Spots and Flares:
Stellar Activity in
the Time Domain Era
James R. A. Davenport
University of Washington
Department of Astronomy
1
"If the Sun did not have a magnetic field,
it would be as uninteresting a star
as most astronomers believe it to be."
-R. B. Leighton
PhD Final Exam
2015-July-10
Slide 2
Slide 2 text
Spots and Flares:
Stellar Activity in
the Time Domain Era
2
First: a short story about spots, flares,
and magnetic activity
6
Raises many
important questions!
• How often do these giant spots appear?
• How long do they live?
• How often do huge flares happen?
• Could they affect life?
• What can spots & flares tell us about a star?
• Are spots & flares on other stars the same?
• How do they change over (astronomical) time?
Slide 7
Slide 7 text
Date
Latitude
7
11-year “butterfly” pattern
Slide 8
Slide 8 text
Differential Rotation Law
8
Rotation Period
(days)
Lat (deg)
0
45
10 15
90
Slide 9
Slide 9 text
9
Stellar Age versus Activity Level
Lyra (2005)
log Ca II Flux
log age (yrs)
ACTIVE!
inactive
Sun
Slide 10
Slide 10 text
10
Kepler
the greatest stellar
mission in decades
Slide 11
Slide 11 text
Topics to Cover
11
Part 1: Starspots
Part 2: Starspots + Transits
Part 3: Flares
Slide 12
Slide 12 text
12
Part 1: Starspots
NASA - SDO
Slide 13
Slide 13 text
13
Slide 14
Slide 14 text
14
Largest sunspot observed in “modern” times
Hoge 1947, Mt. Wilson Observatory
by-eye observation by G.W.
Slide 15
Slide 15 text
Strassmeier (1999)
• Observed across range of
mass, evolutionary phase
• Evolve on timescales from
days to years (perhaps longer!)
• Trace surface B field geometry,
rotation, differential rotation
Starspots:
a generic result of B fields
SDO
Carroll (2012)
15
Slide 16
Slide 16 text
– Stellar rotation rate
– Spot sizes
– Differential rotation rate
– Spot Lifetimes
– Stellar activity stellar cycles
– Evolution of spots with stellar age
16
Starspots:
Parameters/Physics of Interest
• Get rotation period, starspot sizes
• Map back to surface features (at least longitudes)
18
Phase / Longitude
Flux
Starspots with Photometry
Walkowicz+ (2010)
Slide 19
Slide 19 text
• Searching for 2nd order effects in light curve
Rotation Period
Starspot Size
19
Differential Rotation
Slide 20
Slide 20 text
20
4 days
GJ 1243 - M4
Prot=0.5926 days
Flux
Time
Slide 21
Slide 21 text
21
Starspot
Primary feature stable over years
“Shoulder” evolves on
~100 day timescales
4 days
Flux
Time
Slide 22
Slide 22 text
Longitude (deg) or Phase
Time
22
Phase-folded
light curve
over time
Starspot
4 days
Flux
Time
Slide 23
Slide 23 text
pixel
color
Phase–
Flux Map
Time
Phase-folded
light curve
over time
Davenport+ (2015)
23
Longitude (deg) or Phase
Spots moving
in longitude
Spot constant
in longitude
0 1
0 100 200 300 0.5
26
Time (days)
Longitude (deg)
0
360
180
-180
0
540
Chop light curve in to time windows
Davenport+ (2015)
Slide 27
Slide 27 text
Relative Flux
Phase
Fit for spot positions in each window
Rotation
Direction
Phase
27
Davenport+ (2015)
Relative Flux
Slide 28
Slide 28 text
28
Longitude (deg)
360
180
-180
0
540
Time (days)
Davenport+ (2015)
Each pair of points = window with full MCMC solution!
Slide 29
Slide 29 text
29
Longitude (deg)
360
180
-180
0
540
Time (days)
Davenport+ (2015)
Differential Rotation
“Equator-Lap-Pole” times of ~1500 days
10x slower than on Sun!
Slide 30
Slide 30 text
Longitude (deg)
360
180
-180
0
540
Time (days)
Spot lifetimes:150-500 days for 2nd spot
many years for 1st spot
30
Davenport+ (2015)
Part 2: Starspots + Transits
32
Lessons learned from GJ 1243 analysis:
• very hard to constrain latitude of starspots
• can only track 2 (maybe 3) starspots
• can only track very slow evolution
• how dark (cool) to make the spots?
Transits help with many of these problems!
Slide 33
Slide 33 text
33
Out-of-transit modulation
like for GJ 1243
The physical scenario,
many spots!
In-transit full of details!
Slide 34
Slide 34 text
Kepler 63
Sanchis-Ojeda (2013) Béky (2014)
Hat-P-11
Mapping starspots with transits, previous work
TrES-1
(oklo.org)
Rabus (2009)
34
Slide 35
Slide 35 text
Basics of Starspot “bumps”
darker (cooler) spots
35
Slide 36
Slide 36 text
darker (cooler) spots
70-80%
Agrees with solar umbra!
36
Slide 37
Slide 37 text
Phase
Relative Flux
37
Want to recover:
• starspot positions on surface
• differential rotation law
• starspot evolution timescales
First: Test our spot-fitting code using
simulated data!
Slide 38
Slide 38 text
38
Light curve model from Llama (2012),
based on “butterfly pattern”
Time (years)
Latitude (deg)
Flux
Time (years)
Plus solar-like differential rotation,
spot evolution, migration, diffusion…
Slide 39
Slide 39 text
39
Then add eclipsing planet
(hot Jupiter like)
Time (years)
Flux
Slide 40
Slide 40 text
40
A “Kepler” light curve based on Llama (2012)
Time
Flux
• 4 years of data
• 5 min samples
• 10 day rotation (equator)
• 2 day planet orbit
• rp/rs = 0.1
Slide 41
Slide 41 text
41
Use MCMC to fit every starspot
(longitude, latitude, radius) x nspots
planet orbit & stellar rotation fixed
Flux
Time
Slide 42
Slide 42 text
42
Repeat for every time window
Flux
Time
Slide 43
Slide 43 text
43
Longitude (deg)
slower than
mean period
faster
spot lifetime
Time (days)
rotating @ mean period
Goal: measure differential rotation & spot evolution
trace starspot longitudes with time
Slide 44
Slide 44 text
44
Longitude (deg)
slower than
mean period
faster
spot lifetime
Time (days)
rotating @ mean period
Goal: measure differential rotation & spot evolution
trace starspot longitudes with time
Slide 45
Slide 45 text
45
Longitude (deg)
Time (days)
Each set of points = full static MCMC solution!
Point color & size = radius
Slide 46
Slide 46 text
46
Longitude (deg)
Time (days)
Point color & size = radius
Slide 47
Slide 47 text
47
Longitude (deg)
Time (days)
Colors = cluster
grey points = no cluster
Use Python sklearn “DBSCAN” to cluster
DBSCAN = Density-based spatial clustering of applications with noise
Each represents 1 starspot
moving in longitude over time
Slide 48
Slide 48 text
Time (days)
48
Max slope = highest lat spot
Recovers simulated differential
rotation law coefficient, k=1
Longitude (deg)
Slide 49
Slide 49 text
49
Can recover starspot decay profiles!
Time (days)
Largest
sunspots
Work in Progress
related to diffusion timescale
solar decay
m
odel
Area
sunspot decay
Hathaway (2008)
Slide 50
Slide 50 text
50
now some real data!
Slide 51
Slide 51 text
51
Kepler 17 - G2 (same as our Sun!)
Prot= 12.1 days (faster than Sun)
Porb= 1.5 days (super fast)
Mass = 2.5 MJ
Rp/Rs = 0.13
Kepler 17b Désert et al. (2011)
Properties very similar
to the simulated
system!
Slide 52
Slide 52 text
Kepler 17: Examples of Spot Evolution
Every 8th Transit
Prot = 12.1 d
Porb = 1.5 d
52
Slide 53
Slide 53 text
Long Cadence
Short Cadence Short Cadence
Time (days)
Flux
53
Slide 54
Slide 54 text
54
Use MCMC to fit every starspot
(longitude, latitude, radius) x 8 spots
Flux
Time
Slide 55
Slide 55 text
Results from
Kepler 17
reminder: do this for
every time window!
55
Slide 56
Slide 56 text
Bump Evolution Matched!
Every 8th Transit
56
Slide 57
Slide 57 text
57
Longitude (deg)
Time (days)
Starspot evolution with time: more complicated!
Point color & size = radius
slower
faster
mean period
Slide 58
Slide 58 text
58
Longitude (deg)
Time (days)
For Kepler 17 find k=0.8
Compare with solar value of k=0.2
Slide 59
Slide 59 text
59
Lessons from starspots
• Starspots have contrasts similar to sunspots
• With transits, can fit many starspots simultaneously
• Track evolution of at least 100 spot groups over 4 years
• Estimate differential rotation law
• Decay profiles may constrain diffusion timescale
Slide 60
Slide 60 text
60
Flare!
SDO AIA 211
Part 3: Flares
Slide 61
Slide 61 text
Martens & Kuin (1989)
Standard Solar Flare Model
61
N
S
Sunspots
Slide 62
Slide 62 text
Martens & Kuin (1989)
62
Stellar Surface
Sunspots
N
S
Slide 63
Slide 63 text
Kepler: Stellar Flare Machine
• Long continuous light curves
(up to ~4years)
• Very precise photometry
(~0.01%)
• Enormous sample
(>100,000 solar-type stars)
• Complete samples of flares!
(impossible from ground)
• Huge range of flare energy!
(look for Carrington-like events)
63
Slide 64
Slide 64 text
Walkowicz+ (2010)
Flares Observed by Kepler
4 M-dwarfs
64
Slide 65
Slide 65 text
Time (days)
GJ 1243, M4
Prot=0.59 days, ~300days 1-min data
65
Davenport et al. (2014)
Lots of flares!
COLLECT
THEM ALL!
Slide 66
Slide 66 text
Flares By EYE (FBEYE)
66
Davenport et al. (2014)
github.com/jradavenport/FBeye
• Pick flare start/stop times
• Assign classifications
• Help train “autofinder”
Slide 67
Slide 67 text
Interesting to compare users
67
Davenport et al. (2014)
Time (days)
Flux
Slide 68
Slide 68 text
Large Flare Sample!
• 6107 unique flares, spanning 300 days of data
most for any star, besides the Sun!
• 15% flares are “complex”
higher % for large energy flares!
• big energy range: Log E = 28-33 erg
large solar flares around 1E32 erg
68
Hawley et al. (2014)
Davenport et al. (2014)
Slide 69
Slide 69 text
Flare Template: Study Morphology
69
Davenport et al. (2014)
885 “clean” flares
Time (FWHM)
Slide 70
Slide 70 text
Rise Phase 2 Decay Phases
exponentials
Fit with 4th order
polynomial
Energy budget:
rise=20%,
decay1=41%,
decay2=39%
70
Davenport et al. (2014)
Time (FWHM)
Time (FWHM)
Slide 71
Slide 71 text
Complex Flare Fitting
71
Davenport et al. (2014)
Time (days)
works well for
“classical” events
Slide 72
Slide 72 text
Complex Flare Fitting
Use to objectively determine
“complex” vs “classical” events
& decompose events!
72
Davenport et al. (2014)
Time (days)
Slide 73
Slide 73 text
Relative Flux
Some flares not well fit by template
Caused by different physical
morphology (e.g. arcade)?
Active region rolling off limb?
73
Davenport et al. (2014)
Time (days)
Slide 74
Slide 74 text
Hawley et al. (2014)
No correlation between flares & spot
1 month short cadence
74
Slide 75
Slide 75 text
HST
75
Kepler
John Lurie et al. (2015)
Flares from partially resolved
M5 + M5 binary in Kepler!
Extending work:
GJ 1245AB
Slide 76
Slide 76 text
Big questions still await us!
• Dependence of flare morphology
on stellar properties?
• Structure of complex events?
• “Triggered” flares?
“Sympathetic" flares?
• Frequency of “Superflares”?
• Flare rate vs age?
76
Slide 77
Slide 77 text
77
Slide 78
Slide 78 text
78
Age vs. Activity
Lyra (2005)
log Ca II Flux
log age (yrs)
ACTIVE!
inactive
Sun
More Flares
Fewer Flares
Slide 79
Slide 79 text
79
Magnetochronology:
train on Kepler/K2, apply to LSST
Slide 80
Slide 80 text
80
Can we recover solar cycles?
Llama et al. (2012)
Slide 81
Slide 81 text
81
Can we recover solar cycles?
Misaligned orbit/rotation samples more latitudes,
may be better!
Brett Morris et al. (2016)
Slide 82
Slide 82 text
Phase
Relative Flux
(5 min smoothing)
~2.5x
larger
Ongoing work by L.Hebb, M. Gomez,
J. Radigan, and P. McCullough
In vs Out of Transit
Scatter
The Future: study all transiting systems
82
Slide 83
Slide 83 text
83
Phase-tracking: many more stars possible in Kepler!
Slide 84
Slide 84 text
Summary
Use transiting exoplanets to trace
starspot motion & evolution
Measuring differential
rotation & spot lifetimes
for active stars
84
Now is the golden age for statistical studies of stellar activity
Largest sample of flares ever.
New insights on flare morphology
Slide 85
Slide 85 text
85
John R
Nick
Yumi
Eddie
Grace
Brett
John L
Diana
Nell
Practice Talk Guinea Pigs:
Thanks to the
Slide 86
Slide 86 text
New Mexico, the Land of Enchantment!
2007
2006
2010
2011
2012
2008
2014
2015
Slide 87
Slide 87 text
87
Thanks to my committee, collaborators,
and advisors. Especially:
Leslie
Suzanne &
Slide 88
Slide 88 text
88
Slide 89
Slide 89 text
89
Slide 90
Slide 90 text
Summary
Use transiting exoplanets to trace
starspot motion & evolution
Measuring differential
rotation & spot lifetimes
for active stars
90
Now is the golden age for statistical studies of stellar activity
Largest sample of flares ever.
New insights on flare morphology