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Kaneda H.,Nagoya University | Yasuda A.,Nagoya University | Yasuda A.,Japan Aerospace Exploration Agency | Onaka T.,University of Tokyo | And 5 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We investigate the properties of interstellar dust in the Galactic center region toward the Arches and Quintuplet clusters. Methods. With the Fourier Transform Spectrometer of the AKARI/Far-Infrared Surveyor, we performed the far-infrared (60-140 cm -1) spectral mapping of an area of about 10′ × 10′ that includes both clusters to obtain a low-resolution (R = 1.2 cm -1) spectrum at every spatial bin of 30′′ × 30′′. Results. We derive the spatial variations in the dust continuum emission at different wavenumbers, which are compared with those of the [OIII] 88 μm (113 cm -1) emission and the OH 119 μm (84 cm -1) absorption. The spectral fitting shows that two modified blackbody components with temperatures of ∼20 K and ∼50 K can reproduce most of the continuum spectra. For some spectra, however, we find that there is a significant excess on top of a modified blackbody continuum around 80-90 cm -1 (110-130 μm). Conclusions. The warmer dust component is spatially correlated with the [OIII] emission and hence likely to be associated with the highly-ionized gas locally heated by intense radiation from the two clusters. The excess emission probably represents a dust feature, which is found to be spatially correlated with the OH absorption and a CO cloud. We find that a dust model including micron-sized graphite grains can quite closely reproduce the observed spectrum with the dust feature. © 2012 ESO. Source


Kawada M.,Nagoya University | Kawada M.,Japan Aerospace Exploration Agency | Takahashi A.,Nagoya University | Yasuda A.,Nagoya University | And 8 more authors.
Publications of the Astronomical Society of Japan | Year: 2011

We present a distribution map of the far-infrared [OIII] 88μm line emission around the 30 Doradus (30 Dor) region in the Large Magellanic Cloud obtained with the Fourier Transform Spectrometer of the Far-Infrared Surveyor on board AKARI. The map reveals that the [O III] emission is widely distributed by more than 100 around the super star cluster R 136, implying that the 30 Dor region is affluent with interstellar radiation field that is hard enough to ionize O2+. The observed [OIII] line intensities are as high as (1-2) × 106Wm-2 sr-1 on the peripheral regions 4′-5′ away from the center of 30 Dor, which requires gas densities of 60-100cm-3. However, the observed size of the distribution of the [OIII] emission is too large to be explained by massive stars in the 30 Dor region enshrouded by clouds with a constant gas density of 102 cm-3. Therefore, the surrounding structure is likely to be highly clumpy. We also find a global correlation between the [OIII] and the far-infrared continuum emission, suggesting that the gas and dust are well mixed in the highly ionized region where the dust survives in clumpy dense clouds shielded from energetic photons. © 2011. Astronomical Society of Japan. Source


Murakami N.,Bisei Astronomical Observatory | Murakami N.,Nagoya University | Kawada M.,Nagoya University | Kawada M.,Japan Aerospace Exploration Agency | And 20 more authors.
Publications of the Astronomical Society of Japan | Year: 2010

The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a spectroscopic capability provided by a Fourier transform spectrometer (FIS-FTS). FIS-FTS is the first space-borne imaging FTS dedicated to far-infrared astronomical observations. We describe the calibration process of the FIS-FTS, and discuss its accuracy and reliability. The calibration is based on observational data of bright astronomical sources as well as two instrumental sources. We have compared the FIS-FTS spectra with the spectra obtained from the LongWavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO), having a similar spectral coverage. The present calibration method accurately reproduces the spectra of several solar system objects having a reliable spectral model. Under this condition the relative uncertainty of the calibration of the continuum is estimated to be ±?15% for SW, ±10% for 70- 85 cm-1 of LW, and ?20% for 60-70cm-1 of LW; and the absolute uncertainty is estimated to be +35%/-55% for SW, +35%/-55% for 70-85cm-1 of LW, and +40%/-60% for 60-70cm-1 of LW. These values have been confirmed by comparisons with theoretical models and previous observations by the ISO/ LWS. © 2010. Astronomical Society of Japan. Source


Tanaka J.,Kyoto University | Nogami D.,Kyoto University | Fujii M.,Fujii Bisei Observatory | Ayani K.,Bisei Astronomical Observatory | And 4 more authors.
Publications of the Astronomical Society of Japan | Year: 2011

We report on the spectral evolution of the enigmatic, very slow nova V5558 Sgr, based on low-resolution spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical Observatory, Japan during the period of 2007 April 6 to 2008 May 3. V5558 Sgr shows a pre-maximum halt, and then several flare-like rebrightenings, which is similar to another very slow nova, V723 Cas. In our observations, the spectral type of V5558 Sgr evolved from the He/N type toward the Fe II type during a pre-maximum halt, and then toward the He/N type again. This course of spectral transition was observed for the first time in the long history of nova research. In the rebrightening stage after the initial brightness maximum, we could identify many emission lines accompanied by a stronger absorption component of the P Cyg profile at the brightness maxima. We found that the velocity of the P Cyg absorption component, measured from the emission peak, decreased at the brightness maxima. Furthermore, we compared the spectra of V5558 Sgr with V723 Cas, and other novae that exhibited several rebrightenings during the early phase. © 2011. Astronomical Society of Japan. Source


Tanaka J.,Kyoto University | Nogami D.,Kyoto University | Fujii M.,Fujii Bisei Observatory | Ayani K.,Bisei Astronomical Observatory | Kato T.,Kyoto University
Publications of the Astronomical Society of Japan | Year: 2011

We report on the spectral evolution of 6 classical novae (V1186 Sco, V2540 Oph, V4745 Sgr, V5113 Sgr, V458 Vul, and V378 Ser), based on low-resolution spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical Observatory, Japan. In the light curves, these 6 novae show several rebrightenings during the early phase lasting ̃10 d after the first maximum in fast novae, and ̃100 d in slow novae. The early spectra of all of these novae had emission lines with a P-Cygni profile at the maximum brightness. The absorption component of the P-Cygni profiles then disappeared after the maximum, and reappeared when the novae brightened to the next maximum. We suggest that the re-appearance of the absorption component at the rebrightening can be attributable to a re-expansion of the photosphere after it once shifts sufficiently inside. From the light curves, we found that the time intervals of the rebrightenings of these 6 novae show a similar systematic trend, which is applicable to all types of novae; fast and slow, and Fe II type and hybrid type. Moreover, we note the difference between the spectra at the rebrightenings during the early phase and at the rebrightening in V2362 Cyg, and at the oscillation during the transition phase in V1494 Aql, which means a difference in the physical mechanism of the rebrightening during the early phase and the later oscillations. © 2011. Astronomical Society of Japan. Source

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