103,662 research outputs found
Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure
We consider the origin of compact, short-period, Jupiter-mass planets. We
propose that their diverse structure is caused by giant impacts of embryos and
super-Earths or mergers with other gas giants during the formation and
evolution of these hot Jupiters. Through a series of numerical simulations, we
show that typical head-on collisions generally lead to total coalescence of
impinging gas giants. Although extremely energetic collisions can disintegrate
the envelope of gas giants, these events seldom occur. During oblique and
moderately energetic collisions, the merger products retain higher fraction of
the colliders' cores than their envelopes. They can also deposit considerable
amount of spin angular momentum to the gas giants and desynchronize their spins
from their orbital mean motion. We find that the oblateness of gas giants can
be used to infer the impact history. Subsequent dissipation of stellar tide
inside the planets' envelope can lead to runaway inflation and potentially a
substantial loss of gas through Roche-lobe overflow. The impact of super-Earths
on parabolic orbits can also enlarge gas giant planets' envelope and elevates
their tidal dissipation rate over 100 Myr time scale. Since giant
impacts occur stochastically with a range of impactor sizes and energies, their
diverse outcomes may account for the dispersion in the mass-radius relationship
of hot Jupiters.Comment: 19 pages, 7 figures, 7 tables. Accepted for publication in MNRA
Green's function for the Relativistic Coulomb System via Sum Over Perturbation Series
We evaluate the Green's function of the D-dimensional relativistic Coulomb
system via sum over perturbation series which is obtained by expanding the
exponential containing the potential term in the path integral
into a power series. The energy spectra and wave functions are extracted from
the resulting amplitude.Comment: 13 pages, ReVTeX, no figure
Remark on approximation in the calculation of the primordial spectrum generated during inflation
We re-examine approximations in the analytical calculation of the primordial
spectrum of cosmological perturbation produced during inflation. Taking two
inflation models (chaotic inflation and natural inflation) as examples, we
numerically verify the accuracy of these approximations.Comment: 10 pages, 6 figures, to appear in PR
Entanglement creation between two causally-disconnected objects
We study the full entanglement dynamics of two uniformly accelerated
Unruh-DeWitt detectors with no direct interaction in between but each coupled
to a common quantum field and moving back-to-back in the field vacuum. For two
detectors initially prepared in a separable state our exact results show that
quantum entanglement between the detectors can be created by the quantum field
under some specific circumstances, though each detector never enters the
other's light cone in this setup. In the weak coupling limit, this entanglement
creation can occur only if the initial moment is placed early enough and the
proper acceleration of the detectors is not too large or too small compared to
the natural frequency of the detectors. Once entanglement is created it lasts
only a finite duration, and always disappears at late times. Prior result by
Reznik derived using the time-dependent perturbation theory with extended
integration domain is shown to be a limiting case of our exact solutions at
some specific moment. In the strong coupling and high acceleration regime,
vacuum fluctuations experienced by each detector locally always dominate over
the cross correlations between the detectors, so entanglement between the
detectors will never be generated.Comment: 16 pages, 8 figures; added Ref.[7] and related discussion
Path integral for a relativistic Aharonov-Bohm-Coulomb system
The path integral for the relativistic spinless Aharonov-Bohm-Coulomb system
is solved, and the energy spectra are extracted from the resulting amplitude.Comment: 6 pages, Revte
The structure of the central disk of NGC 1068: a clumpy disk model
NGC 1068 is one of the best studied Seyfert II galaxies, for which the
blackhole mass has been determined from the Doppler velocities of water maser.
We show that the standard -disk model of NGC 1068 gives disk mass
between the radii of 0.65 pc and 1.1 pc (the region from which water maser
emission is detected) to be about 7x10 M (for ), more
than four times the blackhole mass, and a Toomre Q-parameter for the disk is
0.001. This disk is therefore highly self-gravitating and is subject to
large-amplitude density fluctuations. We conclude that the standard
-viscosity description for the structure of the accretion disk is
invalid for NGC 1068.
In this paper we develop a new model for the accretion disk. The disk is
considered to be composed of gravitationally bound clumps; accretion in this
clumped disk model arises because of gravitational interaction of clumps with
each other and the dynamical frictional drag exerted on clumps from the stars
in the central region of the galaxy. The clumped disk model provides a
self-consistent description of the observations of NGC 1068. The computed
temperature and density are within the allowed parameter range for water maser
emission, and the rotational velocity in the disk falls off as .Comment: To appear in Ap
Non-Thermal Production of WIMPs and the Sub-Galactic Structure of the Universe
There is increasing evidence that conventional cold dark matter (CDM) models
lead to conflicts between observations and numerical simulations of dark matter
halos on sub-galactic scales. Spergel and Steinhardt showed that if the CDM is
strongly self-interacting, then the conflicts disappear. However, the
assumption of strong self-interaction would rule out the favored candidates for
CDM, namely weakly interacting massive particles (WIMPs), such as the
neutralino. In this paper we propose a mechanism of non-thermal production of
WIMPs and study its implications on the power spectrum. We find that the
non-vanishing velocity of the WIMPs suppresses the power spectrum on small
scales compared to what it obtained in the conventional CDM model. Our results
show that, in this context, WIMPs as candidates for dark matter can work well
both on large scales and on sub-galactic scales.Comment: 6 pages, 2 figures; typo corrected; to appear in PR
Internal Gravity Waves Modulate the Apparent Misalignment of Exoplanets around Hot Stars
We propose that the observed misalignment between extra-solar planets and
their hot host stars can be explained by angular momentum transport within the
host star. Observations have shown that this misalignment is preferentially
around hot stars, which have convective cores and extended radiative envelopes.
This situation is amenable to substantial angular momentum transport by
internal gravity waves (IGW) generated at the convective-radiative interface.
Here we present numerical simulations of this process and show that IGW can
modulate the surface rotation of the star. With these two- dimensional
simulations we show that IGW could explain the retrograde orbits observed in
systems such as HAT-P-6 and HAT-P-7, however, extension to high obliquity
objects will await future three- dimensional simulations. We note that these
results also imply that individual massive stars should show temporal
variations in their v sini measurements.Comment: 6 pages, 2 figures, Accepted for publication in ApJ
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#Bigbirds never die: Understanding social dynamics of emergent hashtag
We examine the growth, survival, and context of 256 novel hashtags during the 2012 U.S. presidential debates. Our analysis reveals the trajectories of hashtag use fall into two distinct classes: âwinnersâ that emerge more quickly and are sustained for longer periods of time than other âalso-ransâ hashtags. We propose a âconversational vibrancyâ framework to capture dynamics of hashtags based on their topicality, interactivity, diversity, and prominence. Statistical analyses of the growth and persistence of hashtags reveal novel relationships between features of this framework and the relative success of hashtags. Specifically, retweets always contribute to faster hashtag adoption, replies extend the life of âwinnersâ while having no effect on âalso-rans.â This is the first study on the lifecycle of hashtag adoption and use in response to purely exogenous shocks. We draw on theories of uses and gratification, organizational ecology, and language evolution to discuss these findings and their implications for understanding social influence and collective action in social media more generally
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
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