arXiv subjects

M. Ishii

Publications and source records attributed to M. Ishii.

At least 37 records · Page 2Linked to original sources

arXiv2012

The Structure of Pre-transitional Protoplanetary Disks I: Radiative Transfer Modeling of the Disk+Cavity in the PDS 70 system

Through detailed radiative transfer modeling, we present a disk+cavity model to simultaneously explain both the SED and Subaru H-band polarized light imaging for the pre-transitional protoplanetary disk PDS 70. Particularly, we are able to match not only the radial dependence, but also the absolute scale, of the surface brightness of the scattered light. Our disk model has a cavity 65 AU in radius, which is heavily depleted of sub-micron-sized dust grains, and a small residual inner disk which produces a weak but still optically thick NIR excess in the SED. To explain the contrast of the cavity edge in the Subaru image, a factor of ~1000 depletion for the sub-micron-sized dust inside the cavity is required. The total dust mass of the disk may be on the order of 1e-4 M_sun, only weakly constrained due to the lack of long wavelength observations and the uncertainties in the dust model. The scale height of the sub-micron-sized dust is ~6 AU at the cavity edge, and the cavity wall is optically thick in the vertical direction at H-band. PDS 70 is not a member of the class of (pre-)transitional disks identified by Dong et al. (2012), whose members only show evidence of the cavity in the millimeter-sized dust but not the sub-micron-sized dust in resolved images. The two classes of (pre-)transitional disks may form through different mechanisms, or they may just be at different evolution stages in the disk clearing process.

astro-ph.EP↗

arXiv2012

High-Resolution Near-Infrared Polarimetry of a Circumstellar Disk around UX Tau A

We present H-band polarimetric imagery of UX Tau A taken with HiCIAO/AO188 on the Subaru Telescope. UX Tau A has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0.15 (21 AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UX Tau A which extends to 120 AU, at a spatial resolution of 0.1 (14 AU). It is inclined by 46 \pm 2 degree as the west side is nearest. Although SED modeling and sub-millimeter imagery suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25 - 30 AU, we detect no evidence of a gap at the limit of our inner working angle (23 AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66 %) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh scattering nor Mie scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with the radii of 30 micron meter is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations and have grown in the circumstellar disk of UX Tau A.

astro-ph.SR↗

arXiv2012

High-Contrast NIR Polarization Imaging of MWC480

One of the key predictions of modeling from the IR excess of Herbig Ae stars is that for protoplanetary disks, where significant grain growth and settling has occurred, the dust disk has flattened to the point that it can be partially or largely shadowed by the innermost material at or near the dust sublimation radius. When the self-shadowing has already started, the outer disk is expected to be detected in scattered light only in the exceptional cases that the scale height of the dust disk at the sublimation radius is smaller than usual. High-contrast imaging combined with the IR spectral energy distribution allow us to measure the degree of flattening of the disk, as well as to determine the properties of the outer disk. We present polarimetric differential imaging in $H$ band obtained with Subaru/HiCIAO of one such system, MWC 480. The HiCIAO data were obtained at a historic minimum of the NIR excess. The disk is detected in scattered light from 0\farcs2-1\farcs0 (27.4-137AU). Together with the marginal detection of the disk from 1998 February 24 by HST/NICMOS, our data constrain the opening half angle for the disk to lie between 1.3$\leqθ\leq 2.2^\circ$. When compared with similar measures in CO for the gas disk from the literature, the dust disk subtends only $\sim$30% of the gas disk scale height (H/R$\sim$0.03). Such a dust disk is a factor of 5-7 flatter than transitional disks, which have structural signatures that giant planets have formed.

astro-ph.SR↗

arXiv2012

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory

We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r~46AU, our observations reveal the presence of scattered light components as close as 0.2" (~28AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0.5" (~70AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h~0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

astro-ph.EP↗

arXiv2012

The missing cavities in the SEEDS polarized scattered light images of transitional protoplanetary disks: a generic disk model

Transitional circumstellar disks around young stellar objects have a distinctive infrared deficit around 10 microns in their Spectral Energy Distributions (SED), recently measured by the Spitzer Infrared Spectrograph (IRS), suggesting dust depletion in the inner regions. These disks have been confirmed to have giant central cavities by imaging of the submillimeter (sub-mm) continuum emission using the Submillimeter Array (SMA). However, the polarized near-infrared scattered light images for most objects in a systematic IRS/SMA cross sample, obtained by HiCIAO on the Subaru telescope, show no evidence for the cavity, in clear contrast with SMA and Spitzer observations. Radiative transfer modeling indicates that many of these scattered light images are consistent with a smooth spatial distribution for micron-sized grains, with little discontinuity in the surface density of the micron-sized grains at the cavity edge. Here we present a generic disk model that can simultaneously account for the general features in IRS, SMA, and Subaru observations. Particularly, the scattered light images for this model are computed, which agree with the general trend seen in Subaru data. Decoupling between the spatial distributions of the micron-sized dust and mm-sized dust inside the cavity is suggested by the model, which, if confirmed, necessitates a mechanism, such as dust filtration, for differentiating the small and big dust in the cavity clearing process. Our model also suggests an inwardly increasing gas-to-dust-ratio in the inner disk, and different spatial distributions for the small dust inside and outside the cavity, echoing the predictions in grain coagulation and growth models.

astro-ph.EP↗

arXiv2011

Images of the Extended Outer Regions of the Debris Ring Around HR 4796 A

We present high-contrast images of HR 4796 A taken with Subaru/HiCIAO in H-band, resolving the debris disk in scattered light. The application of specialized angular differential imaging methods (ADI) allows us to trace the inner edge of the disk with high precision, and reveals a pair of "streamers" extending radially outwards from the ansae. Using a simple disk model with a power-law surface brightness profile, we demonstrate that the observed streamers can be understood as part of the smoothly tapered outer boundary of the debris disk, which is most visible at the ansae. Our observations are consistent with the expected result of a narrow planetesimal ring being ground up in a collisional cascade, yielding dust with a wide range of grain sizes. Radiation forces leave large grains in the ring and push smaller grains onto elliptical, or even hyperbolic trajectories. We measure and characterize the disk's surface brightness profile, and confirm the previously suspected offset of the disk's center from the star's position along the ring's major axis. Furthermore, we present first evidence for an offset along the minor axis. Such offsets are commonly viewed as signposts for the presence of unseen planets within a disk's cavity. Our images also offer new constraints on the presence of companions down to the planetary mass regime (~9 Jupiter masses at 0.5", ~3 Jupiter masses at 1").

astro-ph.SR↗

arXiv2011

Near-Infrared Multi-Band Photometry of the Substellar Companion GJ 758 B

GJ 758 B is a cold (~600K) companion to a Sun-like star at 29 AU projected separation, which was recently detected with high-contrast imaging. Here we present photometry of the companion in seven photometric bands from Subaru/HiCIAO, Gemini/NIRI and Keck/NIRC2, providing a rich sampling of the spectral energy distribution in the 1-5 micron wavelength range. A clear detection at 1.58 micron combined with an upper limit at 1.69 micron shows methane absorption in the atmosphere of the companion. The mass of the companion remains uncertain, but an updated age estimate indicates that the most likely mass range is ~30-40 Mjup. In addition, we present an updated astrometric analysis that imposes tighter constraints on GJ 758 B's orbit and identifies the proposed second candidate companion, "GJ 758 C", as a background star.

astro-ph.SR↗

arXiv2011

The properties of the T8.5p dwarf Ross 458C

We present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints (< 1Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has Teff = 695 \pm 60K, log g = 4.0 - 4.7 and an inferred mass of 5-20 MJup. We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5μm region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of dust opacity in the spectra of Ross 458C that was raised by Burgasser et al (2010). We have found that the BT Settl models which also incorporate a condensate cloud model, provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with fsed = 2, and we briefly discuss the influence of condensate clouds on T dwarf spectra.

astro-ph.SR↗

arXiv2011

Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae

We report high-resolution 1.6 $\micron$ polarized intensity ($PI$) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU ($0."15$) up to 554 AU (3.$"$85), which have been obtained by the high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed complicated and asymmetrical structures in the inner part ($\lesssim$140 AU) of the disk, while confirming the previously reported outer ($r$ $\gtrsim$200 AU) spiral structure. We have imaged a double ring structure at $\sim$40 and $\sim$100 AU and a ring-like gap between the two. We found a significant discrepancy of inclination angles between two rings, which may indicate that the disk of AB Aur is warped. Furthermore, we found seven dips (the typical size is $\sim$45 AU or less) within two rings as well as three prominent $PI$ peaks at $\sim$40 AU. The observed structures, including a bumpy double ring, a ring-like gap, and a warped disk in the innermost regions, provide essential information for understanding the formation mechanism of recently detected wide-orbit ($r$ $>$20 AU) planets.

astro-ph.SR↗

arXiv2010

Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15

We present H- and Ks-band imaging data resolving the gap in the transitional disk around LkCa 15, revealing the surrounding nebulosity. We detect sharp elliptical contours delimiting the nebulosity on the inside as well as the outside, consistent with the shape, size, ellipticity, and orientation of starlight reflected from the far-side disk wall, whereas the near-side wall is shielded from view by the disk's optically thick bulk. We note that forward-scattering of starlight on the near-side disk surface could provide an alternate interpretation of the nebulosity. In either case, this discovery provides confirmation of the disk geometry that has been proposed to explain the spectral energy distributions (SED) of such systems, comprising an optically thick outer disk with an inner truncation radius of ~46 AU enclosing a largely evacuated gap. Our data show an offset of the nebulosity contours along the major axis, likely corresponding to a physical pericenter offset of the disk gap. This reinforces the leading theory that dynamical clearing by at least one orbiting body is the cause of the gap. Based on evolutionary models, our high-contrast imagery imposes an upper limit of 21 Jupiter masses on companions at separations outside of 0.1" and of 13 Jupiter masses outside of 0.2". Thus, we find that a planetary system around LkCa 15 is the most likely explanation for the disk architecture.

astro-ph.SR↗

arXiv2010

Forty seven new T dwarfs from the UKIDSS Large Area Survey

We report the discovery of 47 new T dwarfs in the Fourth Data Release (DR4) from the Large Area Survey (LAS) of the UKIRT Infrared Deep Sky Survey with spectral types ranging from T0 to T8.5. These bring the total sample of LAS T dwarfs to 80 as of DR4. In assigning spectral types to our objects we have identified 8 new spectrally peculiar objects, and divide 7 of them into two classes. H2O-H-early have a H2O-H index that differs with the H2O-J index by at least 2 sub-types. CH4-J-early have a CH4-J index that disagrees with the H20-J index by at least 2 subtypes. We have ruled out binarity as a sole explanation for both types of peculiarity, and suggest that they may represent hitherto unrecognised tracers of composition and/or gravity. Clear trends in z'(AB)-J and Y-J are apparent for our sample, consistent with weakening absorption in the red wing of the KI line at 0.77microns with decreasing effective temperature. We have used our sample to estimate space densities for T6-T9 dwarfs. By comparing our sample to Monte-Carlo simulations of field T dwarfs for various mass functions of the form phi(M) \propto M^-alpha, we have placed weak constraints on the form of the field mass function. Our analysis suggests that the substellar mass function is declining at lower masses, with negative values of alpha preferred. This is at odds with results for young clusters that have been generally found to have alpha > 0.

astro-ph.GA↗

arXiv2008

Exploring the substellar temperature regime down to ~550K

We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ULAS J133553.45+113005.2 (ULAS1335).We detail optical and near-infrared photometry for all three sources, and mid-infrared photometry for ULAS1335. We use near-infrared spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1228) and T9 (ULAS1335) to these objects. We estimate that ULAS1017 has 750 < Teff < 850K, and 5.0 < log g < 5.5, assuming solar metallicity, an age of 1.6-15 Gyr, a mass of 33-70 MJ and lies at a distance of 31-54 pc. We extend the unified scheme of Burgasser et al. (2006) to the type T9 and suggest the inclusion of the WJ index to replace the now saturated J-band indices. ULAS1335 is the same spectral type as ULAS J003402.77-005206.7 and CFBDS J005910.90-011401.3. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600K. We find ULAS1335 to be extremely red in near to mid-infrared colours, with H-[4.49]=4.34+/-0.04. This is the reddest near to mid-infrared colour yet observed for a T dwarf, which supports Teff < 600K, and we estimate Teff ~550-600K for ULAS1335. We estimate that ULAS1335 has an age of 0.6-5.3 Gyr, a mass of 15-31 MJ and lies at a distance of 8-12 pc.

astro-ph↗

arXiv2007

Two T dwarfs from the UKIDSS Early Data Release

We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys. We aim to show that our methodologies for searching the ~27 sq degs of the LAS EDR are successful for finding both L and T dwarfs $via$ cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs. Ultracool dwarfs are selected by combinations of their YJH(K) UKIDSS colours and SDSS DR4 z-J and i-z colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4m and 8m-class facilities. Our main result is the discovery following CH4 imaging and spectroscopy of a T4.5 dwarf, ULASJ 1452+0655, lying ~80pc distant. A further T dwarf candidate, ULASJ 1301+0023, has very similar CH4 colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter L0 dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey J-band limit. Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky.

astro-ph↗

arXiv2003

A Study of the Distribution of Star-Forming Regions in Luminous Infrared Galaxies by Means of H$α$ Imaging Observations

We performed H-alpha imaging observations of 22 luminous infrared galaxies to investigate how the distribution of star-forming regions in these galaxies is related to galaxy interactions. Based on correlation diagrams between H-alpha flux and continuum emission for individual galaxies, a sequence for the distribution of star-forming regions was found: very compact (~100 pc) nuclear starbursts with almost no star-forming activity in the outer regions (type 1), dominant nuclear starbursts < 1 kpc in size and a negligible contribution from the outer regions (type 2), nuclear starbursts > 1 kpc in size and a significant contribution from the outer regions (type 3), and extended starbursts with relatively faint nuclei (type 4). These classes of star-forming region were found to be strongly related to global star-forming properties such as star-formation efficiency, far-infrared color, and dust extinction. There was a clear tendency for the objects with more compact distributions of star-forming regions to show a higher star-formation efficiency and hotter far-infrared color. An appreciable fraction of the sample objects were dominated by extended starbursts (type 4), which is unexpected in the standard scenario of interaction-induced starburst galaxies. We also found that the distribution of star-forming regions was weakly but clearly related to galaxy morphology: severely disturbed objects had a more concentrated distribution of star-forming regions. This suggests that the properties of galaxy interactions, such as dynamical phase and orbital parameters, play a more important role than the internal properties of progenitor galaxies, such as dynamical structure or gas mass fraction. We also discuss the evolution of the distribution of star-forming regions in interacting galaxies.

astro-ph↗

arXiv2001

Upgrade of Spring-8 Beamline Network with Vlan Technology Over Gigabit Ethernet

The beamline network system at SPring-8 consists of three LANs; a BL-LAN for beamline component control, a BL-USER-LAN for beamline experimental users and an OA-LAN for the information services. These LANs are interconnected by a firewall system. Since the network traffic and the number of beamlines have increased, we upgraded the backbone of BL-USER-LAN from Fast Ethernet to Gigabit Ethernet. And then, to establish the independency of a beamline and to raise flexibility of every beamline, we also introduced the IEEE802.1Q Virtual LAN (VLAN) technology into the BL-USER-LAN. We discuss here a future plan to build the firewall system with hardware load balancers.

cs.NI↗

arXiv2001

Development of an Interpreter Program for an Equipment Control

A message oriented software scheme is used for the SPring-8 beamline control. In the early stage, we developed customized control programs for each beamline. As the number of beamlines increased, however, it became necessary to introduce a general software scheme that cloud handle the control sequence for every beamline. We developed a new server program, Command Interpreter (CI), as a software framework. The CI interprets a high-level compound message issued from client programs and decomposes it to a set of primitive control messages. The message are sent to the VME computers afterwards. For flexibility, the operation sequences specific to the individual baemline component are standardized and defined in the interface definition files. Using the CI, the response time overhead is reduced to 26% and the XAFS measurement time also decreases by about 50%.

physics.acc-ph↗

arXiv2001

Toward a Reliable Gigabit Network - an Upgrade of the Spring-8 Network

The SPring-8 controls network has maintained accelerator operations in high reliability and shown good performance during the past years. To cope with the increase of loads on the network due to faster data acquisition and the addition of equipment data, networking hardware has been installed in the last few years. The upgraded network replaces the original FDDI backbone and switches with mixed FDDI/gigabit ethernet and Layer-3 switches. It is necessary to keep the double ring topology for the FDDI and introduce link aggregation technology for the gigabit ethernet to maintain the full redundancy and bandwidth of the system. This paper will discuss the network performance of the gigabit ethernet including its latency and redundancy. We also discuss a future plan for the network including Quality-of-Service over the gigabit ethernet.

physics.acc-ph↗

arXiv2000

XANES study of rare-earth valency in LRu4P12 (L = Ce and Pr)

Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES spectrum suggests that Ce is mainly trivalent, but the 4f state strongly hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We find that the metal-insulator (MI) transition at TMI = 60 K in PrRu4P12 does not originate from Pr valence fluctuation.

cond-mat.str-el↗