Aligned, Tilted, Retrograde Exoplanets
and their Migration Mechanisms
Norio Narita (JSPS Fellow)
National Astronomical Observatory of Japan
am aatransit
transit observer
observer.
IIam
“A transit of the Moon” observed
on July 22, 2009 at Hangzhou, China
Photo by Norio Narita / Canon EOS Kiss X-2
I am working on
• Measurements of the Rossiter-McLaughlin effect for
transiting planetary systems
• High-contrast direct imaging for tilted or eccentric
(transiting) planetary systems
Today’s talk
• Transmission spectroscopy for transiting planets to
detect exoplanetary atmospheres
• Measurements of transit timing variations of HAT-P-13b
Outline
• Brief overview of orbits of Solar System bodies
• Orbits of exoplanets and their migration models
• The Rossiter-McLaughlin effect and observations
• High-contrast direct imaging for tilted or eccentric
planetary systems
• Summary
Orbits of the Solar System Planets
Orbits of the Solar System Planets
 All Solar System planets orbit in the same direction
 small orbital eccentricities
 At a maximum (Mercury) e = 0.2
 small orbital inclinations
 The spin axis of the Sun and the orbital axes of
planets are aligned within 7 degrees
 In almost the same orbital plane (ecliptic plane)
 The configuration is explained by core-accretion models
in a proto-planetary disk
Orbits of Jovian Satellites
Orbits of Solar System Asteroids and Satellites
 Asteroids
 most of asteroids orbits in the ecliptic plane
 significant portion of asteroids have tilted orbits
 dozens of retrograde asteroids have been discovered
 Satellites
 orbital axes of satellites are mostly aligned with the
spin axis of host planets
 dozens of satellites have tilted orbits or even
retrograde orbits (e.g., Triton around Neptune)
 Tilted or retrograde orbits are common for those bodies
and are explained by scattering with other bodies etc
Motivation to study exoplanetary orbits
Orbits of the Solar System bodies reflect
the formation history of the Solar System
How about extrasolar planets?
Planetary orbits would provide us information
about formation histories of exoplanetary systems!
Outline
• Brief overview of orbits of Solar System bodies
• Orbits of exoplanets and their migration models
• The Rossiter-McLaughlin effect and observations
• High-contrast direct imaging for tilted or eccentric
planetary systems
• Summary
Semi-Major Axis Distribution of Exoplanets
Snow line
Jupiter
Need planetary migration mechanisms!
Standard Migration Models
Type I and II migration mechanisms
 consider gravitational interaction between
 proto-planets and proto-planetary disk
• Type I: less than 10 Earth mass proto-planets
• Type II: more massive case (Jovian planets)
 well explain the semi-major axis distribution
 e.g., a series of Ida & Lin papers
 predict small eccentricities and small inclination for
migrated planets
Eccentricity Distribution
Eccentric
Planets
Jupiter
Cannot be explained by Type I & II migration model
Migration Models for Eccentric Planets
 consider gravitational interaction between
 planet-planet (planet-planet scattering models)
 planet-binary companion (Kozai migration)
captured planets
ejected planet
Kozai mechanism
caused by perturbation from a distant companion
and angular momentum conservation
orbit 1: low eccentricity and high inclination
orbit 2: high eccentricity and low inclination
star
binary orbital plane
companion
originally for planet-satellite system (Kozai 1962)
Migration Models for Eccentric Planets
 consider gravitational interaction between
 planet-planet (planet-planet scattering models)
 planet-binary companion (Kozai migration)
 may be able to explain the whole orbital distribution
 e.g., Nagasawa+ 2008, Fabrycky & Tremaine 2007
 predict a variety of eccentricities
 and also predict misalignments between stellar-spin and
planetary-orbital axes
Examples of Obliquity Prediction
Tilted and even retrograde planets are predicted.
Morton & Johnson (2010)
How can we test these models by observations?
Outline
• Brief overview of orbits of Solar System bodies
• Orbits of exoplanets and their migration models
• The Rossiter-McLaughlin effect and observations
• High-contrast direct imaging for tilted or eccentric
planetary systems
• Summary
Planetary transits
transit in the Solar System
transit in exoplanetary systems
(we cannot spatially resolve)
2006/11/9
transit of Mercury
observed with Hinode
slightly dimming
If a planetary orbit passes in front of its host star by chance,
we can observe exoplanetary transits as periodical dimming.
The Rossiter-McLaughlin effect
When a transiting planet hides stellar rotation,
star
planet
planet
the planet hides the approaching side
the planet hides the receding side
→ the star appears to be receding
→ the star appears to be approaching
radial velocity of the host star would have
an apparent anomaly during transits.
What can we learn from RM effect?
The shape of RM effect
depends on the trajectory of a transiting planet.
well aligned
misaligned
Radial velocity during transits = the Keplerian motion and the RM effect
Gaudi & Winn (2007)
Observable parameter
λ： sky-projected angle between
the stellar spin axis and the planetary orbital axis
(e.g., Ohta+ 2005, Gaudi & Winn 2007, Hirano et al. 2010)
Subaru HDS Observations since 2006
HDS
Subaru
Iodine cell
What we got
aligned
TrES-1b: Narita et al. (2007)
aligned
retrograde
aligned
HD17156b: Narita et al. (2009a)
HAT-P-7b: Narita et al. (2009b)
tilted
tilted
XO-4b: Narita et al. (2010c)
TrES-4b: Narita et al. (2010a)
HAT-P-11b: Hirano et al. (2010b)
Discovery of Retrograde Orbit: HAT-P-7b
NN et al. (2009b)
Subaru observation
through UH time
Winn et al. (2009c)
First RM Measurement for
Super-Neptune Planet：HAT-P-11b
Hirano et al. (2010b)
Results of Previous Observations
Our group: Subaru telescope
 13 targets observed
 7 papers published and 3 papers are in prep.
 5 out of 13 planets have tilted or retrograde orbit!
US: Keck telescope, UK, France: HARPS at 3.6m telescope
 over 30 targets observed
 similar percentage planets have tilted or retrograde orbit
 now statistically assured
What we learned from RM measurements
Stellar Spin
Planetary
Orbit
 Tilted or retrograde planets are not rare
 p-p scattering or Kozai mechanism occur in exoplanetary systems
Remaining Problems
Which model is a dominant migration mechanism?
Morton & Johnson (2010)
The number of samples is still insufficient to answer statistically.
Remaining Problems
 One cannot distinguish between p-p scattering and Kozai
migration for each planetary system
 To specify a planetary migration mechanism for each system,
we need to search for counterparts of migration processes
 long term radial velocity measurements (< 10AU)
 direct imaging (> 10-100 AU)
Outline
• Brief overview of orbits of Solar System bodies
• Orbits of exoplanets and their migration models
• The Rossiter-McLaughlin effect and observations
• High-contrast direct imaging for tilted or eccentric
planetary systems
• Summary
Motivation for high-contrast direct imaging
The results of the RM effect encourage direct imaging because
 a significant part of planetary systems may have wide
separation massive bodies (e.g., scattered massive planets or
brown dwarfs, or binary companions)
 direct imaging for tilted or eccentric planetary systems may
allow us to specify a migration mechanism for each planetary
system
An example of this study: Target HAT-P-7
 not eccentric, but retrograde (NN+ 2009b, Winn et al. 2009c)
NN et al. (2009b)
Winn et al. (2009c)
very interesting target to search for outer massive bodies
Subaru’s new instrument: HiCIAO
• HiCIAO: High Contrast Instrument for next
generation Adaptive Optics
• PI: Motohide Tamura (NAOJ)
– Co-PI: Klaus Hodapp (UH), Ryuji Suzuki (TMT)
• 188 elements curvature-sensing AO and will
be upgraded to SCExAO (1024 elements)
• Commissioned in 2009
• Specifications and Performance
– 2048x2048 HgCdTe and ASIC readout
– Observing modes: DI, PDI (polarimetric mode),
SDI (spectral differential mode), & ADI; w/wo
occulting masks (>0.1")
– Field of View: 20"x20" (DI), 20"x10" (PDI), 5"x5"
(SDI)
– Contrast: 10^-5.5 at 1", 10^-4 at 0.15" (DI)
– Filters: Y, J, H, K, CH4, [FeII], H2, ND
– Lyot stop: continuous rotation for spider block
Observations
 Subaru/HiCIAO Observation: 2009 August 6
 Setup: H band, DI mode (FoV: 20’’ x 20’’)
 Total exposure time: 9.75 min
 Angular Differential Imaging (ADI: Marois+ 06) technique with
Locally Optimized Combination of Images (LOCI: Lafreniere+ 07)
 Calar Alto / AstraLux Norte Observation: 2009 October 30
 Setup: I’ and z’ bands, FoV: 12’’ x 12’’
 Total exposure time: 30 sec
 Lucky Imaging technique (Daemgen+ 09)
Result Images
N
NN et al. (2010b)
E
Left: Subaru HiCIAO image, 12’’ x 12’’, Upper Right: HiCIAO LOCI image, 6’’ x 6’’
Lower Right: AstraLux image, 12’’ x 12’’
Characterization of binary candidates
projected separation: ~1000 AU
Based on stellar SED (Table 3) in Kraus and Hillenbrand (2007).
Assuming that the candidates are main sequence stars
at the same distance as HAT-P-7.
Can these candidates cause Kozai migration?
 The perturbation of a binary must be the strongest in the
system to cause the Kozai migration (Innanen et al. 1997)
 If perturbation of another body is stronger
 Kozai migraion refuted
 If such an additional body does not exist
 both Kozai and p-p scattering still survive
An additional body ‘HAT-P-7c’
Winn et al. (2009c)
2007 and 2009 Keck data
2008 and 2010 Subaru data
(unpublished)
HJD - 2454000
Long-term RV trend ~20 m/s/yr is ongoing from 2007 to 2010
constraint on the mass and semi-major axis of ‘c’
(Winn et al. 2009c)
Result for the HAT-P-7 case
 We detected two binary candidates, but the Kozai migration
was excluded because perturbation by the additional body is
stronger than that by companion candidates
 As a result, we conclude that p-p scattering is the most likely
migration mechanism for this system
Ongoing and Future Subaru Observations
 There are numbers of tilted and/or eccentric transiting planets
 These planetary systems are interesting targets that we may be
able to discriminate planetary migration mechanisms
 No detection is still interesting to refute Kozai migration
 Detections of outer massive bodies are very interesting
 but It would take some time to confirm such bodies
Waiting 2nd Epoch and more…
speckle?
Summary
 RM measurements have discovered numbers of tilted and
retrograde planets
 Tilted or eccentric planets are explained by p-p scattering or
Kozai migration --> those mechanisms are not rare
 One problem is that we cannot distinguish between p-p
scattering and Kozai migration from orbital tilt or eccentricity
 High-contrast direct imaging can resolve the problem and may
allow us to specify migration mechanism for each system
 Further results will be reported in the near future!
How to constrain migration mechanism
 Step 1: Is there a binary candidate?
 No
 Kozai migration by a binary companion is excluded
 If a candidate exist → step 2
 both p-p scattering and Kozai migration survive
 need a confirmation of true binary nature
• common proper motion
• common peculiar radial velocity
• common distance (by spectral type)
How to constrain migration mechanism
 Step 2: calculate restricted region for 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
through Kozai mechanism PK,B (Innanen et al. 1997)
 If any additional body exists in the restricted region
 Kozai migraion excluded
 search for long-term RV trend is very important
 If no additional body is found in the region
 both Kozai and p-p scattering still survive
SEEDS Project
 SEEDS: Strategic Exploration of Exoplanets and Disks with Subaru
 First “Subaru Strategic Observations” PI: Motohide Tamura
 Using Subaru’s new instruments: HiCIAO & AO188
 total 120 nights over 5 years (10 semesters) with Subaru
 Direct imaging and census of giant planets and brown dwarfs around
solar-type stars in the outer regions (a few - 40 AU)
 Exploring proto-planetary disks and debris disks for origin of their
diversity and evolution at the same radial regions
 I am working in a sub-category of known planetary systems, especially
targeting for tilted or eccentric planetary systems
Future AO upgrade: SCExAO from 2011
Subaru Coronagraphic Extreme-AO System
AO188 limit
SCExAO limit
Remaining Problems
 Correlation with properties of planet and host star
 Need to observe more targets for statistics.
 One cannot distinguish between p-p scattering and Kozai
migration for each system
 Need to search for counterparts of migration processes
 long term radial velocity measurements (< 10AU)
 direct imaging (> 10-100 AU)