Adriana V. R. Silva CRAAM/Mackenzie COROT /11/2005

 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.

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

 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 are used to test the model.  Silva, ApJ Letters, 585, L147-L150, 2003.

 169 planets detected presently.  9 transiting: HD , TrES-1, OGLE-10, 56, 111, 113, 132, HD , HD  Data from HD : –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.

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

 Star  white light image of the Sun  Planet  opaque disk of radius r/R s  Transit: at each time the planet is centered at a given position in its orbit (a orb /R s and i)  calculate the integrated flux  Search in parameter space for the best values of r /R s, a orb /R s, and i (minimum  2)

 Planet in a circular orbit around HD with a period of days, major semi-axis of AU, and inclination angle, i=86,68.  Planet radius = R Jup, and stellar radius = R Sun.  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.

 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.

Transit with spots without spots

 HST data (Brown et al. 2001) is not well fit by the model, indicating that the limb darkening of HD is not a linear function of , as that of the Sun, instead it is best described by a quadratic function (  =cos  ). quadratic linear quadratic

 Star represented by a quadratic limb darkening with w1= and w2= (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.

Transit with spots without spots

 Starspot temperature, T 0, estimated from blackbody emission, where T e is the stellar surface temperature assumed to be K (Mazeh et al. 2000):  Starspot temperatures between K. SPOTS 26-jul apr-2000 Radius (R p ) Intensity (I star ) Distance to transit line (R p ) R p = km

 This method enables us to estimate the starspots physical parameters.  From modeling HD data, we obtained the starspots characteristics: –sizes of km, being larger than regular sunspots, usually of the order of km (probably a group of starspots, similar to solar active regions). –temperatures of K, being hotter than regular sunspots ( K), however the surface temperature of HD , 6000K, is also hotter than that of the Sun (5780K). Nevertheless, the sunspots seen in the white light image are also about of the solar disk center intensity, similarly to what was obtained from the model. –Location latitude.

sunspot eclipse  Small variations in the lightcurve during the planetary transit caused by the planet occultation of starspots.  Uncertainty of ~ in flux. 1.5 Earth size Planet Jupiter size Planet phase Relative flux

26 April April 2000 starspot

 Subtracting the lightcurve taken 3 days later, measure the  f between the starspot position.  Rotation period of the star:  P s =27.6 days 26 th 29 th phase Relative flux ff I(26 th )-I(29 th )

 Core programme data;  Observations of planetary transits with: –  I/I~ –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)