3,449 research outputs found
Externally-polluted white dwarfs with dust disks
We report Spitzer Space Telescope photometry of eleven externally-polluted
white dwarfs. Of the nine stars for which we have IRAC photometry, we find that
GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be
understood as arising from a flat, opaque, dusty disk. GD 56 also has an
infrared excess characteristic of circumstellar dust, but a flat-disk model
cannot reproduce the data unless there are grains as warm as 1700 K and perhaps
not even then. Our data support the previous suggestion that the metals in the
atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid
with a chondritic composition.Comment: ApJ, in pres
Radiation induced warping of protostellar accretion disks
We examine the consequences of radiatively driven warping of accretion disks
surrounding pre-main-sequence stars. These disks are stable against warping if
the luminosity arises from a steady accretion flow, but are unstable at late
times when the intrinsic luminosity of the star overwhelms that provided by the
disk. Warps can be excited for stars with luminosities of around 10 solar
luminosities or greater, with larger and more severe warps in the more luminous
systems. A twisted inner disk may lead to high extinction towards stars often
viewed through their disks. After the disk at all radii becomes optically thin,
the warp decays gradually on the local viscous timescale, which is likely to be
long. We suggest that radiation induced warping may account for the origin of
the warped dust disk seen in Beta Pictoris, if the star is only around 10-20
Myr old, and could lead to non-coplanar planetary systems around higher mass
stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres
Neural Protection in the Central Nervous System against Nerve Agent Surrogates using Novel Pyridinium Oximes
Organophosphates (OPs), including nerve agents, target the cholinergic system via inhibition of acetylcholinesterase (AChE), with subsequent overstimulation resulting in neural damage and potential detrimental long-term effects. The efficacy of novel pyridinium oxime reactivators, created with moieties to increase blood-brain barrier penetration, was tested using highly relevant sarin and VX surrogates. Glial fibrillary acidic protein (GFAP; an indicator of neural damage) and monoamines (dopamine, serotonin, and their metabolites) were measured in select brain regions via immunohistochemistry and HPLC, respectively. Adult male rats were treated ip with high, sub-lethal doses of surrogates for sarin or VX, nitrophenyl isopropyl methylphosphonate (NIMP) or nitrophenyl ethyl methylphosphonate (NEMP), respectively. Surrogate treatment was followed after 1 hr by im administration of novel oxime. Seizure activity was monitored, and kainic acid (KA) served as a positive control. Administration of KA or surrogate (NIMP or NEMP) significantly increased GFAP expression compared to control animals. Two different formulations of one particular oxime (bromide vs. mesylate salt) attenuated seizures and reduced GFAP levels over NIMP or NEMP treatments alone to levels near those of controls in both the piriform cortex and dentate gyrus region of the hippocampus, while 2-PAM did not provide protection. Serotonergic activity was increased in several brain regions, including the piriform cortex, one hr after NIMP treatment. Markers of oxidative stress (isoprostanes) were also tested. Overall, these results indicate the potential therapeutic efficacy of these oximes and suggest this innovative chemistry may protect against neural damage induced by OP
The alignment of disk and black hole spins in active galactic nuclei
The inner parts of an accretion disk around a spinning black hole are forced
to align with the spin of the hole by the Bardeen-Petterson effect. Assuming
that any jet produced by such a system is aligned with the angular momentum of
either the hole or the inner disk, this can, in principle provide a mechanism
for producing steady jets in AGN whose direction is independent of the angular
momentum of the accreted material. However, the torque which aligns the inner
disk with the hole, also, by Newton's third law, tends to align the spin of the
hole with the outer accretion disk. In this letter, we calculate this alignment
timescale for a black hole powering an AGN, and show that it is relatively
short. This timescale is typically much less than the derived ages for jets in
radio loud AGN, and implies that the jet directions are not in general
controlled by the spin of the black hole. We speculate that the jet directions
are most likely controlled either by the angular momentum of the accreted
material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter
Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations
We model the variability profiles of millisecond period X-ray pulsars. We
performed three-dimensional magnetohydrodynamic simulations of disk accretion
to millisecond period neutron stars with a misaligned magnetic dipole moment,
using the pseudo-Newtonian Paczynski-Wiita potential to model general
relativistic effects. We found that the shapes of the resulting funnel streams
of accreting matter and the hot spots on the surface of the star are quite
similar to those for more slowly rotating stars obtained from earlier
simulations using the Newtonian potential. The funnel streams and hot spots
rotate approximately with the same angular velocity as the star. The spots are
bow-shaped (bar-shaped) for small (large) misalignment angles. We found that
the matter falling on the star has a higher Mach number when we use the
Paczynski-Wiita potential than in the Newtonian case.
Having obtained the surface distribution of the emitted flux, we calculated
the variability curves of the star, taking into account general relativistic,
Doppler and light-travel-time effects. We found that general relativistic
effects decrease the pulse fraction (flatten the light curve), while Doppler
and light-travel-time effects increase it and distort the light curve. We also
found that the light curves from our hot spots are reproduced reasonably well
by spots with a gaussian flux distribution centered at the magnetic poles. We
also calculated the observed image of the star in a few cases, and saw that for
certain orientations, both the antipodal hot spots are simultaneously visible,
as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some
typo
The influence of a foreign body on the induction of tumours in the bladder epithelium of the mouse.
Lense-Thirring precession of accretion disks around compact objects
Misaligned accretion disks surrounding rotating compact objects experience a
torque due to the Lense-Thirring effect, which leads to precession of the inner
disk. It has been suggested that this effect could be responsible for some low
frequency Quasi-Periodic Oscillations observed in the X-ray lightcurves of
neutron star and galactic black hole systems. We investigate this possibility
via time-dependent calculations of the response of the inner disk to impulsive
perturbations for both Newtonian point mass and Paczynski-Wiita potentials, and
compare the results to the predictions of the linearized twisted accretion disk
equations. For most of a wide range of disk models that we have considered, the
combination of differential precession and viscosity causes the warps to decay
extremely rapidly. Moreover, at least for relatively slowly rotating objects,
linear calculations in a Newtonian point mass potential provide a good measure
of the damping rate, provided only that the timescale for precession is much
shorter than the viscous time in the inner disk. The typically rapid decay
rates suggest that coherent precession of a fluid disk would not be observable,
though it remains possible that the damping rate of warp in the disk could be
low enough to permit weakly coherent signals from Lense-Thirring precession.Comment: ApJ, in press. Minor revisions to match accepted version. Animations
showing warp evolution are available at
http://www.cita.utoronto.ca/~armitage/lense_thirring.htm
Tracking the Orbital and Super-orbital Periods of SMC X-1
The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation
of 3.89 days and a super-orbital variation with an average length of 55 days.
As we show here, however, the length of the super-orbital cycle varies by
almost a factor of two, even across adjacent cycles. To study both the orbital
and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing
Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from
Wojdowski et al. (1998) to obtain the average orbital profile, and we show that
this profile exhibits complex modulation during non-eclipse phases.
Additionally, a very interesting ``bounceback'' in X-ray count rate is seen
during mid-orbital eclipse phases, with a softening of the emission during
these periods. This bounceback has not been previously identified in pointed
observations. We then define a super-orbital ephemeris (the phase of the
super-orbital cycle as a function of date) based on the ASM lightcurve and
analyze the trend and distribution of super-orbital cycle lengths. SMC X-1
exhibits a bimodal distribution of these lengths, similar to what has been
observed in other systems (e.g., Her X-1), but with more dramatic changes in
cycle length. There is some hint, but not conclusive evidence, for a dependence
of the super-orbital cycle length upon the underlying orbital period, as has
been observed previously for Her X-1 and Cyg X-2. Using our super-orbital
ephemeris we are also able to create an average super-orbital profile over the
71 observed cycles, for which we witness overall hardening of the spectrum
during low count rate times. We combine the orbital and super-orbital
ephemerides to study the correlation between the orbital and super-orbital
variations in the system.Comment: 10 pages, using emulateapj style. To be published in the
Astrophysical Journa
The Stability of Magnetized Rotating Plasmas with Superthermal Fields
During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated, rotating fluids and
argue that, for these systems, the curvature of toroidal field lines cannot be
neglected even when short wavelength perturbations are considered. We also
comment on the implications of our results for the validity of shearing box
simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and
5 substantially expanded, added Appendix A and 3 figures with respect to
previous version. Animations are available at
http://www.physics.arizona.edu/~mpessah/research
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