N. Narita

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EPJ Web of Conferences

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Good Combination of the Rossiter-McLaughlin Measurements
and Direct Imaging Observations
N. Narita1 and SEEDS/HiCIAO/AO188 teams
1 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo,
181-8588, Japan [norio.narita@nao.ac.jp]
Abstract. We introduce a novel methodology to distinguish planetary migration
mechanisms by a combination of Rossiter-McLaughlin measurements and direct
imaging observations. The methodology is especially useful to specify a planetary
migration mechanism one-by-one for an eccentric or tilted planet. We present an
example of the application of the methodology for HAT-P-7 based on our obser-
vations with the 8.2m Subaru Telescope.
1. Introduction
The discovery of about 500 extrasolar planets and the diversity of their orbital distribu-
tions dramatically changed our perception of planetary systems. To explain the whole
orbital distribution of known exoplanets, a number of planetary migration mechanisms
have been proposed, including disk-planet interaction models (e.g., Lin et al. 1996;
Ida & Lin 2004), planet-planet scattering models considering gravitational interaction
among multiple giant planets (e.g., Nagasawa et al. 2008), or Kozai migration models
considering perturbation by a distant companion and coinstantaneous tidal evolution
(e.g., Fabrycky & Tremaine 2007). These planetary migration models can now be tested
by measurements of the Rossiter-McLaughlin (RM) effect (Rossiter 1924; McLaughlin
1924) for transiting planetary systems, which is an anomalous shift in observed radial
velocities due to a partial occultation of a rotating star. Previous measurements of the
RM effect have revealed that numbers of transiting planets indeed have highly tilted
orbits, which are consistent with predictions of the planet-planet scattering models and
the Kozai migration models. The fact is important evidence that a significant part of
exoplanets have migrated inward through planet-planet scattering or Kozai migration
processes. However, a big problem is that one cannot distinguish the planet-planet
scattering models and the Kozai migration models by orbital eccentricities or spin-
orbit alignment angles alone. We thus need to obtain more clues to discriminate the
two planetary migration models by identifying counterparts of migration processes (i.e.,
scattered outer planets for the planet-planet scattering models, and a binary compan-
ion for the Kozai migration models). For the purpose, direct imaging is very useful to
search for outer massive bodies and give us crucially important information to distin-
guish between the two migration mechanisms of eccentric or tilted exoplanets.
© Owned by the authors, published by EDP Sciences, 2011
DOI:10.1051/epjconf/20111105004
EPJ Web of Conferences 11, 05004 (2011)                                      
This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which 
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Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20101105004
EPJ Web of Conferences
2. Methodology
We have devised a novel procedure to discriminate or at least constrain planetary
migration models for eccentric or tilted exoplanets by direct imaging observations. The
procedure is summarized as follows.
– STEP 1. Check the presence of binary companion candidates.
If there is no binary companion, then we can exclude the Kozai migration mecha-
nism caused by a binary star. If there is a binary candidate(s), proceed to STEP
2. Note that we can confirm the binary nature by checking (i) common dis-
tance inferred by spectral type, (ii) common peculiar velocity by high dispersion
spectrocopy, or (iii) common proper motion by additional direct imaging.
– STEP 2. Calculate the restricted region (in mass and semi-major axis space) of
additional bodies for the Kozai migration.
The Kozai migration cannot occur if the timescale of orbital precession due to
an additional body (PG,c) is shorter than that caused by a binary companion
through Kozai mechanism (PK,B) (Kozai 1962, Innanen et al. 1997). Using this
fact, if a long-term radial velocity trend in the system suggests that an additional
body orbits in the restricted region, we can exclude the Kozai migration. If there
is no additional body in the restricted region, then proceed to STEP 3.
– STEP 3. Calculate the initial planet-binary configuration for the Kozai migration.
In this case we cannot exclude the possibility of the Kozai migration completely.
Instead, we can constrain the initial mutual inclination angle between the plan-
etary orbital axis and the binary orbital axis based on the angular momentum
conservation during the Kozai migration.
3. The Case for HAT-P-7
3.1 Target Properties
We selected the transiting planetary system HAT-P-7 as the first target. HAT-P-7
(also known as Kepler-2) is an F8 star, hosting a very hot Jupiter HAT-P-7b (Pál et
al. 2009). The stellar distance was estimated as 320+50
−40 pc. This star is in the field of
view of the NASA Kepler mission (Borucki et al. 2009). The planet HAT-P-7b has a
mass of 1.82 ± 0.03MJup, with the orbital period of 2.204733 ± 0.000010 days and the
semi-major axis of a = 0.0386±0.0001 AU (Welsh et al. 2010). The orbit of HAT-P-7b
does not have a significant eccentricity. Nevertheless, Narita et al. (2009) and Winn
et al. (2009) found that the planet has an extremely tilted orbit relative to the stellar
rotation axis. In addition, Winn et al. (2009) reported an additional long term RV
trend, implying a massive third body in the planetary system.
3.2 Direct Imaging Observations
We conducted direct imaging observations with the Subaru HiCIAO (High Contrast In-
strument for the Subaru next generation Adaptive Optics; Suzuki et al. 2009), as part
of the SEEDS project (Strategic Explorations of Exoplanets and Disks with Subaru,
PI: Motohide Tamura; Tamura 2009). We observed HAT-P-7 in the H band combined
with the AO188 (188-element curvature sensor adaptive optics system; Hayano et al.
2008), mounted on the Subaru Telescope on UT 2009 August 6. The result image is
05004-p.2
Detection and Dynamics of Transiting Exoplanets
shown in figure 1 (north is up and east is left, and the field of view is 12×12 arcsec,
as a subset of the full 20×20 arcsec frame). Two faint sources at about 3-4 arcsec
away are clearly detected. The apparent H band magnitude, separation angle, and
position angle of each companion star from HAT-P-7 are presented in Narita et al.
(2010). We estimate that projected separation distances of these candidate companion
stars from HAT-P-7 are about 1000 AU. HAT-P-7 was also observed in SDSS i′ and
z′ filter with the AstraLux Norte Lucky Imaging camera (Hormuth et al. 2008) at the
2.2 m telescope at Calar Alto on UT 2009 October 30. Assuming that the candidate
companions are main sequence stars associated with HAT-P-7, we estimate that the
candidate companions are late M stars, based on a comparison of the colors of the can-
didate companions with the spectral energy distribution (SED) standard stars (Kraus
& Hillenbrand 2007; Covey et al. 2007).
Figure 1: A combined image of HAT-P-7 in H band taken with the Subaru HiCIAO on
UT 2009 August 6. North is up and east is left. The field of view is 12×12 arcsec.
05004-p.3
EPJ Web of Conferences
3.3 The planetary migration mechanism of HAT-P-7b
As discussed by Wu & Murray (2003) for HD 80606b, a hypothetical additional body
HAT-P-7c in the HAT-P-7 system could destroy the Kozai migration process (Innanen
et al. 1997), if the timescale of orbital precession of HAT-P-7b caused by the gravi-
tational perturbation from HAT-P-7c (PG,c) is shorter than that caused by the Kozai
mechanism due to the binary companion (PK,B). A conditional equation for a restricted
area of an outer third body at the initial stage is:
Mc >
3
2
Ms
a2ca
a3B
1
b
(1)
3/2
, (1)
where ac and Mc are the semi-major axis and mass of the additional planet, aB is
the semi-major axis of the binary star, and b
(1)
3/2 is the Laplace coefficient (see Wu &
Murray 2003). If the above condition is met, the Kozai migration of HAT-P-7b due
to the candidate companions cannot occur. On the other hand, Winn et al. (2009)
reported that there is indeed a possible third body HAT-P-7c in the HAT-P-7 system.
As a constraint on the mass and semi-major axis of the additional body, they reported
the following relation:
Mc sin ic
ac
2
∼ (0.121 ± 0.014) MJup AU
−2, (2)
where ic is the orbital inclination of HAT-P-7c relative to the line of sight. As a result,
we have found that HAT-P-7c is in the restricted region. Thus in the presence of HAT-
P-7c, it is impossible to explain the tilted orbit of HAT-P-7b by the Kozai migration
caused by the possible binary companions. Thus we conclude that the Kozai migration
is less likely while planet-planet scattering is possible for the migration mechanism of
HAT-P-7b (see Narita et al. 2010 for details).
4. Summary and Future Prospects
We have devised a novel procedure to discriminate or at least constrain planetary mi-
gration models by a combination of the Rossiter-McLaughlin effect and direct imaging
observations. We have applied the methodology to HAT-P-7b, which was reported to
have a highly tilted or even retrograde orbit based on the Rossiter-McLaughlin mea-
surements (Narita et al. 2009; Winn et al. 2009). Based on additional direct imaging
observations, two candidates of distant stellar companions have been found around
HAT-P-7. We have shown that the Kozai migration of HAT-P-7b due to the candi-
date companions is impossible in the presence of HAT-P-7c reported by Winn et al.
(2009). As a result, we conclude that planet-planet scattering is the most likely mi-
gration mechanism for HAT-P-7b. Through the experience for the HAT-P-7 system,
we have demonstrated that the methodology is especially useful to specify planetary
migration mechanisms for eccentric or tilted planets. In addition, we note that direct
imaging observations are also important for non-eccentric and aligned planetary sys-
tems to constrain populations of the Kozai migration. Planetary migration mechanisms
for large numbers of exoplanets can be constrained one-by-one via the combination of
the Rossiter-McLaughlin effect and direct imaging, and thereby we would be able to
specify a dominant mechanism of planetary migration statistically in the near future.
05004-p.4
Detection and Dynamics of Transiting Exoplanets
Acknowledgements. N.N. acknowledges a support and hospitality at OHP by the conference
organizers. N.N. is supported by a Japan Society for Promotion of Science (JSPS) Fellowship
for Research (PD: 20-8141). This work is partly supported by a Grant-in-Aid for Specially
Promoted Research and by the Mitsubishi Foundation. The data for HAT-P-7 were collected at
the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.
N.N. is grateful for the helpful supports and discussions for this study by all members of
SEEDS/HiCIAO/AO188 teams.
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Good Combination of the Rossiter-McLaughlin Measurements and Direct Imaging Observations

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Good Combination of the Rossiter-McLaughlin Measurements and Direct Imaging Observations

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