Search for Transiting Planets around Nearby M Dwarfs Norio Narita (NAOJ)
Outline Introduction of Current Status of Exoplanet Studies Motivation for Transiting Planets around Nearby M Dwarfs Roadmap and Preparations for Discoveries High-Precision Near Infrared Photometry High-Precision Near Infrared Velocimetry Further Studies Summary
The First Discovery of an Exoplanet in 1995
How to Measure Precise RVs of Stars Blue: without cell, Red: with cell Iodine cell Star + Th-Ar
Year of Discovery Histogram (RV)
The First Discovery of a Transiting Planet Charbonneau et al. (2000) HD209458b
How to Search for Transiting Exoplanets Photometric follow-up of RV planets HD209458b, HD149026b 7 transiting planets so far Transit survey and RV follow-up
Transit Survey An image by TrES group
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
What we can learn from transit lightcurve stellar radius, orbital inclination, mid-transit time ratio of planet/star size planet radius stellar limb-darkening
When combined with RVs RVs provide minimum mass: M p sin i Transits provide planetary radius: R p orbital inclination: i Combined information provides planetary mass: M p planetary density: ρ
Inferring Internal Structure of Planets Solid line: H/He dominated Dashed line: 100% H 2 O Dotted line: 75% H 2 O, 22% Si, 3% Fe core Dot-dashed line: Earth-like Charbonneau et al. (2009)
Diversity of Planetary Interior Structure Charbonneau et al. (2006)
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 layer at 0.5 bar Blue: Vapor dominated atmosphere
Inferring Formation History of Planetary Systems captured planets Newton Press ejected planet
Recent Understanding of Planet Migration Stellar Spin Planetary Orbit As for close-in Jovian planets, tilted orbits are not so rare. --> Various migration mechanisms indeed occur in the universe.
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
Pre-Kepler Transiting Planets HAT-P-11 (Kepler-3) HAT-P-7 (Kepler-2) TrES-2 (Kepler-1)
First 4 Month Kepler Planet Candidates <1.25 R E 1235 Planet Candidates
54 candidates are in possible habitable zone. 5 are terrestrial size.
Kepler s Weakness Kepler targets relatively faint and far stars Although over 1000 candidates discovered, RV follow-ups for all 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) PLATO (PLAnetary Transits and Oscillations of stars) Those are in the selection process of NASA Explore mission and ESA Cosmic Vision (late 2010s) Ground-based transit survey for nearby M dwarfs MEarth lead by D. Charbonneau at Harvard IRD transit group (collaborating with UH etc)
Science Merits of Transiting Planets around Nearby M Dwarfs M dwarfs are 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 nearby 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
Some Characteristics of Transiting Planets stellar radius: planetary radius : semi-major axis: orbital period: Toward Earth Transit Probability: Transit Depth: Transit Duration: ~ Rs/a ~ (Rp/Rs) 2 ~ Rs P/a π
G3 V star cf. G3 V star vs M6 V star Rs ~ 1 Rsun ~ 0.005 AU Assuming Rp = 1 Earth radius ~ 0.01 Rsun Transit Probability: Rs/a ~ 0.5%, Transit Depth: (Rp/Rs) 2 ~ 0.01% Transit Duration: Rs P / a π ~ 14hr, Orbital Period: P ~ 365 days M6 V star Rs ~ 0.1 Rsun ~ 0.0005 AU Assuming Rp = 1 Earth radius ~ 0.01 Rsun Typical HZ: a~0.005 AU P~10 hr Transit Probability: Rs/a ~ 10%, Transit Depth: (Rp/Rs) 2 ~ 1% Transit Duration: Rs P / a π ~ 20 min
Difficulties of M Dwarfs M dwarfs are very faint in visible wavelength Typically V > 13 even it is within 10 pc High precision photometry and velocimetry is difficult with current optical instruments (although a small part of targets are bright) Stellar activity (e.g., starspots) will also cause intrinsic noise But M dwarfs are much brighter in near infrared wavelength
Our Strategy We focus on high precision photometry and velocimetry in near infrared wavelength to search for transiting planets around nearby M 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 studies
Ongoing Studies I started a collaboration with a UH group, who makes a transiting planet candidate catalog and a new all-sky nearby M dwarf catalog (Gaidos, Lepine, Hilton, Mann, Chang) We started high precision photometric follow-up of transiting planet candidates provided by the UH group using 188 cm telescope at Okayama Astrophysical Observatory Subaru IRD Transit Group contributes transit observations
Members of the Subaru IRD Transit Group Norio Narita (NAOJ) Akihiko Fukui (NAOJ, Okayama Astrophysical Obs) Teruyuki Hirano (Univ. of Tokyo) Takuya Suenaga (NAOJ, Sokendai) Yasuhiro Takahashi (Univ. of Tokyo) Hiroshi Ohnuki (Tokyo Institute of Technology)
How to Achieve High Precision NIR Photometry High precision (~1mmag or ~0.1%) photometry was considered to be very difficult previously It was because IR detectors tend to have numbers of bad pixels and large unevenness of sensitivity, and thus have a difficulty in precise flat-fielding A solution is to fix stellar position on the detector with image defocus
Example of Detector Image Stars are defocused and kept off from bad pixels.
Example at Okayama (J band) ~1mmag is achieved for J~10 target (Fukui et al. in prep.)
Example at Okayama (Ks band) ~1mmag is achieved for Ks~10 target (Ohnuki et al. in prep.)
Example at South Africa (JHKs) 1(J) - 3(Ks) mmag is achieved for JHKs < 10 target (Narita et al. in prep.)
Target Selection and Our Plan Based on SuperWASP (ground-based transit survey) archive data, transiting planet candidates with SNR > 6 around late-k or M dwarfs have been selected (over 50 candidates are observable from Okayama) Low resolution spectroscopy with UH2.2m telescope is ongoing to estimate characteristics of host stars (T eff etc) by the UH group 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 constrain mass
Okayama Proposal Accepted
Our Current Status We have achieved sufficient photometric precision to find transiting (terrestrial) planets around nearby (J < 10) M dwarfs We have selected transiting planet candidates from SuperWASP archive data Our Okayama proposals were successfully accepted (11B, 12A) A few targets were already observed, but were false positives We will follow-up ~50 candidates at Okayama in the next few years to find true planets (false positive probability is said to be about 90% from previous transit surveys)
Next Step If true transiting planets are discovered, we need to determine their orbits and masses via precise RV measurements Subaru IRD
Subaru IRD Planned Specification Wavelength: Most important 1.20-1.85 (goal 1.1-1.9) μm 1.4-1.8 μm for M stars (based on simulations using model M star spectra; table) Spectral resolution: 70,000(3pixel sampling) Pixel scale: 0.09 arcsec/pixel Slit: 0.27 x 3 arcsec^2 (goal x 5) Fiber-fed: Dispersive optics: Velocity precision: star + reference + sky + comb Echelle (high Blaze angle) & VPH-Grating x 2 1 m/s w/ laser frequency comb Detector: HgCdTe 4k x 4k (or 2k x 2k) Detector temperature: ~80K Optics temperature: -50 Cooler: Tip-Tilt: non-vibrating, LN2 Rlimit=18 & 0.27arcsec slit usable Courtesy: M. Tamura
Key Technology: Laser Frequency Comb Generating ultra-precise wavelength ruler covering J and H band. Courtesy: M. Tamura
Fund and Scheduling Grant-in-Aid for Specially Promoted Research, 22000005 (PI: M. Tamura) FY 1 (2010) Detailed design in each component FY 2 (2011) Fund On-site Progress Review >> Internal review >> CoDR FY 3 (2012) Manufacturing and component testing with prototype(s) FY 4 (2013) Assembling and testing FY 5 (2014) Commissioning and observations Courtesy: M. Tamura
Further Studies after IRD Commissioning RV follow-up for discovered transiting planets to determine their orbits and masses Known planets (currently only GJ1214b) and Kepler planet candidates around M dwarfs are also excellent targets Transit follow-up for planets discovered by RV planet survey using IRD Further Studies for characterizing planets The Rossiter-McLaughlin effect Transit timing variations Transmission spectroscopy / multi-band transit photometry
The Rossiter-McLaughlin Effect When a transiting planet hides stellar rotation, planet star planet the planet hides the approaching side the star appears to be receding the planet hides the receding side the star appears to be approaching radial velocity of the host star would have an apparent anomaly during transits.
On Formation History of M Dwarf Planets Are there any tilted or retrograde terrestrial planets? λ: sky-projected angle between the stellar spin axis and the planetary orbital axis (e.g., Ohta et al. 2005, Gaudi & Winn 2007, Hirano et al. 2010)
Transit Timing Variations (TTV) Another planet Transit timing is not constant if there is another body in the system.
Merits of TTV for M Dwarfs TTV amplitude becomes larger than that around FGK stars because the mass of the host star is smaller Although TTV for hot Jupiters around FGK stars were not so fruitful since hot Jupiters do not tend to have nearby planets, Earth-like planets around M dwarfs tend to be multiple based on theoretical simulations TTV measurements provide clues for additional planets and Subaru IRD can confirm the existence Thus detections of transiting planets outside HZ are still important
Planetary Atmospheric Compositions and Weather de Mooij et al. (2011) Currently GJ1214b is the one and only target, but we can study the diversity of planetary atmospheres in the near future
Summary I think searching for transiting planets around nearby M dwarfs is one of the most important exoplanet studies we should start now We have achieved sufficient photometric sensitivity to detect terrestrial transiting planets around nearby M dwarfs 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!