Engineering topics

Gang, Zhao

Publications and source records attributed to Gang, Zhao.

7 records

DOE OSTI2013

THE GRAVITATIONAL POTENTIAL NEAR THE SUN FROM SEGUE K-DWARF KINEMATICS

To constrain the Galactic gravitational potential near the Sun ({approx}1.5 kpc), we derive and model the spatial and velocity distributions for a sample of 9000 K-dwarfs with spectra from SDSS/SEGUE, which yield radial velocities and abundances ([Fe/H] and [{alpha}/Fe]). We first derive the spatial density distribution for three abundance-selected sub-populations of stars accounting for the survey's selection function. The vertical profiles of these sub-populations are simple exponentials and their vertical dispersion profile is nearly isothermal. To model these data, we apply the 'vertical' Jeans equation, which relates the observable tracer number density and vertical velocity dispersion to the gravitational potential or vertical force. We explore a number of functional forms for the vertical force law, fit the dispersion and density profiles of all abundance-selected sub-populations simultaneously in the same potential, and explore all parameter co-variances using a Markov Chain Monte Carlo technique. Our fits constrain a disk mass scale height {approx}< 300 pc and the total surface mass density to be 67 {+-} 6 M{sub Sun} pc{sup -2} at |z| = 1.0 kpc of which the contribution from all stars is 42 {+-} 5 M{sub Sun} pc{sup -2} (assuming a contribution from cold gas of 13 M{sub Sun} pc{sup -2}). We find significant constraints on the local dark matter density of 0.0065 {+-} 0.0023 M{sub Sun} pc{sup -3} (0.25 {+-} 0.09 GeV cm{sup -3}). Together with recent experiments this firms up the best estimate of 0.0075 {+-} 0.0021 M{sub Sun} pc{sup -3} (0.28 {+-} 0.08 GeV cm{sup -3}), consistent with global fits of approximately round dark matter halos to kinematic data in the outskirts of the Galaxy.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

MECHANISM FOR EXCITING PLANETARY INCLINATION AND ECCENTRICITY THROUGH A RESIDUAL GAS DISK

According to the theory of Kozai resonance, the initial mutual inclination between a small body and a massive planet in an outer circular orbit is as high as {approx}39. Degree-Sign 2 for pumping the eccentricity of the inner small body. Here we show that with the presence of a residual gas disk outside two planetary orbits, the inclination can be reduced to as low as a few degrees. The presence of the disk changes the nodal precession rates and directions of the planet orbits. At the place where the two planets achieve the same nodal processing rate, vertical secular resonance (VSR) occurs so that the mutual inclination of the two planets will be excited, which might further trigger the Kozai resonance between the two planets. However, in order to pump an inner Jupiter-like planet, the conditions required for the disk and the outer planet are relatively strict. We develop a set of evolution equations, which can fit the N-body simulation quite well but can be integrated within a much shorter time. By scanning the parameter spaces using the evolution equations, we find that a massive planet (10 M{sub J} ) at 30 AU with an inclination of 6 Degree-Sign to a massive disk (50 M{sub J} ) can finally enter the Kozai resonance with an inner Jupiter around the snowline. An inclination of 20 Degree-Sign of the outer planet to the disk is required for flipping the inner one to a retrograde orbit. In multiple planet systems, the mechanism can happen between two nonadjacent planets or can inspire a chain reaction among more than two planets. This mechanism could be the source of the observed giant planets in moderate eccentric and inclined orbits, or hot Jupiters in close-in, retrograde orbits after tidal damping.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

PLANETESIMAL DYNAMICS IN INCLINED BINARY SYSTEMS: THE ROLE OF GAS-DISK GRAVITY

We investigate the effects of gas-disk gravity on planetesimal dynamics in inclined binary systems, where the circumprimary disk plane is tilted by a significant angle (i{sub B} ) with respect to the binary disk plane. Our focus is on the Lidov-Kozai mechanism and the evolution of planetesimal eccentricity and inclination. Using both analytical and numerical methods, we find that, on one hand, disk gravity generally narrows down the Kozai-on region, i.e., the Lidov-Kozai effect can be suppressed in certain parts of (or even the whole of) the disk depending on various parameters. In the Kozai-off region, planetesimals would move to orbits close to the midplane of the gas-disk, with the relative angle (i') following a small amplitude periodical oscillation. On the other hand, when we include the effects of disk gravity, we find that the Lidov-Kozai effect can operate even at arbitrarily low inclinations (i{sub B} ), although a lower i{sub B} leads to a smaller Kozai-on region. Furthermore, in the Kozai-on region, most planetesimals' eccentricities can be excited to extremely high values ({approx}1), and such extreme high eccentricities usually accompany orbital flipping, i.e., the planetesimal orbit flips back and forth between anterograde and retrograde. Once a planetesimal reaches very high orbital eccentricity, gas drag damping will shrink the planetesimal orbit, forming a 'hot planetesimal' on a near circular orbit very close to the primary star. Such a mechanism, if replacing the planetesimals and gas drag damping with Jupiters and tidal damping respectively, may lead to a frequent production of hot Jupiters.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

METAL-POOR HYPERVELOCITY STAR CANDIDATES FROM THE SLOAN DIGITAL SKY SURVEY

Hypervelocity stars are believed to be ejected out from the Galactic center (GC) through dynamical interactions of (binary) stars with the central massive black hole(s). In this Letter, we report 13 metal-poor F-type hypervelocity star candidates selected from 370,000 stars of the data release 7 of the Sloan Digital Sky Survey. With a detailed analysis of the kinematics of these stars, we find that seven of them were likely ejected from the GC or the Galactic disk, four neither originated from the GC nor the Galactic disk, and the other two were possibly ejected from either the Galactic disk or other regions. Those candidates that unlikely originated from the GC or the Galactic disk may be explained by other mechanisms, like the tidal disruption of the Milky Way's dwarf galaxies in the Galactic potential, or the gravitational interactions with a massive black hole at the center of M31 or M32.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

PHOTOIONIZATIONAL PLASMAS. I. THEORY

In this paper, an attempt is made to define the subject of photoionizational plasmas through rigorous mathematical formalism. The central results of this paper are the following. (1) A set of recursive equations is introduced for the computation of the charge state distributions in photoionizational plasmas. (2) Quantitative validity limits are given for both the collisional and the photoionizational domains. (3) A parameter that determines the charge state distribution in the photoionizational regime is introduced. (4) We provide detailed discussion about the most important components of the emission spectrum in the different equilibrium domains of the emitting plasma. Some of these components have not been reported so far as included in the analysis of such plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

QUANTIFYING KINEMATIC SUBSTRUCTURE IN THE MILKY WAY'S STELLAR HALO

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative 'close pair distribution' as a statistic in the four-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at r{sub gc} < 20 kpc.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Mode-coupling Cerenkov sum-frequency-generation in a multimode planar waveguide

We present experimental and theoretical studies of the mode-coupling Cerenkov sum-frequency radiations in a multimode LiNbO{sub 3} planar waveguide. The radiations result from the coupling of different guided modes of the fundamental wave, which have the same optical frequencies but different propagation constants. At the same time, scattering-involved Cerenkov sum-frequency-generation was also observed and discussed. Our theoretical predictions are in well accordance with the experimental results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗