arXiv subjects

Teruyuki Hirano

Publications and source records attributed to Teruyuki Hirano.

At least 109 records · Page 6Linked to original sources

arXiv2016

EPIC 219388192 b - an inhabitant of the brown dwarf desert in the Ruprecht 147 open cluster

We report the discovery of EPIC 219388192 b, a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147, the oldest nearby open cluster association, which was photometrically monitored by K2 during its Campaign 7. We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system. EPIC 219388192 b has a radius of $R_\mathrm{b}$=$0.937\pm0.042$~$\mathrm{R_{Jup}}$ and mass of $M_\mathrm{b}$=$36.50\pm0.09$~$\mathrm{M_{Jup}}$, yielding a mean density of $59.0\pm8.1$~$\mathrm{g\,cm^{-3}}$. The host star is nearly a Solar twin with mass $M_\star$=$0.99\pm0.05$~$\mathrm{M_{\odot}}$, radius $R_\star$=$1.01\pm0.04$~$\mathrm{R_{\odot}}$, effective temperature $\mathrm{T_{eff}}$=$5850\pm85$~K and iron abundance [Fe/H]=$0.03\pm0.08$~dex. Its age, spectroscopic distance, and reddening are consistent with those of Ruprecht-147, corroborating its cluster membership. EPIC 219388192 b is the first brown dwarf with precise determinations of mass, radius and age, and serves as benchmark for evolutionary models in the sub-stellar regime.

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arXiv2016

Doppler Monitoring of five K2 Transiting Planetary Systems

In an effort to measure the masses of planets discovered by the NASA {\it K2} mission, we have conducted precise Doppler observations of five stars with transiting planets. We present the results of a joint analysis of these new data and previously published Doppler data. The first star, an M dwarf known as K2-3 or EPIC~201367065, has three transiting planets ("b", with radius $2.1~R_{\oplus}$; "c", $1.7~R_{\oplus}$; and "d", $1.5~R_{\oplus}$). Our analysis leads to the mass constraints: $M_{b}=8.1^{+2.0}_{-1.9}~M_{\oplus}$ and $M_{c}$ < $ 4.2~M_{\oplus}$~(95\%~conf.). The mass of planet d is poorly constrained because its orbital period is close to the stellar rotation period, making it difficult to disentangle the planetary signal from spurious Doppler shifts due to stellar activity. The second star, a G dwarf known as K2-19 or EPIC~201505350, has two planets ("b", $7.7~R_{\oplus}$; and "c", $4.9~R_{\oplus}$) in a 3:2 mean-motion resonance, as well as a shorter-period planet ("d", $1.1~R_{\oplus}$). We find $M_{b}$= $28.5^{+5.4}_{-5.0} ~M_{\oplus}$, $M_{c}$= $25.6^{+7.1}_{-7.1} ~M_{\oplus}$ and $M_{d}$ < $14.0~M_{\oplus} $~(95\%~conf.). The third star, a G dwarf known as K2-24 or EPIC~203771098, hosts two transiting planets ("b", $5.7~R_{\oplus}$; and "c", $7.8~R_{\oplus}$) with orbital periods in a nearly 2:1 ratio. We find $M_{b}$= $19.8^{+4.5}_{-4.4} ~M_{\oplus}$ and $M_{c}$ = $26.0^{+5.8}_{-6.1}~M_{\oplus}$.....

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arXiv2016

The K2-ESPRINT Project IV: A Hot Jupiter in a Prograde Orbit with a Possible Stellar Companion

We report on the detection and early characterization of a hot Jupiter in a 3-day orbit around K2-34 (EPIC~212110888), a metal-rich F-type star located in the K2 Cycle 5 field. Our follow-up campaign involves precise radial velocity (RV) measurements and high-contrast imaging using multiple facilities. The absence of a bright nearby source in our high-contrast data suggests that the transit-like signals are not due to light variations from such a companion star. Our intensive RV measurements show that K2-34b has a mass of $1.773\pm0.086M_J$, confirming its status as a planet. We also detect the Rossiter-McLaughlin effect for K2-34b and show that the system has a good spin-orbit alignment ($λ=-1_{-9}^{+10}$ degrees). High-contrast images obtained by the HiCIAO camera on the Subaru 8.2-m telescope reveal a faint companion candidate ($Δm_H=6.19\pm 0.11$ mag) at a separation of $0\farcs36$. Follow-up observations are needed to confirm that the companion candidate is physically associated with K2-34. K2-34b appears to be an example of a typical "hot Jupiter," albeit one which can be precisely characterized using a combination of K2 photometry and ground-based follow-up.

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arXiv2016

The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf

We validate a $R_p=2.32\pm 0.24R_\oplus$ planet on a close-in orbit ($P=2.260455\pm 0.000041$ days) around K2-28 (EPIC 206318379), a metal-rich M4-type dwarf in the Campaign 3 field of the K2 mission. Our follow-up observations included multi-band transit observations from the optical to the near infrared, low-resolution spectroscopy, and high-resolution adaptive-optics (AO) imaging. We perform a global fit to all the observed transits using a Gaussian process-based method and show that the transit depths in all passbands adopted for the ground-based transit follow-ups ($r'_2, z_\mathrm{s,2}, J, H, K_\mathrm{s}$) are within $\sim 2σ$ of the K2 value. Based on a model of the background stellar population and the absence of nearby sources in our AO imaging, we estimate the probability that a background eclipsing binary could cause a false positive to be $< 2\times 10^{-5}$. We also show that K2-28 cannot have a physically associated companion of stellar type later than M4, based on the measurement of almost identical transit depths in multiple passbands. There is a low probability for a M4 dwarf companion ($\approx 0.072_{-0.04}^{+0.02}$), but even if this were the case, the size of K2-28b falls within the planetary regime. K2-28b has the same radius (within $1σ$) and experiences a similar irradiation from its host star as the well-studied GJ~1214b. Given the relative brightness of K2-28 in the near infrared ($m_\mathrm{Kep}=14.85$ mag and $m_H=11.03$ mag) and relatively deep transit ($0.6-0.7\%$), a comparison between the atmospheric properties of these two planets with future observations would be especially interesting.

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arXiv2015

Characterization of the K2-19 Multiple-Transiting Planetary System via High-Dispersion Spectroscopy, AO Imaging, and Transit Timing Variations

K2-19 (EPIC201505350) is an interesting planetary system in which two transiting planets with radii ~ 7 $R_{Earth}$ (inner planet b) and ~ 4 $R_{Earth}$ (outer planet c) have orbits that are nearly in a 3:2 mean-motion resonance. Here, we present results of ground-based follow-up observations for the K2-19 planetary system. We have performed high-dispersion spectroscopy and high-contrast adaptive-optics imaging of the host star with the HDS and HiCIAO on the Subaru 8.2m telescope. We find that the host star is relatively old (>8 Gyr) late G-type star ($T_{eff}$ ~ 5350 K, $M_s$ ~ 0.9 $M_{Sun}$, and $R_{s}$ ~ 0.9 $R_{Sun}$). We do not find any contaminating faint objects near the host star which could be responsible for (or dilute) the transit signals. We have also conducted transit follow-up photometry for the inner planet with KeplerCam on the FLWO 1.2m telescope, TRAPPISTCAM on the TRAPPIST 0.6m telescope, and MuSCAT on the OAO 1.88m telescope. We confirm the presence of transit-timing variations, as previously reported by Armstrong and coworkers. We model the observed transit-timing variations of the inner planet using the synodic chopping formulae given by Deck & Agol (2015). We find two statistically indistinguishable solutions for which the period ratios ($P_{c}/P_{b}$) are located slightly above and below the exact 3:2 commensurability. Despite the degeneracy, we derive the orbital period of the inner planet $P_b$ ~ 7.921 days and the mass of the outer planet $M_c$ ~ 20 $M_{Earth}$. Additional transit photometry (especially for the outer planet) as well as precise radial-velocity measurements would be helpful to break the degeneracy and to determine the mass of the inner planet.

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arXiv2015

A low stellar obliquity for WASP-47, a compact multiplanet system with a hot Jupiter and an ultra-short period planet

We have detected the Rossiter-Mclaughlin effect during a transit of WASP-47b, the only known hot Jupiter with close planetary companions. By combining our spectroscopic observations with Kepler photometry, we show that the projected stellar obliquity is $λ= 0^\circ \pm 24^\circ$. We can firmly exclude a retrograde orbit for WASP-47b, and rule out strongly misaligned prograde orbits. Low obliquities have also been found for most of the other compact multiplanet systems that have been investigated. The Kepler-56 system, with two close-in gas giants transiting their subgiant host star with an obliquity of at least 45$^\circ$, remains the only clear counterexample.

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arXiv2015

Characterizing Earth-like Planets Using a Combination of High-Dispersion Spectroscopy and High-Contrast Instruments: Doppler-shifted Water and Oxygen Lines

Future radial velocity, astrometric, and direct-imaging surveys will find nearby Earth-sized planets within the habitable zone in the near future. How can we search for water and oxygen in those nontransiting planets? We show that a combination of high-dispersion spectroscopic and coronagraphic techniques is a promising technique to detect molecular lines imprinted in the scattered light of Earth-like planets (ELPs). In this method, the planetary signals are spectroscopically separated from telluric absorption by using the Doppler shift. Assuming a long observing campaign (a 10-day exposure) using a high-dispersion spectrometer (R=50,000) with speckle suppression on a 30-m telescope, we simulate the spectra from ELPs around M dwarfs (whose stellar effective temperature is 2750-3750 K) at 5 pc. Performing a cross-correlation analysis with the spectral template of the molecular lines, we find that raw contrasts of $10^{-4}$ and $10^{-5}$ (using Y, J, and H bands) are required to detect water vapor at the 3 $σ$ and 16 $σ$ levels, respectively, for $T_\star$=3000 K. The raw contrast of $10^{-5}$ is required for a 6 $σ$ detection of the oxygen 1.27 $μ$m band. We also examine possible systematics, incomplete speckle subtraction, and the correction for telluric lines. When those are not perfect, a telluric water signal appears in the cross-correlation function. However, we find the planetary signal is separated from that resulting from the velocity difference. We also find that the intrinsic water lines in the Phoenix spectra are too weak to affect the results for water detection. We conclude that a combination of high-dispersion spectroscopy and high-contrast instruments can be a powerful means to characterize ELPs in the extremely large telescope era.

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arXiv2015

Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894

We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host transiting giant planets, with the Subaru 8.2m telescope and the High Dispersion Spectrograph (HDS). By global modeling of the high-precision radial-velocity data taken with Subaru/HDS and photometric ones taken by Kepler mission taking account of orbital brightness modulations (ellipsoidal variations, reflected/emitted light, etc.) of the host stars, we independently confirmed that Kepler-91 hosts a transiting planet with a mass of 0.66 M_Jup (Kepler-91b), and newly detected an offset of ~20 m s$^{-1}$ between the radial velocities taken at ~1-yr interval, suggesting the existence of additional companion in the system. As for KOI-1894, we detected possible phased variations in the radial velocities and light curves with 2--3 sigma confidence level which could be explained as a reflex motion and ellipsoidal variation of the star caused by the transiting sub-saturn-mass (~0.18 M_Jup) planet.

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arXiv2014

Global Analysis of KOI-977: Spectroscopy, Asteroseismology, and Phase-curve Analysis

We present a global analysis of KOI-977, one of the planet host candidates detected by {\it Kepler}. Kepler Input Catalog (KIC) reports that KOI-977 is a red giant, for which few close-in planets have been discovered. Our global analysis involves spectroscopic and asteroseismic determinations of stellar parameters (e.g., mass and radius) and radial velocity (RV) measurements. Our analyses reveal that KOI-977 is indeed a red giant in the red clump, but its estimated radius ($\gtrsim 20R_\odot=0.093$ AU) is much larger than KOI-977.01's orbital distance ($\sim 0.027$ AU) estimated from its period ($P_\mathrm{orb}\sim 1.35$ days) and host star's mass. RV measurements show a small variation, which also contradicts the amplitude of ellipsoidal variations seen in the light-curve folded with KOI-977.01's period. Therefore, we conclude that KOI-977.01 is a false positive, meaning that the red giant, for which we measured the radius and RVs, is different from the object that produces the transit-like signal (i.e., an eclipsing binary). On the basis of this assumption, we also perform a light-curve analysis including the modeling of transits/eclipses and phase-curve variations, adopting various values for the dilution factor $D$, which is defined as the flux ratio between the red giant and eclipsing binary. Fitting the whole folded light-curve as well as individual transits in the short cadence data simultaneously, we find that the estimated mass and radius ratios of the eclipsing binary are consistent with those of a solar-type star and a late-type star (e.g., an M dwarf) for $D\gtrsim 20$.

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arXiv2014

Multi-band, Multi-epoch Observations of the Transiting Warm Jupiter WASP-80b

WASP-80b is a warm Jupiter transiting a bright late-K/early-M dwarf, providing a good opportunity to extend the atmospheric study of hot Jupiters toward the lower temperature regime. We report multi-band, multi-epoch transit observations of WASP-80b by using three ground-based telescopes covering from optical (g', Rc, and Ic bands) to near-infrared (NIR; J, H, and Ks bands) wavelengths. We observe 5 primary transits, each of which in 3 or 4 different bands simultaneously, obtaining 17 independent transit light curves. Combining them with results from previous works, we find that the observed transmission spectrum is largely consistent with both a solar abundance and thick cloud atmospheric models at 1.7$σ$ discrepancy level. On the other hand, we find a marginal spectral rise in optical region compared to the NIR region at 2.9$σ$ level, which possibly indicates the existence of haze in the atmosphere. We simulate theoretical transmission spectra for a solar abundance but hazy atmosphere, finding that a model with equilibrium temperature of 600 K can explain the observed data well, having a discrepancy level of 1.0$σ$. We also search for transit timing variations, but find no timing excess larger than 50 s from a linear ephemeris. In addition, we conduct 43 day long photometric monitoring of the host star in the optical bands, finding no significant variation in the stellar brightness. Combined with the fact that no spot-crossing event is observed in the five transits, our results confirm previous findings that the host star appears quiet for spot activities, despite the indications of strong chromospheric activities.

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arXiv2014

Tidal evolution of the spin-orbit angle in exoplanetary systems

The angle between the stellar spin and the planetary orbit axes (spin-orbit angle) is supposed to carry valuable information on the initial condition of the planet formation and the subsequent migration history. Indeed current observations of the Rossiter- McLaughlin effect have revealed a wide range of spin-orbit misalignments for transiting exoplanets. We examine in detail the tidal evolution of a simple system comprising a Sun-like star and a hot Jupiter adopting the equilibrium tide and the inertial wave dissipation effects simultaneously. We find that the combined tidal model works as a very efficient realignment mechanism; it predicts three distinct states of the spin-orbit angle (i.e., parallel, polar, and anti-parallel orbits) for a while, but the latter two states eventually approach the parallel spin-orbit configuration. The intermediate spin-orbit angles as measured in recent observations are difficult to be achieved. Therefore the current model cannot reproduce the observed broad distribution of the spin-orbit angles, at least in its simple form. This indicates that the observed diversity of the spin-orbit angles may emerge from more complicated interactions with outer planets and/or may be the consequence of the primordial misalignment between the proto-planetary disk and the stellar spin, which requires future detailed studies.

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arXiv2014

Measurements of Stellar Inclinations for Kepler Planet Candidates II: Candidate Spin-Orbit Misalignments in Single and Multiple-Transiting Systems

We present a test for spin-orbit alignment for the host stars of 25 candidate planetary systems detected by the {\it Kepler} spacecraft. The inclination angle of each star's rotation axis was estimated from its rotation period, rotational line broadening, and radius. The rotation periods were determined using the {\it Kepler} photometric time series. The rotational line broadening was determined from high-resolution optical spectra with Subaru/HDS. Those same spectra were used to determine the star's photospheric parameters (effective temperature, surface gravity, metallicity) which were then interpreted with stellar-evolutionary models to determine stellar radii. We combine the new sample with the 7 stars from our previous work on this subject, finding that the stars show a statistical tendency to have inclinations near 90$^\circ$, in alignment with the planetary orbits. Possible spin-orbit misalignments are seen in several systems, including three multiple-planet systems (KOI-304, 988, 2261). Ideally these systems should be scrutinized with complementary techniques---such as the Rossiter-McLaughlin effect, starspot-crossing anomalies or asteroseismology---but the measurements will be difficult owing to the relatively faint apparent magnitudes and small transit signals in these systems.

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arXiv2013

Characterization of the KOI-94 System with Transit Timing Variation Analysis: Implication for the Planet-Planet Eclipse

The KOI-94 system is a closely-packed, multi-transiting planetary system discovered by the Kepler space telescope. It is known as the first system that exhibited a rare event called a "planet-planet eclipse (PPE)," in which two planets partially overlap with each other in their double-transit phase. In this paper, we constrain the parameters of the KOI-94 system with an analysis of the transit timing variations (TTVs). Such constraints are independent of the radial velocity (RV) analysis recently performed by Weiss and coworkers, and valuable in examining the reliability of the parameter estimate using TTVs. We numerically fit the observed TTVs of KOI-94c, KOI-94d, and KOI-94e for their masses, eccentricities, and longitudes of periastrons, and obtain the best-fit parameters including $m_{\rm c} = 9.4_{-2.1}^{+2.4} M_{\oplus}$, $m_{\rm d} = 52.1_{-7.1}^{+6.9} M_{\oplus}$, $m_{\rm e} = 13.0_{-2.1}^{+2.5} M_{\oplus}$, and $e \lesssim 0.1$ for all the three planets. While these values are mostly in agreement with the RV result, the mass of KOI-94d estimated from the TTV is significantly smaller than the RV value $m_{\rm d} = 106 \pm 11 M_{\oplus}$. In addition, we find that the TTV of the outermost planet KOI-94e is not well reproduced in the current modeling. We also present analytic modeling of the PPE and derive a simple formula to reconstruct the mutual inclination of the two planets from the observed height, central time, and duration of the brightening caused by the PPE. Based on this model, the implication of the results of TTV analysis for the time of the next PPE is discussed.

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arXiv2013

Starspots - Transit Depth Relation of the Evaporating Planet Candidate KIC 12557548b

Violent variation of transit depths and an ingress-egress asymmetry of the transit light curve discovered in KIC 12557548 have been interpreted as evidences of a catastrophic evaporation of atmosphere with dust (M_p gtrsim 1 M_oplus/Gyr) from a close-in small planet. To explore what drives the anomalous atmospheric escape, we perform time-series analysis of the transit depth variation of Kepler archival data for ~ 3.5 yr. We find a ~ 30% periodic variation of the transit depth with P1 = 22.83 pm 0.21 days, which is within the error of the rotation period of the host star estimated using the light curve modulation, Prot = 22.91 pm 0.24 days. We interpret the results as evidence that the atmospheric escape of KIC 12557548b correlates with stellar activity. We consider possible scenarios that account for both the mass loss rate and the correlation with stellar activity. X-ray and ultraviolet (XUV)-driven evaporation is possible if one accepts a relatively high XUV flux and a high efficiency for converting the input energy to the kinetic energy of the atmosphere. Star-planet magnetic interaction is another possible scenario though huge uncertainty remains for the mass loss rate.

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arXiv2013

A Discovery of a Candidate Companion to a Transiting System KOI-94: A Direct Imaging Study for a Possibility of a False Positive

We report a discovery of a companion candidate around one of {\it Kepler} Objects of Interest (KOIs), KOI-94, and results of our quantitative investigation of the possibility that planetary candidates around KOI-94 are false positives. KOI-94 has a planetary system in which four planetary detections have been reported by {\it Kepler}, suggesting that this system is intriguing to study the dynamical evolutions of planets. However, while two of those detections (KOI-94.01 and 03) have been made robust by previous observations, the others (KOI-94.02 and 04) are marginal detections, for which future confirmations with various techniques are required. We have conducted high-contrast direct imaging observations with Subaru/HiCIAO in $H$ band and detected a faint object located at a separation of $\sim0.6''$ from KOI-94. The object has a contrast of $\sim 1\times 10^{-3}$ in $H$ band, and corresponds to an M type star on the assumption that the object is at the same distance of KOI-94. Based on our analysis, KOI-94.02 is likely to be a real planet because of its transit depth, while KOI-94.04 can be a false positive due to the companion candidate. The success in detecting the companion candidate suggests that high-contrast direct imaging observations are important keys to examine false positives of KOIs. On the other hand, our transit light curve reanalyses lead to a better period estimate of KOI-94.04 than that on the KOI catalogue and show that the planetary candidate has the same limb darkening parameter value as the other planetary candidates in the KOI-94 system, suggesting that KOI-94.04 is also a real planet in the system.

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arXiv2013

Multi-Color Transit Photometry of GJ 1214b through BJHKs-Bands and a Long-Term Monitoring of the Stellar Variability of GJ 1214

We present 5 new transit light curves of GJ 1214b taken in BJHKs-bands. Two transits were observed in B-band using the Suprime-Cam and the FOCAS instruments onboard the Subaru 8.2m telescope, and one transit was done in JHKs-bands simultaneously with the SIRIUS camera on the IRSF 1.4m telescope. MCMC analyses show that the planet-to-star radius ratios are, Rp/Rs = 0.11651 \pm 0.00065 (B-band, Subaru/Suprime-Cam), Rp/Rs = 0.11601 \pm 0.00117 (B-band, Subaru/FOCAS), Rp/Rs = 0.11654 \pm 0.00080 (J-band, IRSF/SIRIUS), Rp/Rs = 0.11550 ^{+0.00142}_{-0.00153} (H-band, IRSF/SIRIUS), and Rp/Rs = 0.11547 \pm 0.00127 (Ks-band, IRSF/SIRIUS). The Subaru Suprime-Cam transit photometry shows a possible spot-crossing feature. Comparisons of the new transit depths and those from previous studies with the theoretical models by Howe & Burrows (2012) suggest that the high molecular weight atmosphere (e.g., 1% H$_2$O + 99% N$_2$) models are most likely, however, the low molecular weight (hydrogen dominated) atmospheres with extensive clouds are still not excluded. We also report a long-term monitoring of the stellar brightness variability of GJ 1214 observed with the MITSuME 50cm telescope in g'-, Rc-, and Ic-bands simultaneously. The monitoring was conducted for 32 nights spanning 78 nights in 2012, and we find a periodic brightness variation with a period of Ps = 44.3 \pm 1.2 days and semi-amplitudes of 2.1% \pm 0.4% in g'-band, 0.56% \pm 0.08% in Rc-band, and 0.32% \pm 0.04% in Ic-band.

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arXiv2013

Optical-to-Near-Infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint for Cloud-free Atmosphere

We present optical (g', R_c, and I_c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, a Uranus-mass transiting planet around a nearby M dwarf, by using the 50-cm MITSuME telescope and the 188-cm telescope, both at Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 \pm 1.3 M_earth, 4.32^{+0.21}_{-0.10} R_earth, and 0.94 \pm 0.12 g cm^{-3}, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5-micron photometry (0.72^{+0.13}_{-0.12} g cm^{-3}). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R_p/R_s) in the J band (0.07577^{+0.00072}_{-0.00075}) is smaller than that in the I_c (0.0802 \pm 0.0013) and 4.5-micron (0.07806^{+0.00052}_{-0.00054}) bands by 5.9% \pm 2.0% and 3.0% \pm 1.2%, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed R_p/R_s variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H2O, CH4, and CO, without being prevented by clouds.

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arXiv2012

Planet-Planet Eclipse and the Rossiter-McLaughlin Effect of a Multiple Transiting System: Joint Analysis of the Subaru Spectroscopy and the Kepler Photometry

We report a joint analysis of the Rossiter-McLaughlin (RM) effect with Subaru and the Kepler photometry for Kepler Object of Interest (KOI) 94 system. The system comprises four transiting planet candidates with orbital periods of 22.3 (KOI-94.01), 10.4 (KOI-94.02), 54.3 (KOI-94.03), and 3.7 (KOI-94.04) days from the Kepler photometry. We performed the radial velocity (RV) measurement of the system with the Subaru 8.2 m telescope on August 10, 2012 (UT), covering a complete transit of KOI-94.01 for $\sim 6.7$ hours. The resulting RV variation due to the RM effect spectroscopically confirms that KOI-94.01 is indeed the transiting planet, and implies that its orbital axis is well aligned with the stellar spin axis; the projected spin-orbit angle $λ$ is estimated as $-6_{-11}^{+13}$ deg. This is the first measurement of the RM effect for a multiple transiting system. Remarkably, the archived Kepler lightcurve around BJD=2455211.5 (date in UT January 14/15, 2010) indicates a "ouble transit" event of KOI-94.01 and KOI-94.03, in which the two planets transit the stellar disk simultaneously. Moreover, the two planets partially overlap each other, and exhibit a "planet-planet eclipse" around the transit center. This provides a rare opportunity to put tight constraints on the configuration of the two transiting planets by joint analysis with our Subaru RM measurement. Indeed, we find that the projected mutual inclination of KOI-94.01 and KOI-94.03 is estimated to be $δ= -1.15 \pm 0.55$ deg. Implications for the migration model of multiple planet systems are also discussed.

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