arXiv subjects

Hiroshi Imai

Publications and source records attributed to Hiroshi Imai.

At least 91 records · Page 5Linked to original sources

arXiv2011

H2O maser motions and the distance of the star forming region G192.16-3.84

We present the results of astrometic observations of H2O masers associated with the star forming region G192.16-3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H2O masers seem to be associated with two young stellar objects (YSOs) separated by \sim1200 AU as reported in previous observations. In the present observations, we successfully detected an annual parallax of 0.66 \pm 0.04 mas for the H2 O masers, which corresponds to a distance to G192.16-3.84 of D = 1.52 \pm 0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax distance and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern young stellar object to be 10.0+4.3M\cdot. We also estimate the galactocentric distance and the peculiar motion -3.6 of G192.16-3.84, relative to a circular Galactic rotation: R\star = 9.99 \pm 0.08 kpc, Z\star = -0.10 \pm 0.01 kpc, and (U\star,V\star,W\star)=(-2.8\pm1.0,-10.5\pm0.3,4.9\pm2.7)[kms-1]respectively. The peculiar motion of G192.16-3.84 is within that typically found in recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H2O maser features associated with the northern YSO indicate the existence of a bipolar outflow with a major axis along the northeast-southwest direction.

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arXiv2011

Identification of Bursting Water Maser Features in Orion KL

In February 2011, a burst event of the H$_{2}$O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astrometric observations of the bursting H$_{2}$O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s$^{-1}$ reaches 4.4$\times10^{4}$ Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s$^{-1}$ in May 2011, separated by 12 mas north of the 7.58 km s$^{-1}$ feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We determine the absolute positions of the bursting features for the first time ever with a submilli-arcsecond (mas) accuracy. Their positions are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward southwest direction. It is most likely that the outflow from the radio source I or another young stellar object interacting with the Compact Ridge is a possible origin of the H$_{2}$O maser burst.

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arXiv2011

High Velocity Precessing Jets from the Water Fountain IRAS 18286-0959 Revealed by VLBA Observations

We report the results of multi-epoch VLBA observations of the 22.2GHz water maser emission associated with the "water fountain" IRAS 18286-0959. We suggest that this object is the second example of a highly collimated bipolar precessing outflow traced by water maser emission, the other is W43A. The detected water emission peaks are distributed over a velocity range from -50km/s to 150km/s. The spatial distribution of over 70% of the identified maser features is found to be highly collimated along a spiral jet (jet 1) extended southeast to northwest, the remaining features appear to trace another spiral jet (jet 2) with a different orientation. The two jets form a "double-helix" pattern which lies across ~200 milliarcseconds. The maser distribution is reasonably fit by a model consisting of two bipolar precessing jets. The 3D velocities of jet 1 and jet 2 are derived to be 138km/s and 99km/s, respectively. The precession period of jet 1 is about 56 years. For jet 2, three possible models are tested and they give different values for the kinematic parameters. We propose that the appearance of two jets is the result of a single driving source with significant proper motion.

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arXiv2011

Annual Parallax Measurements of an Infrared Dark Cloud MSXDC G034.43+00.24 with VERA

We have measured the annual parallax of the H2O maser source associated with an infrared dark cloud MSXDC G034.43+00.24 from the observations with VERA (VLBI Exploration of Radio Astrometry). The parallax is 0.643 +/- 0.049 mas, corresponding to the distance of 1.56 +0.12/-0.11 kpc. This value is less than the half of the previous kinematic distance of 3.7 kpc. We revise the core mass estimates of MSXDC G034.43+00.24, based on virial masses, LTE masses and dust masses and show that the core masses decrease from the previous estimations of ~1000 Mo to hundreds of Mo. The spectral type derived from the luminosity also changes from O9.5 to B1 in the case of MM1. This spectral type is still consistent with that of the massive star. The radial velocity derived from the flat rotation model is smaller than the observed velocity, which corresponds to the peculiar motion of ~40 km/s in the line-of-sight direction.

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arXiv2011

Measurement of the Distance and Proper Motions of the H2O Masers in the Young Planetary Nebula K 3-35

In this paper we present the results of very long baseline interferometry (VLBI) ob- servations carried out with the VLBI Exploration of Radio Astrometry (VERA) array and the Very Long Baseline Array (VLBA) toward H2O masers in a young planetary nebula K 3-35. From the VERA observations we measured the annual parallax and proper mo- tion of a bright water maser spot in K 3-35. The resulting distance is D = 3.9+0.7 kpc. -0.5 This is the first time that the parallax of a planetary nebula is obtained by observations of its maser emission. On the other hand, the proper motion of K 3-35 as a whole was esti- mated to be μα = -3.34+/-0.10 mas yr-1, μδ = -5.93+/-0.07 mas yr-1. From these results we determined the position and velocity of K 3-35 in Galactic cylindrical coordinates: (R,θ,z) = (7.11+0.08-0.06 kpc, 27+/-5{\circ}, 140+25-18 pc) and (VR, Vθ, Vz) = (33+/-16, 233+/-11, 11+/-2) km s-1, respectively. Additionally, from our VLBA observations we measured the relative proper motions among the water maser spots located in the central region of the nebula, which have been proposed to be tracing a toroidal structure. The distribution and relative proper motions of the masers, compared with previous reported observed epochs, suggest that such structure could be totally destroyed within a few years, due to the action of high velocity winds and the expansion of the ionization front in the nebula.

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arXiv2010

Astrometry of Galactic Star-Forming Region ON2N with VERA: Estimation of the Galactic Constants

We have performed the astrometry of H2O masers in the Galactic star-forming region Onsala 2 North (ON2N) with the VLBI Exploration of Radio Astrometry.We obtained the trigonometric parallax of 0.261+/-0.009 mas, corresponding to the heliocentric distance of 3.83+/-0.13 kpc. ON2N is expected to be on the solar circle, because its radial velocity with respect to the Local Standard of Rest (LSR) is nearly zero. Using present parallax and proper motions of the masers, the Galactocentric distance of the Sun and the Galactic rotation velocity at the Sun are found to be R_0 = 7.80+/-0.26 kpc and Theta_0 = 213+/-5 km/s, respectively. The ratio of Galactic constants, namely the angular rotation velocity of the LSR can be determined more precisely, and is found to be Omega_0=Theta_0/R_0 = 27.3+/-0.8 km/s/kpc, which is consistent with the recent estimations but different from 25.9 km/s/kpc derived from the recommended values of Theta_0 and R_0 by the International Astronomical Union (1985).

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arXiv2010

Maser Properties of the Enigmatic IRAS Source 19312+1950

The IRAS source, 19312+1950, exhibits SiO maser emission, which is predominantly detected in evolved stars enshrouded by a cold molecular envelope. In fact, the mojority of the observational properties of IRAS 19312+1950 is consistent with the nature of an asymptotic giant branch (AGB) star or post-AGB star. Interestingly, however, some of the observational properties cannot be readily explained within the standard scheme of stellar evolution, and those are rather reminiscent of young stellar objects. In the present research we considered the evolutionary status of IRAS 19312+1950 as revealed by the VLBI and MERLIN observations in SiO, H2O and OH maser lines. The double-peaked profile of the 22 GHz H2O maser line is clearly detected, with the emission regions of its red and blue-shifted components separately located, leaving a space of about 10.9 mas between them. The kinematic properties of H2O maser emission region appear to be more consistent with a bipolar flow rather than other interpretations such as the Keplerian rotation of a disk. The red-shifted component of the SiO maser emission, which exhibits a double-peak profile in previous single-dish observations, is clearly detected in the present interferometry, while the 1612 MHz OH maser line exhibits a complicated line profile consisting of a single strong peak and many weak, high-velocity spikes. The structure of OH maser emission region is partially resolved, and the kinematic properties of the OH maser emission region are reminiscent observations of a spherically expanding shell, even though the evidence is scant. Collectively, the maser observations described here provide additional support for the evolved star hypothesis for IRAS 19312+1950.

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arXiv2010

Astrometry of H$_{2}$O Masers in Nearby Star-Forming Regions with VERA --- IV. L1448C

We have carried out multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22~GHz H$_{2}$O masers associated with a Class 0 protostar L1448C in the Perseus molecular cloud. The maser features trace the base of collimated bipolar jet driven by one of the infrared counter parts of L1448C named as L1448C(N) or L1448-mm A. We detected possible evidences for apparent acceleration and precession of the jet according to the three-dimensional velocity structure. Based on the phase-referencing VLBI astrometry, we have successfully detected an annual parallax of the H$_{2}$O maser in L1448C to be 4.31$\pm$0.33~milliarcseconds (mas) which corresponds to a distance of 232$\pm$18~pc from the Sun. The present result is in good agreement with that of another H$_{2}$O maser source NGC~1333 SVS13A in the Perseus molecular cloud, 235~pc. It is also consistent with the photometric distance, 220~pc. Thus, the distance to the western part of the Perseus molecular cloud complex would be constrained to be about 235~pc rather than the larger value, 300~pc, previously reported.

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arXiv2009

CO J=3-2 Emission from the "Water Fountain" Sources IRAS 16342-3814 and IRAS 18286-0959

We observed CO J=3-2 emission from the "water fountain" sources, which exhibit high-velocity collimated stellar jets traced by water maser emission, with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We detected the CO emission from two sources, IRAS 16342-3814 and IRAS 18286-0959. The IRAS 16342-3814 CO emission exhibits a spectrum that is well fit to a Gaussian profile, rather than to a parabolic profile, with a velocity width (FWHM) of 158+/-6 km/s and an intensity peak at VLSR = 50+/-2 km/s. The mass loss rate of the star is estimated to be ~2.9x10^-5 M_sun/yr. Our morpho-kinematic models suggest that the CO emission is optically thin and associated with a bipolar outflow rather than with a (cold and relatively small) torus. The IRAS 18286-0959 CO emission has a velocity width (FWHM) of 3.0+/-0.2 km/s, smaller than typically seen in AGB envelopes. The narrow velocity width of the CO emission suggests that it originates from either an interstellar molecular cloud or a slowly-rotating circumstellar envelope that harbors the water maser source.

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arXiv2008

Fourier Analytic Approach to Phase Estimation

For a unified analysis on the phase estimation, we focus on the limiting distribution. It is shown that the limiting distribution can be given by the absolute square of the Fourier transform of $L^2$ function whose support belongs to $[-1,1]$. Using this relation, we study the relation between the variance of the limiting distribution and its tail probability. As our result, we prove that the protocol minimizing the asymptotic variance does not minimize the tail probability. Depending on the width of interval, we derive the estimation protocol minimizing the tail probability out of a given interval. Such an optimal protocol is given by a prolate spheroidal wave function which often appears in wavelet or time-limited Fourier analysis. Also, the minimum confidence interval is derived with the framework of interval estimation that assures a given confidence coefficient.

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arXiv2008

VLBI Astrometry of AGB Variables with VERA -- A Semiregular Variable S Crateris --

We present a distance measurement for the semiregular variable S Crateris (S Crt) based on its annual parallax. With the unique dual beam system of the VLBI Exploration for Radio Astrometry (VERA) telescopes, we measured the absolute proper motion of a water maser spot associated with S Crt, referred to the quasar J1147-0724 located at an angular separation of 1.23$^{\circ}$. In observations spanning nearly two years, we have detected the maser spot at the LSR velocity of 34.7 km s$^{-1}$, for which we measured the annual parallax of 2.33$\pm$0.13 mas corresponding to a distance of 430$^{+25}_{-23}$ pc. This measurement has an accuracy one order of magnitude better than the parallax measurements of HIPPARCOS. The angular distribution and three-dimensional velocity field of maser spots indicate a bipolar outflow with the flow axis along northeast-southwest direction. Using the distance and photospheric temperature, we estimate the stellar radius of S Crt and compare it with those of Mira variables.

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arXiv2008

Distance to VY Canis Majoris with VERA

We report astrometric observations of H2O masers around the red supergiant VY Canis Majoris (VY CMa) carried out with VLBI Exploration of Radio Astrometry (VERA). Based on astrometric monitoring for 13 months, we successfully measured a trigonometric parallax of 0.88 +/- 0.08 mas, corresponding to a distance of 1.14 +0.11/-0.09 kpc. This is the most accurate distance to VY CMa and the first one based on an annual parallax measurement. The luminosity of VY CMa has been overestimated due to a previously accepted distance. With our result, we re-estimate the luminosity of VY CMa to be (3 +/- 0.5) x 10^5 L_sun using the bolometric flux integrated over optical and IR wavelengths. This improved luminosity value makes location of VY CMa on the Hertzsprung-Russel (HR) diagram much closer to the theoretically allowable zone (i.e. the left side of the Hayashi track) than previous ones, though uncertainty in the effective temperature of the stellar surface still does not permit us to make a final conclusion.

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arXiv2008

Astrometry of H$_{2}$O Masers in Nearby Star-Forming Regions with VERA. III. IRAS 22198+6336 in L1204G

We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS 22198+6336 in a dark cloud L1204G. Based on the phase-referencing VLBI astrometry, we derive an annual parallax of IRAS 22198+6336 to be 1.309$\pm$0.047 mas, corresponding to the distance of 764$\pm$27 pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduce the errors in the derived annual parallax by employing the position measurements for all of the 26 detected maser spots. Based on this result, we reanalyze the spectral energy distribution (SED) of IRAS 22198+6336 and find that the bolometric luminosity and total mass of IRAS 22198+6336 are 450$L_{\odot}$ and 7$M_{\odot}$, respectively. These values are consistent with an intermediate-mass YSO deeply embedded in the dense dust core, which has been proposed to be an intermediate-mass counterpart of a low-mass Class 0 source. In addition, we obtain absolute proper motions of the H$_{2}$O masers for the most blue-shifted components. We propose that the collimated jets aligned along the east-west direction are the most plausible explanation for the origin of the detected maser features.

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arXiv2008

Distance to NGC 281 in a Galactic Fragmenting Superbubble: Parallax Measurements with VERA

We have used the Japanese VLBI array VERA to perform high-precision astrometry of an H2O maser source in the Galactic star-forming region NGC 281 West, which has been considered to be part of a 300-pc superbubble. We successfully detected a trigonometric parallax of 0.355+/-0.030 mas, corresponding to a source distance of 2.82+/-0.24 kpc. Our direct distance determination of NGC 281 has resolved the large distance discrepancy between previous photometric and kinematic studies; likely NGC 281 is in the far side of the Perseus spiral arm. The source distance as well as the absolute proper motions were used to demonstrate the 3D structure and expansion of the NGC 281 superbubble, ~650 pc in size parallel to the Galactic disk and with a shape slightly elongated along the disk or spherical, but not vertically elongated, indicating the superbubble expansion may be confined to the disk. We estimate the expansion velocity of the superbubble as ~20 km/s both perpendicular to and parallel to the Galactic disk with a consistent timescale of ~20 Myr.

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arXiv2008

Molecular Line Observations of the SiO Maser Source IRAS 19312+1950

IRAS 19312+1950 is a unique SiO maser source, exhibiting a rich set of molecular radio lines, although SiO maser sources are usually identified as oxygen-rich evolved stars, in which chemistry is relatively simple comparing with carbon-rich environments. The rich chemistry of IRAS 19312+1950 has raised a problem in circumstellar chemistry if this object is really an oxygen-rich evolved star, but its evolutional status is still controversial. In this paper, we briefly review the previous observations of IRAS 19312+1950, as well as presenting preliminary results of recent VLBI observations in maser lines.

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arXiv2007

Stellar molecular jets traced by maser emission

Highly collimated jets found in AGB and post-AGB stars are expected to play an important role for shaping planetary nebulae. Recent VLBI observations of H2O maser sources have revealed that some of the spatio-kinematical structures of H2O maser sources exhibit stellar jets with extremely spatially and kinematically high collimation. Such stellar H2O maser jets, so-called "water fountain" sources, have been identified in about 10 sources to date. Here we review recent VLBI observations and relevant observational results of the water fountain sources. They have revealed a typical dynamical age and the detailed kinematical structures of the water fountains, possibility of the existence of "equatorial flows", and the evolutionary status of the host stars. The location and kinematics of one of the water fountain sources in the Galaxy is also revealed and shown here.

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arXiv2007

Astrometry of Water Maser Sources in Nearby Molecular Clouds with VERA - II. SVS 13 in NGC 1333

We report on the results of multi-epoch 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 have carried out phase-referencing VLBI astrometry and measured an annual parallax of the maser features in SVS 13 of 4.25+/-0.32 mas, corresponding to the 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 results provide 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 the LSR (local standard of rest) velocities of 7-8 km s-1 are most likely associated with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed proper motions of the maser features are currently difficult to attribute to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

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arXiv2007

Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve

We have performed high-precision astrometry of H2O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's center) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.

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