arXiv subjects

Yasuhiro Hirahara

Publications and source records attributed to Yasuhiro Hirahara.

8 recordsLinked to original sources

arXiv2026

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $μ$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

astro-ph.IM↗

arXiv2025

Measurement of the Temperature Dependence of the Refractive Index of CdZnTe

We have been developing a CdZnTe immersion grating for a compact high-dispersion mid-infrared spectrometer (wavelength range 10--18 $μ$m, spectral resolution $R = λ/Δλ> 25,000$, operating temperature $T < 20$ K). Using an immersion grating, the spectrometer size can be reduced to $1/n$ ($n$: refractive index) compared to conventional diffraction gratings. CdZnTe is promising as a material for immersion gratings for the wavelength range. However, the refractive index $n$ of CdZnTe has not been measured at $T < 20$ K. We have been developing a system to precisely measure $n$ at cryogenic temperatures ($T \sim 10$ K) in the mid-infrared wavelength range. As the first result, this paper reports the temperature dependence of $n$ of CdZnTe at the wavelength of 10.68 $μ$m. This system employs the minimum deviation method. The refractive index $n$ of CdZnTe is measured at temperatures of $ T = 12.57, 22.47, 50.59, 70.57, \text{ and } 298 \, \text{K} $. We find that $n$ of CdZnTe at $λ=$ 10.68 $μ$m is $2.6371 \pm 0.0022$ at $12.57 \pm 0.14$ K, and the average temperature dependence of $n$ between 12.57 $\pm$ 0.14 K and 70.57 $\pm$ 0.23 K is $Δn/ΔT = (5.8 \pm 0.3) \times 10^{-5}$ K$^{-1}$.

astro-ph.IM↗

arXiv2025

Mid-Infrared Imaging Spectroscopy of N2O Solid Simulating the haze of trans-Neptunian objects

\ Nitrous oxide (N$_2$O) ice is likely to exist in trans-Neptunian objects such as Pluto and Triton, potentially formed through ultraviolet (UV) radiation from the Sun or cosmic ray irradiation of N$_2$ and CO ices. However, the mid-infrared spectral characteristics of N$_2$O ice in higher temperature regions (90-110 K), changes in mid-infrared spectra during UV irradiation, and the chemical network of nitrogen oxide (N$_x$O$_y$) ices remain insufficiently understood. This study aims to elucidate these aspects through in-situ mid-infrared spectral measurements of cryogenic particles using two-dimensional imaging Fourier transform infrared spectroscopy. Spectroscopic imaging confirmed strong absorption at 7.75 $μ$m (N$_2$O $ν_1$ vibrational mode), with weaker vibrational modes observed at 8.60 $μ$m (N$_2$O 2$ν_2$), 7.27 $μ$m (N$_2$O torsion), and 5.29 $μ$m (N$_2$O $ν_1$+$ν_2$). Annealing experiments simulating high-temperature conditions demonstrated that all vibrational modes irreversibly intensified with increasing temperature, indicating progressive crystallization. New spectral features appeared at approximately 12 $μ$m and 14 $μ$m at the condensed sample. N$_2$O ice was exposed to ultraviolet radiation (190-340 nm) using a D$_2$ lamp for 8.5 hours to investigate spectral changes during UV irradiation. After 60-90 minutes of irradiation, all N$_2$O vibrational modes disappeared, while absorption intensities of various nitrogen oxides, including NO, NO$_2$, N$_2$O$_3$, and O$_3$ increased. Beyond 180 minutes, vibrational modes of multiple nitrogen oxide ices exhibited intensity variations across different wavelengths, corresponding to other species such as cis-(NO)$_2$, N$_2$O$_4$, and N$_2$O$_5$.

astro-ph.EP↗

arXiv2023

GREX-PLUS Science Book

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA's strategic L-class mission to be launched in the 2030s. Its primary sciences are two-fold: galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2-8 $μ$m wavelength band and a high resolution spectrometer with a wavelength resolution of 30,000 in the 10-18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high resolution spectrometer aims to identify the location of the water ``snow line'' in proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on.

astro-ph.CO↗

arXiv2022

Simulations of the Spectral Resolving Power of a Compact Space-Borne Immersion-Echelle Spectrometer Using Mid-Infrared Wave Tracing

We performed wave-optics-based numerical simulations at mid-infrared wavelengths to investigate how the presence or absence of entrance slits and optical aberrations affect the spectral resolving power $R$ of a compact, high-spectral-resolving-power spectrometer containing an immersion-echelle grating. We tested three cases of telescope aberration (aberration-free, astigmatism and spherical aberration), assuming the aberration budget of the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), which has a 20-$\mathrm{μm}$-wavelength diffraction limit. In cases with a slit, we found that the value of $R$ at around 10--20 $\mathrm{μm}$ is approximately independent of the assumed aberrations, which is significantly different from the prediction of geometrical optics. Our results also indicate that diffraction from the slit improves $R$ by enlarging the effective illuminated area on the grating window and that this improvement decreases at short wavelengths. For the slit-less cases, we found that the impact of aberrations on $R$ can be roughly estimated using the Strehl ratio.

astro-ph.IM↗

arXiv2021

Infrared Absorption and its Sources of CdZnTe at Cryogenic Temperature

To reveal the infrared absorption causes in the wavelength region between electronic and lattice absorptions, we measured the temperature dependence of the absorption coefficient of $p$-type low-resistivity ($\sim 10^2~{\rm Ωcm}$) CdZnTe crystals. We measured the absorption coefficients of CdZnTe crystals in four-wavelength bands ($λ=6.45$, 10.6, 11.6, 15.1$~μ$m) over the temperature range of $T=8.6-300$ K with an originally developed system. The CdZnTe absorption coefficient was measured to be $α=0.3-0.5$ ${\rm cm^{-1}}$ at $T=300$ K and $α=0.4-0.9$ ${\rm cm^{-1}}$ at $T=8.6$ K in the investigated wavelength range. With an absorption model based on transitions of free holes and holes trapped at an acceptor level, we conclude that the absorption due to free holes at $T=150-300$ K and that due to trapped-holes at $T<50$ K are dominant absorption causes in CdZnTe. We also discuss a method to predict the CdZnTe absorption coefficient at cryogenic temperature based on the room-temperature resistivity.

astro-ph.IM↗

arXiv2016

ASTE Observation of Jupiter's Stratospheric Composition: Detection of Carbon Monosulfide ($J$=7--6) in 19 Years After the Cometary Impact

In Jupiter's stratosphere, gaseous carbon monosulfide (CS) was first discovered in 1994 by millimeter and ultraviolet observations as a product induced by the collision of comet Shoemaker-Levy 9 (SL9). {To constrain sulfur chemistry, in 2013, 19 years after the SL9 event, we observed Jupiter's stratospheric CS $J$=7 -- 6 rotational transition at 0.8 mm wavelength by using the Atacama Submillimeter Telescope Experiment (ASTE) 10-m single dish telescope. The CS molecular line was successfully detected with 120 mK intensity in the antenna temperature scale. The obtained CS total mass shows $\sim$90$\%$ decrease relative to that observed in 1998. From the line shape analysis, CS is suggested to be present above the 0.2$^{+0.4}_{-0.15}$ mbar pressure level, which is comparable to that of determined in 1998 which was provided by SL9 along with CS, was observed up to the 2.0 mbar pressure region. The discrepancy of CS vertical distribution with \ce{H2O} and the decrease in its total mass may be attributed to the chemical loss of CS in Jupiter's stratosphere lower than the 0.2 mbar altitude region.

astro-ph.EP↗

arXiv1993

Sequential Star Formation in Taurus Molecular Cloud 1

We discuss the fragmentation of a filamentary cloud on the basis of a 1-dimensional hydrodynamical simulation of a self-gravitating gas cloud. The simulation shows that dense cores are produced with a semi-regular interval in space and time from one edge to the other. At the initial stage the gas near one of the edges is attracted inwards by gravity and the accumulation of the gas makes a dense core near the edge. When the dense core grows in mass up to a certain amount, it gathers gas from the other direction. Accordingly the dense core becomes isolated from the main cloud and the parent filamentary cloud has a new edge. This cycle repeats and the fragmentation process propagates towards the other edge. The propagation speed is a few tens of percent larger than the sound speed. According to the theory, the age difference for the northwest-most and southeast-most cores in TMC-1 is estimated to be 0.68~pc/0.6~km~s$^{-1}$ = $ 10 ^6 $~y. The estimated age difference is consistent with that obtained from the chemical chronology.

astro-ph↗