410 research outputs found
High-energy emissions from neutron star mergers
In 2017, LIGO-Virgo collaborations reported detection of the first neutron
star merger event, GW170817, which is accompanied by electromagnetic
counterparts from radio to gamma rays. Although high-energy neutrinos were not
detected from this event, mergers of neutron stars are expected to produce such
high-energy particles. Relativistic jets are launched when neutron stars merge.
If the jets contain protons, they can emit high-energy neutrinos through
photomeson production. In addition, neutron star mergers produce massive and
fast ejecta, which can be a source of Galactic high-energy cosmic rays above
the knee. We briefly review what we learned from the multi-messenger event,
GW170817, and discuss prospects for multi-messenger detections and hadronic
cosmic-ray production related to the neutron star mergers.Comment: 9 pages, 4 figures, 5 tables, conference proceedings of UHECR 201
General form of entropy on the horizon of the universe in entropic cosmology
Entropic cosmology assumes several forms of entropy on the horizon of the
universe, where the entropy can be considered to behave as if it were related
to the exchange (the transfer) of energy. To discuss this exchangeability, the
consistency of the two continuity equations obtained from two different methods
is examined, focusing on a homogeneous, isotropic, spatially flat, and
matter-dominated universe. The first continuity equation is derived from the
first law of thermodynamics, whereas the second equation is from the Friedmann
and acceleration equations. To study the influence of forms of entropy on the
consistency, a phenomenological entropic-force model is examined, using a
general form of entropy proportional to the -th power of the Hubble horizon.
In this formulation, the Bekenstein entropy (an area entropy), the
Tsallis--Cirto black-hole entropy (a volume entropy), and a quartic entropy are
represented by , , and , respectively. The two continuity equations
for the present model are found to be consistent with each other, especially
when , i.e., the Bekenstein entropy. The exchange of energy between the
bulk (the universe) and the boundary (the horizon of the universe) should be a
viable scenario consistent with the holographic principle.Comment: Final version accepted for publication in PRD. Several pasragraphs
and references are added and corrected. [10 pages
Conditions for Gravitational Instability in Protoplanetary Disks
Gravitational instability is one of considerable mechanisms to explain the
formation of giant planets. We study the gravitational stability for the
protoplanetary disks around a protostar. The temperature and Toomre's Q-value
are calculated by assuming local equilibrium between viscous heating and
radiative cooling (local thermal equilibrium). We assume constant
viscosity and use a cooling function with realistic opacity. Then, we derive
the critical surface density that is necessary for a disk to
become gravitationally unstable as a function of . This critical surface
density is strongly affected by the temperature dependence of
the opacity. At the radius AU, where ices form, the value of
changes discontinuously by one order of magnitude. This
is determined only by local thermal process and criterion of
gravitational instability. By comparing a given surface density profile to
, one can discuss the gravitational instability of
protoplanetary disks. As an example, we discuss the gravitational instability
of two semi-analytic models for protoplanetary disks. One is the steady state
accretion disk, which is realized after the viscous evolution. The other is the
disk that has the same angular momentum distribution with its parent cloud
core, which corresponds to the disk that has just formed. As a result, it is
found that the disks tend to become gravitationally unstable for because ices enable the disks to become low temperature. In the region
closer to the protostar than , it is difficult for a typical
protoplanetary disk to fragment because of the high temperature and the large
Coriolis force. From this result, we conclude that the fragmentation near the
central star is possible but difficult.Comment: accepted for publication in PASJ. Draft version with 26 pages, 8
figures, 1 tabl
Effect of lift force on the aerodynamics of dust grains in the protoplanetary disk
We newly introduce lift force into the aerodynamics of dust grains in the
protoplanetary disk. Although many authors have so far investigated the effects
of the drag force, gravitational force and electric force on the dust grains,
the lift force has never been considered as a force exerted on the dust grains
in the gas disk. If the grains are spinning and moving in the fluid, then the
lift force is exerted on them. We show in this paper that the dust grains can
be continuously spinning due to the frequent collisions so that the lift force
continues to be exerted on them, which is valid in a certain parameter space
where the grain size is larger than ~ 1 m and where the distance from the
central star is larger than 1 AU for the minimum mass solar nebula. In
addition, we estimate the effects of the force on the grain motion and obtain
the result that the mean relative velocity between the grains due to the lift
force is comparable to the gas velocity in the Kepler rotational frame when the
Stokes number and lift-drag ratio are both ~ 1. This estimation is performed
under the assumptions of the steady state and the isotropic spin angular
momentum. We also estimate the mean relative velocity when the grains keep
spinning and conclude that the lift force marginally affects the mean relative
velocity in the minimum mass solar nebula. If there is a grain-concentrated
part in the disk, the relative velocity due to the lift force may dominate
there because of high collision rate.Comment: 9 pages, 4 figures. Accepted for publication in Earth, Planets and
Spac
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