Spot detection on solar like stars

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Spot detection on solar like stars

• Adriana V. R. Silva
• CRAAM/Mackenzie

COROT 2005 01/11/2005
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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.

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Transits

• Mercury transit on November 15, 1999, that
  lasted about 1 hour.

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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 planet-ary
  transits is proposed.
• Observations of HD 209458 are used to test the
  model.
• Silva, ApJ Letters, 585, L147-L150, 2003.

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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.

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Data

• Two observations with bumps in the light curve
  were used
• Deeg et al. (2001) Brown et al. (2001)
  - HST

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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)

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Transit Simulation
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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, i86,68.
• 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 i86,68).
• 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.

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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.

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HD209458 (Deeg et al. 2001)
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Limb darkening
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?).

linear
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Model star

• Star represented by a quadratic limb darkening
  with w10.2925 and w20.3475 (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.

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HD209458 (Brown et al. 2001)
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Results
Rp9.4 104 km

• Starspot temperature, T0, estimated from
  blackbody emission, where Te is the stellar
  surface temperature assumed to be 600050 K
  (Mazeh et al. 2000)
• Starspot temperatures between 4900-5000 K.

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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.

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CoRoTobservational requirements,feasability,
andexpectations
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Simulation results

• Small variations in the lightcurve during the
  planetary transit caused by the planet
  occultation of starspots.
• Uncertainty of 0.0001 in flux.

Relative flux
Jupiter size Planet
Relative flux
1.5 Earth size Planet
phase
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Stellar rotation
26 April 2000
29 April 2000
starspot
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Rotation period

• Subtracting the lightcurve taken 3 days later,
  measure the ?f between the starspot position.
• Rotation period of the star
• Ps27.6 days

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Summary

• Core programme data
• Observations of planetary transits with
• ?I/I0.0001
• 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)