Spot detection Spot detection on solar like starson solar like stars

Adriana V. R. SilvaCRAAM/Mackenzie

COROT 2005 01/11/2005

SunspotsSunspots

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.

TransitsTransits

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

Goal of accepted APGoal 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.

Extra-solar planetsExtra-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.

DataData

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

HST

ModelModel 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 SimulationTransit Simulation

HD 209458 transitHD 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 parametersSpot 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)HD209458 (Deeg et al. 2001)

Transit with spots without spots

Limb darkeningLimb darkening

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).

quadratic

linear

linearquadratic

221 )1()1(1

)1(

)( ww

I

I

Model starModel 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)HD209458 (Brown et al. 2001)

Transit with spots without spots

ResultsResults

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.

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

1exp

1exp

o

e

e

o

KTh

KTh

I

I

ConclusionsConclusions

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.

CoRoTCoRoTobservational requirements,observational requirements,

feasability, andfeasability, andexpectationsexpectations

Simulation resultsSimulation results

sunspot eclipse

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

Jupiter size Planet

phase

Rela

tive fl

ux

Rela

tive fl

ux

Stellar rotationStellar rotation

26 April 2000 29 April 2000

starspot

Rotation periodRotation period

Subtracting the lightcurve taken 3 days later, measure the f between the starspot position.

Rotation period of the star:

Ps=27.6 days

26th29th

phase

Rela

tive fl

ux

f

I(26th)-I(29th)

s

s

Ra

f

tP

SummarySummary 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)