Confirmation of a Retrograde Orbit for Exoplanet WASP-17b

Abstract

We present high-precision radial velocity observations of WASP-17 throughout the transit of its close-in giant planet, using the MIKE spectrograph on the 6.5m Magellan Telescope at Las Campanas Observatory. By modeling the Rossiter-McLaughlin effect, we find the sky-projected spin-orbit angle to be lambda = 167.4 \pm 11.2 deg. This independently confirms the previous finding that WASP-17b is on a retrograde orbit, suggesting it underwent migration via a mechanism other than just the gravitational interaction between the planet and the disk. Interestingly, our result for lambda differs by 45 \pm 13 deg from the previously announced value, and we also find that the spectroscopic transit occurs 15 \pm 5 min earlier than expected, based on the published ephemeris. The discrepancy in the ephemeris highlights the need for contemporaneous spectroscopic and photometric transit observations whenever possible. Comment: 4 pages, 3 figures, accepted for publication in ApJ Letters

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arXiv:1009.5061v1 [astro-ph.EP] 26 Sep 2010
Draft version September 28, 2010
Preprint typeset using LATEX style emulateapj v. 11/10/09
CONFIRMATION OF A RETROGRADE ORBIT FOR EXOPLANET WASP-17B
Daniel D. R. Bayliss1, Joshua N. Winn2, Rosemary A. Mardling3, and Penny D. Sackett1
Draft version September 28, 2010
ABSTRACT
We present high-precision radial velocity observations of WASP-17 throughout the transit of its
close-in giant planet, using the MIKE spectrograph on the 6.5m Magellan Telescope at Las Campanas
Observatory. By modeling the Rossiter-McLaughlin effect, we find the sky-projected spin-orbit angle
to be λ = 167.4±11.2 deg. This independently confirms the previous finding that WASP-17b is on a
retrograde orbit, suggesting it underwent migration via a mechanism other than just the gravitational
interaction between the planet and the disk. Interestingly, our result for λ differs by 45±13 deg from
the previously announced value, and we also find that the spectroscopic transit occurs 15 ± 5 min
earlier than expected, based on the published ephemeris. The discrepancy in the ephemeris highlights
the need for contemporaneous spectroscopic and photometric transit observations whenever possible.
Subject headings: planetary systems: formation - stars: individual (WASP-17) - techniques: radial
velocities
1. INTRODUCTION
Knowledge of the angle ψ between a star’s spin vector
and a planet’s orbital angular momentum vector con-
tains information about the planetary formation process.
This is especially so for Hot Jupiters, which are thought
to have “migrated” inward from large orbital distances
(Lin et al. 1996). One possible migration mechanism in-
volves the gravitational interaction between the planet
and the gas in the protoplanetary disk (e.g. Lin et al.
1996; Ida & Lin 2004).Such interactions would pre-
serve a prograde orbit, and could not account for Hot
Jupiters with retrograde orbits (ψ > 90◦) unless the pro-
toplanetary disk was initially misaligned with the star
(Bate et al. 2010).Other migration mechanisms, in-
volving planet-planet scattering and Kozai cycles, (e.g.
Nagasawa et al. 2008) can produce retrograde orbits.
Therefore by measuring ψ one gains insight into the mi-
gration history of a particular planetary system.
TheRossiter-McLaughlin
McLaughlin 1924) occurs when part of the rotating
stellar photosphere is eclipsed by a companion star
or planet.This removes a velocity component of the
observed rotationally- broadened line profiles, causing
a pattern of anomalous radial velocities to be observed
throughout the eclipse. Although this effect does not
reveal ψ, it does give a measure of λ, the angle between
the sky projection of the stellar spin vector and the
planetary orbital angular momentum.
McLaughlin effect has now been used to measure λ for
28 transiting planet systems (see Triaud et al. (2010) or
Winn et al. (2010)).
The transiting Hot Jupiter WASP-17b, which was dis-
covered by Anderson et al. (2010), is reported to have a
effect(Rossiter1924;
The Rossiter-
1Research School of Astronomy and Astrophysics, The Aus-
tralian National University, Mt Stromlo Observatory, Cotter Rd,
Weston Creek, ACT 2611, Australia
daniel@mso.anu.edu.au
2Department of Physics, and Kavli Institute for Astrophysics
and Space Research, Massachusetts Institute of Technology,
Cambridge, MA 02139
3School of Mathematical Sciences, Monash University, VIC
3800, Australia
planetary radius of RP = 1.74 RJ and a host star ro-
tational velocity of v sini = 9km s−1. The deep transit
and fast stellar rotation make it a propitious target for
Rossiter-McLaughlin effect observations.
Anderson et al. (2010) reported that the planet ap-
peared to be in a retrograde orbit, based on three radial-
velocity measurements that were obtained during tran-
sits. Subsequently, the same group announced further
radial velocity measurements of WASP-17, with denser
time sampling (Triaud et al. 2010).
much stronger support to the claim of a retrograde or-
bit, and indeed gave a value for λ nearly identical to the
value derived by Anderson et al. (2010). In this Letter
we present new observations and an independent confir-
mation of the likely retrograde orbit of the WASP-17 sys-
tem, though with a significantly different derived value
for λ.
Those data gave
2. OBSERVATIONS
On the night of 2010 May 11, we obtained high-
resolution spectra of WASP-17 using the Magellan In-
amori Kyocera Echelle (MIKE) spectrograph on the 6.5m
Magellan II (Clay) telescope at Las Campanas Obser-
vatory. MIKE was used in its standard configuration
with a 0.35′′slit and fast readout mode, which deliv-
ered 32 red echelle orders with wavelengths from 5000-
9500˚ A and with a resolving power of R ≈ 48,000. Spec-
tra were taken continuously for 9 hours, with an interrup-
tion of approximately 30 minutes when the star crossed
the meridian near the zenith. A total of 33 spectra of
WASP-17 were obtained, with 600 s exposures. ThAr
calibration frames bracketed each spectrum. Observing
conditions were good, although some high, thin clouds
were present in the latter half of the night. The moon
was down throughout the WASP-17 observations, and
the seeing was 0.5-0.7′′.
The spectra were reduced using the Carnegie MIKE
pipeline developed by Dan Kelson. Radial velocities were
determined via cross correlation with respect to a tem-
plate spectrum. We tested different choices for the tem-
plate spectrum, including a radial velocity standard ob-
served on the same night and various individual WASP-

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2Bayliss et al.
17 spectra. The lowest inter-order scatter was obtained
when using the highest signal-to-noise WASP-17 spec-
trum as the template. This spectrum was also obtained
at low airmass (1.005). For the cross-correlation analysis
we selected 13 echelle orders that were free of obvious
telluric lines and had a signal-to-noise ratio exceeding
100 pixel−1. Cross-correlation was performed using the
IRAF4task fxcor. We used the O2absorption features
from 6870-6900˚ A to provide a constant reference for our
wavelength solutions for each spectrum. These absorp-
tion bands have been shown to be stable at the 5 m s−1
level over short timescales (Figueira et al. 2010).
MIKE lacks an atmospheric dispersion corrector, and
this resulted in an airmass-dependent systematic trend
in the radial velocity measurements, with an amplitude
that increased for bluer echelle orders. The continuous
nine-hour monitoring allowed us to track this systematic
trend and account for it during the model-fitting pro-
cedure, as described in Section 3. The order-averaged,
airmass-corrected radial velocities are given in Table 1,
with uncertainties taken to be the standard deviation of
the mean of the results of all 13 orders.
3. ANALYSIS
Our model for the radial velocity data has the form:
Vcalc,n(t) = Vorb(t)+VRM(t)+c0+c1n+(c2+c3n)X, (1)
where Vcalc,n(t) is the calculated radial velocity at time
t in echelle order n (ranging from 1 to 13), Vorb is the
radial velocity due to the star’s orbital motion (assumed
to be circular), VRM is the transit-specific “anomalous
velocity” due to the Rossiter-McLaughlin effect, and
{c0,c1,c2,c3} are constants specifying the offset between
the relative barycentric velocity of WASP-17 and the ar-
bitrary template spectrum that was used to calculate ra-
dial velocities. To account for the wavelength-dependent
airmass trend mentioned in the previous section, the off-
set was allowed to be a linear function of both order
number n (effectively a wavelength index) and airmass
X.
Many approaches have been taken to model the
Rossiter-McLaughlin effect, such as the pixellated photo-
spheric model of Queloz et al. (2000), the first-moment
approach of Ohta et al. (2005) and Gim´ enez (2006),
the forward-modeling approach of Winn et al. (2005) or
the spectral line fitting of Albrecht et al. (2009) and
Collier Cameron et al. (2010). In our case the most ap-
plicable and convenient model is the analytic formula of
Hirano et al. (2010),
VRM(t) = −δ(t) vp(t)
?2v2
2v2
?
0+ 2(vsini)2
0+ (v sini)2
vp(t)2
2v2
?3/2
1 −
0+ (v sini)2
?
,(2)
where δ(t) is the loss of light during the transit, vp(t) is
the mean radial velocity of the small portion of the pho-
tosphere that is hidden by the planet, v sini is the stellar
projected rotation rate, and v0is the velocity width of the
4IRAF is distributed by the National Optical Astronomy Ob-
servatories, which are operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with
the National Science Foundation.
Fig. 1.— The spectroscopic transit of WASP-17b. Top.—
Radial velocity variation observed on the night of 2010 May 11-
12, along with the best-fitting model. The small gray points are
the order-by-order radial velocities that were fitted.
blue points are averages of the results from all orders. Bottom.—
Residuals between the data and the best-fitting model.
The larger
spectral lines that would be seen from a small portion of
the photosphere (i.e. due to mechanisms other than rota-
tion). This formula relates the phase of the transit to the
radial velocity anomaly derived from cross-correlation.
To calculate vp(t) we assumed that the stellar photo-
sphere rotates uniformly with an angle λ between the
sky projections of the spin vector and the orbital an-
gular momentum vector (see, e.g., Ohta et al. (2005) or
Gaudi & Winn (2007)). We took v sini to be a free pa-
rameter, and v0 was constrained by the condition that
the quadrature sum of v0 and v sini should equal the
observed total linewidth of 11 km s−1(Anderson et al.
2010).
To calculate δ(t), we assumed a linear limb-darkening
law with a fixed coefficient of 0.7, and used the analytic
formulas of Mandel & Agol (2002), as implemented by
P´ al (2008). The parameters of the photometric model
were the planet-to-star radius ratio Rp/R⋆, orbital in-
clination i, and normalized stellar radius R⋆/a (where a
is the orbital distance). Some of the transit character-
istics are more tightly constrained by previous observa-
tions of photometric transits than by the radial velocity
data presented here. Hence, we used Gaussian priors for
the transit depth (Rp/R⋆)2, total duration tIV− tI, par-
tial duration tIV−tIII, orbital period P, and time of tran-
sit Tc, based on the results presented by Anderson et al.
(2010) (see their Table 4, case 3). We also used those
results to set a Gaussian prior for K, the orbital velocity
semi-amplitude, since our coverage of the spectroscopic
orbit is sparse.
All together there are 11 adjustable parameters in our
model fit, of which 7 are mainly determined by our new
radial velocity data: λ, v sini, c0, c1, c2, c3, and the
time of conjunction Tc. The other 4 parameters, Rp/R⋆,
i, R⋆/a, and K, are controlled mainly by the priors.
We determined the best-fitting parameter values and
their 68.3% confidence limits with a Monte Carlo Markov
Chain (MCMC) algorithm that we have described else-
where (see, e.g., Winn et al. (2007)). The likelihood was
taken to be exp(−χ2/2) with
χ2=
33
?
i=1
13
?
n=1
?vobs(ti,n) − vcalc(ti,n)
σ
?2
, (3)
where vobs(ti,n) is the radial velocity measured at time
ti in echelle order n, and vcalc(ti,n) are the calculated

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Retrograde Orbit for WASP-17b3
Fig. 2.— Illustration of airmass correction. The radial ve-
locities for echelle orders 61, 59, and 57 (from top to bottom), along
with the best-fitting model that includes both an airmass correc-
tion and the Rossiter-McLaughlin effect. Central wavelengths of
the orders are indicated on the plot. The lower panel shows the
time dependence of the airmass.
radial velocities. We found that the choice σ = 92 m s−1
gave χ2= Ndof for the best-fitting model, and is also
approximately equal to the scatter observed in the resid-
uals to the best-fitting model, so we used this value in
the MCMC analysis.
Table 2 gives the resulting best-fit parameter values.
Figure 1 shows the airmass-corrected radial velocities as
a function of time, along with the best-fitting model and
the residuals. Figure 2 illustrates the airmass correction
for three separate echelle orders. Figure 3 displays the
marginalized, joint a posteriori probability distributions
involving λ and the other model parameters.
The radial velocity data clearly exhibit an “upside-
down” Rossiter-McLaughlin
blueshift for the first half of the transit, followed by
an anomalous redshift for the second half.
sult for the projected stellar rotation velocity, v sini⋆=
8.61 ± 0.45 km s−1, is in agreement with the results of
Anderson et al. (2010) and Triaud et al. (2010). How-
ever, our finding of λ = 167.4 ± 11.2 deg does not
agree with either of the previously reported results.5
Anderson et al. (2010) found λ = 210.7+11.5
though this result was based on only 3 data points.
More significant is the result by Triaud et al. (2010) of
λ = 212.7+5.5
−5.9deg, based on two data sets densely sam-
pling the transit. The difference between our result for
λ and that of Triaud et al. (2010) is 45±13 deg, or 3.5σ
away from zero.
We do not know the reason for the discrepancy. We
have sparser coverage of the spectroscopic orbit than did
Triaud et al. (2010), but this did not bias our results: we
used the results of Anderson et al. (2010) to set a prior
for K, and also found that the results for K and λ are un-
correlated. We also checked to see if the gap in our time
coverage when WASP-17 was at zenith has biased our
results: we fitted fake data with λ = 210.◦7 and the same
time coverage and velocity errors as our MIKE data, and
found λ = 206◦± 9◦, i.e., the result was not biased. We
also fitted the Triaud et al. (2010) data ourselves and
found λ = 208◦±12◦. Although this reduces the discrep-
ancy in results to 2.5σ, it indicates that the differences
effect:an anomalous
Our re-
−8.9
deg, al-
5For ease of comparison, we added 360 deg to the result
quoted by Anderson et al. (2010), and we converted the result of
Triaud et al. (2010) onto our coordinate system using λ = −β.
in λ values cannot be attributed solely to differences in
fitting procedures. An important step in our data anal-
ysis was fitting and removing the airmass trend in our
radial velocity data. We fitted for the airmass terms si-
multaneously with the Rossiter-McLaughlin model, and
therefore our results do take into account any correlations
between the airmass parameters and the other parame-
ters including λ. However, these errors are internal to the
choice of our model, which is linear in airmass and or-
der number. This model seems reasonable and provides
a satisfactory fit to the data; however it is impossible
to exclude the possibility that the true dependence is
more subtle and that this has altered the shape of the
Rossiter-McLaughlin effect.
According to the ephemeris of Anderson et al. (2010),
the predicted time of conjunction for the event we ob-
served is HJD 2,455,328.6814 ± 0.0017, which is ear-
lier by 15 ± 5 min than the time we measured. Either
the transits are nonperiodic, or the uncertainties in at
least some of the transit times were underestimated. A
straight-line fit to the epochs and transit times in Table 5
of Anderson et al. (2010) gives χ2= 36 with 11 degrees
of freedom, i.e., statistically different than random. For
this reason, it is likely that the true uncertainty in the
period is larger than the previously reported uncertainty,
by a factor of about
?36/11 or 1.8.
Consequently, we conclude that our timing offset from
the ephemeris of Anderson et al. (2010) is at most a 1.5σ
discrepancy. The uncertainty in the time of conjunc-
tion would be best addressed in the future by obtain-
ing high-quality photometric data simultaneously with
spectroscopic observations. However we note that this
ephemeris discrepancy cannot account for the difference
between our result for λ and that of Triaud et al. (2010).
Our result for λ is consistent with a retrograde orbit,
but it must be remembered that λ is a sky-projected
quantity. The true angle ψ between the vectors is given
by
cosψ = cosi⋆cosi + sini⋆sinicosλ,(4)
where i and i⋆ are the line-of-sight inclinations of the
orbital and stellar angular momentum vectors, respec-
tively. Using i = 86.95+0.87
−0.63deg from Anderson et al.
(2010), and supposing i⋆to be drawn from an “isotropic”
distribution (uniform in cosi⋆), we find ψ > 92.◦6 with
99.73% confidence. In this sense, the WASP-17b orbit is
very likely to be retrograde (ψ > 90◦), although nearly
pole-on configurations (ψ ≈ 90◦) are possible.
The assumption of an isotropic distribution in i⋆ ne-
glects our prior knowledge that main-sequence stars have
somewhat predictable rotation rates, and therefore that
the measured value of v sini⋆ also bears information
about i⋆. Recently, Schlaufman (2010) used this insight
to seek evidence for small values of sini⋆(and therefore
large spin-orbit misalignments along the line of sight)
among all the transit hosts. He did not identify WASP-
17 as one of the systems with a likely small value of sini⋆.
In light of this analysis, our observation of a large λ fa-
vors a more nearly retrograde orbits over polar orbits.
4. DISCUSSION
It is now ten years since the first reported detection of
the Rossiter-McLaughlin effect due to a transiting planet
(Queloz et al. 2000). As with many other properties of

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4 Bayliss et al.
Fig. 3.— Results for the model parameters, based on our
MCMC analysis of the radial velocity data, for parameters which
show a degree of correlation with λ. The contours represent 68%,
95%, and 99.73% confidence limits. The quantity ∆Tc is defined
as Tc minus the optimized value of HJD 2,455,328.6918.
extrasolar planets, the Rossiter-McLaughlin effect obser-
vations have thrown up surprises, with systems that ap-
pear very different from those in our own Solar System.
The Hot Jupiter HAT-P-7b was found to be on a po-
lar or retrograde orbit, by independent measurements
of Narita et al. (2009) and Winn et al. (2009). Similarly
WASP-17b has now been confirmed to be in a retrograde
orbit by two independent groups (Triaud et al. 2010, this
work).It now seems that at least a fraction of Hot
Jupiters are migrating by mechanisms other than disk
migration, or that proto-planetary disks are frequently
misaligned with their stars.
The work of Fabrycky & Winn (2009) suggested that
we may be seeing two different populations of Hot
Jupiters that have migrated via different mechanisms.
The results of this work adds weight to that suggestion,
however a larger statistical sample will be needed before
this can be robustly confirmed. Only a larger sample
of planets with known λ values will allow correlations
with parameters such as planetary mass, planetary ra-
dius, metallicity, and stellar mass to be robustly tested.
Winn et al. (2010) have proposed that misaligned Hot
Jupiters occur preferentially around hot stars (Teff >
6250 K), of which WASP-17 is a supporting example.
Fortunately the discovery rate of transiting planets or-
biting bright stars is set to rise due to large-scale ground-
based surveyssuch as SuperWASP Cameron et al. (2007)
and HAT-South (Bakos et al. 2009), and proposed space-
based projects such as TESS (Deming et al. 2009) and
PLATO (Catala & PLATO Consortium 2008).
Australian access to the Magellan Telescopes was sup-
ported through the National Collaborative Research In-
frastructure Strategy of the Australian Federal Govern-
ment. DDRB and RAM acknowledge financial support
from the Access to Major Research Facilities Programme,
which is a component of the International Science Link-
ages Programme established under the Australian Gov-
ernments innovation statement, Backing Australias Abil-
ity.JNW gratefully acknowledges support from the
NASA Origins program through award NNX09AD36G,
and from a generous gift of the MIT Class of 1942. We
thank Amaury Triaud for providing the radial veloc-
ity data for WASP-17 presented by Triaud et al. (2010).
DDRB and RAM would also like to thank St´ ephane Udry
for his helpful discussions on subjects relating to this
work.
Facilities: Magellan: Clay (MIKE)
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Retrograde Orbit for WASP-17b5
TABLE 1
WASP-17 Radial Velocities (order averaged)
Time
HJD
RadialRadial velocity
uncertainty (m s−1)velocity (m s−1)
2455328.57331
2455328.58273
2455328.59219
2455328.60194
2455328.61135
2455328.62035
2455328.62971
2455328.63921
2455328.64876
2455328.65817
2455328.66761
2455328.67700
2455328.68633
2455328.69570
2455328.70557
2455328.71481
2455328.74077
2455328.75012
2455328.75939
2455328.76863
2455328.77785
2455328.78711
2455328.79641
2455328.80593
2455328.81546
2455328.82536
2455328.83495
2455328.84430
2455328.85358
2455328.86289
2455328.87214
2455328.88145
2455328.89069
−9.88
−4.73
9.70
26.88
−49.67
−87.48
−68.18
−110.63
−141.71
−124.57
−92.68
−135.62
−42.40
−13.34
50.30
37.16
164.59
86.38
86.77
31.27
6.61
−32.55
−30.80
−15.31
−23.01
−25.39
−18.66
6.76
50.02
−15.15
−45.57
13.95
−44.50
38.75
32.47
27.08
27.51
37.43
24.96
20.08
40.87
30.59
35.23
27.30
27.72
34.15
24.05
23.05
15.00
17.62
23.37
38.93
30.88
39.38
18.04
24.08
24.74
32.34
32.21
20.94
26.41
32.97
25.22
30.49
15.26
24.20