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Measurements of Spin-Orbit Alignment Angles of Transiting Exoplanets
with the Subaru HDS
Norio NaritaNational Astronomical Observatory of
Japan
1/15
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Semi-Major Axis Distribution of Exoplanets
2/15Need planetary migration mechanisms!
Snow line
Jupiter
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Standard Migration Models
3/15
consider gravitational interaction between proto-planetary disk planets• Type I: less than 10 Earth mass proto-
planet• Type II: more massive case
well explain the semi-major axis distribution e.g., a series of Ida & Lin papers
predict small eccentricities for migrated planets
Type I and II migration mechanisms
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Eccentricity Distribution
4/15
Jupiter
Cannot be explained by Type I & II migration model.
Eccentric Planets
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Migration Models for Eccentric Planets
5/15
consider gravitational interaction between planet-planet (planet-planet scattering
models) planet-binary companion (the Kozai migration)
• Cf. Kozai = the first NAOJ directormay be able to explain eccentricity distribution
e.g., Nagasawa+ 2008, Chatterjee+ 2008predict a variety of eccentricities and
misalignments between stellar spin and planetary orbital axes
How can we confirm these models by observations?
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The Rossiter Effect for Transiting Planets
6/15
hide approaching side→ appear to be receding
hide receding side→ appear to be
approaching
planet planetstar
When a transiting planet hides stellar rotation,
radial velocity of the host star would show an anomaly.
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What can we learn from the effect?
Gaudi & Winn (2007)
The shape of the Rossiter effectdepends on the trajectory of the transiting
planet.well aligned misaligned
7/15
Radial velocity = Keplerian motion + Rossiter effect
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Spin-Orbit Alignment
λ : sky-projected angle betweenthe stellar spin axis and the planetary orbital axis(e.g., Ohta et al. 2005, Gimentz 2006, Gaudi & Winn 2007)
8/15
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Subaru Observations
9/15Iodine cell
HDS
Subaru
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TrES-1: Slow Rotation and Faint Target
10/15
Our first observation with Subaru/HDS – difficult target
Thanks to Subaru, clear detection of the Rossiter effect.
We confirmed a prograde orbit and alignment of the planet.
NN et al. (2007)
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TrES-3: Slower and Fainter Target
11/15
More difficult target
NN et al. in prep.Very weak detection of the Rossiter effect.
This planet also orbits in a prograde and aligned way.
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TrES-4: Fast Rotating Binary Target
12/15
NN et al. in prep.
The Rossiter effect is greater than the Keplerian motion!
Well aligned in spite of its binarity.NN et al. in prep.
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HD17156: Eccentric Target
13/15
NN et al. (2009)
Aligned in spite of its eccentricity.
Eccentric planet with the orbital period of 21.2
days.
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cf: Another Eccentric Target XO-3
14/15
Winn et al. (2009)λ = 37.3 ± 3.7 deg
Hebrard et al. (2008)λ = 70 ± 15 deg
Large misalignments were reported,
but not yet confirmed.
(still 2σ discrepancy)
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Statistical Study
15/15
Fabrycky and Winn (2009) considered two models, A perfectly aligned (with large dispersion) distribution
Corresponding to Type II migration model Aligned distribution + partly isotropic distribution
Corresponding to a combination of migration modelsWinn et al. (2009) reported
The second model is preferred with a confidence of 96.6%
But it is not robust because of few misaligned samples More samples still needed!