Transcript Spot detection on solar like stars

Spot detection
on solar like stars
Adriana V. R. Silva
CRAAM/Mackenzie
COROT 2005 01/11/2005
Sunspots
 Regions of high concentration of magnetic fields;
 Indicators of magnetic activity cycle;
 Understanding of solar activity:
– solar flares, coronal mass ejections, etc;
 Currently it is not possible to detect, let alone monitor
the behavior of solar like spots on other stars due to
their very small sizes.
Transits
 Mercury transit
on November 15,
1999, that lasted
about 1 hour.
Goal of accepted AP
 During one of its transits,
an exoplanet may pass in
front of a stellar group of
spots.
 A method for studying the
physical characteristics of
starspots based on planetary transits is proposed.
 Observations of HD 209458
are used to test the model.
 Silva, ApJ Letters, 585,
L147-L150, 2003.
Extra-solar planets
 169 planets detected presently.
 9 transiting: HD 209458, TrES-1, OGLE-10,
56, 111, 113, 132, HD 189733, HD 149026.
 Data from HD 209458:
– April 25, 2000 (Brown et al. 2001) with the Hubble
Space Telescope (HST);
– July 26, 2000 (Deeg et al. 2001) with the 0.9
telescope of the Observatorio Sierra Nevada.
Data
 Two observations with “bumps” in the light
curve were used:
Deeg et al. (2001)
Brown et al. (2001) - HST
Model
 Star  white light image of
the Sun
 Planet  opaque disk of
radius r/Rs
 Transit: at each time the
planet is centered at a
given position in its orbit
(aorb/Rs and i)  calculate
the integrated flux
 Search in parameter space
for the best values of r /Rs,
aorb /Rs, and i (minimum 2)
Transit Simulation
HD 209458 transit
 Planet in a circular orbit around HD 209458 with
a period of 3.5247 days, major semi-axis of
0.0467 AU, and inclination angle, i=86,68.
 Planet radius = 1.347 RJup, and stellar radius =
1.146 RSun.
 The planet is represented by an opaque disk
that crosses the stellar disk at 30.45° latitude
(corresponding to i=86,68).
 The planet position is calculated every two
minutes.
 Lightcurve intensity at every two minutes is the
sum of all the pixels values in the image.
Spot parameters
The spots were modeled by three
parameters:
Intensity, as a function of stellar
intensity at disk center (max);
Size, as a function of planet radius;
Position, as a distance to the transit line
in units of planet radius.
HD209458 (Deeg et al. 2001)
Transit
with spots
without spots
Limb darkening
quadratic
linear
linear
quadratic
 HST data (Brown et al.
2001) is not well fit by
the model, indicating that
the limb darkening of
HD209458 is not a linear
function of , as that of
the Sun, instead it is best
described by a quadratic
function (=cos).
I ( )
 1  w1 (1   )  w2 (1   ) 2
I (1)
Model star
 Star represented by a
quadratic limb darkening
with
w1=0.2925
and
w2=0.3475 (Brown et al.
2001).
 Spot modeled by three
parameters:
– Intensity, as a function
of stellar intensity at
disk center (max);
– Size, as a function of
planet radius;
– Position, as a distance
to the transit line in
units of planet radius.
HD209458 (Brown et al. 2001)
Transit
with spots
without spots
Results
SPOTS
26-jul-2000
25-apr-2000
Radius (Rp)
0.4-0.6
0.3-0.4
Intensity (Istar)
0.4-0.6
0.5-0.7
Distance to transit
line (Rp)
0.5-0.8
0.7-0.9
Rp=9.4 104 km
 Starspot temperature, T0, estimated from blackbody
emission, where Te is the stellar surface temperature
assumed to be 6000+50 K (Mazeh et al. 2000):
 Starspot temperatures between 4900-5000 K.
 h 
  1
exp 
Io
 KTe 

Ie
 h 
  1
exp 
 KTo 
Conclusions
 This method enables us to estimate the starspots
physical parameters.
 From modeling HD208458 data, we obtained the
starspots characteristics:
– sizes of 3-6 104 km, being larger than regular sunspots,
usually of the order of 11000 km (probably a group of
starspots, similar to solar active regions).
– temperatures of 4900 - 5500 K, being hotter than regular
sunspots (3800-4400K), however the surface temperature of
HD 209458, 6000K, is also hotter than that of the Sun
(5780K). Nevertheless, the sunspots seen in the white light
image are also about 0.4-0.7 of the solar disk center intensity,
similarly to what was obtained from the model.
– Location latitude.
CoRoT
observational requirements,
feasability, and
expectations
Simulation results
Jupiter size Planet
Relative flux
sunspot
eclipse
Relative flux
Small variations in the lightcurve during the planetary
transit caused by the planet occultation of starspots.
Uncertainty of ~0.0001 in flux.
1.5 Earth size Planet
phase
Stellar rotation
26 April 2000
29 April 2000
starspot
 Subtracting the
lightcurve taken 3
days later, measure
the f between the
starspot position.
 Rotation period of
the star:
Ps 
Relative flux
Rotation period
26th 29
th
I(26th)-I(29th)
f
t
 a 
f  
 Rs 
 Ps=27.6 days
phase
Summary
 Core programme data;
 Observations of planetary transits with:
– I/I~0.0001
– Temporal resolution of few minutes
 Results expected:
– Starspot characteristics (size, temperature, location,
evolution);
– Starspot structure for Earth size planets;
– Limb darkening  temperature gradient of the stellar
photosphere;
– Stellar rotation (solar-like stars: 150 days ~ 5 periods)
 Extra:
– Differential rotation (planets at different latitudes);
– Activity cycles (for short cycles)