• 천문학회지
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• 제31권2호
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• pp.95-99
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• 1998

We report preliminary results of an on-going survey of optically selected barred galaxies with $^{12}CO$(J=1-0) line. The entire sample is composed of about 100 bright barred galaxies ($B_T{\le}13$) with small inclination angle. Most of the galaxies are relatively nearby with receding speed less than 10,000 km/sec. In the first observing run, we have observed central parts of 18 galaxies and detected CO emissions from 5 galaxies (NGC521, 2525, 4262, 4900, and 7479). Most of these galaxies are not observed with CO previously, except for NGC7479 which has been studied at various wavelengths. The peak antenna temperature of detected galaxies ranges from about 30 to 300 mK.

• Journal of Astronomy and Space Sciences
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• 제33권4호
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• pp.257-264
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• 2016

A planet revolving around binary star system is a familiar system. Studies of these systems are important because they provide precise knowledge of planet formation and orbit evolution. In this study, a method to determine the evolution of an exoplanet revolving around a binary star system using different rates of stellar mass loss will be introduced. Using a hierarchical triple body system, in which the outer body can be moved with the center of mass of the inner binary star as a two-body problem, the long period evolution of the exoplanet orbit is determined depending on a Hamiltonian formulation. The model is simulated by numerical integrations of the Hamiltonian equations for the system over a long time. As a conclusion, the behavior of the planet orbital elements is quite affected by the rate of the mass loss from the accompanying binary star.

• 천문학회보
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• 제46권2호
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• pp.64.1-64.1
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• 2021

Accretion flows around black holes and neutron stars emit high energy radiation with varying spectral and timing properties. Observed timing variations, both short and long-term, point to the existence of a mechanism, dictated by the flow dynamics, and not by the stellar surface or magnetic fields, that is common in both. Spectral energy distributions of multiple sources indicate that the Comptonization process, the dominant mechanism for changing states in X-ray, takes place inside the flow that has similar physical properties in both the objects. In a series of observational and numerical studies, we enquire about the following: 1. Is there a steady state configuration for accreting matter around black holes that can explain spectral and timing properties? 2. Could a similar formalism explain spectral and timing properties of accretion around neutron stars? 3. Could there be a generalized flow configuration for accreting matter around such compact objects? Furthermore, we show that a unified spectral model can be constructed based on the generalized flow configuration, common to black holes and neutron stars.

• Monthly Notices of the Royal Astronomical Society
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• 제491권3호
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• pp.4057-4068
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• 2020

Elliptical galaxies today appear aligned with the large-scale structure of the Universe, but it is still an open question when they acquire this alignment. Observational data are currently insufficient to provide constraints on the time evolution of intrinsic alignments, and hence existing models range from assuming that galaxies gain some primordial alignment at formation, to suggesting that they react instantaneously to tidal interactions with the large-scale structure. Using the cosmological hydrodynamical simulation Horizon-AGN, we measure the relative alignments between the major axes of galaxies and eigenvectors of the tidal field as a function of redshift. We focus on constraining the time evolution of the alignment of the main progenitors of massive z = 0 elliptical galaxies, the main weak-lensing contaminant at low redshift. We show that this population, which at z = 0 has a stellar mass above 10^10.4 M_⊙, transitions from having no alignment with the tidal field at z = 3, to a significant alignment by z = 1. From z = 0.5, they preserve their alignment at an approximately constant level until z = 0. We find a mass dependence of the alignment signal of elliptical progenitors, whereby ellipticals that are less massive today (10^10.4 < M/M_⊙ < 10^10.7) do not become aligned till later redshifts (z < 2), compared to more massive counterparts. We also present an extended study of progenitor alignments in the parameter space of stellar mass and galaxy dynamics, the impact of shape definition, and tidal field smoothing.

• 천문학회지
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• 제46권4호
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• pp.173-181
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• 2013

We trace the dynamical evolution of dark matter (DM) content in NGC 6397, one of the native Galactic globular clusters (GCs). The relatively strong tidal field (Galactocentric radius of ~ 6 kpc) and short relaxation timescale (~0.3 Gyr) of the cluster can cause a significant amount of DM particles to evaporate from the cluster in the Hubble time. Thus, the cluster can initially contain a non-negligible amount of DM. Using the most advanced Fokker-Planck (FP) method, we calculate the dynamical evolution of GCs for numerous initial conditions to determine the maximum initial DM content in NGC 6397 that matches the present-day brightness and velocity dispersion profiles of the cluster. We find that the maximum allowed initial DM mass is slightly less than the initial stellar mass in the cluster. Our findings imply that NGC 6397 did not initially contain a significant amount of DM, and is similar to that of NGC 2419, the remotest and the most massive Galactic GC.

• 천문학회지
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• 제29권1호
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• pp.19-30
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• 1996

We have examined consequences of strong tidal encounters between a neutron star and a normal star using SPH as a possible formation mechanism of isolated recycled pulsars in globular clusters. We have made a number of SPH simulations for close encounters between a main-sequence star of mass ranging from 0.2 to 0.7 $M_\bigodot$ represented by an n=3/2 poly trope and a neutron star represented by a point mass. The outcomes of the first encounters are found to be dependent only on the dimensionless parameter $\eta'{\equiv}(m/(m+ M))^{1/2}(\gamma_{min}/R_{MS})^{3/2}(m/M)^{{1/6)}$, where m and M are the mass of the main-sequence star and the neutron star, respectively, $\gamma_{min}$ the minimum separation between two stars, and $R_{MS}$ the size of the main-sequence star. The material from the (at least partially) disrupted star forms a disk around the neutron star. If all material in the disk is to be acctreted onto the neutron star's surface, the mass of the disk is enough to spin up the neutron star to spin period of 1 ms.

• 천문학회지
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• 제37권3호
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• pp.91-117
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• 2004

We analyze the dependence of the mass-to-light ratio of spiral galaxies on the present star formation rate (SFR), and find that galaxies with high present star formation rates have low mass-to-light ratios, presumably as a result of the enhanced luminosity. On this basis we argue that variations in the stellar content of galaxies result in a major source of intrinsic scatter in the Tully-Fisher relation (TF relation). Ideally one should use a 'population-corrected' luminosity. We have also analyzed the relation between the (maximum) luminous mass and rotational velocity, and find it to have a small scatter. We therefore propose that the physical basis of the Tully-Fisher relation lies in a relationship between the luminous mass and rotational velocity, in combination with a 'well-behaved' relation between luminous and dark matter. This implies that the Tully-Fisher relation is a combination of two independent relations: (i) a relation between luminosity and (luminous) mass, based mainly on the star formation history in galaxies, and (ii) a relation between mass and rotation velocity, which is the outcome of the process of galaxy formation. In addition to a 'population-corrected' Tully-Fisher relation, one may also use the relation between mass and luminosity, and the relation between luminous mass and rotation velocity as distance estimators.

• 천문학회보
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• 제43권2호
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• pp.32.1-32.1
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• 2018

The galactic warp is almost ubiquitous among disk galaxies and suspected to be an imprint of recent interactions with other galaxies. The detailed evolutionary course, however, is still uncertain due to the lack of observational evidence. To address this issue, we construct a new extensive catalog of 412 conspicuously warped disks at z = 0.01 ~ 0.05, based on SDSS DR7. We classify the warp morphology through a visual inspection from the Zooniverse Project and our new automated scheme for the warp measurement. We find an interesting color difference between S-and U-shaped warps. The U-type warp galaxies exhibits considerable color offset towards blue compared to both the S-type warps and the control sample of un-warped galaxies. The effect is even more pronounced for galaxies (a) with the greater warp amplitude and (b) with lower luminosity. This is the first piece of observational evidence that the S- and U-shaped warps are on different evolutionary phases in terms of not only dynamics but stellar populations as well. We discuss the implications in the context of the warp evolution theory.

• 천문학회지
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• 제49권5호
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• pp.193-198
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• 2016

Early-type galaxies (ETGs) are supposed to follow the virial relation $M=k_e{\sigma}^2R_e/G$, with M being the mass, σ_* being the stellar velocity dispersion, R_e being the effective radius, G being Newton's constant, and k_e being the virial factor, a geometry factor of order unity. Applying this relation to (a) the ATLAS^3D sample of Cappellari et al. (2013) and (b) the sample of Saglia et al. (2016) gives ensemble-averaged factors 〈k_e〉 = 5.15 ± 0.09 and 〈k_e〉 = 4.01 ± 0.18, respectively, with the difference arising from different definitions of effective velocity dispersions. The two datasets reveal a statistically significant tilt of the empirical relation relative to the theoretical virial relation such that $M{\propto}({\sigma}^2_*R_e)^{0.92}$. This tilt disappears when replacing R_e with the semi-major axis of the projected half-light ellipse, a. All best-fit scaling relations show zero intrinsic scatter, implying that the mass plane of ETGs is fully determined by the virial relation. Whenever a comparison is possible, my results are consistent with, and confirm, the results by Cappellari et al. (2013). The difference between the relations using either a or R_e arises from a known lack of highly elliptical high-mass galaxies; this leads to a scaling (1 - ϵ ) ∝ M^0.12, with ϵ being the ellipticity and $R_e=a\sqrt[]{1-{\epsilon}}$. Accordingly, a, not R_e, is the correct proxy for the scale radius of ETGs. By geometry, this implies that early-type galaxies are axisymmetric and oblate in general, in agreement with published results from modeling based on kinematics and light distributions.