Search for New Transiting Exoplanets around
Nearby Cool Dwarfs and their Characterization
Norio Narita (NAOJ Fellow)
Special Thanks to IRD Transit Team Members
©NAOJ
Outline
• Motivation for Searching Transiting Exoplanets around
Nearby Cool Dwarfs
• Roadmap and Preparations for Discoveries
– High-Precision Near Infrared Photometry
– High-Precision Near Infrared Velocimetry
• Follow-up Studies
• Summary
Science Merits of Transiting Planets
• One can learn precise size of planets
– The information is only available for transiting planets
• Also, transit observations enable us to infer
– internal structure of planets
– atmospheric composition of planets
– orbital migration history of planets
When combined with RVs
RVs provide

minimum mass: Mp sin i
Transits provide

planetary radius: Rp

orbital inclination: i
Combined information provides

planetary mass: Mp

planetary density: ρ
Year of Discovery Histogram (Transit)
The number of transiting planets is rapidly growing.
Dedicated Space Mission for Transiting Planets
CoRoT
launched 2006/12/27
Kepler
launched 2009/3/6
First 4 Month Kepler Planet Candidates
<1.25 RE
1235 Planet Candidates
~50 candidates are in
possible habitable zone.
5 are terrestrial size.
Kepler’s Weakness
• Kepler targets relatively faint and far stars
• Although over 2000 candidates discovered
including possible habitable terrestrial planets,
RV follow-ups for the targets are difficult
• Further characterization studies are also difficult
• Kepler is good for statistical studies, but not for
detailed studies for each planet
Strategy of Future Transit Survey
• Future transit surveys will target nearby stars and
aim to detect terrestrial planets in habitable zone
• Space-based all-sky transit survey for bright stars
– TESS (Transiting Exoplanet Survey Satellite)
– It is in the selection process of NASA Explore mission
• Ground-based transit survey for nearby cool dwarfs
– MEarth lead by D. Charbonneau at Harvard
– IRD transit group (collaborating with UH etc)
Science Merits of Transiting Planets
around Nearby Cool (M-type) Dwarfs
• Good targets for searching transitng planets in HZ
– M dwarfs are cool and HZ of M dwarfs is relatively close-in
– Geometric transit probability becomes higher
– M dwarfs are small and thus transit depth becomes deeper
– M dwarfs are less massive and thus RV signal becomes larger
– Orbital period of planets in HZ is shorter
• Plenty of M dwarfs exist around the Solar System
– over 500 within 15 pc
• If detected, such planets are very interesting targets for further
follow-up studies using IRD, TMT, SPICA etc
Difficulties of Cool Dwarfs
• Cool dwarfs are very faint in visible wavelength
• High precision photometry and velocimetry is
difficult with current optical instruments (although
a small part of targets are bright)
• But cool dwarfs are much brighter in near infrared
wavelength
– 6>V-J>3 for M dwarfs, V-J>2 for late-K
Our Strategy
• We focus on high precision photometry and velocimetry in
near infrared wavelength to search for transiting planets
around nearby cool dwarfs
• Subaru will equip new IR Doppler instrument in near future
(IRD: PI, Motohide Tamura)
• Subaru IRD aims to achieve ~1 m/s precision for J < 10 targets
• Our strategy
– Before IRD -> Transit Survey for J < 10 targets
– After IRD -> RV follow-up and further characterization studies
Ongoing Studies
• We have started high precision photometric follow-up of
transiting planet candidates using 188 cm telescope at
Okayama Astrophysical Observatory (OAO)
©NAOJ
Target Selection and Our Plan
• Based on SuperWASP (ground-based transit survey)
archive data, transiting planet candidates around late-K or
M dwarfs have been selected (about 50 candidates are
observable from OAO)
• We aim to follow-up transiting planet candidates via high
precision NIR photometry so as to eliminate false positives
• If confirmed, we plan to conduct RV measurements with
current instruments (e.g., Subaru HDS) to confirm or
refute its planetary nature by its mass
Okayama Proposal Accepted from 2011
Current Status of Transit Survey
• 6 clear nights among 19 allocated nights
– This is near average ratio of clear nights
• 4 targets have been observed
• 3 targets turned out to be false positives
Current Status of Transit Survey
One candidate shows a transit-like dimming consistent with
Rp/Rs ~ 0.042 ± 0.002 (~20σ detection)
Further Studies
• RV follow-up for discovered transiting planet candidates to
determine their orbits and masses
• Transit follow-up for planets discovered by RV planet survey
using IRD
• Further Studies for characterizing planets
– The Rossiter-McLaughlin effect
– Transmission spectroscopy / multi-band transit photometry
– Known planets and Kepler planet candidates around cool dwarfs
(e.g., GJ1214b, KOI-254b) are also excellent targets
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.
(cf. Teruyuki Hirano’s talk yesterday)
Transit Spectroscopy / Band Photometry
star
Transit depth depends on lines or bands.
Inferring Atmospheric Composition of GJ1214b
de Mooij et al. (2011)
Green: H dominated with solar metallicity
Red: H dominated with sub-solar metallicity and cloud/haze layer at 0.5 bar
Blue: Vapor dominated atmosphere
Useful Signs for Discriminating Models
• Super-earths around cool dwarfs are good targets
• NIR region
– multi-band (especially narrow-band) photometry
• IRSF/SIRIUS, OAO/ISLE etc are useful
– spectro-photometry is the most efficient
• Subaru/FMOS, MOIRCS are very useful
• Blue optical region
– detecting Rayreigh Scattering
• Subaru/Suprime-Cam is useful
Summary
• Searching for transiting planets around nearby cool dwarfs is
one of the most important exoplanet studies we should start
now
• We have started high precision photometric follow-up of
transiting planet candidates at Okayama
• We aim to find transiting planets before commissioning of IRD
and take an advantage of the Subaru IRD over other groups
• Stay tuned for new results!