Mitsuhiko Honda

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 30u2009au (1u2009au 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.

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.

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.

The 10 Micron Spectra of Comet C/2002 V1 (NEAT) and C/2001 RX14 (LINEAR)*

We have carried out mid-infrared 8–13 � m spectroscopic observations of C/2002 V1 (NEAT) and C/2001 RX14 (LINEAR) on 2003 January 10–11 UT using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope. The spectra of C/2002 V1 showed the broad silicate feature with the 11.2 � m peak, indicating the presence of crystalline olivine grains. The spectra of C/2001 RX14 also showed the broad trapezoidal silicate feature. The silicate feature profile of C/2002 V1 is explained by a combination of small (0.1 � m) amorphous olivine and pyroxene, large (2.0 � m) amorphous silicate, and small crystalline forsterite grains, while that of C/2001 RX14 is explained by small and large amorphous silicate grains without crystalline silicate grains. The ubiquity of large grains and crystalline silicate grains among these Oort Cloud comets indicates processing of the primordial interstellar matter in the early solar system and incorporation into the region where these cometary nuclei were formed. Subject headings: comets: individual (C/2002 V1, C/2001 RX14) — infrared: solar system

Search for 17 μm H2 Pure Rotational Emission from Circumstellar Disks

We report the negative detection of the S(1) pure rotational line emission of molecular hydrogen at 17.035 μm for four young stars, HD 163296, MWC 863, CQ Tau, and LkCa 15, for which Infrared Space Observatory (ISO) observations detected the S(1) emission, with the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope. We did not detect the line emission toward the central star nor within 15 of the central star along the slit for any of the present targets. Upper limits of the present observations are much smaller than previous line flux estimates based on ISO Short Wavelength Spectrometer ( SWS) observations, irrespective of the intrinsic line width. The present results indicate that the emission detected by the ISO SWS is not concentrated within the disk. Our detection limit of the H2 S(1) emission corresponds to upper limits of the disk masses of (0.72-3.8) × 10-4 M☉ within a radius of ~30-45 AU for the optically thin emission from the gas of 150 K. The upper limits of the disk masses are significantly lower than those of the warm molecular hydrogen mass predicted by the model (Chiang & Goldreich 1997), suggesting that the optically thick emission from dust dominates in the radiation from the disks in the mid-infrared wavelength. We point out that detectability of the H2 emission in young stars depends on the evolution of the disks, particularly in the sedimentation and growth of dust grains in the disk.

No high-mass protostars in the silhouette young stellar object M17-SO1

The birth of very massive stars is not well understood, in contrast to the formation process of low-mass stars like our Sun. It is not even clear that massive stars can form as single entities; rather, they might form through the mergers of smaller ones born in tight groups. The recent claim of the discovery of a massive protostar in M17 (a nearby giant ionized region) forming through the same mechanism as low-mass stars has therefore generated considerable interest. Here we show that this protostar has an intermediate mass of only 2.5 to 8 solar masses (M[circdot]), contrary to the earlier claim of 20M[circdot] (ref. 8). The surrounding circumstellar envelope contains only 0.09M[circdot] and a much more extended local molecular cloud has 4–9M[circdot].