204 research outputs found
On the evolution of the radio pulsar PSR J1734−3333
Recent measurements showed that the period derivative of the ‘hig
h-B’ radio pulsar PSR J1734−3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period,the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^12 − 10^13 G on the pole of the neutron star. When the model source
reaches the current state properties of PSR J1734−3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734−3333 is at an age of ∼3×10^4 −2×10^5years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk
XMM-Newton observations of two black hole X-ray transients in quiescence
We report on XMM-Newton observations of GRO J1655-40 and GRS 1009-45, which
are two black hole X-ray transients currently in their quiescent phase. GRO
J1655-40 was detected with a 0.5 - 10 keV luminosity of 5.9 10^{31} erg/s. This
luminosity is comparable to a previous Chandra measurement, but ten times lower
than the 1996 ASCA value, most likely obtained when the source was not yet in a
true quiescent state. Unfortunately, XMM-Newton failed to detect GRS 1009-45. A
stringent upper limit of 8.9 10^{30} erg/s was derived by combining data from
the EPIC-MOS and PN cameras.
The X-ray spectrum of GRO J1655-40 is very hard as it can be fitted with a
power law model of photon index ~ 1.3 +/- 0.4. Similarly hard spectra have been
observed from other systems; these rule out coronal emission from the secondary
or disk flares as the origin of the observed X-rays. On the other hand, our
observations are consistent with the predictions of the disc instability model
in the case that the accretion flow forms an advection dominated accretion flow
(ADAF) at distances less than a fraction ~ 0.1 - 0.3) of the circularization
radius. This distance corresponds to the greatest extent of the ADAF that is
thought to be possible.Comment: 6 pages, 4 figures. Submitted to Astronomy and Astrophysic
Hot Accretion With Conduction: Spontaneous Thermal Outflows
Motivated by the low-collisionality of gas accreted onto black holes in Sgr
A* and other nearby galactic nuclei, we study a family of 2D advective
accretion solutions with thermal conduction. While we only impose global
inflow, the accretion flow spontaneously develops bipolar outflows. The role of
conduction is key in providing the extra degree of freedom (latitudinal energy
transport) necessary to launch these rotating thermal outflows. The sign of the
Bernoulli constant does not discriminate between inflowing and outflowing
regions. Our parameter survey covers mass outflow rates from ~ 0 to 13% of the
net inflow rate, outflow velocities from ~0 to 11% of the local Keplerian
velocity and outflow opening angles from ~ 0 to 60 degs. As the magnitude of
conduction is increased, outflows can adopt a conical geometry, pure inflow
solutions emerge, and the limit of 2D non-rotating Bondi-like solutions is
eventually reached. These results confirm that radiatively-inefficient, hot
accretion flows have a hydrodynamical propensity to generate bipolar thermal
outflows.Comment: 38 pages, 10 figures, accepted for publication in Ap
The Giant X-Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?
We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 as the
tidal disruption of a star by a supermassive black hole. From the observed
rapid decline of the luminosity, over a timescale of a few years, we argue that
the flare was powered by the fallback of debris rather than subsequent
accretion via a thin disk. The fallback model allows constraints to be set on
the black hole mass and the mass of debris. The latter must be very much less
than a solar mass to explain the very low luminosity of the flare. The
observations can be explained either as the partial stripping of the outer
layers of a low-mass main sequence star or as the disruption of a brown dwarf
or a giant planet. We find that the X-ray emission in the flare must have
originated within a small patch rather than over the entire torus of
circularized material surrounding the black hole. We suggest that the patch
corresponds to the ``bright spot'' where the stream of returning debris impacts
the torus. Interestingly, although the peak luminosity of the flare was highly
sub-Eddington, the peak flux from the bright spot was close to the Eddington
limit. We speculate on the implications of this result for observations of
other flare events.Comment: 25 pages, including 5 figure
Type I Planet Migration in Nearly Laminar Disks
We describe 2D hydrodynamic simulations of the migration of low-mass planets
() in nearly laminar disks (viscosity parameter ) over timescales of several thousand orbit periods. We consider disk
masses of 1, 2, and 5 times the minimum mass solar nebula, disk thickness
parameters of and 0.05, and a variety of values and
planet masses. Disk self-gravity is fully included. Previous analytic work has
suggested that Type I planet migration can be halted in disks of sufficiently
low turbulent viscosity, for . The halting is due to a
feedback effect of breaking density waves that results in a slight mass
redistribution and consequently an increased outward torque contribution. The
simulations confirm the existence of a critical mass () beyond which migration halts in nearly laminar disks. For \alpha
\ga 10^{-3}, density feedback effects are washed out and Type I migration
persists. The critical masses are in good agreement with the analytic model of
Rafikov (2002). In addition, for \alpha \la 10^{-4} steep density gradients
produce a vortex instability, resulting in a small time-varying eccentricity in
the planet's orbit and a slight outward migration. Migration in nearly laminar
disks may be sufficiently slow to reconcile the timescales of migration theory
with those of giant planet formation in the core accretion model.Comment: 3 figures, accepted to ApJ
Low-Mass Proto-Planet Migration in T-Tauri Alpha-Disks
We present detailed estimates of ''type-I'' migration rates for low-mass
proto-planets embedded in steady-state T-Tauri alpha-disks, based on Lindblad
torque calculations ignoring feedback on the disk. Differences in migration
rates for several plausible background disk models are explored and we contrast
results obtained using the standard two dimensional formalism of spiral density
wave theory with those obtained from a simple treatment of three-dimensional
effects. Opacity transitions in the disk result in sudden radial variations of
the migration rates. Regions with minimal migration rates may be preferred
sites of gravitational interactions between proto-planets. Three-dimensional
torques are significantly weaker than two-dimensional ones and they are
sensitive to the surface density profile of the background disk. We find that
migration times in excess of runaway envelope accretion times or T-Tauri disk
lifetimes are possible for Earth-mass proto-planets in some background disk
models, even at sub-AU distances. We conclude that an understanding of the
background disk structure and ''viscosity'', as well as a proper treatment of
three-dimensional effects in torque calculations, are necessary to obtain
reliable estimates of ``type-I'' migration rates.Comment: Minor revisions, 31 pages, 7 figures, accepted for publication in Ap
Motion of charged particles around a rotating black hole in a magnetic field
We study the effects of an external magnetic field, which is assumed to be
uniform at infinity, on the marginally stable circular motion of charged
particles in the equatorial plane of a rotating black hole. We show that the
magnetic field has its greatest effect in enlarging the region of stability
towards the event horizon of the black hole. Using the Hamilton-Jacobi
formalism we obtain the basic equations governing the marginal stability of the
circular orbits and their associated energies and angular momenta. As
instructive examples, we review the case of the marginal stability of the
circular orbits in the Kerr metric, as well as around a Schwarzschild black
hole in a magnetic field. For large enough values of the magnetic field around
a maximally rotating black hole we find the limiting analytical solutions to
the equations governing the radii of marginal stability. We also show that the
presence of a strong magnetic field provides the possibility of relativistic
motions in both direct and retrograde innermost stable circular orbits around a
Kerr black hole.Comment: 25 pages, 2 figure
СОВРЕМЕННОЕ СОСТОЯНИЕ И ПРОБЛЕМы ОБОГАЩЕНИЯ ТИТАНОЦИРКОНИЕВОГО СыРЬЯ В УКРАИНЕ
В природе известно 70 минералов, в различных количествах содержащих титан. На сегодняшний день промышленными источниками титанового сырья являются месторождения, содержащие ильменит, рутил, лейкоксен и, в послед-нее время, анатаз. Львиная (около 90%) часть ильменитовых, лейкоксеновых и рутиловых концентратов используются для производства диоксида титана. Из ильменитовых концентратов получают синтетический рутил и титановые шла-ки, которые можно использовать как для производства губчатого титана, так и диоксида титана. На производство металлического титана используется 7-10% сырья. Природный рутил, кроме того, используется частично и для обмазки сварочных электродов. Наиболее богатыми по содержанию диоксида титана являются рутиловые концентраты (93-96%), ильменитовые содержат 44-70% диоксида титана, а концентраты из лейкоксеновых руд содержат до 90% TiO
Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
The bright M dwarf K2-18 at 34 pc is known to host a transiting
super-Earth-sized planet orbiting within the star's habitable zone; K2-18b.
Given the superlative nature of this system for studying an exoplanetary
atmosphere receiving similar levels of insolation as the Earth, we aim to
characterize the planet's mass which is required to interpret atmospheric
properties and infer the planet's bulk composition. We obtain precision radial
velocity measurements with the HARPS spectrograph and couple those measurements
with the K2 photometry to jointly model the observed radial velocity variation
with planetary signals and a radial velocity jitter model based on Gaussian
process regression. We measure the mass of K2-18b to be
M with a bulk density of g/cm which may correspond
to a predominantly rocky planet with a significant gaseous envelope or an ocean
planet with a water mass fraction %. We also find strong evidence
for a second, warm super-Earth K2-18c at days with a semi-major axis
2.4 times smaller than the transiting K2-18b. After re-analyzing the available
light curves of K2-18 we conclude that K2-18c is not detected in transit and
therefore likely has an orbit that is non-coplanar with K2-18b. A suite of
dynamical integrations with varying simulated orbital eccentricities of the two
planets are used to further constrain each planet's eccentricity posterior from
which we measure and at 99% confidence. The discovery
of the inner planet K2-18c further emphasizes the prevalence of multi-planet
systems around M dwarfs. The characterization of the density of K2-18b reveals
that the planet likely has a thick gaseous envelope which along with its
proximity to the Solar system makes the K2-18 planetary system an interesting
target for the atmospheric study of an exoplanet receiving Earth-like
insolation.Comment: 13 pages, 8 figures including 4 interactive figures best viewed in
Adobe Acrobat. Submitted to Astronomy & Astrophysics. Comments welcom
Type I planet migration in nearly laminar disks - long term behavior
We carry out 2-D high resolution numerical simulations of type I planet
migration with different disk viscosities. We find that the planet migration is
strongly dependent on disk viscosities. Two kinds of density wave damping
mechanisms are discussed. Accordingly, the angular momentum transport can be
either viscosity dominated or shock dominated, depending on the disk
viscosities. The long term migration behavior is different as well. Influences
of the Rossby vortex instability on planet migration are also discussed. In
addition, we investigate very weak shock generation in inviscid disks by small
mass planets and compare the results with prior analytic results.Comment: Accepted for publication in Ap
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