Figure 2.
ics of these detectors were a
big boon to our program.
Spectroscopic light curves
of WASP-12b (points)
and best-fit models
(lines) for the January
25th observations. The
numbers on the left side
give the wavelength
range of the channels in
units of nanometers.
The so-called “white” light
curves for WASP-12b —
made from the Gemini data
by summing over all of the
wavelengths — is shown in
Figure 1. The data exhibit an
unexpected instrument systematic. The effect is correlated with the rotation angle
of the Cassegrain instrument
support structure. (Note that
this angle changes smoothly
over the course of a transit
observation as the structure
rotates to keep the GMOS slit
mask aligned on the stars.)
The origin of this effect is unknown and is currently being investigated. However,
we found that it can be modeled (black lines in the top
two panels of Figure 1) and
removed from the data.
Figure 3.
Derived transmission
spectrum of WASP-12b
(blue circles with error bars)
with the transit depth (left
y-axis) and relative number
of scale heights (right
y-axis). The different lines
represent different models
for the planet’s atmosphere,
and the diamonds are the
models binned over the
data bandpasses. The red
line is a model with only
hydrogen and potassium.
The feature in this model
at 0.775 micron is due to
the potassium resonance
doublet. The black line is a
model for an atmosphere
that has solar elemental
abundances (i.e., oxygen
rich). The gold line is a
model with a carbonrich composition.
optical (720 1008 nanometers [nm]) and
-
took advantage of the new e2v deep depletion CCDs that had been installed in GMOS-N
just a few months beforehand. The excellent
red optical quantum efficiencies and cosmet-
In Figure 2, we show the corrected highprecision spectroscopic light curves that we
obtained with GMOS-N by binning the data
for the first night over 15-nm-wide spectral
channels. The measured transit depths vary
as a function of wavelength; this gives us
the planet’s transmission spectrum, which
in turn, tells us about its atmospheric
composition.
The transmission spectrum of WASP-12b
along with three atmospheric models is
shown in Figure 3. The GMOS-N data rule
out the possibility of an atmosphere with
only hydrogen and potassium. For an oxygen-rich atmosphere, the data can be explained by the presence of metal oxides.
However, the presence of metal hydrides
in a carbon-rich atmosphere can also explain the data.
10
GeminiFocus 2013 Year in Review
January2014