HST PanCET program: A Cloudy Atmosphere for the promising JWST target WASP-101b

3.3 MB·

by  H. R. Wakeford

Accepted to ApJ Letters January 3rd 2017
PreprinttypesetusingLATEXstyleemulateapjv.01/23/15
HST PanCET PROGRAM: A CLOUDY ATMOSPHERE FOR THE PROMISING JWST TARGET WASP-101b
H.R. Wakeford1, K.B. Stevenson2, N.K. Lewis2, D.K. Sing3, M. Lo´pez-Morales4, M. Marley5, T. Kataria6, A.
Mandell1, G.E. Ballester7, J. Barstow8, L. Ben-Jaffel9, V. Bourrier10, L.A. Buchhave11, D. Ehrenreich10,
T.M. Evans3, A. Garc´ıa Mun˜oz12, G. Henry13, H. Knutson14, P. Lavvas15, A. Lecavelier des Etangs9, N.
Nikolov3, J. Sanz-Forcada16.
1. PlanetarySystemsLab,NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA
2. SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218,USA
3. AstrophysicsGroup,PhysicsBuilding,UniversityofExeter,StockerRoad,ExeterEX44QL,UK
4. Harvard-SmithsonianCenterforAstrophysics,Cambridge,MA02138,USA
5. NASAAmesResearchCenter,MS245-5,MoffettField,CA94035,USA
6. NASAJetPropulsionLaboratory,4800OakGroveDr,Pasadena,CA91109,USA
7 7. Dept. ofPlanetarySciences&Lunar&PlanetaryLab.,UniversityofArizona,1541EUniv. Blvd.,Tucson,AZ85721,USA
1 8. PhysicsandAstronomy,UniversityCollegeLondon,London,UK
0 9. InstitutdAstrophysiquedeParis,CNRS,UMR7095&SorbonneUniversit´es,UPMCParis6,98bisbdArago,75014Paris,France
10. Observatoiredel’Universit´edeGen`eve,51chemindesMaillettes,1290Sauverny,Switzerland
2
11. CentreforStarandPlanetFormation,NielsBohrInstituteandNaturalHistoryMuseum&,UniversityofCopenhagen,Øster

n Voldgade5-7,DK-1350CopenhagenK,Denmark
12. Zentrumfu¨rAstronomieundAstrophysik,TechnischeUniversit¨atBerlin,D-10623Berlin,Germany
a
13. CenterofExcellenceinInformationSystems,TennesseeStateUniversity,Nashville,TN37209,USA
J
14. DivisionofGeologicalandPlanetarySciences,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA

3 15. GroupedeSpectroscopieMol´eculaireetAtmosph´erique,Universit´edeReims,Champagne-Ardenne,CNRSUMR7331,Franceand
16. CentrodeAstrobiolog´ıa(CSIC-INTA),ESACCampus,P.O.Box78,E-28691VillanuevadelaCan˜ada,Madrid,Spain

] Accepted to ApJ Letters January 3rd 2017
P
E ABSTRACT
. We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic
h
Comparative Exoplanet Treasury (PanCET) program for WASP-101b, a highly inflated hot Jupiter
p
and one of the community targets proposed for the James Webb Space Telescope (JWST) Early
-
o Release Science (ERS) program. From a single HST Wide Field Camera 3 (WFC3) observation,
r we find that the near-infrared transmission spectrum of WASP-101b contains no significant H O
t 2
s absorption features and we rule out a clear atmosphere at 13σ. Therefore, WASP-101b is not an
a optimumtargetforaJWSTERSprogramaimedatobservingstrongmoleculartransmissionfeatures.
[
We compare WASP-101b to the well studied and nearly identical hot Jupiter WASP-31b. These

twinplanetsshowsimilartemperature-pressureprofilesandatmosphericfeaturesinthenear-infrared.
1
v We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit
3 cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests
4 that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted
8 instrumental systematics.
0 Subjectheadings: techniques: spectroscopic,planetsandsatellites: atmospheres,planetsandsatellites:
0 individual: WASP101b
.
1
0
1. INTRODUCTION atmosphere key constraints can be placed on the abun-
7
dance of both water and carbon-species, important in
1 Observations of hot Jupiters with Hubble Space Tele-
breaking the C/O-metallicity dependence. This in turn
: scope (HST) have revealed a diversity of atmospheres
v can give insights into planetary formation (e.g., Fortney
from clear to cloudy (Sing et al. 2016). In the near-
i etal.2013;Kreidbergetal.2014;Benneke2015). There-
X infrared with Wide Field Camera 3 (WFC3), significant
fore, understanding the presence or absence of clouds in
waterabsorptionfeatureshavebeenmeasuredforanum-
r the atmosphere of exoplanets is important in the lead
a ber of exoplanets (e.g., Deming et al. 2013; Wakeford
up to the launch of the James Webb Space Telescope
et al. 2013; Stevenson et al. 2014; Evans et al. 2016).
(JWST).
However, a significant number also show little or no evi-
TheHSTPanchromaticComparativeExoplanetTrea-
dence of water absorption (e.g., Line et al. 2013; Ehren-
sury (PanCET) program has targeted twenty planets
reich et al. 2014; Knutson et al. 2014; Sing et al. 2015),
for the first large-scale simultaneous UVOIR compara-
whichisplausiblyduetocloudshighintheplanetaryat-
tive study of exoplanets. A major aim of PanCET is
mosphere (Sing et al. 2016). A number of studies across
to produce the first, complete and statistically rigorous
temperature, mass, and metallicity ranges for exoplan-
comparative study of clouds and hazes in exoplanet at-
ets have shown that clouds and aerosols will likely play
mospheres over a wide range of parameters such as tem-
a critical role in exoplanetary atmospheres (e.g., Mor-
perature, metallicity, mass, and radius. As part of the
ley et al. 2015; Wakeford & Sing 2015; Parmentier et al.
program WASP-101b was selected as it would have a
2016; Wakeford et al. 2017). Several studies, such as by
large predicted transmission signal with strong molecu-
Greene et al. (2016), have shown that with a cloud-free
lar features. The planet is a highly inflated hot Jupiter
(Rp=1.4R and Mp=0.5M ) in orbit around a rela-
J J
[email protected]
2 Wakeford et al.

a) TABLE 1
Raw light curve
Transmission spectrum of WASP-101b measured with HST
WFC3 G141 grism and analysed with using common-mode
x correction.
u
Fl
We avelength, λ ∆λ Rp/R∗ Uncertainty
v
ati (µm) (µm)
el 1.14215.1425 μm0.035 1.1738 0.0082 1.3125 μm 1.4925 μm
R
1.1775 0.035 1.1810 0.0067 S
e Flux 11..22142755 00..003355 11..11781751 00..00007605 R/Rp* cale He
b) elativ 11..238127155.1775 μm00..003355 11..11885000 00..00006638 1.3525 μm 1.5275 μm null-hypothesis (BIC=36) ight
R 1.3525 0.035 1.1786 0.0063 T=1250K, flat x100 (BIC=34)
T=1250K, equilibrium chemistry (BIC=211)
ux 1.3875 0.035 1.1804 0.0063
e Fl 1.4225 0.035 1.1997 0.0072 Wavelength (μm)
v 1.4575 0.035 1.1655 0.0067
Orbit 1 elati 1.49215.2125 μm0.035 1.1936 0.0072 1.3875 μm 1.5625 μm
Orbit 2 R 1.5275 0.035 1.1857 0.0071 Common-mode
Orbit 3 x 1.5625 0.035 1.1579 0.0071 Marginalization
u
Orbit 4 e Fl 1.5975 0.035 1.1671 0.0077 Sc
v 1.6325 0.035 1.1708 0.0077 a
c) Corrected light curve havewRelatiellcons1t.r2a4i7n5e dμmparameters,arelativelybrighthost 1.4225 μm 1.5975 μm R/Rp* le Heigh
star,auxndatransmissionspectrumwithmeasurablespec- null-hypothesis (BIC=36) t
troscoFlpic features. The ERS proposal submission dead- T=1250K, flat x100 (BIC=34)
line isve expected to be in the summer of 2017; as such, it T=1250K, equilibrium chemistry (BIC=211)
iosnvtihtaeRelatilbtehsatttar1er.cg2oe8nt2s5n aiμnimssaatnicmeeolybsmeravnantieorn.s be carried out 1.4575 μm 1.6325 μm Wavelength (μm)
WASP-101b is expected to have a predominantly
cloudy atmosphere, usinBgJDthTBeD s- u2r4f5a7c6e6g3r.0avity and tem- BJDTBD - 2457663.0 BJDTBD - 2457663.0
perature as predictive parameters outlined in Stevenson
(2016s). However,otherobservedhotJupitersinthesame
d) paramualeter regime have already shown contrasting prop-
d
ertiessi(Singetal.2016): WASP-17bandHD209458bare
clear ewhile WASP-31b is cloudy. As such, it is not yet
R
clear to what extent simplified predictive strategies will
be accurate and exceptions to general trends can bring
aboutalsadditional insight.
WAuSP-101wasoneofthefirstobservationsundertaken
d
Fig. 1.—Band-integratedlightcurveandresiduals. a)rawlight intheesinewHSTcycle. Inthisletterwepresentthenear-
curve(blackpoints)andmodel(red). b)raworbit-to-orbitresidu- infrarRed observations conducted using HST and the re-
alsshowingthegradientdifferencebetweenHSTorbit1andorbits sultant transmission section with discussion on clouds
2–4. c)lightcurvecorrectedbythesystematicmodelandthebest and JWST.
s
fit transit model (red). d) residuals in parts per million (ppm) of al
corrected light curve compared to the transit model (red dashed u
d 2. OBSERVATIONSANDANALYSIS
line). si
Obseervations of WASP-101 were conducted with HST
R
tively bright V Mag=10.3 F6 host star (Hellier et al. WFC3G141grismaspartoftheHSTPanCETprogram
2014). GO-14767(PIsSingandL´opez-Morales)onOctober2nd
WASP-101b was also presented as one of the most fa- 2016.sObservations were conducted in forward spatial
al
vorable targets for JWST Early Release Science (ERS) scan muode, using the 512×512 subarray, in SPARS25,
d
program (Stevenson et al. 2016). The JWST ERS pro- with 7sireads per exposure, resulting in an exposure time
gram focuses on testing specific observing modes in an of 138Re.38 seconds. We use a scan rate of ∼0.65 pixels
efforttoquicklysupplythecommunitywithdataandex- per second with a final spatial scan covering ∼90 pixels
perienceinreductionandanalysisofJWSTobservations. in the cross-dispersion direction on the detector. This
As such, the targets suitable for such observations were resultalss in a maximum count rate of 22,000e− per pixel,
limited to targets that fit the following criteria (Steven- whichuis quite low but importantly does not enter the
d
son et al. 2016): Be in or near the continuous viewing non-lisinearity regime of the detector.
e
zone (CVZ) of JWST (i.e. near the ecliptic poles) and WeRusetheIMAoutputfilesfromtheCalWF3pipeline
have a short orbital period such that multiple transit which are in e−, each are calibrated for zero-read bias,
opportunities arise and long visibility windows are open and dark current subtracBtJioDnT.BD -T 2o45e7x6t6ra3c.0t the spectra BJDTBD - 2457663.0 BJDTBD - 2457663.0
for scheduling. Additionally, the planetary system must we followed a similar method to Evans et al. (2016).
A Cloudy Atmosphere for WASP-101b 3
Raw light curve
x
u
Fl
e
v
ati
el 1.1425 μm 1.3125 μm 1.4925 μm
R
x
u
Fl
e
v
elati 1.1775 μm 1.3525 μm 1.5275 μm
R
x
u
Fl
e
v
ati
Orbit 1 Rel 1.2125 μm 1.3875 μm 1.5625 μm
Orbit 2
Orbit 3 x
u
Orbit 4 Fl
e
v
ati
el 1.2475 μm 1.4225 μm 1.5975 μm
R
Corrected light curve
x
u
Fl
e
v
ati
el 1.2825 μm 1.4575 μm 1.6325 μm
R
BJDTBD - 2457663.0 BJDTBD - 2457663.0 BJDTBD - 2457663.0
s
al
u
d
si
e
R
s
al
u
d
si
e
R
s
al
u
d
si
e
R
s
al
u
d
si
e
R
s
al
u
d
si
e
R
BJDTBD - 2457663.0 BJDTBD - 2457663.0 BJDTBD - 2457663.0
Fig. 2.—Spectroscopiclightcurvesandresidualsforthecommon-modemethod. Top: normalizedandsystematics-correcteddata(points)
and best-fit transit model (line) in 15 spectroscopic channels between 1.125–1.65µm. Bottom: corresponding residuals (ppm) from the
systematiccorrectedlightcurveswiththeir1σ errorbars.
In each exposure the difference between sucessive non- 2.1. Band-integrated light curve
destructive reads was taken and a top-hat filter applied
We monitored the transit with HST over a total of
around the target spectrum. The full image is then
five orbits with observations occurring prior-, during-,
reconstructed by adding the individual reads back to-
and post-transit. We discard the zeroth orbit as it con-
gether. We then extract the stellar spectrum from each
tains vastly different systematics to the subsequent or-
reconstructed image for a series of apertures. We de-
bits. This is an effect observed in many previous studies
termine the best aperture across the whole spectrum by
of WFC3 exoplanet transits data (e.g., Deming et al.
reducingthescatterintheout-of-transitband-integrated
2013; Mandell et al. 2013; Wakeford et al. 2013; Sing
residuals. Wefindabestapertureof±44pixelsforG141
et al. 2013; Ehrenreich et al. 2014; Sing et al. 2016).
aroundacenteringprofilewhichisfoundtobeconsistent
We additionally remove the first exposure of each orbit
across the spectrum for each exposure.
following the buffer dump as they contain significantly
a)
Raw light curve
x
u
l
F

e
v
i
t
a
l 1.1425 μm 1.3125 μm 1.4925 μm
e
R
S
c
x
a
u * l
R e
l
F
/ H
p
e R
e
v
i
i g
t h
a
1.1775 μm 1.3525 μm 1.5275 μm t
b) l null-hypothesis (BIC=36)
e
R T=1250K, flat x100 (BIC=34)
T=1250K, equilibrium chemistry (BIC=211)
x
u
l
F
Wavelength (μm)
e
v
i 4 Wakeford et al.
t
a
l 1.2125 μm 1.3875 μm 1.5625 μm
Orbit 1 e
R
Common-mode
Orbit 2
Marginalization
Orbit 3 x
u
Orbit 4 l
F
S

e c
v a
i * l
t R e
la 1.2475 μm 1.4225 μm 1.5975 μm /p H
e R
e
R
c) i
g
Corrected light curve h
x t
u null-hypothesis (BIC=36)
l
F T=1250K, flat x100 (BIC=34)

e T=1250K, equilibrium chemistry (BIC=211)
v
i
t
a
l 1.2825 μm 1.4575 μm 1.6325 μm Wavelength (μm)
e
R
Fig. 3.—TransmissionspectrumofWASP-101bfromourcommon-modeanalysis(blackpoints)andusingmarginalization(greysquares).
Weusethecommon-modetransmissionspectrumandfitforthebest1Disothermalmodel(Fortneyetal.2010),whichhasT=1250Kand
a flat cloud deck x100 the hydrogen cross-section (red). The model in blue shows the expected absorption feature for a clear atmosphere
BJD - 2457663.0 BJD - 2457663.0 BJD - 2457663.0 atthesametemperature. Thedashedlineshowsthenullhypothesis.
TBD TBD TBD
lower count levels than the following exposures. This re- (a/R ). We find an inclination of 84.73±0.05 and an
∗
sults in a total of 48 exposures over four orbits. We first improved a/R of 8.173±0.046. We fix each parameter,
∗
analyze the band-integrated light curve, to obtain the excluding R /R , for each spectroscopic light curve.
p ∗
s broad-bandplanet-to-starradiusratio(R /R ),bysum-
al mingthefluxbetween1.125–1.65µmineapche∗xposureto 2.2. Transmission Spectrum
d)
u create the band-integrated light curve (see Fig.1a). To measure the atmospheric transmission spectrum
d To analyze the band-integrated light curve we first we divide the spectrum into 15 bins, each with
i havetounderstandtheinstrumentalsystematicsimpact- ∆λ=0.035µm, and measure Rp/R∗ in each spectro-
s
ing the observed flux. We test a number of systematic scopic light curve (Fig.2). We perform common-mode
e
model corrections for visit-long and orbit-long trends in correction on each spectroscopic light curve using the
R
the data. To correct for visit-long trends we test linear, residuals from the band-integrated light curve in addi-
quadratic, and a rising exponential plus linear function tion to a linear trend in time (e.g., Deming et al. 2013;
(Stevenson et al. 2014; Line et al. 2016). We find that a Stevensonetal.2014). Thisremoves“common”system-
risingexponentialpluslinearfunctioninplanetaryphase atics and assumes wavelength independent trends. The
combined with a rising exponential plus linear function measured R /R from each spectroscopic light curve are
s p ∗
in HST orbital phase produced the best fit for the data. listed in Table 1. We additionally measure the transmis-
l
a As can be seen in Fig. 1b the first orbit, following the sion spectrum using marginalization, which accounts for
u removal of the zeroth orbit, exhibits a different system- systematic trends by calculating a weight based on the
d atic trend compared to the subsequent orbits. We see evidenceoffittoalltestedmodels(seeTable2Wakeford
i this in other unpublished datasets and appears to be re- et al. 2016) and marginalizes over all results to get the
s
lated to low counts per pixel resulting from the chosen measured parameter and uncertainty (Wakeford et al.
e
scan rates. This potentially introduces not only an up- 2016). We find both transmission spectra are in good
R
per count limit needed for HST exoplanet studies (e.g., agreement with each other (see Fig.3) suggesting that
Berta et al. 2012; Wilkins et al. 2014; Wakeford et al. there are limited or no wavelength-dependent systemat-
2016), but also a lower count limit that needs to be ex- ics.
ceeded per pixel on the detector (i.e. 25,000e−<per We test the measured transmission spectrum against
s pixel count<35,000e−). a suite of isothermal models calculated from a grid for-
l From the band-integrated light curve we measure mulated in Fortney et al. (2010). Each one-dimensional
a
a center of transit time of 2457663.51944±0.000080 isothermal model assumes solar metallicity, and includes
u
(BJD ), converted from MJD using Eastman a self-consistent treatment of radiative transfer and
d TBD
(2012), and compute a band-integrated R /R of thermo-chemicalequilibriumofneutralandionicspecies.
i p ∗
s 0.10860±0.00019. As the HST orbital phasing also in- Thesemodelsaccountforcondensationandthermalion-
e cludes the full ingress we also fit for the system incli- ization, though do not include photochemistry (Lodders
R nation and the ratio of semi-major axis to stellar radius 1999; Lodders & Fegley 2002; Lodders 2002; Visscher
s
l
a
u
d
i
s
e
R
s
l
a
u
d
i
s
e
R
BJD - 2457663.0 BJD - 2457663.0 BJD - 2457663.0
TBD TBD TBD
A Cloudy Atmosphere for WASP-101b 5
a) a) a) a) etal.2006;Lodders&Fegley2006;Freedmanetal.2008;
Ti-bearinAgl-bearing WWWAAASSSPPP---1310171bbb Ti-bearinAgl-bearing WWWAAASSSPPP---1310171bbb Ti-bearinAgl-bearing WWWAAASSSPPP---1310171bbb Ti-bearinAgl-bearing WWWAAASSSPPP---1310171bbb Ti-bearinAgl-bearing WWWAAASSSPPP---1310171bbb Ti-bearinAgl-bearing WWAASSPP--13011bb L10oW1dbdeesrfihsno2dw0s1t0hn)ao.tetvhideentrcaensfomrisHsi2oOn sapbescotrrputmiono.f WWeAScaPn-
rule out spectral features at 13σ significance. As it is
well known, there is degeneracy in the interpretation of
a flat transmission spectrum at the WFC3 G141 wave-
lengths. Such flat spectra could be produced either by a
cloudy atmosphere, or by an atmosphere with very low
H2O content. However, based on the recent large HST
C Fe C Fe C Fe C Fe C Fe C Fe hot Jupiter survey results by Sing et al. (2016), clouds
r r r r r r
ZnSKCl Na2S MnS MgSiOMg2SiO ZnSKCl Na2S MnS MgSiOMg2SiO ZnSKCl Na2S MnS MgSiOMg2SiO ZnSKCl Na2S MnS MgSiOMg2SiO ZnSKCl Na2S MnS MgSiOMg2SiO ZnSKCl Na2S MnS MgSiOMg2SiO aWthpAepteSraPar-n1ts0om1ibbs.esiTotnhoesmpmoecdotsertlutmlhikewelleyacusksceeonafamurniooilfeofcroumrla,arwpfaelvaaetnlueerntegsltikhine-
3 4 3 4 3 4 3 4 3 4 3 4
independent, grey-scattering cloud with scattering ap-
proximated by increasing the Rayleigh cross-section of
b) b) b) b) hydrogen at 350nm by 100x, which likely corresponds
Al2O3 WASP-101b WASP-101b (T=1498 K) WASP-101b to clouds formed with large particle sizes (>1µm). This
FMCerg SiO3 WWAASSPP--3117bb WWAASSPP--3117bb ((TT==11562726 KK)) (WTA=1S4P9-83 1Kb, gp=6 ms-2) saibgsnoirfipctainotnlyferaetduurecse.sHthoewaemveprl,ittuhdisehoafstohnelyexapmecatregdinHa2llOy
MnS (T=1522 K, gp=5 ms-2) betterfitcomparedtothenull-hypothesisofacompletely
featureless spectrum and is consistent with a flat spec-
WASP-101b Comparison of MgSiO3 Comparison of MgSiO3 trum at the resolution of the data (see Fig.3).
WASP-31b cloud over transmission cloud with an fsed=0.1
WASP-17b pressures. over transmission pressures. 3. DISCUSSION
WASP-101b
Obscuringcondensatecloudsinplanetaryatmospheres
canremovelowabundanceabsorbersfromthegasphase,
c) c) AFMCMelrgn2 OSSi3O3 c) AFMCMelrgn2 OSSi3O3 MAglS2OiMFO3en 3( 1S((C221 (044r0 868(.12316 ggggg/////mmmmm22222))))) c) MAglS2OiMFO3e n(3 1 S((C362 (459r0 401(.34185 ggggg/////mmmmm22222))))) o(tWaShbpieaspnikcrrgueofrxoeoewitrmdanaabelt.asteob2larys0lpo.1t5r26ip0o0)t%n,1io7sfn)oce.faaftAtmetuaesertraweussiarnuesfcrsresoohdm(mWoiewnxgangoakpseirenflaoasdSrnadiieantttg&idtoerentaSe,npainasle.gmrn(di2l2se00sva1i1edo65lnds);
spectra show some source of aerosol or cloud opacity in
WASP-101b the atmosphere.
WASP-31b WASP-101b Out of the previously well studied hot Jupiters (e.g.,
WASP-101b
WASP-17b Total optical depth = 137 Deming et al. 2013; Kreidberg et al. 2014; Sing et al.
WASP-31b WASP-101b Total optical depth = 35.9
2016) WASP-101b is most similar to WASP-31b. Both
planetsorbitanF6starwithanorbitalperiodofapprox-
imately 3.5 days. WASP-101b is marginally denser with
d) Al2O3 d) Al2O3 Al2FOe3 ((16366015 gg//mm22)) d) Al2OF3e ( 1(10808606 gg//mm22)) Rp=1.41RJ and Mp=0.50MJ and gp=6.22ms−2, com-
Fe Fe pared to WASP-31b with R =1.53R and M =0.48M
MCrg SiO3 MCrg SiO3 MgSiO3 (C17r 5(15 gg//mm22)) MgSiO3 (C24r 7(28 gg//mm22)) with a lower gp=4.95ms−2p. BothJplanetspalso havJe
MnS MnS MnS (6 g/m2) MnS (0.1 g/m2) Teq ≈1560K and modest atmospheric scale heights,
around 900km and 1100km respectively, making them
good targets for transmission spectral characterization.
WASP-31b WASP-31b Tofurtherexploretheroleofcloudopacityintheatmo-
WASP-17b WASP-31b Total optical depth = 18.6 Total optical depth = 72 sphere we compute a temperature-pressure (TP) profile
for WASP-101b following the methods of Fortney et al.
(2008) and compare it to WASP-31b (Fig. 4). WASP-
31b was found to have a grey cloud that sits reasonably
e) Al2O3 Al2FOe3 ((213671 gg//mm22)) thFisigw.or4k.—),wai)thTWemApSePra-3tu1bre(-bPlruees,suSrinegpertofialle.(o2f01W6)A)S.PT-h1e01cbon(dreend-, low in the atmosphere with a spectral signature that ex-
Fe tends from the optical to the near-IR (see Sing et al.
MgSiO3 MgSiO3C (3r 2(25 gg//mm22)) sthateioantmcuorsvpehse(rdeawshheedrelatbheelyedc)roinssditchaeteTtPhepdriofffielree.nTtchleougdresyperecgieisonins 2015, and Barstow et al. 2016). We approximate the
Cr
MnS (9 g/m2) in each panel show the general pressure levels probed by trans- condensate cloud forming species from multiple conden-
MnS
mission spectral observations. b) comparison between the slant sation curves from Visscher et al. (2010), Morley et al.
optical depth of MgSiO3 at the pressures probed by transmission (2012), and Wakeford et al. (2017). The Global Circula-
spectroscopy. c and d) slant optical depth of each cloud forming
WASP-17b species for a grey scattering cloud with fsed=0.1 in each atmo- tion Model (GCM) for WASP-31b (Kataria et al. 2016)
WASP-17b Total optical depth = 0.91 stophcearlec.ulaWteetuhseeotphteicfaolrmdeuplathtioannsdintotAaclkoeprmticaanl&depMtharolefyal(l20c0o1n)- sehrnowasndthwatesttheerrnelsihmobultdembepleirtatlteudreiffs.erWenecethbeertewfoereenuesaesta-
densatesintheatmospherewiththeindicatedcloudmassforeach
single global TP profile for both planets as an approxi-
species.
mation. TheTPprofilesofbothWASP-101b(thiswork)
and WASP-31b (Sing et al. 2016) cross multiple conden-
sation curves with aluminium- and titanium-oxides deep
intheatmosphere,followedbysilicates,metals,andMnS
6 Wakeford et al.
in or near the observable atmosphere. transit event. As one of the community targets pro-
We use the atmospheric TP profiles with the Acker- posed for the JWST ERS program WASP-101 was a top
man & Marley (2001) cloud code, which calculates the priority for the exoplanet community to conduct a pre-
cumulative geometric scattering optical depth by cloud liminary atmospheric characterization. We measure the
particles through the atmosphere following Mie theory transmission spectrum and show that we can rule out
(e.g., Morley et al. 2012; Wakeford & Sing 2015; Morley spectral features at 13σ significance and the planet is
et al. 2015). We convert this to the slant optical depth mostlikelycloudy. Therefore,withthecurrentsobserva-
(Fortney 2005) for each species and compare their cloud tions WASP-101b does not appear to be an ideal target
mass. We use a sedimentation efficiency f =0.1 (Mor- to be used for JWST ERS programs aimed at observing
sed
ley et al. 2015) for both atmospheres and compare the large-amplitude molecular transmission features, as it is
optical depth of enstatite clouds over the critical pres- likely to have some cloud or aerosol opacity at longer
sures probed in transmission (Fig.4b). This shows that wavelengths.
at pressures higher than 0.001bar, WASP-101b has a Tofurthercharacterizethecloudopacitysourcesinthe
higher optical depth, while at lower pressures higher in atmosphere we compare WASP-101b to the well studied
the atmosphere both WASP-101b and WASP-31b have hot Jupiter WASP-31b which is near identical in plane-
similarly significant opacity caused by enstatite clouds, taryandsystemparameters. Wepredictthesetwinplan-
wheresmallchangesarelikelyduetotheslightdifference etsbothhavecloudsmostlikelycomposedofenstatiteas
intemperatureandgravity. Weshowthedeconstruction it shows the highest opacity and mass. However, obser-
of the total opacity of both atmospheres in Fig.4c for vationsintheopticalareneededtocharacterizethepar-
WASP-101b and Fig.4d for WASP-31b, with each con- ticle sizes comprising the cloud and predict any specific
tribution from the different cloud forming species. The absorption signatures from the clouds themselves. If the
cloud model shows that in both atmospheres enstatite clouds are in fact composed of small particles (<1µm)
andironcausethehighestopacityatpressuresprobedby thenitislikelythecloudopacitywillsignificantlyreduce
transmission spectral observations. Similar to the cloud with increasing wavelength making molecular detection
masses shown in Wakeford et al. (2017) MnS and Cr with JWST possible.
are not significant contributors to the overall opacity of
the atmosphere which is dominated by Al O , Fe, and
2 3
MgSiO . It will be interesting to determine if exoplan- 5. ACKNOWLEDGEMENTS
3
ets in the same parameter regime as WASP-101b and This research has made use of NASAs Astrophysics
WASP-31b will also exhibit cloudy transmission spec- Data System, and components of the IDL astronomy li-
tral features. These models can be used to approximate brary.Based on observations made with the NASA/ESA
the cloud species and opacities expected in transmission HubbleSpaceTelescope,obtainedfromtheDataArchive
spectral studies in preparation for JWST, however, we at the Space Telescope Science Institute, which is oper-
note that no thermal feedback from the clouds them- ated by the Association of Universities for Research in
selves are included. Astronomy, Inc., under NASA contract NAS 5-26555.
To fully characterize the cloud in the atmosphere of These observations are associated with program GO-
WASP-101b we require the information from future op- 14767. H.R. Wakeford acknowledges support by an
tical observations with HST Space Telescope Imaging appointment to the NASA Postdoctoral Program at
Spectrograph (STIS) which will be taken as part of the Goddard Space Flight Center, administered by USRA
HST PanCET program. These observations can be used through a contract with NASA. D.K. Sing, N. Nikolov,
to constrain the nature and particle size of the opacity and T. Evans acknowledge funding from the European
source(e.g.,Wakeford&Sing2015;Singetal.2016;Par- Research Council under the European Unions Seventh
mentier et al. 2016; Wakeford et al. 2017). Additionally, Framework Programme (FP7/2007-2013) / ERC grant
if the particle size is small, <1µm, vibrational mode agreementno. 336792. JKBissupportedbyaRoyalAs-
features associated with the cloud composition may be tronomical Society Research Fellowship. D.E. acknowl-
distinguishable in JWST MIRI wavelengths (Wakeford edges the financial support of the National Centre for
& Sing 2015; Wakeford et al. 2017). CompetenceinResearch‘PlanetSsupportedbytheSwiss
National Science Foundation (SNSF). L. Ben-Jaffel, P.
4. CONCLUSION Lavvas&A.LecavelieracknowledgesupportfromCNES
We present the first results from the HST PanCET (France) under project PACES. The authors would like
program GO-14767 (PIs Sing and L´opez-Morales). Ob- to thank the referee for their careful examination of this
servationsofthehotJupiterWASP-101bwereconducted letter.
in the near-infrared using HST WFC3 G141 over one Facilities: HST (WFC3).
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