A ground-based Ks-band detection of the thermal emission from the transiting exoplanet WASP-4b

Abstract

Secondary eclipses are a powerful tool to measure directly the thermal
emission from extrasolar planets, and to constrain their type and physical
parameters. We started a project to obtain reliable broad-band measurements of
the thermal emission of transiting exoplanets. Ground-based high-cadence
near-infrared relative photometry was used to obtain sub-millimagnitude
precision light curve of a secondary eclipse of WASP-4b -- a 1.12 M_J hot
Jupiter on a 1.34 day orbit around G7V star. The data show a clear ~10-\sigma
detection of the planet's thermal emission at 2.2 \mu m. The calculated thermal
emission corresponds to a fractional eclipse depth of 0.185^{+0.014}_{-0.013}%,
with a related brightness temperature in Ks of T_B = 1995 \pm 40 K, centered at
T_C = 2455102.61162^{+0.00071}_{-0.00077} HJD. We could set a limit on the
eccentricity of e cos \omega=0.0027 \pm 0.0018, compatible with a near-circular
orbit. The calculated brightness temperature, as well as the specific models
suggest a highly inefficient redistribution of heat from the day-side to the
night-side of the planet, and a consequent emission mainly from the day-side.
The high-cadence ground-based technique is capable of detecting the faint
signal of the secondary eclipse of extrasolar planets, making it a valuable
complement to space-based mid-IR observations.

Full text

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arXiv:1104.0041v1 [astro-ph.EP] 31 Mar 2011
Astronomy & Astrophysics manuscript no. paper2.astroph
April 4, 2011
c ? ESO 2011
A ground-based KS-band detection of the thermal emission from
the transiting exoplanet WASP-4b⋆
C. C´ aceres1,2, V. D. Ivanov2, D. Minniti1, A. Burrows3, F. Selman2, C. Melo2, D. Naef4, E. Mason5, and G. Pietrzynski6
1Department of Astronomy and Astrophysics, P. Universidad Cat´ olica de Chile. Av. Vicu˜ na Mackenna 4860, 7820436 Macul,
Santiago, Chile
2European Southern Observatory, Av. Alonso de C´ ordova 3107, Casilla 19, Santiago 19001, Chile
3Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
4Observatoire de Gen` eve, Universit´ e de Gen` eve, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
5ESA-STScI, 3700 San Martin Drive, Batlimore, MD 21218, U.S.A.
6Department of Astronomy, Universidad de Concepci´ on, Casilla 160-C, Concepci´ on, Chile
ABSTRACT
Context. Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain
their type and physical parameters.
Aims. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets.
Methods. Ground-based high-cadence near-infrared relative photometry was used to obtain sub-millimagnitude precision light curve
of a secondary eclipse of WASP-4b – a 1.12MJhot Jupiter on a 1.34day orbit around G7V star.
Results. The data show a clear ≥10σ detection of the planet’s thermal emission at 2.2µm. The calculated thermal emission corre-
sponds to a fractional eclipse depth of 0.185+0.014
TC = 2455102.61162+0.00071
circular orbit.
Conclusions. The calculated brightness temperature, as well as the specific models suggest a highly inefficient redistribution of heat
from the day-side to the night-side of the planet, and a consequent emission mainly from the day-side. The high-cadence ground-based
technique is capable of detecting the faint signal of the secondary eclipse of extrasolar planets, making it a valuable complement to
space-based mid-IR observations.
−0.013%, with a related brightness temperature in KS of TB= 1995 ± 40K, centered at
−0.00077HJD. We could set a limit on the eccentricity of ecosω = 0.0027 ± 0.0018, compatible with a near-
Key words. Planetary systems - Eclipses - Stars: individual: WASP-4 - Techniques: photometric
1. Introduction
Photometryofextrasolarplanetarytransits allowstomeasurethe
planetary radii and density, and to infer the nature of the planet -
gaseous or rocky. The detection of the thermal emission during
the secondary eclipses when the planet passes behind the star
givesaccess toadditionalphysicalparameterslike thebrightness
temperature,animportantquantityforthecomparisonwithmore
sophisticated planetary atmospheric models with chemistry, and
dynamics (i.e. Burrows et al. 2008). While a few secondary
eclipses have been secured from space (e.g. Charbonneau et al.
2005; Deming et al. 2005), the detection of secondary eclipses
has been very challenging from the ground, with only a
few recent secure detections (see Sing & L´ opez-Morales 2009;
Rogers et al. 2009; Croll et al. 2010a,b; and references therein).
We obtained sub-millimagnitude precision ground-based rela-
tive photometry, based on ultra-fast high-cadence near-infrared
(NIR) observations,which can detect transits down to a few mil-
limagnitudes (C´ aceres et al. 2009; hereafter Paper I). In this pa-
per we report the detection of a secondary eclipse of WASP-4b.
WASP-4b was discovered by Wilson et al. (2008). It is
a 1.12MJ hot Jupiter on a 1.34day orbit around a G7V
star, with a heavily irradiated atmosphere and large radius.
⋆Based on observations collected at the European Southern
Observatory, Chile. Programme 083.C-0528.
Correspondence to: cccacere@astro.puc.cl
Additional orbital and physical parameters were measured also
by Gillon et al. (2009), Winn et al. (2009), and Southworth et al.
(2009).Recently,Beerer et al.(2011)performedphotometry,us-
ing the IRAC instrument on the Warm Spitzer, of the planet
WASP-4b in the 3.6 and 4.5µm bands. Their data suggests the
WASP-4b atmosphere lacks a strong thermal inversion on the
day-side of the planet, an unexpected result for an highly irradi-
ated atmosphere.
WASP-4b was one of the pilot targets for our program be-
cause of its short period and large size with respect to most
transiting exoplanets, with a relatively large predicted secondary
eclipse amplitude of about a millimagnitude, placing it well
within our expected range of detection. In Section 2 we discuss
the observations and their analysis. Section 3 presents the detec-
tion of the secondary eclipse, and gives the measured brightness
temperatureforWASP4-b,comparingit withtheoreticalmodels.
Finally, Section 4 summarizes the results and lists some conclu-
sions.
2. Observations and data reduction
The KS band WASP-4b observations were obtained with
the Infrared Spectrometer And Array Camera (ISAAC
Moorwood et al. 1998) at the ESO VLT, on September 27,
2009. The observations were carried out in the FastPhot
mode, as in Paper I. The InSb 1024×1024px Aladdin detec-

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2C. C´ aceres et al.: KS-band thermal emission from WASP-4b.
Fig.1. Upper panel: The range of airmass covered during the
run. Mid and bottom panels: The variation around the median
values of the position of the centroid of the stellar psf on the de-
tector. The positions of those images discardedbecause of point-
ing jumps whose amplitude is larger than the plotted range are
not shown, for the sake of clarity.
tor was windowed down to 304×608px, yielding a field of
view of ∼45×90arcsec. The field of view includes WASP-
4 and one reference star – R.A.233418.4, Dec.-420451.0
(J2000). 2MASS (Skrutskie et al. 2006) lists for them respec-
tively: J=11.2, H=10.8,and KS=10.7mag and J=11.7, H=11.3,
and KS=11.2mag.
This observation mode generates a series of data-cubes with
a fixed number of frames. Each frame is an individual detector
integration, and there is nearly zero “dead” time between the
integrations.The cubes are separated by an interval of ∼6sec for
fits header merging, file transfer, and saving on a hard disk. For
these observations the cubes had 250 frames, corresponding to
integrations of 0.6sec each. In total, we collected 30,003 frames
during a continuous 310min long run.
Standard procedures for NIR data reduction as dark subtrac-
tion and flat fielding were performedon the data set. We applied
aperturephotometry,usingtheIRAF1packageDaophot,tomea-
sure the apparent fluxes of the two stars in the field. An aper-
ture radius of 1.92arcsec was selected empirically to have the
best compromise between the r.m.s. on the transit light curve,
determined as ratio of the target-vs-reference fluxes, and the
systematic noise introduced by a varying sky. The sky back-
ground was estimated in a circular annulus with an inner radius
of 3.10arcsec and outer radius of 7.5arcsec, from the centroid
of the point-spread function.
Occasionally, during the observations the central peak of the
stars exceeded the linearity limit of the detector of ∼6500ADU.
To avoid the non-linearityeffects that may distort the occultation
depth measurement we discarded those frames. Moreover,at the
end of the run the detector was moved yielding in jumps in the
position of the stars. All the points after the first jump in posi-
tion were also discarded. The starting points showed also a jump
that could not be corrected, and these points were also removed
to avoid including a systematic error to the determined param-
eters. After cleaning the light curve, we leave a total of 24,070
measurements, that were used during the subsequent analysis.
The light curve shows a smooth trend during the period of
observation, most likely due to airmass variation, important dur-
1IRAF
Observatories, which are operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with the
National Science Foundation.
isdistributed bythe NationalOpticalAstronomy
Fig.2. The effect of rednoise on the obtained light curve is
shown. The total noise for a given bin (solid line), is com-
pared with the expected Poisson noise (dashed line). The resid-
ual points for the complete light curve were used in this calcula-
tion.
ing the first half of the night, and due to target drift across the
detector, which dominated the end of the run, as shown in Fig.
1. To remove these effects we modeled the base-line of the light
curve with a polynomial that includes a linear dependency on
time (t), airmass (secz), and positions of the centroid of the star
on the detector (xCand yC) as follows:
fBline= a0+ a1t + a2sec z + a3xC+ a4yC.
(1)
The coefficients in Eq. 1 were determined using the en-
tire light curve, with the fitting procedure described below. The
form of the polynomial was selected to flatten the out-of-eclipse
lightcurve after multiple experiments. We also attempted to in-
cludeotherparameters,i.e.thefull-width-half-maximumofstars
on each frame, but the correlation of the signal with this param-
eter was negligible.
3. Analysis and discussion
Various parameters can be measured from the secondary eclipse
lightcurve.Thethermalplanetaryemissionis proportionaltothe
eclipse depthd, the orbitaleccentricitye andargumentofperias-
tron w could be inferred through the analysis of both the eclipse
central time TC, and the secondary eclipse length τocc. We create
anoccultationmodelfollowingthealgorithmforuniformillumi-
nated transit light curve (i.e. neglecting the limb-darkening con-
tribution) from Mandel & Agol (2002), and we scaled the model
transit depth to fit the observed eclipse depth. The stellar and
planetary parameters for WASP-4 and WASP-4b were adopted
from Winn et al. (2009): period P=1.33823214d, planet-to-star
radius ratio p=0.15375,semi-major axis a=5.473R⋆, orbital in-
clination i=88.56deg, and stellar radius R⋆=0.912R⊙.
Past experience with NIR data showed that correlated noise
could degrade photometric accuracy in these wavelengths. To
evaluatethecontributionofthis kindof noiseonourphotometry,
we analyzed the behavior of the light curve r.m.s. for different
binning factors, after removingthe best-fitting model. It shows a
small deviation from the expected curve for pure Poisson noise
(Fig.2)overrangeofbinningcorrespondingtotimesoftheorder
of tens of minutes – similar to the duration of the ingress-egress
for the secondary eclipse.
The best fitting model parameters were found with the mul-
tidimensional minimization algorithm AMOEBA (Press et al.
1992), where the free parameters were the eclipse depth, cen-
tral time, length, and the coefficients of the base-line in Eq. 1,

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C. C´ aceres et al.: KS-band thermal emission from WASP-4b.3
Fig.3. The upper panel shows the 2-min bin uncorrected light
curve, with the overlaid best fitting model considering the oc-
cultation model and the base-line model. The corrected KSlight
curve of WASP-4b is shown in the middle panel, including the
complete data set, where the resulting rms in the out-of-eclipse
pointsis 0.0069.Inthebottompaneleachdatapointcorresponds
to a 2-minute bin, along with its error, and the best-fitting oc-
cultation model. The residuals for this curve are shown with an
offset of 0.006, and present a r.m.s. of 0.00072. A color version
of this figure is available in the electronic form.
on the 2-min binned light curve, yielding 131 data points. The
function to minimize was the χ2statistic, for a model that is:
mod = mocc× fBline,
(2)
where mocccorresponds to the occultation model described
above. To determine the errors in the fitted parameters, we per-
formedthebootstrappingproceduredescribedinPaperI:wefirst
subtracted the best fitting model from the data set, then we took
thesetofresidualsandshiftedtheithresidualtobecomethei+1th
residual, and the last to become the first. Next, we added the re-
ordered residuals to the best fitting model and run it through the
χ2minimizationprocedureinsteadof thedata toobtaina newset
of fitting results. This procedure was repeated until a full circle
over the “good” points was completed, and the reported uncer-
tainties were defined by the 68.3% level around the median val-
ues of the distributions. Therefore, our errors properly account
for the correlated noise in the data set.
The final corrected light curve is shown in Fig. 3. Table 1
lists the individual flux measurements, with their time-stamps
andPoissonuncertainties.Theresultingparametersandtheirun-
certainties for WASP-4b are listed in Table 2.
ThesignificanceofourdetectioncanbeassessedfromFig.4,
where the distribution of the relative flux (with respect to the
reference star) during the eclipse and outside of the eclipse are
compared. The bin width is equal to the calculated eclipse depth
d = 0.185%, and each distribution curve was normalized for
comparison purposes. A K-S test applied on these distributions
shows they are identical with a ∼ 99% of probability, but dis-
placed exactly one bin.
Fig.4. Distribution of the flux during the eclipse (dashed line)
andoutoftheeclipse(solidline).Thebinwidthis 0.185%,equal
to the calculated eclipse depth.
Table1.RelativephotometryofWASP-4b.Extradigitsaregiven
to avoid round up errors.
HJDRelative flux
0.99951
0.99732
1.00339
0.99452
1.00043
Uncertainty
0.00427
0.00427
0.00421
0.00424
0.00426
2455102.5150000
2455102.5150069
2455102.5150138
2455102.5150277
2455102.5150347
Only a small portion of the data set is presented in this
table, to exemplify its presentation format. The complete
set can be found in the electronic version of the Journal.
Table 2. Derived parameters of WASP-4b.
Parameter Value 68.3% Confidence LimitsUnit
TC
d
τocc
ecosω
TB
2455102.61162
0.185
0.0881
0.0027
1995
-0.00071, +0.00077
-0.014, +0.013
-0.0017, +0.0022
± 0.0018
± 40
HJD
%
d
-
K
Base-line parameters
1.6933
-0.0364
-0.0112
-0.00025
-0.00175
a0
a1
a2
a3
a4
-0.0169, +0.0214
-0.0027, +0.0042
-0.0014, +0.0013
-0.00074, +0.00062
-0.00060, +0.00078
The center of the secondary eclipse occurs at φ = 0.49933±
0.00059, assuming the ephemeris of Southworth et al. (2009).
For zero eccentricity (Wilson et al. 2008; Gillon et al. 2009;
Winn et al. 2009; Southworth et al. 2009), and a light travel time
of 23.2sec for this system (Loeb 2005), the secondary eclipse
is expected to occur at phase φexp = 0.5002. The phase dif-
ference δφ implies a non-zero eccentricity, that could be deter-
mined knowingthe primary transit length τtr, from the equations
(Charbonneau et al. 2005):
ecosω ≃ π δφ ,
(3)
esinω ≃τtr− τocc
τtr+ τocc
.
(4)

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4C. C´ aceres et al.: KS-band thermal emission from WASP-4b.
The accuracy of our eclipse length measurement prevents us
from using Eq. 4, so we only use the former equation, to put a
upperlimit onthe eccentricityecosω = 0.0027±0.0018.Within
the 3-σ level, this results argues in favor of a nearly circular
orbit.
Previous planet-to-star contrast measurements for WASP-
4b have been obtained by Beerer et al. (2011) in the 3.6 and
4.5µm broad band filters with the Spitzer Space Telescope.
As a zero-order approximation, we represent the planet with
a black body with temperature TB, and the host star with a
Hauschildt et al. (1999) stellar atmosphere model, with param-
eters Tef f = 5500K, logg = 4.5, and [Fe/H]=0.0 (Gillon et al.
2009). They are divided to obtain the planet-to-star flux con-
trast as a function of wavelength. The expected depth was calcu-
lated as weighted average of the contrast curve, weighted by the
KS filter transmission curve, the atmospheric transparency, and
the detector efficiency curve. The best-fitting black body curve
yielded a planetary brightness temperature of TB= 1995± 40K
at ∼2.2micron.
The observational data of highly irradiated planets indicate
very low Bond albedo ABvalues (e.g. Charbonneau et al. 1999;
Rowe et al. 2008; Rogers et al. 2009), implying that the atmo-
spheres of those planets are highly heated by the stellar ra-
diation, and probably bloated. The temperature at which the
planet re-emits the energy absorbed from the stellar flux, at a
given distance a from the star, in units of the stellar radius, is
given by Teq= Teffa1/2(f(1 − AB))1/4. Here, the f factor refers
to the fraction of energy that is re-radiated from the planet to
the observer. If the stellar radiation absorbed by the planet is
re-radiated isotropically then f = 1/4, and we could deter-
mine for a zero Bond albedo an equilibrium temperature for
the planet of Teq = 1662K. The difference between the equi-
librium and the brightness temperatures suggests a poor energy
redistribution in the atmosphere of WASP-4b. In fact, to ob-
tain the calculated brightness temperature the re-radiation fac-
tor should be f = 0.52, indicating a planet with re-radiation
only from the day-side (Burrows et al. 2008), when assuming
a zero Bond-albedo. This result is in agreement with the results
in Beerer et al. (2011), who found that the Fortney et al. (2008)
models with a small redistribution of heat, and an absence of
TiO in the WASP-4b stratosphere,are best fits for their measure-
ments.
Planetary atmosphere models tuned for WASP-4b are shown
in Fig. 5. The modeled atmospheres assume radiative and
chemical equilibrium, and employ the chemical composi-
tions and thermo-chemistry found in Burrows & Sharp (1999),
and Sharp & Burrows (2007), and the opacities described in
Sharp & Burrows (2007), and references therein. These mod-
els were calculated for different redistribution factors Pn, where
Pn= 0 implies no redistribution, and Pn= 0.5 means maximum
redistribution (Burrows et al. 2008). This parameter represents
the fraction of energy that is transferred from the day-side to the
night-side of the planet. Moreover,some highly irradiated atmo-
spheresrequiretheinclusionofanextraabsorbertofit calculated
eclipse depths, this accounting for the presence of inversion lay-
ers in the upper atmosphere (Burrows et al. 2007; Knutson et al.
2009). This scenario was tested by introducing an opacity pa-
rameter κefor this extra absorber as a second parameter to ex-
plore.
Our observations favor lower PNparameter values, arguing
in favor of an inefficient redistribution of the received radiation
from the day-side to the night-side of the planet, as shown in
Fig. 5. The modelsbest suited with ourmeasurementcorrespond
to atmospheres with null or low amounts of a stratospheric ab-
sorber, this leading to non inverted, or scarcely inverted atmo-
spheres. A similar result was found in Beerer et al. (2011), for
observations in the mid-IR. When considering the three mea-
surements, i.e. the ISAAC one at 2.2µm, and the Spitzer ones at
3.6 and 4.5µm, the best fitting models is the one with an ineffi-
cient redistribution of heat, and an small quantity of absorber in
the planet stratosphere. Thus, the lack of a strong thermal inver-
sion in the atmosphere of WASP-4b could be inferred from this
IR detection.
4. Conclusions
The ultrafast photometry technique developed in Paper I is able
to detect very low amplitude secondary eclipses of extraso-
lar planets. We have measured the secondary eclipse depth of
WASP-4b which has an amplitude of dKS= 0.185% at ≥10σ
level. This secure detection strengthenourconfidencein the new
method, and indicates that it could be fruitful to attempt the ob-
servation of secondary eclipses at shorter wavelengths, where
the eclipses are shallower, but the NIR signal is still within our
observational capabilities.
For the stellar and planetary parameters, this yields a bright-
ness temperature of T = 1995K in the KS, which agrees with
the specific models for WASP-4b, and argues in favor of ineffi-
cientheatredistributionfromtheday-sidetothenight-sideofthe
planet. The absence of an strong thermal inversion in the strato-
sphere of WASP-4b is inferred from our near-IR measurement,
and mid-IR measurements in the literature.
Acknowledgements. This work is supported by ESO, by BASAL Center for
Astrophysics and Associated Technologies PFB-06, by FONDAP Center for
Astrophysics 15010003, and by Ministry for the Economy, Development, and
Tourism’s Programa Inicativa Cient´ ıfica Milenio through grant P07-021-F,
awarded to The Milky Way Millennium Nucleus. A.B would like to acknowl-
edge support in part by NASA grant NNX07AG80G and through JPL/Spitzer
Agreements 1328092, 1348668, and 1312647. The authors would like to ac-
knowledge David Anderson by his useful comments and suggestions.
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C. C´ aceres et al.: KS-band thermal emission from WASP-4b.5
Fig.5. The planet-to-star flux contrast as a function of wavelength. The solid lines show three models with κe= 0.00− 0.03cm2/g,
and Pn= 0.1 − 0.3 for the planet WASP-4b. The squares are the estimated band-weighted measurements for these models, and the
black point at 2.16µm represents our measurement, with its error bars. Note that the red model point at 2.16µm is overlapped by
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