Yoshiko K. Okamoto

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

We report high-resolution 1.6 μm polarized intensity (PI) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU (015) up to 554 AU (385), 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 (140 AU) of the disk while confirming the previously reported outer (r 200 AU) spiral structure. We have imaged a double ring structure at ~40 and ~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 ~45 AU or less) within two rings, as well as three prominent PI peaks at ~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.

An early extrasolar planetary system revealed by planetesimal belts in β Pictoris

β Pictoris (β Pic) is a main-sequence star with an edge-on dust disk that might represent a state of the early Solar System. The dust does not seem to be a remnant from the original protoplanetary disk, but rather is thought to have been generated from large bodies like planetesimals and/or comets. The history and composition of the parent bodies can therefore be revealed by determining the spatial distribution, grain size, composition and crystallinity of the dust through high-resolution mid-infrared observations. Here we report that the sub-micrometre amorphous silicate grains around β Pic have peaks in their distribution around 6, 16 and 30 au (1 au is the Sun–Earth distance), whereas the crystalline and micrometre-sized amorphous silicate grains are concentrated in the disk centre. As sub-micrometre grains are blown quickly out from the system by radiation pressure from the central star, the peaks indicate the locations of ongoing dust replenishment, which originates from ring-like distributions of planetesimals or ‘planetesimal belts’.

Detection of Crystalline Silicates around the T Tauri Star Hen 3-600A*

We have carried out mid-infrared N-band spectroscopic observations of the T Tauri star Hen 3-600A in the TW Hydra association with the Cooled Mid-Infrared Camera and Spectrometer on the 8.2 m Subaru Telescope and found structured features in its spectrum. These structured features are well explained by a combination of crystalline forsterite, crystalline enstatite, silica, and glassy olivine grains. Among intermediate-mass young stellar objects (YSOs), crystalline silicates have already been detected, but no firm detection has been reported so far for low-mass YSOs such as T Tauri stars. This is the first clear detection of crystalline silicates in low-mass YSOs, and it shows that the crystallization event occurs even in the protoplanetary disk of low-mass YSOs in the T Tauri phase. The physical processes leading to the inferred dust composition in the Hen 3-600A system may be analogous to those that occurred in the early epoch of the solar system.

The asymmetric thermal emission of the protoplanetary disk surrounding HD 142527 seen by Subaru/COMICS

Mid-infrared (MIR) images of the Herbig Ae star HD 142527 were obtained at 18.8 and 24.5 μm with the Subaru/COMICS. Bright extended arclike emission (outer disk) is recognized at r = 085 together with a strong central source (inner disk) and a gap around r = 06 in both images. The thermal emission on the eastern side is much brighter than that on the western side in the MIR. We estimate the dust size to be a few microns from the observed color of the extended emission and the distance from the star. The dust temperature T and the optical depth τ of the MIR-emitting dust are also derived from the two images as T = 82 ± 1 K, τ = 0.052 ± 0.001 for the eastern side and T = 85 ± 3 K, τ = 0.018 ± 0.001 for the western side. The observed asymmetry in the brightness can be attributed to the difference in the optical depth of the MIR-emitting dust. To account for the present observations, we propose an inclined disk model, in which the outer disk is inclined along the east-west direction with the eastern side being on the far side while the inner rim of the outer disk on the eastern side is directly exposed to us. The proposed model can successfully account for the MIR observations as well as the near-infrared images of the scattering light, in which the asymmetry is seen in the opposite sense and in which the forward scattering light (near side-western side) is brighter.

Resolved 24.5 micron emission from massive young stellar objects

Context. Massive young stellar objects (MYSO) are surrounded by massive dusty envelopes, whose physical structure and geometry are determined by the star formation process. Aims. Our principal aim is to establish the density structure of MYSO envelopes on scales of ∼1000 AU. This constitutes an increase of a factor ∼10 in angular resolution compared to similar studies performed in the (sub)mm. Methods. We have obtained diffraction-limited (0.6 �� ) 24.5 μm images (field of view of 40 �� × 30 �� ) of 14 well-known massive star formation regions with the COMICS instrument mounted on the 8.2 m Subaru telescope. We construct azimuthally averaged intensity profiles of the resolved MYSO envelopes and build spectral energy distributions (SEDs) from archival data and the COMICS 24.5 μm flux density. The SEDs range from near-infrared to millimeter wavelengths. Self-consistent 1-D radiative transfer models described by a density dependence of the form n(r) ∝ r −p are used to simultaneously compare the intensity profiles and SEDs to model predictions. Results. The images reveal the presence of discrete MYSO sources which are resolved on arcsecond scales, and, to first-order, the observed emission is circular on the sky. For many sources, the spherical models are capable of satisfactorily reproducing the 24.5 μm intensity profile, the 24.5 μm flux density, the 9.7 μm silicate absorption feature, and the submm emission. They are described by density distributions with p = 1.0 ± 0.25. Such distributions are shallower than those found on larger scales probed with single-dish (sub)mm studies. Other sources have density laws that are shallower/steeper than p = 1.0 and there is evidence that these are viewed near edge-on or near face-on respectively. In these cases spherical models fail to provide good fits to the data. The images also reveal a diffuse component tracing somewhat larger scale structures, particularly visible in the regions S 140, AFGL 2136, IRAS 20126+4104, Mon R2, and Cep A. Conclusions. We find a flattening of the MYSO density law going from scales probed with single-dish submm observations down to scales of ∼1000 AU probed with the observations presented here. We propose that this may be evidence of rotational support of the envelope. This finding will be explored further in a future paper using 2-D axisymmetric radiative transfer models.

Subaru/COMICS Study on Silicate Dust Processing around Young Low-Mass Stars*

We have obtained 8-13 ?m spectra of 30 young (1-10?Myr) low-mass pre-main-sequence stars using COMICS on the 8.2?m Subaru Telescope to examine dust evolution in protoplanetary disks. Most spectra show silicate emission features of various strengths and shapes, indicative of dust processing during the different stages of protoplanetary disk evolution. We have analyzed the observed silicate emission features using a simple model previously applied to the more massive and luminous Herbig Ae/Be systems. We determined the feature strength and shape and derived the composition and typical size of the silicate dust grains. We confirm the previously reported dependency of the silicate feature strength and shape on the grain size of the amorphous silicate dust. We examine the relation between the derived dust properties and stellar and circumstellar disk parameters, such as systemic age, luminosity of H? (LH?), disk mass, and opacity power-law index ? at radio wavelengths. A possible relation is found between silicate feature strength (grain size indicator) and the LH?, which may be an indicator of accretion activity. It implies that the turbulence induced by accretion activity may be important for grain size evolution in the disk. No clear correlation between the crystallinity and the stellar/disk parameters is found. We find that on average 5%-20% in mass of the silicate dust grains is in crystalline form, irrespective of systemic age. This latter finding supports the idea that crystalline silicate is formed at an early evolutionary phase, probably at the protostellar phase, and is remaining during the later stages.

Crystalline Silicate Feature of the Vega-like Star HD 145263*

We have observed the 8-13 μm spectrum (R ~ 250) of the Vega-like star candidate HD 145263 using Subaru/COMICS. The spectrum of HD 145263 shows the broad trapezoidal silicate feature with the shoulders at 9.3 and 11.44 μm, indicating the presence of crystalline silicate grains. This detection implies that crystalline silicate may also be commonly present around Vega-like stars. The 11.44 μm feature is slightly shifted to a longer wavelength compared to the usual 11.2-3 μm crystalline forsterite feature detected toward Herbig Ae/Be stars and T Tauri stars. Although the peak shift due to the effects of the grain size cannot be ruled out, we suggest that Fe-bearing crystalline olivine explains the observed peak wavelength fairly well. Fe-bearing silicates are commonly found in meteorites and most interplanetary dust particles, which originate from planetesimal-like asteroids. According to studies of meteorites, Fe-bearing silicate must have been formed in asteroidal planetesimals, supporting the scenario that dust grains around Vega-like stars are of planetesimal origin, if the observed 11.44 μm peak is due to Fe-bearing silicates.

Improved performances and capabilities of the Cooled Mid-Infrared Camera and Spectrometer (COMICS) for the Subaru Telescope

COMICS is an observatory and mid-infrared instrument for the 8.2 m Subaru Telescope. It is designed for imaging and spectroscopic observations in the N- (8-13 micron) and Q-bands (16-25 micron) atmospheric windows. The design and very preliminary performances at the first light observations in December 1999 were reported at the SPIE meeting in 2000. We describe here the improved performances of COMICS and capability of high spectral resolution spectrocopy which became available from December 2001. We will also briefly report preliminary scientific results.

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 0.2r1.2, or 29r174 AU, is successfully detected. The azimuthally-averaged radial profile of the PI shows a double power-law distribution, in which the PIs in r = 2952 AU and r = 81.2145 AU respectively show r3-dependence. These two power-law regions are connected smoothly with a transition zone (TZ), exhibiting an apparent gap in r = 4070 AU. The PI in the inner power-law region shows a deep minimum whose location seems to coincide with the point source at lambda = 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.

Improvements in Operating the Raytheon 320 × 240 Pixel Si:As Impurity Band Conduction Mid‐Infrared Array

ABSTRACT We have developed a new operating method for the Raytheon \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape