur once in 5000 years !

Shibata et al. 2013

C M X X10 X100

X10000 flare

Fundamental Question

• Why and how can superflares occuron 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) ?

What is the spot area statistics ? Teff=5500-6000 K

Period (day)

Am

plit

ud

e

Maehara+ (2017)

Spo

t A

rea

(in

un

it o

f to

tal a

rea

of

a st

ar)

Solar Rotation

Many stars without superflaresShow evidence of large spots !

0.5Ｇyr<= Tu+ 2015

1Ｇyr4.6Ｇyr0.1Ｇyr

Size distributions of sunspots

• Our Sun did not show very large spot , i.e. “superflare spot” (with area > 10^(-2) solar hemisphere) during the last 140 years.

Maehara+ (2017)

1000 in 1 year100 in 1 year10 in 1 year1 in 1 year1 in 10 year1 in 100 year1 in 1000 year1 in 10000 year

Size distributions of sunspots and starpots• Size distribution of starspots on Sun-like stars and that of

large sunspots are roughly on the same power-law line.

• Hence if we observe the Sun more than 10000 years, we may observe very large spots as in Sun-like stars !

Sun like stars

Maehara+ (2017)

1000 in 1 year100 in 1 year10 in 1 year1 in 1 year1 in 10 year1 in 100 year1 in 1000 year1 in 10000 year

Necessary time to generate magnetic flux producing superflares (Shibata et al. 2013)

yearsHzMxMx

t

rRBdt

d

pt

pppt

1

7

1

2224 106.5101040

222

only 8 years (< 11 years) to generate2x1023 Mx producing superflares of 1034 erg

The necessary time to generate magnetic flux of 1024 Mx that can produce superflares of 1035 ergare 40 years （<< 5000 years) (but > 11 years)

Is it possible to store such huge magnetic flux below the base of convection zone ?

=> big challenge to dynamo theorist !

=> easily occur !?

Why and how can large spots be generated on the present Sun ? (Shibata, Choudhuri et al. 2013)

Upper limit of CME speed associated with Superflares

We should Consider there Is Large scatter

Takahashi , Mizuno,Shibata 2016, ApJL

Based on Yashiro+(2004), Gopalswamy+(2010)

superflares

Upper Limit of Proton Flux Associated with Superflare CMEs

We should Consider there Is Large scatter

Takahashi , Mizuno,Shibata 2016, ApJL

Based on Yashiro+(2004), Gopalswamy+(2010)

superflares

Is there evidence of superflareson the Sun ?

Miyake+ 2012, 2013

Hayakawa+ 2017 ApJL

Evidence of a superflare ?

Corresponding to 10^34 erg superflareIf this is due to a solar flare

(Miyake et al. Nature , 2012, June, 486, 240)

Another evidence ?

Another event!

Miyake 2012

Miyake 2013

AD993

AD775

From Miyake et al. (2013)Nature Communications 2783

Aurora observed in Nagoya, Japan Sep 17, 1770 (Hayakawa et al., ApJL, 2017)

The auroras wereObserved in manyPlaces of lowLatitudes in East Asia.

This magnetic storm may be comparable with or larger than the Carringtonevent

Large Spots Leading to 1770 aurora (Hayakawa et al., ApJL, 2017)

A = 6000msh

Hayakawa et al . (2017) ApJL

First ApJ Paper including Japanese characters !

Okayama 3.8m New Technology Telescope of Kyoto Univ

(under construction)

courtesy of Prof. Nagata (Department of Astronomy , Kyoto University)

High speed photometricand spectroscopicobservation of Transient objects

Gamma ray burstsExoplanetsStellar flares

(superflares)

First light ~ 2018

Spectroscopic Observations of Solar type stars causing superflares will be extremely important

New Technology1. Making Mirrors with

Grinding 2. Segmented mirror3. Ultra Light mounting

Budget for spectrographIs still lacking.Please support us !

Summary• Using Kepler data, we found many superflares (10^33-10^36

erg) on solar type stars (Maehara+ 2012, Shibayama+ 2013).

• Superflares occur on Sun-like stars with frequency such that superflares with energy 10^34-10^35 erg occur once in 800-5000 years, suggesting possibility of solar superflares(Shibata+ 2013, Nogami+ 2014）

• These stars have large star spot (Notsu+ 2013, 2015), which are observed in many Sun-like stars with low frequency (Maehara+ 2017).

• There are evidence of solar superflares in tree ring (14C) on 775 and 993 (Miyake+ 2012, 13) and old literatures on 1770 (Hayakawa+ 2017).

Thank you for your attention