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Author

T. Fujii

Bio: T. Fujii is an academic researcher from Kagoshima University. The author has contributed to research in topics: Maser & Stars. The author has an hindex of 8, co-authored 11 publications receiving 249 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry) were presented, and the results demonstrate the high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.
Abstract: We present the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The observations of a pair of H2O maser sources, W 49N and OH 43.8−0.1, were carried out on 2002 May 29 and July 23, and fringes of the H2O maser lines at 22GHz were successfully detected. While the residual fringe phases of both sources showed rapid variations over 360 ◦ due to the atmospheric fluctuation, the differential phase between the two sources remained constant for 1 hour with an r.m.s. of 8 ◦ , demonstrating that the atmospheric phase fluctuation was effectively removed by dual-beam phase referencing. An analysis based on the Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for an arbitrarily long time. These results demonstrate VERA’s high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.

67 citations

Journal ArticleDOI
TL;DR: The first dual-beam observations of a pair of H2O maser sources W49N and OH43.1 have been carried out on 2002 May 29 and July 23 as discussed by the authors.
Abstract: We present the results of first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The first dual-beam observations of a pair of H2O maser sources W49N and OH43.8-0.1 have been carried out on 2002 May 29 and July 23, and fringes of H2O maser lines at 22 GHz have been successfully detected. While the residual fringe phases of both sources show rapid variations over 360 degree due to the atmospheric fluctuation, the differential phase between the two sources remains constant for 1 hour with r.m.s. of 8 degree, demonstrating that the atmospheric phase fluctuation is removed effectively by the dual-beam phase referencing. The analysis based on Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for arbitrarily long time. These results demonstrate VERA's high capability of phase referencing, indicating that VERA is a promising tool for phase referencing VLBI astrometry at 10 microarcsec level accuracy.

58 citations

Journal ArticleDOI
TL;DR: In this paper, the authors surveyed the 7 degrees x 2 degrees area of the galactic center in the 43 GHz SiO J = 1-0 v = I and 2 maser lines, obtaining accurate radial velocities of 163 detected objects.
Abstract: We surveyed 291 MSX/2MASS infrared objects in the 7 degrees x 2 degrees area of the galactic center in the 43 GHz SiO J = 1-0 v = I and 2 maser lines, obtaining accurate radial velocities of 163 detected objects. The surveyed area is the region where the IRAS catalog is incomplete due to contamination by high source density. The objects in the present MSX/2MASS sample were chosen to have similar infrared characteristics to those of the previous SiO maser-survey samples based on the color-selected IRAS sources. The sampling based on the 2MASS catalog causes a bias to the foreside objects of the bulge due to heavy obscuration by interstellar dust; the detections considerably leaned on the V-lsr < 0 side. The l-V diagram reveals two conspicuous features, which were not present or tenuous in the previous studies: one feature indicating a linear velocity increase with longitude with I I I < 1.degrees 5, which is likely to be associated with the inner bar, and the other feature having considerably eccentric velocities, more than those of the normal x(1)-orbit family feature. The extinction-corrected K magnitudes (if used as a distance modulus) tend to show a sequential deposition of these objects along the line of sight toward the galactic center, depending on their radial velocities. The tendency that appeared in the distance measures is consistent with the bulge-bar dynamical model utilizing the periodic orbit families in the bar potential.

33 citations

Journal ArticleDOI
TL;DR: In this paper, a SiO maser survey in the J = 1 0, v = 1 and 2 transitions has been made for IRAS sources in the North Galactic Cap (b > 30) with the Nobeyama 45 m radio telescope.
Abstract: A SiO maser survey in the J =1 0, v = 1 and 2 transitions has been made for IRAS sources in the North Galactic Cap (b > 30) with the Nobeyama 45 m radio telescope. The sources were selected on the basis of their IRAS 12/25-m and 25/60-m flux ratios as likely oxygen-rich AGB candidates. SiO masers were detected from 24 out of the 97 selected sources, where 17 were new detections. The distances and heights above the Galactic plane are calculated. The Galactic distribution of detected and undetected stars indicates that metallicity is likely to govern the detection rate. The Galactocentric angular velocities of the subsampled stars are derived and their variation with the Galactic height is discussed.

27 citations

Journal ArticleDOI
TL;DR: In this paper, the authors report on a strong outburst of the W 49N H2O maser observed with VERA, which had a duration of ∼ 100 days and a peak intensity of 7.9 × 10 4 Jy, being one of the strongest outbursts in W 49 N observed so far.
Abstract: We report on a strong outburst of the W 49N H2O maser observed with VERA. Single-dish monitoring with VERA 20m telescopes detected a strong outburst of the maser feature at VLSR = −30.7kms −1 in 2003 October. The outburst had a duration of ∼ 100 days and a peak intensity of 7.9 ×10 4 Jy, being one of the strongest outbursts in W 49N observed so far. VLBI observations with the VERA array were also carried out near to the maximum phase of the outburst, and the outburst spot was identified in the VLBI map. While the map was in good agreement with previous studies, showing three major concentrations of maser spots, we found a newly formed arc-like structure in the central maser concentration, which may be a shock front powered by a forming star or a star cluster. The outburst spot was found to be located on the arc-like structure, indicating a possible connection of the present outburst to a shock phenomenon.

22 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, the authors measured the annual parallax of the maser features in SVS 13 of 4.25 ˙ 0.32 mas, corresponding to a distance of 235 ˘ 18 pc from the Sun.
Abstract: We report on the results of multiepoch very long baseline interferometry (VLBI) observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We carried out phase-referencing VLBI astrometry, and measured the annual parallax of the maser features in SVS 13 of 4.25 ˙ 0.32 mas, corresponding to a distance of 235 ˙ 18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc, previously reported. Even though the maser features were detectable only for 6 months, the present result provides the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with LSR (local standard of rest) velocities of 7–8 km s � 1 are most likely associated with VLA 4A, which is a radio counterpart of SVS 13. It is currently difficult to attribute the observed proper motions of the maser features to either the jet or the rotating circumstellar disk associated with VLA 4A, which should be investigated through future high-resolution astrometric observations of VLA 4A and other radio sources in NGC 1333.

332 citations

Journal ArticleDOI
TL;DR: In this paper, the fundamental parameters (temperature and luminosity) of 107 619 Hipparcos stars and place these stars on a true Hertzsprung-Russell diagram were derived by comparing bt-settl model atmospheres to spectral energy distributions.
Abstract: We derive the fundamental parameters (temperature and luminosity) of 107 619 Hipparcos stars and place these stars on a true Hertzsprung–Russell diagram. This is achieved by comparing bt-settl model atmospheres to spectral energy distributions (SEDs) created from Hipparcos, Tycho, Sloan Digital Sky Survey, DENIS, Two Micron All Sky Survey, MSX, AKARI, IRAS and Wide-field Infrared Survey Explorer data. We also identify and quantify from these SEDs any infrared excesses attributable to circumstellar matter. We compare our results to known types of objects, focusing on the giant branch stars. Giant star dust production (as traced by infrared excess) is found to start in earnest around 680 L⊙.

275 citations

Journal ArticleDOI
Ulrich Heber1
TL;DR: Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars are reviewed in this paper, where the resulting atmospheric parameters and abundances are used to obtain constraints on the evolutionary status of different classes of stars.
Abstract: Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars are reviewed. These classes of stars trace stellar evolution from the horizontal branch towards the white dwarf cooling sequence. The resulting atmospheric parameters and abundances are used to obtain constraints on the evolutionary status of the different classes of stars. The sdB stars form a homogeneous group and can be identified with models of the extended horizontal branch. Abundance anomalies (deficiency of helium and some metals, enrichment of 3He) observed in Horizontal Branch stars and sdB stars are caused by atmospheric diffusion. The class of subluminous O stars is much less homogenous and two subclasses can be defined: the “compact” sdO stars probably evolved from the extended horizontal branch and are hence successors of the sdBs, whereas some sdOs of relatively low gravity are in a post-AGB stage of evolution. Hot subdwarfs in binary systems can be formed by case B or case C mass transfer.

268 citations

Journal ArticleDOI
TL;DR: In this article, the authors presented the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry).
Abstract: We present the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we carried out phase-referencing VLBI astrometry, and successfully detected the annual parallax of Orion KL to be 2.29 ˙ 0.10 mas, corresponding to a distance of 437 ˙ 19 pc from the Sun. The distance to Orion KL was determined for the first time with the trigonometric parallax method in these observations. Although this value is consistent with that previously reported, 480 ˙ 80 pc, which was estimated from a statistical parallax method using the proper motions and radial velocities of the H2O maser features, our new results provide a much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.

235 citations

Journal ArticleDOI
TL;DR: In this article, the authors derived the trigonometric parallax of Orion-KL to be 2.39˙0.03mas, corresponding to a distance of 418˙6pc.
Abstract: We present results of phase-referencing VLBI observations of SiO masers in the Orion-KL region made with VERA. Using a strong maser spot in the 43 GHz v = 2 J = 1–0 emission, we derived the trigonometric parallax of Orion-KL to be 2.39˙0.03mas, corresponding to a distance of 418˙6pc, with the highest accuracy among existing parallax measurements of the source. We made a superimposed image of v = 1 J = 1–0 and v = 2 J = 1–0 maser features in Orion-KL based on absolute positions obtained from the phase-referencing astrometry with a common reference source. The maser features of both transitions show similar X-shaped distributions centered at Source I. However, in each of the four arms of the X-shape, the SiO v = 2 features tend to lie closer to Source I than the SiO v = 1 features. The radial velocities of the maser emission decrease with the distance from Source I. The spatial and radial velocity distributions of the SiO masers suggest that the SiO masers lie in the rotating materials associated with a disk around Source I, rather than a decelerating outflow.

224 citations