Post on 22-Jun-2018
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