arXiv subjects

Nobuyuki Sakai

Publications and source records attributed to Nobuyuki Sakai.

At least 37 records · Page 2Linked to original sources

arXiv2020

Local gravity test of unified models of inflation and dark energy in $f(R)$ gravity

We consider $f(R)$ gravity theories which unify $R^n$ inflation and dark energy models. First, from the final Planck data of the cosmic microwave background, we obtain a condition, $1.977 < n < 2.003$. Next, under this constraint, we investigate local-gravity tests for three models. We find that the $R^n$ term can dominate over the dark energy term even at the Earth's curvature scale, contrary to intuition; however, the $R^n$ term does not relax or tighten the constraints on the three models.

astro-ph.CO↗

arXiv2020

The First VERA Astrometry Catalog

We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric annual parallax and proper motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported while the rest of 78 sources are referred to previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry are revisited and compared with those from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould's Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameter of $R_{0}=7.92\pm0.16_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~kpc and $Ω_{\odot}=30.17\pm0.27_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~km~s$^{-1}$~kpc$^{-1}$, where $R_{0}$ and $Ω_{\odot}$ are the distance from the Sun to the Galactic center and the Sun's angular velocity of the Galactic circular rotation, respectively.

astro-ph.GA↗

arXiv2019

Vertical Structure and Kinematics of the Galactic Outer Disk

We report measurements of parallax and proper motion for four 22 GHz water maser sources as part of VERA Outer Rotation Curve project. All sources show Galactic latitudes of $>$ 2$^{\circ}$ and Galactocentric distances of $>$ 11 kpc at the Galactic longitude range of 95$^{\circ}$ $< l <$ 126$^{\circ}$. The sources trace the Galactic warp reaching to 200$\sim$400 pc, and indicate the signature of the warp to 600 pc toward the north Galactic pole. The new results along with previous results in the literature show the maximum height of the Galactic warp is increased with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found an oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of the bending (vertical density) waves, possibly induced by a perturbing satellite (e.g. passage of the Sagittarius dwarf galaxy).

astro-ph.GA↗

arXiv2019

Outer rotation curve of the Galaxy with VERA IV: Astrometry of IRAS 01123+6430 and the possibility of cloud-cloud collision

As part our investigation into the Galactic rotation curve, we carried out Very Long Baseline Interferometry (VLBI) observations towards the star-forming region IRAS 01123+6430 using VLBI Exploration of Radio Astrometry (VERA) to measure its annual parallax and proper motion. The annual parallax was measured to be 0.151+/-0.042 mas, which corresponds to a distance of D=6.61^{+2.55}_{-1.44} kpc, and the obtained proper motion components were (μ_αcosδ, μ_δ)=(-1.44+/-0.15, -0.27+/-0.16) mas yr^{-1} in equatorial coordinates. Assuming Galactic constants of (R_0, Θ_0)=(8.05+/-0.45 kpc, 238+/-14 km s^{-1}), the Galactocentric distance and rotation velocity were measured to be (R, Θ)=(13.04+/-2.24 kpc, 239+/-22 km s^{-1}), which are consistent with a flat Galactic rotation curve. The newly estimated distance provides a more accurate bolometric luminosity of the central young stellar object, L_Bol=(3.11+/-2.86)\times 10^3 L_solar, which corresponds to a spectral type of B1-B2. The analysis of 12CO(J=1-0) survey data obtained with the Five College Radio Astronomical Observatory (FCRAO) 14 m telescope shows that the molecular cloud associated with IRAS 01123+6430 consists of arc-like and linear components, which well matches a structure predicted by numerical simulation of the cloud-cloud collision (CCC) phenomenon. The coexistence of arc-like and linear components implies that the relative velocity of initial two clouds was as slow as 3-5 km s^{-1}, which meets the expected criteria of massive star formation where the core mass is effectively increased in the presence of low relative velocity (~3-5 km s^{-1}), as suggested by Takahira et al.(2014).

astro-ph.GA↗

arXiv2019

Stellar Overdensity in the Local Arm in Gaia DR2

Using the cross-matched data of Gaia DR2 and 2MASS Point Source Catalog, we investigated the surface density distribution of stars aged ~1 Gyr in the thin disk in the range of 90° <= l <= 270°. We selected 4,654 stars above the turnoff corresponding to the age ~1 Gyr, that fall within a small box region in the color-magnitude diagram, (J-Ks)0 versus M(Ks), for which the distance and reddening are corrected. The selected sample shows an arm-like overdensity at 90° <= l <= 190°.This overdensity is located close to the Local arm traced by high-mass star forming regions (HMSFRs), but its pitch angle is slightly larger than that of the HMSFR-defined arm. Although the significance of the overdensity we report is marginal, its structure poses questions concerning both of the competing scenarios of spiral arms, the density-wave theory and the dynamic spiral arm model. The offset between the arms traced by stars and HMSFRs, i.e., gas, is difficult to be explained by the dynamic arm scenario. On the other hand, the pitch angle of the stellar Local arm, if confirmed, larger than that of the Perseus arm is difficult to be explained with the classical density-wave scenario. The dynamic arm scenario can explain it if the Local arm is in a growing up phase, while the Perseus arm is in a disrupting phase. Our result provides a new and complex picture of the Galactic spiral arms, and encourages further studies.

astro-ph.GA↗

arXiv2019

Parallaxes for star forming regions in the inner Perseus spiral arm

We report trigonometric parallax and proper motion measurements of 6.7-GHz CH3OH and 22-GHz H2O masers in eight high-mass star-forming regions (HMSFRs) based on VLBA observations as part of the BeSSeL Survey. The distances of these HMSFRs combined with their Galactic coordinates, radial velocities, and proper motions, allow us to assign them to a segment of the Perseus arm with ~< 70 deg. These HMSFRs are clustered in Galactic longitude from ~30 deg to ~50, neighboring a dirth of such sources between longitudes ~50 deg to ~90 deg.

astro-ph.GA↗

arXiv2019

Non-circular Motions in the Outer Perseus Spiral Arm

We report measurements of parallax and proper motion for five 6.7-GHz methanol maser sources in the outer regions of the Perseus arm as part of the BeSSeL Survey of the Galaxy. By combining our results with previous astrometric results, we determine an average spiral arm pitch angle of $9.2\pm1.5$ deg and an arm width of 0.39 kpc for this spiral arm. For sources in the interior side of the Perseus arm, we find on average a radial inward motion in the Galaxy of $13.3\pm5.4$ km s$^{-1}$ and counter to Galactic rotation of $6.2\pm3.2$ km s$^{-1}$. These characteristics are consistent with models for spiral arm formation that involve gas entering an arm to be shocked and then forming stars. However, similar data for other spiral arms do not show similar characteristics.

astro-ph.GA↗

arXiv2019

Metallicity dependence of the Hercules stream in Gaia/RAVE data -- explanation by non-closed orbits

The origin of the Hercules stream, the most prominent velocity substructure in the Solar neighbour disc stars, is still under debate. Recent accurate measurements of position, velocity, and metallicity provided by Tycho Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiments (RAVE) have revealed that the Hercules stream is most clearly seen in the metal-rich region ([Fe/H] > 0), while it is not clearly seen in lower metallicity region ([Fe/H] < -0.25). By using a large number of chemo-dynamical 2D test-particle simulations with a rotating bar and/or spiral arms, we find that the observed [Fe/H] dependence of the Hercules stream is a natural consequence of the inside-out formation of the stellar disc and the existence of highly non-closed orbits in the rotating frame of the bar or spiral arms. Our successful models that reproduce the observed properties of the Hercules stream include not only fast-bar-only and fast-bar+spiral models, but also slow-bar+spiral models. This indicates that it is very difficult to estimate the pattern speed of the bar or spiral arms based only on the observations of the Hercules stream in the Solar neighbourhood. As a by-product of our simulations, we make some predictions about the locations across the Galactic plane where we can observe velocity bimodality that is not associated with the Hercules stream. These predictions can be tested by the Gaia Data Release 2, and such a test will improve our understanding of the evolution of the Milky Way stellar disc.

astro-ph.GA↗

arXiv2017

Wormhole Shadows

We propose a new method of detecting Ellis wormholes by use of the images of wormholes surrounded by optically thin dust. First, we derive steady solutions of dust and more general medium surrounding the wormhole by solving relativistic Euler equations. We find two types of dust solutions: one is a static solution with arbitrary density profile, and the other is a solution of dust which passes into the wormhole and escapes into the other side with constant velocity. Next, solving null geodesic equations and radiation transfer equations, we investigate the images of the wormhole surrounded by dust for the above steady solutions. Because the wormhole spacetime possesses unstable circular orbits of photons, a bright ring appears in the image, just as in Schwarzschild spacetime. This indicates that the appearance of a bright ring solely confirms neither a black hole nor a wormhole. However, we find that the intensity contrast between the inside and the outside of the ring are quite different. Therefore, we could tell the difference between an Ellis wormhole and a black hole with high-resolution very-long-baseline-interferometry observations in the near future.

gr-qc↗

arXiv2017

Wormhole Shadows in Rotating Dust

As an extension of our previous work, which investigated the shadows of the Ellis wormhole surrounded by nonrotating dust, in this paper we study wormhole shadows in rotating dust flow. First, we derive steady-state solutions of slowly rotating dust surrounding the wormhole by solving relativistic Euler equations. Solving null geodesic equations and radiation transfer equations, we investigate the images of the wormhole surrounded by dust for the above steady-state solutions. Because the Ellis wormhole spacetime possesses unstable circular orbits of photons, a bright ring appears in the image, just as in Schwarzschild spacetime. The bright ring looks distorted due to rotation. Aside from the bright ring, there appear weakly luminous complex patterns by the emission from the other side of the throat. These structure could be detected by high-resolution very-long-baseline-interferometry observations in the near future.

gr-qc↗

arXiv2016

The local spiral structure of the Milky Way

The nature of the spiral structure of the Milky Way has long been debated. Only in the last decade have astronomers been able to accurately measure distances to a substantial number of high-mass star-forming regions, the classic tracers of spiral structure in galaxies. We report distance measurements at radio wavelengths using the Very Long Baseline Array for eight regions of massive star formation near the Local spiral arm of the Milky Way. Combined with previous measurements, these observations reveal that the Local Arm is larger than previously thought, and both its pitch angle and star formation rate are comparable to those of the Galaxy's major spiral arms, such as Sagittarius and Perseus. Toward the constellation Cygnus, sources in the Local Arm extend for a great distance along our line of sight and roughly along the solar orbit. Because of this orientation, these sources cluster both on the sky and in velocity to form the complex and long enigmatic Cygnus X region. We also identify a spur that branches between the Local and Sagittarius spiral arms.

astro-ph.GA↗

arXiv2015

Outer Rotation Curve of the Galaxy with VERA III: Astrometry of IRAS 07427-2400 and Test of the Density-Wave Theory

We report the trigonometric parallax of IRAS 07427-2400 with VERA to be 0.185 $\pm$ 0.027 mas, corresponding to a distance of 5.41$^{+0.92}_{-0.69}$ kpc. The result is consistent with the previous result of 5.32$^{+0.49}_{-0.42}$ kpc obtained by Choi et al. (2014) within error. To remove the effect of internal maser motions (e.g., random motions), we observed six maser features associated with IRAS 07427-2400 and determined systematic proper motions of the source by averaging proper motions of the six maser features. The obtained proper motions are ($μ_α$cos$δ$, $μ_δ$) = ($-$1.79 $\pm$ 0.32, 2.60 $\pm$ 0.17) mas yr$^{-1}$ in equatorial coordinates, while Choi et al. (2014) showed ($μ_α$cos$δ$, $μ_δ$) = ($-$2.43 $\pm$ 0.02, 2.49 $\pm$ 0.09) mas yr$^{-1}$ with one maser feature. Our astrometry results place the source in the Perseus arm, the nearest main arm in the Milky Way. Using our result with previous astrometry results obtained from observations of the Perseus arm, we conducted direct (quantitative) comparisons between 27 astrometry results and an analytic gas dynamics model based on the density-wave theory and obtained two results. First is the pitch angle of the Perseus arm determined by VLBI astrometry, 11.1 $\pm$ 1.4 deg, differing from what is determined by the spiral potential model (probably traced by stars), $\sim$ 20 deg. The second is an offset between a dense gas region and the bottom of the spiral potential model. The dense gas region traced by VLBI astrometry is located downstream of the spiral potential model, which was previously confirmed in the nearby grand-design spiral galaxy M51 in Egusa et al. (2011).

astro-ph.GA↗

arXiv2015

Outer Rotation Curve of the Galaxy with VERA II: Annual Parallax and Proper Motion of the Star-Forming Region IRAS21379+5106

We conducted astrometric VLBI observations of water-vapor maser emission in the massive star forming region IRAS 21379+5106 to measure the annual parallax and proper motion, using VERA. The annual parallax was measured to be $0.262 \pm 0.031$ mas corresponding to a trigonometric distance of $3.82^{+0.51}_{-0.41}$ kpc. The proper motion was $(μ_α\cosδ, μ_δ)=(-2.74 \pm 0.08, -2.87 \pm 0.18)$ mas yr$^{-1}$. Using this result, the Galactic rotational velocity was estimated to be $V_θ=218\pm 19$ km s$^{-1}$ at the Galactocentric distance $R=9.22\pm0.43$ kpc, when we adopted the Galactic constants $R_0=8.05\pm 0.45$ kpc and $V_0=238\pm 14$ km s$^{-1}$. With newly determined distance, {the bolometric luminosity of the central young stellar object was re-evaluated to $(2.15\pm 0.54)\times 10^3 L_\odot$, which corresponds to spectral type of} B2--B3. Maser features were found to be distributed along a straight line from south-west to north-east. In addition, a vector map of the internal motions constructed from the residual proper motions implies that maser features trace a bipolar flow and that it cannot be explained by simple ballistic motion.

astro-ph.GA↗

arXiv2014

Gravastar Shadows

Direct observation of black holes is one of the grand challenges in astronomy. If there are super-compact objects which possess unstable circular orbits of photons, however, it may be difficult to distinguish them from black holes by observing photons. As a model of super-compact objects, we consider a gravastar (gravitational-vacuum-star) which was originally proposed by Mazur and Mottola. For definiteness, we adopt a spherical thin-shell model of a gravastar developed by Visser and Wiltshire, which connects interior de-Sitter geometry and exterior Schwarzschild geometry. We find that unstable circular orbits of photons can appear around the gravastar. Then, we investigate the optical images of the gravastar possessing unstable circular orbits, with assuming the optically transparent surface of it and two types of optical sources behind the gravastar: (i) an infinite optical plane and (ii) a companion star. The main feature of the image of (i) is that a bright disk and a dark thick ring surrounding the disk appear in the center of the region which would be completely dark if the compact object was not the gravastar but Schwarzschild black hole. Also in the case (ii), a small disk and arcs around the disk appear in the region which would be completely dark for the lensing image by Schwarzschild black hole. Because characteristic images appear inside the gravastar in both cases, we could tell the difference between a black hole and a gravastar with high-resolution VLBI observations near future.

gr-qc↗

arXiv2014

Trigonometric Parallax of IRAS 22555+6213 with VERA: 3-Dimensional View of Sources Along the Same Line of Sight

We report the results of the measurement of the trigonometric parallax of an H$_2$O maser source in \r22 with the VLBI Exploration of Radio Astrometry (VERA). The annual parallax was determined to be 0.314$\pm$0.070~mas, corresponding to a distance of 3.18$^{+0.90}_{-0.66}$~kpc. Our results confirm \r22 to be located in the Perseus arm. We computed the peculiar motion of \r22 to be $(U_{src}, V_{src}, W_{src}) = (4\pm4, -32\pm6, 8\pm6)$~\kms, where $U_{src}$, $V_{src}$, and $W_{src}$ are directed toward the Galactic center, in the direction of Galactic rotation and toward the Galactic north pole, respectively. \r22, NGC\,7538 and Cepheus A lie along the same line of sight, and are within 2$^{\circ}$ on the sky. Their parallax distances with which we derived their absolute position in the Milky Way show that \r22 and \ngc~are associated with the Perseus Arm, while \cepa~is located in the Local Arm. We compared the kinematic distances of \iras, \ga, \gb, \gc, \gd, \cepa, \ngc, \ge, \gf~and \r22~derived with flat and non-flat rotation curve with its parallax distance and found the kinematic distance derived from the non-flat rotation assumption ($-$5 to $-$39 \kms~lag) to be consistent with the parallax distance.

astro-ph.SR↗

arXiv2014

Gauged Q-balls in the Affleck-Dine mechanism

We consider gauged Q-balls in the gravity-mediation-type model in the Affleck-Dine mechanism, which is described by the potential $V_{\rm grav.}(ϕ):=(m_{\rm grav.}^2/2)ϕ^2\left[1+K\ln(ϕ/M)^2\right]$ with $K<0$. In many models of gauged Q-balls, which were studied in the literature, there are upper limits for charge and size of Q-balls due to repulsive Coulomb force. In the present model, by contrast, our numerical calculation strongly suggests that stable solutions with any amount of charge and size exist. As the electric charge $Q$ increases, the field configuration of the scalar field becomes shell-like; because the charge is concentrated on the surface, the Coulomb force does not destroy the Q-ball configuration. These properties are analogous to those in the V-shaped model, which was studied by Arodź and Lis. We also find that for each $K$ there is another sequence of unstable solutions, which is separated from the other sequence of the stable solutions. As $|K|$ increases, the two sequences approach; eventually at some point in $-1.07<K<-1.06$, the "recombination" of the two sequences takes place.

hep-th↗

arXiv2013

Absolute Proper Motion of IRAS 00259+5625 with VERA : Indication of Superbubble Expansion Motion

We present the first measurement of the absolute proper motions of IRAS 00259+5625 (CB3, LBN594) associated with the HI loop called the "NGC281 superbubble" that extends from the Galactic plane over ~300 pc toward decreasing galactic latitude. The proper motion components measured with VERA are (mu_alpha cos(delta), mu_delta) = (-2.48 +/- 0.32, -2.85 +/- 0.65) mas yr^{-1}, converted into (mu_l cos(b), mu_b) = (-2.72 +/- 0.32, -2.62 +/- 0.65) mas yr^{-1} in the Galactic coordinates. The measured proper motion perpendicular to the Galactic plane (mu_b) shows vertical motion away from the Galactic plane with a significance of about ~4-sigma. As for the source distance, the distance measured with VERA is marginal, 2.4^{+1.0}_{-0.6} kpc. Using the distance, an absolute vertical motion (v_{b}) of -17.9 +/- 12.2 km s^{-1} is determined with ~1.5-sigma significance. The tendency of the large vertical motion is consistent with previous VLBI results for NGC 281 associated with the same superbubble. Thus, our VLBI results indicate the superbubble expansion motion whose origin is believed to be sequential supernova explosions.

astro-ph.GA↗

arXiv2012

The outer rotation curve project with VERA: Trigonometric parallax of IRAS 05168+3634

We present a measurement of the trigonometric parallax of IRAS 05168+3634 with VERA. The parallax is 0.532 +/- 0.053 mas, corresponding to a distance of 1.88 +0.21/-0.17 kpc. This is significantly closer than the previous distance estimate of 6 kpc based on a kinematic distance measurement. This drastic change in the source distance implies the need for revised values of not only the physical parameters of IRAS 05168+3634, but it also implies a different location in the Galaxy, placing it in the Perseus arm rather than the Outer arm. We also measured the proper motion of the source. A combination of the distance and proper motion with the systemic velocity yields a rotation velocity Θ = 227 +9/-11 km s^-1 at the source position, assuming Θ_0 = 240 km s^-1. Our result, combined with previous VLBI results for six sources in the Perseus arm, indicates that the sources rotate systematically more slowly than the Galactic rotation velocity at the local standard of rest. In fact, we derive peculiar motions in the disk averaged over the seven sources in the Perseus arm of (U_mean, V_mean) = (11 +/- 3, -17 +/- 3) km s^-1, which indicates that these seven sources are moving systematically toward the Galactic Center and lag behind the overall Galactic rotation.

astro-ph.GA↗