Yoshiko Kataza Okamoto

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.

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.

A Spectroscopic Study of Dust around 18 Oxygen-rich Mira Variables in the N Band. I. Dust Profiles

We have obtained N-band spectra for 18 M Mira variables with the Mid-Infrared Camera and Spectrometer (MICS) to investigate the dust materials formed around M Mira variables. The observed spectra show a wide variety; 15 of them show a strong feature at 10 μm, and eight and seven stars show 11 and 13 μm bumps, respectively. We have made an analysis of oxygen-rich dust spectra by using two representative spectra empirically derived. One is the 10 μm feature, and the other is the 11 μm feature. Linear combinations of the two dust spectra together with the stellar continuum can describe all the observed spectra quite successfully, indicating the presence of two kinds of dust materials around M Mira variables. The 10 μm feature is ascribed to silicate grains, while we propose to attribute the 11 μm feature to amorphous alumina grains. We found that crystalline silicates do not appear necessary to fit any component of the spectrum. We have compared the strengths of the dust emissions with the parameters of stellar variability. We found that the strength of the silicate emission increases as the light curve becomes asymmetric, while that of the alumina emission stays constant against the change of the asymmetry factor. This can be interpreted in terms of the differences in the dust-forming region.

Design for an aberration-corrected concave grating for a mid-infrared long-slit spectrometer

A new design for an aberration-corrected concave grating for the spectral region near 10 microm is presented. It was designed for use in the ground-based astronomical medium-resolution (lambda/Dlambda approximately 100) Mid-Infrared Camera and Spectrometer (MICS). It provides a flat focal plane for a wide spectral range (7.5-13.5 microm) with small aberrations, permitting efficient long-slit observations in the mid-infrared region. It permits a simple design of the spectrometer without collimator and camera mirrors, which is quite advantageous for cryogenic instruments. The grating has variable spacing grooves to reduce aberrations. In addition, the grating surface figure is designed to be toroidal and in the direction perpendicular to the grooves, aspherical, to suppress the aberrations further over a wide spectral range. The angle of the grooves is also varied to yield better efficiency near the blaze angle. The grating was fabricated by high-quality ultraprecision machining, which made these features possible. Test observations confirmed that the designed spectral resolution was achieved.

MICHI: a thermal-infrared instrument for the TMT

With the imminent launch of the JWST, the field of thermal-infrared (TIR) astronomy will enjoy a revolution. It is easy to imagine that all areas of infrared (IR) astronomy will be greatly advanced, but perhaps impossible to conceive of the new vistas that will be opened. To allow both follow-up JWST observations and a continuance of work started on the ground-based 8m’s, we continue to plan the science cases and instrument design for a TIR imager and spectrometer for early operation on the TMT. We present the current status of our science cases and the instrumentation plans, harnessing expertise across the TMT partnership. This instrument will be proposed by the MICHI team as a second-generation instrument in any upcoming calls for proposals.

Spatially Resolved Spectra of the Red Rectangle in the N-Band

We have observed the Red Rectangle by our newly developed instrument COMICS attached on the SUBARU 8.2 m telescope, and obtained images and spectra in the N-band (8–13.2µm). Diffraction limited spatial resolution (~ 0.35 arcsec) was almost achieved, revealing differences of dust emission profiles with its distance from the central star. In the nebula region (distance > 260 AU), UIR emissions at 7.7, 8.6, 11.27, and 12.7 µm were prominent, while spectra around the central star (distance < 260 AU) are dominated by strong continuum emissions.

Mid-infrared camera and spectrometer (MICS) and sky noise measurement in the N-band

We have developed the mid-IR Camera and Spectrometer (MICS), which optimized for ground based observations in the N-band atmospheric window. The MICS has two observing capabilities, imaging and long slit low-resolution spectroscopy. The major characteristics of the MICS are nearly diffraction-limited performance, both in imaging and in spectroscopy and the capability to take a spectrum of the whole N-band range with a spectral resolving power of 100 under one exposure. The MICS employs a 2D array of 128 by 128 Si:As BIB detector, an aberration-corrected concave grating, and a high-speed read out system of a compact design with high sensitivity. In mid-IR observations form the ground, there is a large background radiation from the telescope and the sky. The fluctuation of the background radiation is not well understood so far. We measured the sky fluctuation in the N- band region with the MICS on the UKIRT. These measurements have revealed that (1) the sky noise was dominant below 0.5 Hz when the sky condition was good, and (2) the sky noise has strong excess at the positions of atmospheric water vapor lines than those without water vapor lines. In this paper, we describe the design of the MICS, including optics, cryogenics, and electronics, and its performance when used on the UKIRT. We also discuss sky noise measured by the MICS in the N-band region.