• 천문학회보
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• 제46권2호
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• pp.66.3-67
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• 2021

Low-mass stars form by the gravitational collapse of dense molecular cores. Observations and theories of low-mass protostars both suggest that accretion bursts happen in timescales of ~100 years with high accretion rates, so called episodic accretion. One mechanism that triggers accretion bursts is infalling fragments from the outer disk. Such fragmentation happens when the disk is massive enough, preferentially activated during the embedded phase of star formation (Class 0 and I). Most observations and models focus on the gas structure of the protostars undergoing episodic accretion. However, the dust and ice composition are poorly understood, but crucial to the chemical evolution through thermal and energetic processing via accretion burst. During the burst phase, the surrounding material is heated up, and the chemical compositions of gas and ice in the disk and envelope are altered by sublimation of icy molecules from grain surfaces. Such alterations leave imprints in the ice composition even when the temperature returns to the pre-burst level. Thus, chemical compositions of gas and ice retain the history of past bursts. Infrared spectral observations of the Spitzer and AKARI revealed a signature caused by substantial heating, toward many embedded protostars at the quiescent phase. We present the AKARI IRC 2.5-5.0 ㎛ spectra for embedded protostars to trace down the characteristics of accretion burst across the evolutionary stages. The ice compositions obtained from the absorption features therein are used as a clock to measure the timescale after the burst event, comparing the analyses of the gas component that traced the burst frequency using the different refreeze-out timescales. We discuss ice abundances, whose chemical change has been carved in the icy mantle, during the different timescales after the burst ends.

• 천문학회보
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• 제43권1호
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• pp.47.1-47.1
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• 2018

The NISS (Near-infrared Imaging Spectrometer for Star formation history) onboard NEXTSat-1 have successfully developed by KASI. The capability of both imaging and spectroscopy is a unique function of the NISS. At first, it have realized the low-resolution spectroscopy (R~20) with a wide field of view of $2{\times}2deg$. in a wide near-infrared range from 0.95 to $2.5{\mu}m$. The major scientific mission is to study the cosmic star formation history in local and distant universe. It will also demonstrate the space technologies related to the infrared spectro-photometry in space. Now, the NISS is ready to launch in late 2018. After the launch, the NISS will be operated during 2 years. As an extension of the NISS, the SPEHREx (Spectro-Photometer for the History of the Universe Epoch of Reionization, and Ices Explorer) is the NASA MIDEX (Medium-class Explorer) mission proposed together with KASI (PI Institute: Caltech). It will perform the first all-sky infrared spectro-photometric survey to probe the origin of our Universe, to explore the origin and evolution of galaxies, and to explore whether planets around other stars could harbor life. Compared to the NISS, the SPHEREx is designed to have much more wide FoV of $3.5{\times}11.3deg$. as well as wide spectral range from 0.75 to $5.0{\mu}m$. After passing the first selection process, the SPHEREx is under the Phase-A study. The final selection will be made in the end of 2018. Here, we report the status of the NISS and SPHEREx missions.

• 천문학회보
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• 제41권1호
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• pp.58.2-58.2
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• 2016

The NISS (Near-infrared Imaging Spectrometer for Star formation history) onboard NEXTSat-1 is the near-infrared instrument optimized to the first small satellite of NEXTSat series. The capability of both imaging and low spectral resolution spectroscopy with the Field of View of $2{\times}2deg.$ in the near-infrared range from 0.9 to $3.8{\mu}m$ is a unique function of the NISS. The major scientific mission is to study the cosmic star formation history in local and distant universe. The Flight Model of the NISS is being developed and tested. After an integration into NEXTSat-1, it will be tested under the space environment. The NISS will be launched in 2017 and it will be operated during 2 years. As an extension of the NISS, SPEHREx (Spectro-Photometer for the History of the Universe Epoch of Reionization, and Ices Explorer) is the NASA SMEX (SMall EXploration) mission proposed together with KASI (PI Institute: Caltech). It will perform an all-sky near-infrared spectral survey to probe the origin of our Universe; explore the origin and evolution of galaxies, and explore whether planets around other stars could harbor life. The SPHEREx is designed to have wider FoV of $3.5{\times}7deg.$ as well as wider spectral range from 0.7 to $4.8{\mu}m$. After passing the first selection process, SPHEREx is under the Phase-A study. The final selection will be made in the end of 2016. Here, we report the current status of the NISS and SPHEREx missions.

• 천문학회보
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• 제44권1호
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• pp.57.2-57.2
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• 2019

The NISS (Near-infrared Imaging Spectrometer for Star formation history) onboard NEXTSat-1 was successfully launched on last December and is now under the operation phase. The capability of both imaging and spectroscopy is a unique function of the NISS. It has realized the imaging spectroscopy (R~20) with a wide field of view of $2{\times}2deg$. in a wide near-infrared range from 0.95 to $2.5{\mu}m$. The major scientific mission is to study the cosmic star formation history in the local and distant universe. It also demonstrated the space technologies related to the infrared spectro-photometry in space. The NISS is performing the imaging spectroscopic survey for local star-forming galaxies, clusters of galaxies, star-forming regions, ecliptic deep fields and so on. As an extension of the NISS, the SPEHREx (Spectro-Photometer for the History of the Universe Epoch of Reionization, and Ices Explorer) was selected as the NASA MIDEX (Medium-class Explorer) mission (PI Institute: Caltech). As an international partner, KASI will participate in the development and the science for SPHEREx. It will perform the first all-sky infrared spectro-photometric survey to probe the origin of our Universe, to explore the origin and evolution of galaxies, and to explore whether planets around other stars could harbor life. Compared to the NISS, the SPHEREx is designed to have a much wider FoV of $3.5{\times}11.3deg$. as well as wider spectral range from 0.75 to $5.0{\mu}m$. Here, we introduce the status of the two space missions.

• 천문학논총
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• 제32권1호
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• pp.225-230
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• 2017

The extragalactic background suggests half the energy generated by stars was reprocessed into the infrared (IR) by dust. At z~1.3, 90% of star formation is obscured by dust. To fully understand the cosmic star formation history, it is critical to investigate infrared emission. AKARI has made deep mid-IR observation using its continuous 9-band filters in the NEP field ($5.4deg^2$), using ~10% of the entire pointed observations available throughout its lifetime. However, there remain 11,000 AKARI infrared sources undetected with the previous CFHT/Megacam imaging (r ~25.9ABmag). Redshift and IR luminosity of these sources are unknown. These sources may contribute significantly to the cosmic star-formation rate density (CSFRD). For example, if they all lie at 1< z <2, the CSFRD will be twice as high at the epoch. We are carrying out deep imaging of the NEP field in 5 broad bands (g, r, i, z, and y) using Hyper Suprime-Camera (HSC), which has 1.5 deg field of view in diameter on Subaru 8m telescope. This will provide photometric redshift information, and thereby IR luminosity for the previously-undetected 11,000 faint AKARI IR sources. Combined with AKARI's mid-IR AGN/SF diagnosis, and accurate midIR luminosity measurement, this will allow a complete census of cosmic star-formation/AGN accretion history obscured by dust.

• 천문학논총
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• 제27권4호
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• pp.111-116
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• 2012

Far-infrared observations provide crucial data for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since most of its energy is emitted between ~ 100 and $200{\mu}m$. We present the first all-sky image from a sensitive all-sky survey using the Japanese AKARI satellite, in the wavelength range $50-180{\mu}m$. Covering > 99% of the sky in four photometric bands with four filters centred at $65{\mu}m$, $90{\mu}m$, $140{\mu}m$, and $160{\mu}m$ wavelengths, this achieved spatial resolutions from 1 to 2 arcmin and a detection limit of < 10 MJy $sr^{-1}$, with absolute and relative photometric accuracies of < 20%. All-sky images of the Galactic dust continuum emission enable astronomers to map the large-scale distribution of the diffuse ISM cirrus, to study its thermal dust temperature, emissivity and column density, and to measure the interaction of the Galactic radiation field and embedded objects with the surrounding ISM. In addition to the point source population of stars, protostars, star-forming regions, and galaxies, the high Galactic latitude sky is shown to be covered with a diffuse filamentary-web of dusty emission that traces the potential sites of high latitude star formation. We show that the temperature of dust particles in thermal equilibrium with the ambient interstellar radiation field can be estimated by using $90{\mu}m$, $140{\mu}m$, and $160{\mu}m$ data. The FIR AKARI full-sky maps provide a rich new data set within which astronomers can investigate the distribution of interstellar matter throughout our Galaxy, and beyond.

• 천문학회보
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• 제42권1호
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• pp.32.1-32.1
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• 2017

To dynamically and chemically understand how filaments, dense cores, and stars form under different environments, we are conducting a systematic mapping survey of nearby molecular clouds using the TRAO 14 m telescope with high ($N_2H^+$ 1-0, $HCO^+$ 1-0, SO 32-21, and $NH_2D$ v=1-0) and low ($^{13}CO$ 1-0, $C^{18}O$ 1-0) density tracers. The goals of this survey are to obtain the velocity distribution of low dense filaments and their dense cores for the study of their origin of the formation, to understand whether the dense cores form from any radial accretion or inward motions toward dense cores from their surrounding filaments, and to study the chemical differentiation of the filaments and the dense cores. Until Feb. 2017, the real OTF observation time is 460 hours. We have almost completed mapping observation with four molecular lines ($^{13}CO$ 1-0, $C^{18}O$ 1-0, $N_2H^+$ 1-0, and $HCO^+$ 1-0) on the five regions of molecular clouds (L1251 of Cepheus, Perseus west, Polaris south, BISTRO region of Serpense, California, and Orion B). The maps of a total area of $7.38deg^2$ for both $^{13}CO$ and $C^{18}O$ lines and $2.19deg^2$ for both $N_2H^+$ and $HCO^+$ lines were obtained. All OTF data were regridded to a cell size of 22 by 22 arcseconds. The $^{13}CO$ and $C^{18}O$ data show the RMS noise level of about 0.22 K and $N_2H^+$ and $HCO^+$ data show about 0.14 K at the velocity resolution of 0.06 km/s. Additional observations will be made on some regions that have not reached the noise level for analysis. We are refining the process for a massive amount of data and the data reduction and analysis are underway. This presentation introduces the overall progress from observations to data processing and the initial analysis results to date.

• Journal of Astronomy and Space Sciences
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• 제26권4호
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• pp.621-634
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• 2009

조선 초기의 보천가(步天歌)와 천문류초(天文類抄)에 대해 연구하였다. 이 연구에서는, 다음과 같은 근거로부터 이 보천가(步天歌)의 가결(歌訣)이 A.D. 1161년에 중국에서 출간된 통지(通志)에 실려 있는 보천가(步天歌)에 큰 영향을 받았음을 알았다. 두 보천가(步天歌)의 별자리들은 서술 순서가 같고, 태미원(太微垣)의 가결(歌訣)의 첫 구철이 일곱 글자가 아니라 다섯 글자로 되어 있으며, 특히 별자리들에 대한 은하수의 위치를 기술한 천하기몰(天河起沒)은 통지(通志)의 저자인 정초(鄭樵)가 특별히 작성하여 보충한 것인데 그것이 조선 보천가(步天歌)에 들어 있다는 점이 그 근거들이다. 통지(通志)는 A.D. 1364년에 고려에 들어왔으므로, 조선 초기의 보천가(步天歌)는 그 후에야 성립 될 수 있다. 조선 초기의 보천가(步天歌)는, 중국의 보천가(步天歌)들과 비교할 때, 그 가결(歌訣)에 별자리들의 색깔이 유난히 강조되어 있다. 이것은 석신(石申), 감덕(甘德), 무함(巫咸)으로부터 기원한 별자리를 색깔로 구분하려는 의도이다. 조선 초기에 출간된 보천가(步天歌)와 천상열차분야지도(天象列次分野之圖)의 성도(星圖)들은 별자리들의 이름, 별개수, 모양 등이 일치한다. 이 사설은 조선 보천가(步天歌)의 성도(星圖)가 천상열차분야지도(天象列次分野之圖)의 모본(母本)이나 천상열차분야지도(天象列次分野之圖) 자체에서 기원했음을 뜻한다. 천상열차분야지도(天象列次分野之圖)의 모본(母本)은 A.D. 1392년에 발견되어 A.D. 1396년에 비석에 새겨졌기 때문에, 보천가(步天歌)의 성립시기의 상한은 1392년이 된다. 천문류초(天文類抄)는 보천가(步天歌)에 중국의 점성술 문헌의 내용을 덧붙인 책이다. 이 책의 성립 시기는 그 저자인 이순지(李純之)의 생애 등을 근거로 A.D. 1440-1450년으로 추론되었다. 더군다나, 보천가(步天歌)는 A.D. 1430년에 관상감의 천문학자를 뽑는 취재(取才) 시험의 과목으로 선정되었다. 이러한 사설들로부터, 이 연구에서는 조선의 보천가(步天歌)가, 송(宋)의 선진 체제를 받아들임으로써 조선 왕조의 문화적 정치적 기초를 다지기 위해, A.D. 1392년에서 A.D. 1430 사이에 출간되었을 것으로 추론하였다.