arXiv subjects

Lyu Abe

Publications and source records attributed to Lyu Abe.

At least 73 records · Page 4Linked to original sources

arXiv2016

Near-Infrared Imaging Polarimetry of LkCa 15: A Possible Warped Inner Disk

We present high-contrast H-band polarized intensity images of the transitional disk around the young solar-like star LkCa 15. By utilizing Subaru/HiCIAO for polarimetric differential imaging, both the angular resolution and the inner working angle reach 0.07" and r=0.1", respectively. We obtained a clearly resolved gap (width <~ 27 AU) at ~ 48 AU from the central star. This gap is consistent with images reported in previous studies. We also confirmed the existence of a bright inner disk with a misaligned position angle of 13+/-4 degree with respect to that of the outer disk, i.e., the inner disk is possibly warped. The large gap and the warped inner disk both point to the existence of a multiple planetary system with a mass of <~1Mjup.

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arXiv2016

A Substellar Companion to Pleiades HII 3441

We find a new substellar companion to the Pleiades member star, Pleiades HII 3441, using the Subaru telescope with adaptive optics. The discovery is made as part of the high-contrast imaging survey to search for planetary-mass and substellar companions in the Pleiades and young moving groups. The companion has a projected separation of 0".49 +/- 0".02 (66 +/- 2 AU) and a mass of 68 +/- 5 M_J based on three observations in the J-, H-, and K_S-band. The spectral type is estimated to be M7 (~2700 K), and thus no methane absorption is detected in the H band. Our Pleiades observations result in the detection of two substellar companions including one previously reported among 20 observed Pleiades stars, and indicate that the fraction of substellar companions in the Pleiades is about 10.0 +26.1/-8.8 %. This is consistent with multiplicity studies of both the Pleiades stars and other open clusters.

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arXiv2016

Constraining the Movement of the Spiral Features and the Locations of Planetary Bodies within the AB Aur System

We present new analysis of multi-epoch, H-band, scattered light images of the AB Aur system. We used a Monte Carlo, radiative transfer code to simultaneously model the system's SED and H-band polarized intensity imagery. We find that a disk-dominated model, as opposed to one that is envelope dominated, can plausibly reproduce AB Aur's SED and near-IR imagery. This is consistent with previous modeling attempts presented in the literature and supports the idea that at least a subset of AB Aur's spirals originate within the disk. In light of this, we also analyzed the movement of spiral structures in multi-epoch H-band total light and polarized intensity imagery of the disk. We detect no significant rotation or change in spatial location of the spiral structures in these data, which span a 5.8 year baseline. If such structures are caused by disk-planet interactions, the lack of observed rotation constrains the location of the orbit of planetary perturbers to be >47 AU.

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arXiv2016

Extreme Asymmetry in the Disk of V1247 Ori

We present the first near-infrared scattered-light detection of the transitional disk around V1247 Ori, which was obtained using high-resolution polarimetric differential imaging observations with Subaru/HiCIAO. Our imaging in the H band reveals the disk morphology at separations of ~0.14"-0.86" (54-330 au) from the central star. The polarized intensity (PI) image shows a remarkable arc-like structure toward the southeast of the star, whereas the fainter northwest region does not exhibit any notable features. The shape of the arm is consistent with an arc of 0.28" $\pm$ 0.09" in radius (108 au from the star), although the possibility of a spiral arm with a small pitch angle cannot be excluded. V1247 Ori features an exceptionally large azimuthal contrast in scattered, polarized light; the radial peak of the southeastern arc is about three times brighter than the northwestern disk measured at the same distance from the star. Combined with the previous indication of an inhomogeneous density distribution in the gap at $\lesssim$46 au, the notable asymmetry in the outer disk suggests the presence of unseen companions and/or planet-forming processes ongoing in the arc.

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arXiv2016

High-Contrast Imaging of Intermediate-Mass Giants with Long-Term Radial Velocity Trends

A radial velocity (RV) survey for intermediate-mass giants has been operated for over a decade at Okayama Astrophysical Observatory (OAO). The OAO survey has revealed that some giants show long-term linear RV accelerations (RV trends), indicating the presence of outer companions. Direct imaging observations can help clarify what objects generate these RV trends. We present the results of high-contrast imaging observations or six intermediate-mass giants with long-term RV trends using the Subaru Telescope and HiCIAO camera. We detected co-moving companions to $γ$ Hya B ($0.61^{+0.12}_{-0.14} M_\odot$), HD 5608 B ($0.10 \pm 0.01 M_\odot$), and HD 109272 B ($0.28 \pm 0.06 M_\odot$). For the remaining targets($ι$ Dra, 18 Del, and HD 14067) we exclude companions more massive than 30-60 $M_\mathrm{Jup}$ at projected separations of 1arcsec-7arcsec. We examine whether these directly imaged companions or unidentified long-period companions can account for the RV trends observed around the six giants. We find that the Kozai mechanism can explain the high eccentricity of the inner planets $ι$ Dra b, HD 5608 b, and HD 14067 b.

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arXiv2016

Experimental demonstration of binary shaped pupil mask coronagraphs for telescopes with obscured pupils

We present the fabrication and experimental demonstration of three free-standing binary shaped pupil mask coronagraphs, which are applicable for telescopes with partially obscured pupils. Three masks, designed to be complementary (labeled Mask-A, Mask-B, and Mask-C), were formed in 5 micron thick nickel. The design of Mask-A is based on a one-dimensional barcode mask. The design principle of Mask-B is similar, but has a smaller inner working angle and a lower contrast than Mask-A. Mask-C is based on a concentric ring mask and provides the widest dark region and a symmetric point spread function. Mask-A and Mask-C were both designed to produce a flexibly tailored dark region (i.e., non-uniform contrast). The contrast was evaluated using a light source comprising a broadband super-luminescent light-emitting diode with a center wavelength of 650 nm, and the measurements were carried out in a large vacuum chamber. Active wavefront control was not applied in this work. The coronagraphic images obtained by experiment were mostly consistent with the designs. The contrast of Mask-A within the ranges 3.3 - 8 lambda/D and 8 - 12 lambda/D was ~10^{-4} - 10^{-7} and ~10^{-7}, respectively, where lambda is the wavelength and D is the pupil diameter. The contrast close to the center of Mask-B was ~10^{-4} and that of Mask-C over an extended field of view (5 - 25 lambda/D) was ~10^{-5} - 10^{-6}. The effect of tilting the masks was investigated, and found to be irrelevant at the ~10^{-7} contrast level. Therefore the masks can be tilted to avoid ghosting. These high-contrast free-standing masks have the potential to enable coronagraphic imaging over a wide wavelength range using both ground-based and space-borne general-purpose telescopes with pupil structures not specifically designed for coronagraphy.

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arXiv2015

Coronagraphic demonstration experiment using aluminum mirrors for space infrared astronomical observations

For future space infrared astronomical coronagraphy, we perform experimental studies on the application of aluminum mirrors to a coronagraph. Cooled reflective optics is required for broad-band mid-infrared observations in space, while high-precision optics is required for coronagraphy. For the coronagraph instrument originally proposed for the next-generation infrared astronomical satellite project SPICA (SCI: SPICA Coronagraph Instrument), we fabricated and evaluated the optics consisting of high-precision aluminum off-axis mirrors with diamond-turned surfaces, and conducted a coronagraphic demonstration experiment using the optics with a coronagraph mask. We first measured the wave front errors (WFEs) of the aluminum mirrors with a He-Ne Fizeau interferometer to confirm that the power spectral densities of the WFEs satisfy the SCI requirements. Then we integrated the mirrors into an optical system and evaluated the overall performance of the system. As a result, we estimate the total WFE of the optics to be 33 nm (rms), each mirror contributing 10-20 nm (rms) for the central 14 mm area of the optics, and obtain a contrast of 10^(-5.4) as a coronagraph in the visible light. At a wavelength of 5 um, the coronagraphic system is expected to achieve a contrast of ~10^(-7) based on our model calculation with the measured optical performance. Thus our experiment demonstrates that aluminum mirror optics is applicable to a highly WFE-sensitive instrument such as a coronagraph in space.

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arXiv2015

Near-Infrared Polarimetry of the GG Tauri A Binary System

A high angular resolution near-infrared polarized-intensity image of the GG Tau A binary system was obtained with the Subaru Telescope. The image shows the circumbinary disk scattering the light from the central binary. The azimuthal profile of the polarized intensity of the circumbinary disk is roughly reproduced by a simple disk model with the Henyey-Greenstein function and the Rayleigh function, indicating small dust grains at the surface of the disk. Combined with a previous observation of the circumbinary disk, our image indicates that the gap structure in the circumbinary disk orbits anti-clockwise, while material in the disk orbit clockwise. We propose a shadow of material located between the central binary and the circumbinary disk. The separations and position angles of the stellar components of the binary in the past 20 years are consistent with the binary orbit with a = 33.4 AU and e = 0.34.

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arXiv2015

Near-IR Polarized Scattered Light Imagery of the DoAr 28 Transitional Disk

We present the first spatially resolved polarized scattered light H-band detection of the DoAr 28 transitional disk. Our two epochs of imagery detect the scattered light disk from our effective inner working angle of 0.10" (13 AU) out to 0.50" (65 AU). This inner working angle is interior to the location of the system's gap inferred by previous studies using SED modeling (15 AU). We detected a candidate point source companion 1.08" northwest of the system; however, our second epoch of imagery strongly suggests that this object is a background star. We constructed a grid of Monte Carlo Radiative Transfer models of the system, and our best fit models utilize a modestly inclined (50 deg), 0.01 Msun disk that has a partially depleted inner gap from the dust sublimation radius out to ~8 AU. Subtracting this best fit, axi-symmetric model from our polarized intensity data reveals evidence for two small asymmetries in the disk, which could be attributable to variety of mechanisms.

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arXiv2015

Thermalizing a telescope in Antarctica: Analysis of ASTEP observations

The installation and operation of a telescope in Antarctica represent particular challenges, in particular the requirement to operate at extremely cold temperatures, to cope with rapid temperature fluctuations and to prevent frosting. Heating of electronic subsystems is a necessity, but solutions must be found to avoid the turbulence induced by temperature fluctua- tions on the optical paths. ASTEP 400 is a 40 cm Newton telescope installed at the Concordia station, Dome C since 2010 for photometric observations of fields of stars and their exoplanets. While the telescope is designed to spread star light on several pixels to maximize photometric stability, we show that it is nonetheless sensitive to the extreme variations of the seeing at the ground level (between about 0.1 and 5 arcsec) and to temperature fluctuations between --30 degrees C and --80 degrees C. We analyze both day-time and night-time observations and obtain the magnitude of the seeing caused by the mirrors, dome and camera. The most important effect arises from the heating of the primary mirror which gives rise to a mirror seeing of 0.23 arcsec K--1 . We propose solutions to mitigate these effects.

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arXiv2015

Detailed structure of the outer disk around HD 169142 with polarized light in H-band

Coronagraphic imagery of the circumstellar disk around HD 169142 in H-band polarized intensity (PI) with Subaru/HiCIAO is presented. The emission scattered by dust particles at the disk surface in 0.2" <= r <= 1.2", or 29 <= r <= 174 AU, is successfully detected. The azimuthally-averaged radial profile of the PI shows a double power-law distribution, in which the PIs in r=29-52 AU and r=81.2-145 AU respectively show r^{-3}-dependence. These two power-law regions are connected smoothly with a transition zone (TZ), exhibiting an apparent gap in r=40-70 AU. The PI in the inner power-law region shows a deep minimum whose location seems to coincide with the point source at λ= 7 mm. This can be regarded as another sign of a protoplanet in TZ. The observed radial profile of the PI is reproduced by a minimally flaring disk with an irregular surface density distribution or with an irregular temperature distribution or with the combination of both. The depletion factor of surface density in the inner power-law region (r< 50 AU) is derived to be <= 0.16 from a simple model calculation. The obtained PI image also shows small scale asymmetries in the outer power-law region. Possible origins for these asymmetries include corrugation of the scattering surface in the outer region, and shadowing effect by a puffed up structure in the inner power-law region.

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arXiv2015

Near-IR High-Resolution Imaging Polarimetry of the SU Aur Disk: Clues for Tidal Tails?

We present new high-resolution ($\sim$0\farcs09) $H$-band imaging observations of the circumstellar disk around the T Tauri star SU Aur. Our observations with Subaru-HiCIAO have revealed the presence of scattered light as close as 0\farcs15 ($\sim$20 AU) to the star. Within our image, we identify bright emission associated with a disk with a minimum radius of $\sim$90 AU, an inclination of $\sim$35\degr from the plane of the sky, and an approximate P.A. of 15\degr for the major axis. We find a brightness asymmetry between the northern and southern sides of the disk due to a non-axisymmetric disk structure. We also identify a pair of asymmetric tail structures extending east and west from the disk. The western tail extends at least 2\farcs5 (350 AU) from the star, and is probably associated with a reflection nebula previously observed at optical and near-IR wavelengths. The eastern tail extends at least 1\arcsec (140 AU) at the present signal-to-noise. These tails are likely due to an encounter with an unseen brown dwarf, but our results do not exclude the explanation that these tails are outflow cavities or jets.

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arXiv2014

SEEDS Adaptive Optics Imaging of the Asymmetric Transition Disk Oph IRS 48 in Scattered Light

We present the first resolved near infrared imagery of the transition disk Oph IRS 48 (WLY 2-48), which was recently observed with ALMA to have a strongly asymmetric sub-millimeter flux distribution. H-band polarized intensity images show a $\sim$60AU radius scattered light cavity with two pronounced arcs of emission, one from Northeast to Southeast and one smaller, fainter and more distant arc in the Northwest. K-band scattered light imagery reveals a similar morphology, but with a clear third arc along the Southwestern rim of the disk cavity. This arc meets the Northwestern arc at nearly a right angle, revealing the presence of a spiral arm or local surface brightness deficit in the disk, and explaining the East-West brightness asymmetry in the H-band data. We also present 0.8-5.4$μ$m IRTF SpeX spectra of this object, which allow us to constrain the spectral class to A0$\pm$1 and measure a low mass accretion rate of 10$^{-8.5}$M$_{\odot}$/yr, both consistent with previous estimates. We investigate a variety of reddening laws in order to fit the mutliwavelength SED of Oph IRS 48 and find a best fit consistent with a younger, higher luminosity star than previous estimates.

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arXiv2014

Indications of M-dwarf Deficits in the Halo and Thick Disk of the Galaxy

We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20+/-13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52+/-13%) or the scale height must be decreased (~600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities.

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arXiv2014

Surface Geometry of Protoplanetary Disks Inferred From Near-Infrared Imaging Polarimetry

We present a new method of analysis for determining the surface geometry of five protoplanetary disks observed with near-infrared imaging polarimetry using Subaru-HiCIAO. Using as inputs the observed distribution of polarized intensity (PI), disk inclination, assumed properties for dust scattering, and other reasonable approximations, we calculate a differential equation to derive the surface geometry. This equation is numerically integrated along the distance from the star at a given position angle. We show that, using these approximations, the local maxima in the PI distribution of spiral arms (SAO 206462, MWC 758) and rings (2MASS J16042165-2130284, PDS 70) are associated with local concave-up structures on the disk surface. We also show that the observed presence of an inner gap in scattered light still allows the possibility of a disk surface that is parallel to the light path from the star, or a disk that is shadowed by structures in the inner radii. Our analysis for rings does not show the presence of a vertical inner wall as often assumed in studies of disks with an inner gap. Finally, we summarize the implications of spiral and ring structures as potential signatures of ongoing planet formation.

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arXiv2014

DISCO: a Spatio-Spectral Recombiner for Pupil Remapping Interferometry

Pupil-remapping is a new high-dynamic range imaging technique that has recently demonstrated feasibility on sky. The current prototypes present however deceiving limiting magnitude, restricting the current use to the brightest stars in the sky. We propose to combine pupil-remapping with spatio-spectral encoding, a technique first applied to the VEGA/CHARA interferometer. The result is an instrument proposal, called "Dividing Interferometer for Stars Characterizations and Observations" (DISCO). The idea is to take profit of wavelength multiplexing when using a spectrograph in order to pack as much as possible the available information, yet providing a potential boost of 1.5 magnitude if used in existing prototypes. We detail in this paper the potential of such a concept.

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arXiv2014

High-Resolution Submillimeter and Near-Infrared Studies of the Transition Disk around Sz 91

To reveal the structures of a transition disk around a young stellar object in Lupus, Sz 91, we have performed aperture synthesis 345 GHz continuum and CO(3--2) observations with the Submillimeter Array ($\sim1\arcsec$--3$\arcsec$ resolution), and high-resolution imaging of polarized intensity at the $K_s$-band by using the HiCIAO instrument on the Subaru Telescope ($0\farcs25$ resolution). Our observations successfully resolved the inner and outer radii of the dust disk to be 65 AU and 170 AU, respectively, which indicates that Sz 91 is a transition disk source with one of the largest known inner holes. The model fitting analysis of the spectral energy distribution reveals an H$_2$ mass of $2.4\times10^{-3}$ $M_\sun$ in the cold ($T<$30 K) outer part at $65 3\times10^{-9}$ $M_\sun$) of hot ($T\sim$180 K) dust possibly remains inside the inner hole of the disk. The structure of the hot component could be interpreted as either an unresolved self-luminous companion body (not directly detected in our observations) or a narrow ring inside the inner hole. Significant CO(3--2) emission with a velocity gradient along the major axis of the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an evolutionary stage immediately after the formation of protoplanets because of the large inner hole and the lower disk mass than other transition disks studied thus far.

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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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