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

CoRoTlaunched 2006/12/27

Keplerlaunched 2009/3/6

<1.25 RE

First 4 Month Kepler Planet Candidates

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 Planetsaround 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 withRp/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

the planet hides the approaching side→ the star appears to be receding

the planet hides the receding side→ the star appears to be approaching

planet planetstar

When a transiting planet hides stellar rotation,

radial velocity of the host star would havean 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

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

de Mooij et al. (2011)

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!