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Nakano H.,AMMSYS Inc | Nakano H.,Tokyo Institute of Technology | Shimizu T.,Utsunomiya University | Ohno T.,Japan Communication Equipment Co. | And 3 more authors.
Publications of the Astronomical Society of Japan | Year: 2012

This paper describes cryogenic 36-45GHz InP low-noise amplifier monolithic microwave integrated circuits (MMIC's) with an improved noise temperature by eliminating parasitic parallel-plate resonance modes. These MMIC's are used for a Radio Astronomical receiver, which needs the ultimate super low-noise and wide-band frequency characteristics, such as those in ALMA Band 1. The MMIC chips were designed in the coplanar waveguide (CPW), and mounted to the AlN substrate with a flip-chip assembly, which was promising compared to wire bonding. The flip-chip assemblies, however, are prone to cause the parasitic parallel plate resonance mode (PPM). The relationship between the S-parameters and the PPM was investigated by using a 3D-electromagnetic simulation of the simple transmission-line test-chip with the same chip size as that of the actual MMIC. In order to eliminate the PPM, additional bumps were mounted on the simple transmission-line test-chip, and the effect of these bumps was confirmed by the simulation. These results obtained from the simple transmission-line test-chip were applied to an actual MMIC chip assembly. The MMIC assembled with the additional bumps had no abnormality in the measured S-parameters, and the PPM had been eliminated up to 65 GHz. Moreover, the stability factor, K, became more than 2.4 over 36-45GHz. This InP low-noise amplifier MMIC exhibited a gain of 15 dB and a noise temperature of 180-240K at room temperature in the frequency range of 36-45GHz. When cooled to 28K, a gain of 17 dB and a noise temperature of 22-35K were obtained at a power consumption of 4.7mW over 36-45GHz. A high-gain amplifier module consisting of two cascaded chips, exhibited a gain of 27-30 dB and a noise temperature of 25-30K at the ambient temperature of 22K in the frequency range of 41-45GHz. © 2012. Astronomical Society of Japan. Source

Tsujimoto T.,National Astoronomical Observatory of Japan | Bland-HAWTHORN J.,University of Sydney | Freeman K.C.,Australian National University
Publications of the Astronomical Society of Japan | Year: 2010

Large-scale homogeneous surveys of Galactic stars may indicate that the elemental abundance gradient evolves with cosmic time, which is a phenomenon that was not foreseen in existing models of Galactic chemical evolution (GCE). If this phenomenon is confirmed in future studies, we can show that this effect, at least in part, is due to large-scale winds that once enriched the disk. These set up a steep abundance gradient in the inner disk (RGC ≲ 14 kpc). At the close of the wind phase, chemical enrichment through accretion of metal-poor material from the halo to the disk gradually reduced the metallicity of the inner region, whereas a slight increase in the metallicity proceeded beyond the solar circle. Our "wind+infall" model accounts for a flattening of the abundance gradient in the inner disk, in agreement with observations. Accordingly, we propose that enrichment by large-scale winds is a crucial factor in chemical evolution in the disk. We anticipate that rapid flattening of the abundance gradient is the hallmark of disk galaxies with significant central bulges. © 2010. Astronomical Society of Japan. Source

Koyama S.,University of Tokyo | Kino M.,Japan Aerospace Exploration Agency | Nagai H.,National Astoronomical Observatory of Japan | Hada K.,National institute for astrophysics | And 2 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

We investigated observational properties of a kilo-parsec scale knot in the radio-loud quasar 3C 380 by using two epoch archival data obtained by Very Long Baseline Interferometry (VLBI) at 5 GHz on 1998 July and 2001 April. We succeed to obtain the highest spatial resolution image of the bright knot K1 located at 732 mas, or ≥20 kpc deprojected, downstream from the nucleus three-times better than the previously obtained highest-resolution image by Papageorgiou et al. (2006, MNRAS, 373, 449). Our images reveal, with new clarity, an "inverted bow-shock" structure in K1 facing the nucleus; its morphology resembles a conical shock wave. By comparing the two epoch images directly, we explored the kinematics of K1, and obtained the upper limit of the apparent velocity, 0.25 mas yr-1 or 9.8c of K1 for the first time. The upper limit of the apparent velocity is marginally smaller than superluminal motions seen in the core region. Further new epoch VLBI observations are necessary to measure the proper motion at K1. © 2013 Astronomical Society of Japan. Source

Akiyama E.,National Astoronomical Observatory of Japan | Momose M.,Ibaraki University | Kitamura Y.,Japan Aerospace Exploration Agency | Tsukagoshi T.,Ibaraki University | And 3 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

This paper presents observations of a protoplanetary disk around a Herbig Ae star, MWC480, in 12CO(J = 1-0), 12CO(J = 3-2), 13CO(J = 1-0), and C18O (J = 1-0) emission lines. Double-peaked emission profiles originating from the rotating circumstellar disk were detected in all of the lines. The vertical temperature and radial surface density structures of the outer region of the disk were derived by applying the similarity solution in the standard accretion disk model. Taking advantage of differences in the height of the photosphere among the CO lines, the temperature in the uppermost 12CO(J = 3-2) emitting layer was shown to be about 3-times higher than that of any other CO emitting region, suggesting that there are at least two distinct temperature regions. Our modeling succeeds in describing all of the observational results obtained in the four CO lines, particularly different emission extents at different frequencies, by a single set of the parameters for a disk model. Since the similarity solution model could be the most suitable for the radial surface density structure it is likely that the disk around MWC480 evolves by transferring angular momentum outward via viscous diffusion. Although further quantitative studies are required for identifying what disk model is the best for describing physical disk structures, our results suggest the potential advantage of the similarity solution model, indicating that disks around Herbig Ae/Be stars likely have diffused gas in the outer regions, and that the disk surface density exponentially decreases with increasing radial distance. © 2013. Astronomical Society of Japan. Source

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