Shigeyuki Sako

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.

COMICS: the cooled mid-infrared camera and spectrometer for the Subaru telescope

In this paper, we present the design and test performance of the COMICS, the mid-IR instrument for the 8.2 m Subaru Telescope at Mauna Kea. The instrument has both imaging and long slit grating spectroscopy capabilities in the 8-26 micrometers wavelength range. In the camera section, there are selectable three sets of lens assembly, one for the 10 micrometers imaging, another for the 20 micrometers imaging, and the other for the 10 micrometers pupil imaging. This camera section has an SBRC 320 X 240 Si:As IBC array and serves as a slit viewer and as a camera pixel scale of 0.130 arcsec. The spectrograph section is designed to have fiber SBRC 320 X 240 Si:As IBC arrays. Five arrays will cover 8-13 micrometers wavelength range in two positions of the grating with resolving power around 2500. So far, two arrays are installed for the spectrograph section and full spectral region is covered with tilting the grating. Selectable four sets of gratings provide spectral resolution ranging from 250 to 10000 in the N band and around 2500 in the Q band.

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.

Kiso Supernova Survey (KISS) : Survey strategy

The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2.2x2.2 deg field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a three-minute exposure in g-band once every hour in our survey, reaching magnitude g~20-21. About 100 nights of telescope time per year have been spent on the survey since April 2012. The number of the shock breakout detections is estimated to be of order of 1 during our 3-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.

K3-50A: An Ultracompact H II Region Ionized by a Massive Stellar Cluster*

We have made imaging and spectroscopic observations of the ultracompact H II region K3-50A with a spatial resolution of 04 using a new mid-infrared instrument, the Cooled Mid-Infrared Camera and Spectrometer, on the 8.2 m Subaru Telescope. The spectra show thermal dust emission, 9.7 ?m silicate absorption, and fine-structure line emissions of [Ne II] at 12.8 ?m, [Ar III] at 8.99 ?m, and [S IV] at 10.51 ?m. From the maps of the continuum, line emissions, and/or derived dust parameters, we identify eight mid-infrared sources in K3-50A. Especially the central [Ne II] emission has been resolved into two peaks clearly. The ionization condition is investigated with the line flux ratios I([Ar III])/I([Ne II]) and I([S IV])/I([Ne II]). It is suggested that the spectral types of the ionizing stars in K3-50A correspond to B0-O8 V, which is much later than O5.5 V, the type estimated from radio continuum observations under a single-star assumption. The three line fluxes suggest a number of Ne+ ions greater than what is ionized by a single star of any spectral type, but the numbers of Ar2+ and S3+ are similar to that formed by a single O8-O9 V star and that by a single O7-O9 V star, respectively. From these features as well as the dust temperature and the correspondence of each identified source with the near-infrared source, we propose that K3-50A is excited by at least two (possibly three) ionizing stars. This is the first convincing example that a massive stellar cluster is ionizing an ultracompact H II region.