Transits and Starspots
Jeremy Tregloan-Reed
Ph.D. Research Student
jtr@astro.keele.ac.uk
Supervisor:
John Southworth
High Precision Photometry
Orbital Based
Photometry
Credit: NASA
Ground Based Defocused
Photometry
Credit: 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)
PRISM
 PRISM (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.
PRISM
 PRISM 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 Data
We 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 AU
Stellar 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 data
sets containing spot anomalies.
Stellar Rotation Period
 With 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 Alignment
 The 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
Evolution
 Winn 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.
Summary
 PRISM 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 spinorbit 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.