arXiv subjects

Nobuyuki Sakai

Publications and source records attributed to Nobuyuki Sakai.

At least 55 records · Page 3Linked to original sources

arXiv2012

Fundamental Parameters of the Milky Way Galaxy Based on VLBI astrometry

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA and EVN. We model the Galaxy's structure with a set of parameters including the Galaxy center distance R_0, the angular rotation velocity at the LSR Omega_0, mean peculiar motion of the sources with respect to Galactic rotation (U_src, V_src, W_src), rotation-curve shape index, and the V component of the Solar peculiar motions V_sun. Based on a Markov chain Monte Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R_0 = 8.05 +/- 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion U_src and W_src are fairly small compared to the Galactic rotation velocity, being U_src = 1.0 +/- 1.5 km/s and W_src = -1.4 +/- 1.2 km/s. Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between V_src and V_sun as V_src = V_sun -19 (+/- 2) km/s, suggesting that the value of V_src is fully dependent on the adopted value of V_sun. Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Omega_0 and V_sun. We find that the angular velocity of the Sun, Omega_sun, which is defined as Omega_sun = Omega_0 + V_sun/R_0, can be well constrained with the best estimate of Omega_sun = 31.09 +/- 0.78 km/s/kpc. This corresponds to Theta_0 = 238 +/- 14 km/s if one adopts the above value of R_0 and recent determination of V_sun ~ 12 km/s.

astro-ph.GA↗

arXiv2012

Annual Parallax Distance to the K-type star system IRAS 22480+6002 measured with VERA

We present the astrometric VLBI observations of water masers associated with IRAS 22480+6002 (IRC+60370, hereafter I22480) with the VLBI Exploration of Radio Astrometry (VERA). The stellar type of I22480 looks unusual as a stellar maser source and has been debated since the 1970s. We successfully determined the annual parallax of a group of the water maser spots, pi=0.400+/-0.025 mas, corresponding to a distance to I22480 of D=2.50 /+0.17/-0.15 kpc. This suggests that the estimated bolometric luminosity of I22480 should be revised to 35 000 L_sun, favoring a K-type supergiant rather than an RV Tau-type variable star previously suggested. Although the spectral type is unusual as a stellar maser source, the internal motions of the water maser features suggest that the water masers are associated with the circumstellar envelope of this star. Taking into account a possible stellar motion with respect to the maser feature motions, we derived a secular proper motion of I22480, (mu_alpha, mu_delta)=(-2.58+/-0.33, -1.91+/-0.17) [mas yr^-1]. The derived motion of I22480 in the Milky Way has a deviation by ~ -30 km/s in the galactic azimuthal direction from a circular motion estimated from the galactocentric distance to I22480 and assumption of a flat Galactic rotation curve. This peculiar motion is still comparable to those typically seen in the water maser sources located in the Perseus spiral arm. Taking into account the peculiar motion and the proximity to the Galactic midplane (z ~60 pc), I22480 may be a member of the Galactic thin disk.

astro-ph.SR↗

arXiv2012

Unified pictures of Q-balls and Q-tubes

While Q-balls have been investigated intensively for many years, another type of nontopological solutions, Q-tubes, have not been understood very well. In this paper we make a comparative study of Q-balls and Q-tubes. First, we investigate their equilibrium solutions for four types of potentials. We find, for example, that in some models the charge-energy relation is similar between Q-balls and Q-tubes while in other models the relation is quite different between them. To understand what determines the charge-energy relation, which is a key of stability of the equilibrium solutions, we establish an analytical method to obtain the two limit values of the energy and the charge. Our prescription indicates how the existent domain of solutions and their stability depends on their shape as well as potentials, which would also be useful for a future study of Q-objects in higher-dimensional spacetime.

gr-qc↗

arXiv2012

What happens to Q-balls if $Q$ is so large?

In the system of a gravitating Q-ball, there is a maximum charge $Q_{\rm max}$ inevitably, while in flat spacetime there is no upper bound on $Q$ in typical models such as the Affleck-Dine model. Theoretically the charge $Q$ is a free parameter, and phenomenologically it could increase by charge accumulation. We address a question of what happens to Q-balls if $Q$ is close to $Q_{\rm max}$. First, without specifying a model, we show analytically that inflation cannot take place in the core of a Q-ball, contrary to the claim of previous work. Next, for the Affleck-Dine model, we analyze perturbation of equilibrium solutions with $Q\approx Q_{\rm max}$ by numerical analysis of dynamical field equations. We find that the extremal solution with $Q=Q_{\rm max}$ and unstable solutions around it are "critical solutions", which means the threshold of black-hole formation.

gr-qc↗

arXiv2012

Outer Rotation Curve of the Galaxy with VERA I: Trigonometric parallax of IRAS 05168+3634

We report measurement of 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 result is significantly smaller than the previous distance estimate of 6 kpc based on kinematic distance. This drastic change in the source distance revises not only physical parameters of IRAS 05168+3634, but also its location of the source, placing it in the Perseus arm rather than the Outer arm. We also measure proper motions of the source. A combination of the distance and the proper motions with systemic velocity yields rotation velocity (Θ) of 227+9/-11 km s-1 at the source, assuming Θ0 = 240 km s-1. Our result combined with previous VLBI results for six sources in the Perseus arm indicates that the sources rotate systematically slower than the Galactic rotation velocity at the LSR. In fact, we show observed disk peculiar motions averaged over the seven sources in the Perseus arm as (Umean, Vmean) = (11 +/- 3, -17 +/- 3) km s-1, indicating that these seven sources are systematically moving toward the Galactic center, and lag behind the Galactic rotation.

astro-ph.GA↗

arXiv2011

Q-tubes and Q-crusts

We explore equilibrium solutions of non-topological solitons in a general class of scalar field theories which include global U(1) symmetry. We find new types of solutions, tube-shaped and crust-shaped objects, and investigate their stability. Like Q-balls, the new solitons can exist in supersymmetric extensions of the Standard Model, which may responsible for baryon asymmetry and dark matter. Therefore, observational signals of the new solitons would give us more informations on the early universe and supersymmetric theories.

hep-th↗

arXiv2011

What are universal features of gravitating Q-balls?

We investigate how gravity affects Q-balls by exemplifying the case of the Affleck-Dine potential $V(ϕ):=m^4 \ln (1+\frac{ϕ^2}{m^2})$. Surprisingly, stable Q-balls with arbitrarily small charge exist, no matter how weak gravity is, contrary to the case of flat spacetime. We also show analytically that this feature holds true for general models as long as the leading order term of the potential is a positive mass term in its Maclaurin series.

gr-qc↗

arXiv2011

Gravitating Q-balls in the Affleck-Dine mechanism

We investigate how gravity affects "Q-balls" with the Affleck-Dine potential $V_{AD}(ϕ):=\frac{m^2}{2}ϕ^2[ 1+K\ln (\fracϕ{M})^2]$. Contrary to the flat case, in which equilibrium solutions exist only if $K<0$, we find three types of gravitating solutions as follows. In the case that $K<0$, ordinary Q-ball solutions exist; there is an upper bound of the charge due to gravity. In the case that K=0, equilibrium solutions called (mini-)boson stars appear due to gravity; there is an upper bound of the charge, too. In the case that $K>0$, equilibrium solutions appear, too. In this case, these solutions are not asymptotically flat but surrounded by Q-matter. These solutions might be important in considering a dark matter scenario in the Affleck-Dine mechanism.

gr-qc↗

arXiv2011

How does gravity save or kill Q-balls?

We explore stability of gravitating Q-balls with potential $V_4(ϕ)={m^2\over2}ϕ^2-λϕ^4+\frac{ϕ^6}{M^2}$ via catastrophe theory, as an extension of our previous work on Q-balls with potential $V_3(ϕ)={m^2\over2}ϕ^2-μϕ^3+λϕ^4$. In flat spacetime Q-balls with $V_4$ in the thick-wall limit are unstable and there is a minimum charge $Q_{\rm min}$, where Q-balls with $Q<Q_{\rm min}$ are nonexistent. If we take self-gravity into account, on the other hand, there exist stable Q-balls with arbitrarily small charge, no matter how weak gravity is. That is, gravity saves Q-balls with small charge. We also show how stability of Q-balls changes as gravity becomes strong.

gr-qc↗

arXiv2011

Unified picture of Q-balls and boson stars via catastrophe theory

We make an analysis of Q-balls and boson stars using catastrophe theory, as an extension of the previous work on Q-balls in flat spacetime. We adopt the potential $V_3(ϕ)={m^2\over2}ϕ^2-μϕ^3+λϕ^4$ for Q-balls and that with $μ=0$ for boson stars. For solutions with $|g^{rr}-1|\sim 1$ at its peak, stability of Q-balls has been lost regardless of the potential parameters. As a result, phase relations, such as a Q-ball charge versus a total Hamiltonian energy, approach those of boson stars, which tell us an unified picture of Q-balls and boson stars.

gr-qc↗

arXiv2011

Dynamics of Void and its Shape in Redshift Space

We investigate the dynamics of a single spherical void embedded in a Friedmann-Lemaître universe, and analyze the void shape in the redshift space. We find that the void in the redshift space appears as an ellipse shape elongated in the direction of the line of sight (i.e., an opposite deformation to the Kaiser effect). Applying this result to observed void candidates at the redshift z~1-2, it may provide us with a new method to evaluate the cosmological parameters, in particular the value of a cosmological constant.

astro-ph.CO↗

arXiv2010

Evidence of Quasi-linear Super-Structures in the Cosmic Microwave Background and Galaxy Distribution

Recent measurements of hot and cold spots on the cosmic microwave background (CMB) sky suggest a presence of super-structures on (>100 h^{-1}Mpc) scales. We develop a new formalism to estimate the expected amplitude of temperature fluctuations due to the integrated Sachs-Wolfe (ISW) effect from prominent quasi-linear structures. Applying the developed tools to the observed ISW signals from voids and clusters in catalogs of galaxies at redshifts z<1, we find that they indeed imply a presence of quasi-linear super-structures with a comoving radius 100~300 h^{-1}Mpc and a density contrast ~O(0.1). We find that the observed ISW signals are at odd with the concordant Λcold dark matter (CDM) model that predicts Gaussian primordial perturbations at equal to or larger than 3 sigma level. We also confirm that the mean temperature around the CMB cold spot in the southern Galactic hemisphere filtered by a compensating top-hat filter deviates from a mean value at ~3 sigma level, implying that a quasi-linear supervoid or an underdensity region surrounded by a massive wall may reside at low redshifts z<0.3 and the actual angular size (16^\circ-17^\circ) may be larger than the apparent size (4^\circ-10^\circ) discussed in literature. Possible solutions are briefly discussed.

astro-ph.CO↗

arXiv2008

Cosmic Microwave Background Anisotropy from Nonlinear Structures in Accelerating Universes

We study the cosmic microwave background (CMB) anisotropy due to spherically symmetric nonlinear structures in flat universes with dust and a cosmological constant. By modeling a time-evolving spherical compensated void/lump by Lemaitre-Tolman-Bondi spacetimes, we numerically solve the null geodesic equations with the Einstein equations. We find that a nonlinear void redshifts the CMB photons that pass through it regardless of the distance to it. In contrast, a nonlinear lump blueshifts (or redshifts) the CMB photons if it is located near (or sufficiently far from) us. The present analysis comprehensively covers previous works based on a thin-shell approximation and a linear/second order perturbation method and the effects of shell thickness and full nonlinearity. Our results indicate that, if quasi-linear and large ($>100$Mpc) voids/lumps would exist, they could be observed as cold or hot spots with temperature variance $>10^{-5}$K in the CMB sky.

astro-ph↗

arXiv2008

Stability of Q-balls and Catastrophe

We propose a practical method for analyzing stability of Q-balls for the whole parameter space, which includes the intermediate region between the thin-wall limit and thick-wall limit as well as Q-bubbles (Q-balls in false vacuum), using the catastrophe theory. We apply our method to the two concrete models, $V_3=m^2ϕ^2/2-μϕ^3+λϕ^4$ and $V_4=m^2ϕ^2/2-λϕ^4+ϕ^6/M^2$. We find that $V_3$ and $V_4$ Models fall into {\it fold catastrophe} and {\it cusp catastrophe}, respectively, and their stability structures are quite different from each other.

hep-ph↗

arXiv2008

The universe out of a breathing bubble

We consider the model of a false vacuum bubble with a thin wall where the surface energy density is composed of two different components, "domain-wall" type and "dust" type, with opposite signs. We find stably oscillating solutions, which we call "breathing bubbles". By decay to a lower mass state, such a breathing bubble could become either i) a child universe or ii) a bubble that "eats up" the original universe, depending on the sign of the surface energy of the "domain-wall" component. We also discuss the effect of the finite-thickness corrections to the thin-wall approximation and possible origins of the energy contents of our model.

gr-qc↗

arXiv2006

The universe out of a monopole in the laboratory?

To explore the possibility that an inflationary universe can be created out of a stable particle in the laboratory, we consider the classical and quantum dynamics of a magnetic monopole in the thin-shell approximation. Classically there are three types of solutions: stable, collapsing and inflating monopoles. We argue that the transition from a stable monopole to an inflating one could occur either by collision with a domain wall or by quantum tunneling.

gr-qc↗

arXiv2005

Optimal supply against fluctuating demand

Sornette et al. claimed that the optimal supply does not agree with the average demand, by analyzing a bakery model where a daily demand fluctuates with a uniform distribution. In this note, we extend the model to general probability distributions, and obtain the formula of the optimal supply for Gaussian distribution, which is more realistic. Our result is useful in a real market to earn the largest income on average.

physics.soc-ph↗

arXiv2003

Generality of Topological Inflation

Many authors claimed that a large initial inhomogeneity prevents the onset of inflation and therefore inflation takes place only if the scalar field is homogeneous or appropriately chosen over the horizon scale. We show that those arguments do not apply to topological inflation. The core of a defect starts inflation even if it has much smaller size than the horizon and much larger gradient energy than the potential, as long as the vacuum expectation value is large enough ($\gtrsim0.3\mpl$) and the core is not contracting initially. This is due to stability of false vacuum.

gr-qc↗