Observations and Modelling of WASP-19

Transits and StarspotsJeremy Tregloan-Reed

Ph.D. Research Student

jtr@astro.keele.ac.uk

Supervisor:John Southworth

High Precision PhotometryOrbital Based

PhotometryGround Based

Defocused PhotometryCredit:

NASACredit: ESO

WASP-50 An Example of Ground Based Defocused Photometry

Collected from the ESO NTT La Silla, Chile.

Highest achieved ground based photometric precision to date.

(Tregloan-Reed et al. In Preparation)

High Precision Transit Photometry Around Active Stars

A Transit of WASP-19 modelled using JKTEBOP. The transit parameters were fixed to the values found by Hellier et al. (2011).

The transit data points suggest that the planet/star radii ratio is greater than what the model predicts.

This change in transit depth is caused by the change in overall flux from the star, caused by the starspot.

High Precision Transit Photometry Around Active Stars

Kepler data of HATP-11. The red data points indicate potential star spot transit events.(Sanchis-Ojeda et al. 2011)

PRISMPRISM (Planetary Retrospective Integrated

Starspot Model) uses a pixellation approach to create the modelled star on a 2D Cartesian array.

Allows us to model the transit, limb darkening and starspots on the stellar disc simultaneously.

PRISM is set to use the standard quadratic limb darkening law and uses the stellar and planetary radii scaled by the semi major axis rs,p = Rs,p /a.

PRISMPRISM requires 10

parameters to model a planetary transit with a single starspot.

Output model of a transit coupled with a starspot using PRISM. The transit cord is represented by the two horizontal black lines. The dark disk to the left is the planet. A starspot has been placed on the curved stellar surface to show how PRISM projects a elliptical shape onto the stellar disc for a circular spot.

Fitting PRISM to the WASP-19 DataWe used GEMC (Genetic Evolution Markov Chain) to fit PRISM to the WASP-19 data sets.

The best fitted models are shown to the left together with the transit data.

Using the best fitting parameters and literature spectroscopic values we find:-

Stellar Mass = 0.92 +/- 0.04 MSun

Planet Mass = 1.12 +/- 0.03 MJup

Separation = 0.016 +/- 0.002 AUStellar Radius = 0.99 +/- 0.02 RSun

Planet Radius = 1.38 +/- 0.02 RJup

Planet Equ Temperature = 2058 +/- 24 K

This agrees with both Hebb et al. (2010) and Hellier et al. (2011)

Modelled Starspots

Surface plot of the function used to test GEMC. N=9 to generate 81 local maxima.

Output models of the starspots on the stellar disc for the two datasets containing spot anomalies.

Stellar Rotation PeriodWith the two sets of positions for the spot known it was

possible to calculate the latitudinal rotation period.

The latitudinal rotation period was found to be (10.4 ± 0.1) days. Hebb et al. (2010) found the rotation period to be (10.5 ± 0.2) days.

When coupled with the star’s radius we can calculate the surface velocity to be:-

v = (4.82 ± 0.15) km s-1

Sky-Projected Spin-Orbit AlignmentThe position of the spot at two different time periods

allows us to determine the alignment between the star’s rotation axis and the planet's orbit in the plane of the sky λ.

We calculate that λ = (1.0 ± 1.5)°. While Hellier et al. (2011) found λ = (4.6 ± 5.2)°.

This agrees with previous results but is more precise.

WASP-19 Planetary System EvolutionWinn et al. (2010) showed that a low obliquity

would follow the idea that the planet formed many AU away from the host star and through tidal interactions with the interplanetary disc the orbit of the planet decayed.

While a large obliquity would seem to suggest that the orbital decay was by gravitational interactions from other bodies in the system (Schlaufman 2010).

Future Work When a precise value for v sin(I) has been

determined it would be possible to find the inclination of the stellar spin axis towards the observer I.

If enough observations of the planet crossing the starspot are made, it would be possible to make a direct measurement of the spin-orbit alignment of the system ψ.

If λ is large it can be possible to map out rotation periods at different latitudes across the star and hence measure the differential rotation function of the star.

SummaryPRISM can simultaneously model the planetary transit, limb

darkening and starspots. To allow us to correctly measure the system parameters.

High precision photometry can allow us to drive down the uncertainties of the parameters of the transiting planetary system.

If the host star is active, then with enough transit data sets it is possible to measure the rotation period and sky-projected spin-orbit alignment of the system, without the need to rely on spectrographs.

Because of this high precision photometry can help us better understand the planetary system’s evolution.