427 research outputs found
Amplification of MHD waves in swirling astrophysical flows
Recently it was found that helical magnetized flows efficiently amplify
Alfv\'en waves (Rogava et al. 2003, A&A, v.399, p.421). This robust and
manifold nonmodal effect was found to involve regimes of transient algebraic
growth (for purely ejectional flows), and exponential instabilities of both
usual and parametric nature. However the study was made in the incompressible
limit and an important question remained open - whether this amplification is
inherent to swirling MHD flows per se and what is the degree of its dependence
on the incompressibility condition. In this paper, in order to clear up this
important question, we consider full compressible spectrum of MHD modes:
Alfv\'en waves (AW), slow magnetosonic waves (SMW) and fast magnetosonic waves
(FMW). We find that helical flows inseparably blend these waves with each other
and make them unstable, creating the efficient energy transfer from the mean
flow to the waves. The possible role of these instabilities for the onset of
the MHD turbulence, self-heating of the flow and the overall dynamics of
astrophysical flows are discussed.Comment: 8 pages, 9 figures, accepted for publication (18.03.2003) in the
"Astronomy and Astrophysics
KH15D: a star eclipsed by a large scale dusty vortex?
We propose that the large photometric variations of KH15D are due to an
eclipsing swarm of solid particles trapped in giant gaseous vortex rotating at
\~0.2 AU from the star. The efficiency of the capture-in-vortex mechanism
easily explains the observed large optical depth. The weaker opacity at
mid-eclipse is consistent with a size segregation of the particles toward the
center of the vortex. This dusty structure must extend over ~1/3 of an orbit to
account for the long eclipse duration. The estimated size of the trapped
particles is found to range from 1 to 10cm, consistent with the gray extinction
of the star. The observations of KH15D support the idea that giant vortices can
grow in circumstellar disks and play a central role in planet formation.Comment: Accepted in ApJ Letters - 4 pages - 2 figure
Two years of INTEGRAL monitoring of GRS 1915+105 Part 1: multiwavelength coverage with INTEGRAL, RXTE, and the Ryle radio Telescope
(Abridged) We report the results of monitoring observations of the Galactic
microquasar GRS 1915+105 performed simultaneously with INTEGRAL and RXTE Ryle .
We present the results of the whole \integral campaign, report the sources that
are detected and their fluxes and identify the classes of variability in which
GRS 1915+105 is found. The accretion ejection connections are studied in a
model independent manner through the source light curves, hardness ratio, and
color color diagrams. During a period of steady ``hard'' X-ray state (the
so-called class chi) we observe a steady radio flux. We then turn to 3
particular observations during which we observe several types of soft X-ray
dips and spikes cycles, followed by radio flares. During these observations GRS
1915+105 is in the so-called nu, lambda, and beta classes of variability. The
observation of ejections during class lambda are the first ever reported. We
generalize the fact that a (non-major) discrete ejection always occurs, in GRS
1915+105, as a response to an X-ray sequence composed of a spectrally hard
X-ray dip terminated by an X-ray spike marking the disappearance of the hard
X-ray emission above 18 keV. We also identify the trigger of the ejection as
this X-ray spike. A possible correlation between the amplitude of the radio
flare and the duration of the X-ray dip is found in our data. In this case the
X-ray dips prior to ejections could be seen as the time during which the source
accumulates energy and material that is ejected later.Comment: 17 pages, 14 figures. Accepted for publication in ApJ, scheduled for
the March 20, 2008, vol676 issue. Table 3 has been degrade
A General Relativistic Magnetohydrodynamics Simulation of Jet Formation
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion
disk around a Schwarzschild black hole with a free-falling corona. The initial
simulation results show that a bipolar jet (velocity ) is created as
shown by previous two-dimensional axisymmetric simulations with mirror symmetry
at the equator. The 3-D simulation ran over one hundred light-crossing time
units ( where ) which is
considerably longer than the previous simulations. We show that the jet is
initially formed as predicted due in part to magnetic pressure from the
twisting the initially uniform magnetic field and from gas pressure associated
with shock formation in the region around . At later times,
the accretion disk becomes thick and the jet fades resulting in a wind that is
ejected from the surface of the thickened (torus-like) disk. It should be noted
that no streaming matter from a donor is included at the outer boundary in the
simulation (an isolated black hole not binary black hole). The wind flows
outwards with a wider angle than the initial jet. The widening of the jet is
consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This
evolution of disk-jet coupling suggests that the jet fades with a thickened
accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better
resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf
Vortex generation in protoplanetary disks with an embedded giant planet
Vortices in protoplanetary disks can capture solid particles and form
planetary cores within shorter timescales than those involved in the standard
core-accretion model. We investigate vortex generation in thin unmagnetized
protoplanetary disks with an embedded giant planet with planet to star mass
ratio and . Two-dimensional hydrodynamical simulations of a
protoplanetary disk with a planet are performed using two different numerical
methods. The results of the non-linear simulations are compared with a
time-resolved modal analysis of the azimuthally averaged surface density
profiles using linear perturbation theory. Finite-difference methods
implemented in polar coordinates generate vortices moving along the gap created
by Neptune-mass to Jupiter-mass planets. The modal analysis shows that unstable
modes are generated with growth rate of order for azimuthal
numbers m=4,5,6, where is the local Keplerian frequency.
Shock-capturing Cartesian-grid codes do not generate very much vorticity around
a giant planet in a standard protoplanetary disk. Modal calculations confirm
that the obtained radial profiles of density are less susceptible to the growth
of linear modes on timescales of several hundreds of orbital periods.
Navier-Stokes viscosity of the order (in units of )
is found to have a stabilizing effect and prevents the formation of vortices.
This result holds at high resolution runs and using different types of boundary
conditions. Giant protoplanets of Neptune-mass to Jupiter-mass can excite the
Rossby wave instability and generate vortices in thin disks. The presence of
vortices in protoplanetary disks has implications for planet formation, orbital
migration, and angular momentum transport in disks.Comment: 14 pages, 15 figures, accepted for publication in A&
A cosmic ray current driven instability in partially ionised media
We investigate the growth of hydromagnetic waves driven by streaming cosmic
rays in the precursor environment of a supernova remnant shock. It is known
that transverse waves propagating parallel to the mean magnetic field are
unstable to anisotropies in the cosmic ray distribution, and may provide a
mechanism to substantially amplify the ambient magnetic field. We quantify the
extent to which temperature and ionisation fractions modify this picture. Using
a kinetic description of the plasma we derive the dispersion relation for a
collisionless thermal plasma with a streaming cosmic ray current. Fluid
equations are then used to discuss the effects of neutral-ion collisions. We
calculate the extent to which the environment into which the cosmic rays
propagate influences the growth of the magnetic field, and determines the range
of possible growth rates. If the cosmic ray acceleration is efficient, we find
that very large neutral fractions are required to stabilise the growth of the
non-resonant mode. For typical supernova parameters in our galaxy, thermal
effects do not significantly alter the growth rates. For weakly driven modes,
ion-neutral damping can dominate over the instability at more modest ionisation
fractions. In the case of a supernova shock interacting with a molecular
clouds, such as in RX J1713.7-3946, with high density and low ionisation, the
modes can be rapidly damped.Comment: 5 pages, 2 figures, accepted to A&A. Corrections made. Applications
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Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow
We observed the bright phase of the 2003 outburst of the Galactic black hole
candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four
occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV
and (H-like) Fe XXVI resonance absorption lines. In the first observation, the
Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200
km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe
XXV line is observed to vary significantly on a timescale of a few hundred
seconds in the first observation, which corresponds to the Keplerian orbital
period at approximately 1 E+4 gravitational radii. Our models for the
absorption geometry suggest that a combination of geometric effects and
changing ionizing flux are required to account for the large changes in line
flux observed between observations, and that the absorption likely occurs at a
radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the
absorption geometry include cyclic absorption due to an accretion disk
structure, absorption in a clumpy outflowing disk wind, or possibly a
combination of these two. If the wind in H 1743-322 has unity filling factor,
the highest implied mass outflow rate is 20 percent of the Eddington mass
accretion rate. This wind may be a hot precursor to the Seyfert-like,
outflowing "warm absorber" geometries recently found in the Galactic black
holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of
ionized Fe absorption lines found in the spectra of other Galactic sources, and
connections to warm absorbers, winds, and jets in other accreting systems.Comment: 18 pages, 7 figures, 5 in color, subm. to ApJ. Uses emulateapj.sty
and apjfonts.st
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18 Nov 2013 In situ detection of electrified aerosols in the upper troposphere and stratosphere
Electrified aerosols have been observed in the
lower troposphere and in the mesosphere, but have never
been detected in the stratosphere and upper troposphere. We
present measurements of aerosols obtained during a balloon
flight to an altitude of 24 km. The measurements were per-
formed with an improved version of the Stratospheric and
Tropospheric Aerosol Counter (STAC) aerosol counter dedi-
cated to the search for charged aerosols. It is found that most of the aerosols are charged in the upper troposphere for altitudes below 10 km and in the stratosphere for altitudes above 20 km. Conversely, the aerosols seem to be uncharged between 10 km and 20 km. Model calculations are used to quantify the electrification of the aerosols with a stratospheric aerosol–ion model. The percentages of charged aerosols obtained with model calculations are in excellent agreement with the observations below 10 km and above 20 km. However, the model cannot reproduce the absence of electrification found in the lower stratosphere, as the processes leading to neutralisation in this altitude range are unknown. The presence of sporadic transient layers of electrified aerosol in the upper troposphere and in the stratosphere could have significant implications for sprite formatio
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