arXiv subjects

Norio Narita

Publications and source records attributed to Norio Narita.

At least 199 records · Page 11Linked to original sources

arXiv2011

XO-2b: a Prograde Planet with a Negligible Eccentricity, and an Additional Radial Velocity Variation

We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2$σ$ ($e=0.045\pm0.024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $λ=10^{\circ}\pm72^{\circ}$). The poor constraint of $λ$ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90$^{\circ}$). The data also provide an improved estimate of the mass of XO-2b as $0.62\pm0.02$ $M_{\rm Jup}$. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.

astro-ph.EP↗

arXiv2011

Further Observations of the Tilted Planet XO-3: A New Determination of Spin-Orbit Misalignment, and Limits on Differential Rotation

We report on observations of the Rossiter-McLaughlin (RM) effect for the XO-3 exoplanetary system. The RM effect for the system was previously measured by two different groups, but their results were statistically inconsistent. To obtain a decisive result we observed two full transits of XO-3b with the Subaru 8.2-m telescope. By modeling these data with a new and more accurate analytic formula for the RM effect, we find the projected spin-orbit angle to be λ=37.3 deg \pm 3.0 deg, in good agreement with the previous finding by Winn et al. (2009). In addition, an offset of ~22 m/s was observed between the two transit datasets. This offset could be a signal of a third body in the XO-3 system, a possibility that should be checked with future observations. We also attempt to search for a possible signature of the stellar differential rotation in the RM data for the first time, and put weak upper limits on the differential rotation parameters.

astro-ph.EP↗

arXiv2011

Improved Modeling of the Rossiter-McLaughlin Effect for Transiting Exoplanets

We present an improved formula for the anomalous radial velocity of the star during planetary transits due to the Rossiter-McLaughlin (RM) effect. The improvement comes from a more realistic description of the stellar absorption line profiles, taking into account stellar rotation, macroturbulence, thermal broadening, pressure broadening, and instrumental broadening. Although the formula is derived for the case in which radial velocities are measured by cross-correlation, we show through numerical simulations that the formula accurately describes the cases where the radial velocities are measured with the iodine absorption-cell technique. The formula relies on prior knowledge of the parameters describing macroturbulence, instrumental broadening and other broadening mechanisms, but even 30% errors in those parameters do not significantly change the results in typical circumstances. We show that the new analytic formula agrees with previous ones that had been computed on a case-by-case basis via numerical simulations. Finally, as one application of the new formula, we reassess the impact of the differential rotation on the RM velocity anomaly. We show that differential rotation of a rapidly rotating star may have a significant impact on future RM observations.

astro-ph.EP↗

arXiv2011

Two Upper Limits on the Rossiter-McLaughlin Effect, with Differing Implications: WASP-1 has a High Obliquity and WASP-2 is Indeterminate

We present precise radial-velocity measurements of WASP-1 and WASP-2 throughout transits of their giant planets. Our goal was to detect the Rossiter-McLaughlin (RM) effect, the anomalous radial velocity observed during eclipses of rotating stars, which can be used to study the obliquities of planet-hosting stars. For WASP-1 a weak signal of a prograde orbit was detected with ~2sigma confidence, and for WASP-2 no signal was detected. The resulting upper bounds on the RM amplitude have different implications for these two systems, because of the contrasting transit geometries and the stellar types. Because WASP-1 is an F7V star, and such stars are typically rapid rotators, the most probable reason for the suppression of the RM effect is that the star is viewed nearly pole-on. This implies the WASP-1 star has a high obliquity with respect to the edge-on planetary orbit. Because WASP-2 is a K1V star, and is expected to be a slow rotator, no firm conclusion can be drawn about the stellar obliquity. Our data and our analysis contradict an earlier claim that WASP-2b has a retrograde orbit, thereby revoking this system's status as an exception to the pattern that cool stars have low obliquities.

astro-ph.EP↗

arXiv2011

A comparison of spectroscopic methods for detecting starlight scattered by transiting hot Jupiters, with application to Subaru data for HD 209458b and HD 189733b

The measurement of the light scattered from extrasolar planets informs atmospheric and formation models. With the discovery of many hot Jupiter planets orbiting nearby stars, this motivates the development of robust methods of characterisation from follow up observations. In this paper we discuss two methods for determining the planetary albedo in transiting systems. First, the most widely used method for measuring the light scattered by hot Jupiters (Collier Cameron et al.) is investigated for application for typical echelle spectra of a transiting planet system, showing that detection requires high signal-to-noise ratio data of bright planets. Secondly a new Fourier analysis method is also presented, which is model-independent and utilises the benefits of the reduced number of unknown parameters in transiting systems. This approach involves solving for the planet and stellar spectra in Fourier space by least-squares. The sensitivities of the methods are determined via Monte Carlo simulations for a range of planet-to-star fluxes. We find the Fourier analysis method to be better suited to the ideal case of typical observations of a well constrained transiting system than the Collier Cameron et al. method. We apply the Fourier analysis method for extracting the light scattered by transiting hot Jupiters from high resolution spectra to echelle spectra of HD 209458 and HD 189733. Unfortunately we are unable to improve on the previous upper limit of the planet-to-star flux for HD 209458b set by space-based observations. A 1σupper limit on the planet-to-star flux of HD 189733b is measured in the wavelength range of 558.83-599.56 nm yielding ε < 4.5 \times 10-4. Improvement in the measurement of the upper limit of the planet-to-star flux of this system, with ground-based capabilities, requires data with a higher signal-to-noise ratio, and increased stability of the telescope.

astro-ph.EP↗

arXiv2010

Measurements of Transit Timing Variations for WASP-5b

We have observed 7 new transits of the `hot Jupiter' WASP-5b using a 61 cm telescope located in New Zealand, in order to search for transit timing variations (TTVs) which can be induced by additional bodies existing in the system. When combined with other available photometric and radial velocity (RV) data, we find that its transit timings do not match a linear ephemeris; the best fit χ^2 values is 32.2 with 9 degrees of freedom which corresponds to a confidence level of 99.982 % or 3.7 σ. This result indicates that excess variations of transit timings has been observed, due either to unknown systematic effects or possibly to real TTVs. The TTV amplitude is as large as 50 s, and if this is real, it cannot be explained by other effects than that due to an additional body or bodies. From the RV data, we put an upper limit on the RV amplitude caused by the possible secondary body (planet) as 21 m s^{-1}, which corresponds to its mass of 22-70 M_{Earth} over the orbital period ratio of the two planets from 0.2 to 5.0. From the TTVs data, using the numerical simulations, we place more stringent limits down to 2 M_{Earth} near 1:2 and 2:1 mean motion resonances (MMRs) with WASP-5b at the 3 σlevel, assuming that the two planets are co-planer. We also put an upper limit on excess of Trojan mass as 43 M_{Earth} (3 σ) using both RV and photometric data. We also find that if the possible secondary planet has non- or a small eccentricity, its orbit would likely be near low-order MMRs. Further follow-up photometric and spectroscopic observations will be required to confirm the reality of the TTV signal, and results such as these will provide important information for the migration mechanisms of planetary systems.

astro-ph.EP↗

arXiv2010

A Possible Tilted Orbit of the Super-Neptune HAT-P-11b

We report the detection of the Rossiter-McLaughlin effect for the eccentric, super-Neptune exoplanet HAT-P-11b, based on radial velocity measurements taken with HDS mounted on the Subaru 8.2m telescope, and simultaneous photometry with the FTN 2.0m telescope, both located in Hawai'i. The observed radial velocities during a planetary transit of HAT-P-11b show a persistent blue-shift, suggesting a spin-orbit misalignment in the system. The best-fit value for the projected spin-orbit misalignment angle is $λ= 103_{-19}^{+23}$ deg. Our result supports the notion that eccentric exoplanetary systems are likely to have significant spin-orbit misalignment (e.g., HD 80606, WASP-8, WASP-14, WASP-17, and XO-3). This fact suggests that not only hot-Jupiters but also super-Neptunes like HAT-P-11b had once experienced dynamical processes such as planet-planet scattering or the Kozai migration.

astro-ph.EP↗

arXiv2010

The Rossiter-McLaughlin Effect of the Transiting Exoplanet XO-4b

We report photometric and radial velocity observations of the XO-4 transiting planetary system, conducted with the FLWO 1.2m telescope and the 8.2m Subaru Telescope. Based on the new light curves, the refined transit ephemeris of XO-4b is $P = 4.1250828 \pm 0.0000040$ days and $T_c [BJD_TDB] = 2454485.93323 \pm 0.00039$. We measured the Rossiter-McLaughlin effect of XO-4b and estimated the sky-projected angle between the stellar spin axis and the planetary orbital axis to be $λ= -46.7^{\circ} ^{+8.1^{\circ}}_{-6.1^{\circ}}$. This measurement of $λ$ is less robust than in some other cases because the impact parameter of the transit is small, causing a strong degeneracy between $λ$ and the projected stellar rotational velocity. Nevertheless, our finding of a spin-orbit misalignment suggests that the migration process for XO-4b involved few-body dynamics rather than interaction with a gaseous disk. In addition, our result conforms with the pattern reported by Winn et al. (2010, ApJL, 718, L145) that high obliquities are preferentially found for stars with effective temperatures hotter than 6250~K.

astro-ph.EP↗

arXiv2010

The HAT-P-13 Exoplanetary System: Evidence for Spin-Orbit Alignment and a Third Companion

We present new radial-velocity measurements of HAT-P-13, a star with two previously known companions: a transiting giant planet "b" with an orbital period of 3 days, and a more massive object "c" on a 1.2 yr, highly eccentric orbit. For this system, dynamical considerations would lead to constraints on planet b's interior structure, if it could be shown that the orbits are coplanar and apsidally locked. By modeling the Rossiter-McLaughlin effect, we show that planet b's orbital angular momentum vector and the stellar spin vector are well-aligned on the sky (lambda = -0.9 +/- 8.5 deg), suggesting that the planetary orbits are also well-aligned. The refined orbital solution favors a slightly eccentric orbit for planet b (e = 0.0142_{-0.0044}^{+0.0052}), although it is not clear whether it is apsidally locked with c's orbit (Δω= 48_{-38}^{+25} deg). We find a long-term trend in the star's radial velocity and interpret it as evidence for an additional body "d", which may be another planet or a low-mass star. The next inferior conjunction of c, when a transit may happen, is expected on JD 2,455,315.2 +/- 1.9 (centered on UT 17h 2010 April 28).

astro-ph.EP↗

arXiv2010

Search for Outer Massive Bodies around Transiting Planetary Systems: Candidates of Faint Stellar Companions around HAT-P-7

We present results of direct imaging observations for HAT-P-7 taken with the Subaru HiCIAO and the Calar Alto AstraLux. Since the close-in transiting planet HAT-P-7b was reported to have a highly tilted orbit, massive bodies such as giant planets, brown dwarfs, or a binary star are expected to exist in the outer region of this system. We show that there are indeed two candidates for distant faint stellar companions around HAT-P-7. We discuss possible roles played by such companions on the orbital evolution of HAT-P-7b. We conclude that as there is a third body in the system as reported by Winn et al. (2009, ApJL, 763, L99), the Kozai migration is less likely while planet-planet scattering is possible.

astro-ph.EP↗

arXiv2010

Spin-Orbit Alignment of the TrES-4 Transiting Planetary System and Possible Additional Radial Velocity Variation

We report new radial velocities of the TrES-4 transiting planetary system, including observations of a full transit, with the High Dispersion Spectrograph of the Subaru 8.2m telescope. Modeling of the Rossiter-McLaughlin effect indicates that TrES-4b has closely aligned orbital and stellar spin axes, with $λ= 6.3^{\circ} \pm 4.7^{\circ}$. The close spin-orbit alignment angle of TrES-4b seems to argue against a migration history involving planet-planet scattering or Kozai cycles, although there are two nearby faint stars that could be binary companion candidates. Comparison of our out-of-transit data from 4 different runs suggest that the star exhibits radial velocity variability of $\sim$20 ms^-1 in excess of a single Keplerian orbit. Although the cause of the excess radial velocity variability is unknown, we discuss various possibilities including systematic measurement errors, starspots or other intrinsic motions, and additional companions besides the transiting planet.

astro-ph.EP↗

arXiv2009

Analytic Description of the Rossiter-McLaughlin Effect for Transiting Exoplanets: Cross-Correlation Method and Comparison with Simulated Data

We obtain analytical expressions for the velocity anomaly due to the Rossiter- McLaughlin effect, for the case when the anomalous radial velocity is obtained by cross-correlation with a stellar template spectrum. In the limit of vanishing width of the stellar absorption lines, our result reduces to the formula derived by Ohta et al. (2005), which is based on the first moment of distorted stellar lines. Our new formula contains a term dependent on the stellar linewidth, which becomes important when rotational line broadening is appreciable. We generate mock transit spectra for four existing exoplanetary systems (HD17156, TrES-2, TrES- 4, and HD209458) following the procedure of Winn et al. (2005), and find that the new formula is in better agreement with the velocity anomaly extracted from the mock data. Thus, our result provides a more reliable analytical description of the velocity anomaly due to the Rossiter-McLaughlin effect, and explains the previously observed dependence of the velocity anomaly on the stellar rotation velocity.

astro-ph.EP↗

arXiv2009

First Evidence of a Retrograde Orbit of Transiting Exoplanet HAT-P-7b

We present the first evidence of a retrograde orbit of the transiting exoplanet HAT-P-7b. The discovery is based on a measurement of the Rossiter-McLaughlin effect with the Subaru HDS during a transit of HAT-P-7b, which occurred on UT 2008 May 30. Our best-fit model shows that the spin-orbit alignment angle of this planet is λ= -132.6 (+10.5, -16.3) degrees. The existence of such a retrograde planet have been predicted by recent planetary migration models considering planet-planet scattering processes or the Kozai migration. Our finding provides an important milestone that supports such dynamic migration theories.

astro-ph.EP↗

arXiv2009

Gifts from Exoplanetary Transits

The discovery of transiting extrasolar planets has enabled us a number of interesting stduies. Transit photometry reveals the radius and the orbital inclination of transiting planets, and thereby we can learn the true mass and the density of respective planets by the combined information of radial velocity measurements. In addition, follow-up observations of transiting planets such as secondary eclipse, transit timing variations, transmission spectroscopy, and the Rossiter-McLaughlin effect provide us information of their dayside temperature, unseen bodies in systems, planetary atmospheres, and obliquity of planetary orbits. Such observational information, which will provide us a greater understanding of extrasolar planets, is available only for transiting planets. Here I briefly summarize what we can learn from transiting planets and introduce previous studies.

astro-ph.EP↗

arXiv2009

Improved Measurement of the Rossiter-McLaughlin Effect in the Exoplanetary System HD 17156

We present an improved measurement of the Rossiter-McLaughlin effect for the exoplanetary system HD 17156, based on radial-velocity data gathered with the Subaru 8.2m telescope throughout the planetary transit of UT 2008 November 7. The data allow for a precise and independent determination of the projected spin-orbit angle of this system: $λ= 10.0^{\circ} \pm 5.1^{\circ}$. This result supersedes the previous claim of $λ= 62^{\circ} \pm 25^{\circ}$ by Narita et al., which was based on lower-precision data with poor statistics. Thus the stellar spin and planetary orbital axes of the HD 17156 system are likely to be well-aligned, despite the planet's large orbital eccentricity suggesting a history of strong dynamical interactions.

astro-ph.EP↗

arXiv2009

On the Spin-Orbit Misalignment of the XO-3 Exoplanetary System

We present photometric and spectroscopic observations of the 2009 Feb. 2 transit of the exoplanet XO-3b. The new data show that the planetary orbital axis and stellar rotation axis are misaligned, as reported earlier by Hebrard and coworkers. We find the angle between the sky projections of the two axes to be 37.3 +/- 3.7 deg, as compared to the previously reported value of 70 +/- 15 deg. The significance of this discrepancy is unclear because there are indications of systematic effects. XO-3b is the first exoplanet known to have a highly inclined orbit relative to the equatorial plane of its parent star, and as such it may fulfill the predictions of some scenarios for the migration of massive planets into close-in orbits. We revisit the statistical analysis of spin-orbit alignment in hot-Jupiter systems. Assuming the stellar obliquities to be drawn from a single Rayleigh distribution, we find the mode of the distribution to be 13^{+5}_{-2} deg. However, it remains the case that a model representing two different migration channels--in which some planets are drawn from a perfectly-aligned distribution and the rest are drawn from an isotropic distribution--is favored over a single Rayleigh distribution.

astro-ph.EP↗

arXiv2008

Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b

We present new spectroscopic and photometric observations of the HAT-P-1 planetary system. Spectra obtained during three transits exhibit the Rossiter-McLaughlin effect, allowing us to measure the angle between the sky projections of the stellar spin axis and orbit normal, λ= 3.7 +/- 2.1 degrees. The small value of λfor this and other systems suggests that the dominant planet migration mechanism preserves spin-orbit alignment. Using two new transit light curves, we refine the transit ephemeris and reduce the uncertainty in the orbital period by an order of magnitude. We find a upper limit on the orbital eccentricity of 0.067, with 99% confidence, by combining our new radial-velocity measurements with those obtained previously.

astro-ph↗

arXiv2008

The Prograde Orbit of Exoplanet TrES-2b

We monitored the Doppler shift of the G0V star TrES-2 throughout a transit of its giant planet. The anomalous Doppler shift due to stellar rotation (the Rossiter-McLaughlin effect) is discernible in the data, with a signal-to-noise ratio of 2.9, even though the star is a slow rotator. By modeling this effect we find that the planet's trajectory across the face of the star is tilted by -9 +/- 12 degrees relative to the projected stellar equator. With 98% confidence, the orbit is prograde.

astro-ph↗