Starspot Mapping of HAT-P-11
We only know as much about an exoplanet as we know about its host star, which is why I
collaborate with Professors
(HWSC), Suzanne Hawley (UW) and
Dr. James Davenport (WWU) on Kepler observations
of spotted host stars of transiting planets. Transits allow us to break degeneracies and locate
star spots on the surface of the star with precision from photometry alone.
The K4 dwarf HAT-P-11 hosts a hot Neptune planet, which we use as a mini-coronograph to study the
starspots of the host star. The spots are distributed in latitude similar to sunspots at solar activity
maximum, though the area covered by spots is 100x greater than the typical solar spot coverage.
Our analysis of starspots on HAT-P-11 has been published in the Astrophysical Journal,
read the paper here, or watch my presentation on HAT-P-11 at Cool Stars 20 here.
HAT-P-11 seems to have a Sun-like spot distribution, but is the overall amount of activity on HAT-P-11
normal among stars of its mass and rotation period/age? To find out, I calibrated the APO Echelle Spectrograph
to measure Mount Wilson S-indices of chromospheric activity. It seems HAT-P-11 is more active than planet hosts of
similar mass and rotation period, and we suggest that the tides induced by the close-in planet may be to blame.
Our analysis of the chromospheric activity of HAT-P-11 was recently
publishied in the Astrophysical Journal,
Reparameterizing the transit light curve with robin
If a star is sufficiently contaminated by bright or dark regions, the transit depth that we observe with photometry is likely not
equivalent to the square-root of the ratio of planet to stellar radii. However, the planet's radius is still encoded in the transit light curve
in the ingress and egress durations. In a paper recently accepted by AJ, we introduce a simple reparameterization
to the Mandel and Agol (2002) transit model which allows you to fit for the planet radius, even in the presence of significant contamination
by bright or dark regions on the stellar surface.
Ground-based photometry of HAT-P-11 b
As the stellar activity cycle of HAT-P-11 progresses, I am monitoring HAT-P-11's starspots from the ground with the ARC 3.5 m Telescope at APO, using a holographic diffuser for precision photometry (see details here). In this Research Note, we find that HAT-P-11 was more spotted than ever before in late 2017.
Spotting activity with Gaia Astrometry
As spots rotate into and out of view, the apparent centroid of the star appears to shift by tiny fractions of a stellar radius.
The Gaia mission will measure stellar centroids with sufficient precision to resolve centroid shifts due to activity. We build a
small sample of candidate targets for astrometric activity observations with Gaia in a paper recently accepted by MNRAS.
The plot above shows reconstructed observations of the Sun from Mount Wilson Observatory near solar maximum, and the associated
photocenter shifts due to spots and rotation.
The Stellar Variability of TRAPPIST-1
By comparing the out-of-transit rotational variability of TRAPPIST-1 in the Kepler and Spitzer bandpasses, we can
constrain the properties of its starspots. In a paper recently accepted by ApJ, we show that bright active regions
can explain the variability observed with both K2 and Spitzer. We also find that flares seem correlated with increasing stellar brightness,
perhaps suggesting that the variability in the Kepler band is not a rotational signal after all.