1,129 research outputs found
Accretion disk reversal and the spin-up/spin-down of accreting pulsars
We numerically investigate the hydrodynamics of accretion disk reversal and
relate our findings to the observed spin-rate changes in the accreting X-ray
pulsar GX~1+4. In this system, which accretes from a slow wind, the accretion
disk contains two dynamically distinct regions. In the inner part viscous
forces are dominant and disk evolution occurs on a viscous timescale. In the
outer part dynamical mixing of material with opposite angular momentum is more
important, and the externally imposed angular momentum reversal timescale
governs the flow. In this outer region the disk is split into concentric rings
of material with opposite senses of rotation that do not mix completely but
instead remain distinct, with a clear gap between them. We thus predict that
torque reversals resulting from accretion disk reversals will be accompanied by
minima in accretion luminosity.Comment: 13 pages, 7 figures, accepted for publication in Ap
Simulating currency substitution bias
The sign and size of estimates of the elasticity of currency substitution critically depend on the definition of the oppurtunity costs of holding money. We investigate possible biases by means of Monte Carlo experiments, as sufficient real data are not available
Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common Envelope Binaries
We present FUSE observations of the hot white dwarfs in the post-common
envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The
spectra show numerous photospheric absorption lines which trace the white dwarf
orbital motion. We report the detection of C III, O VI, P V, and Si IV in the
spectra of Feige 24, EUVE J0720-317 and EUVE J2013+400, and the detection of C
III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance
measurements support the possibility that white dwarfs in post-common envelope
binaries accrete material from the secondary star wind. The FUSE observations
of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE
spectra to measure the radial velocities traced by the white dwarf in the four
binaries, where the zero-point velocity were fixed using the ISM velocities in
the line of sight of the stellar systems. For BPM 6502 we determined a white
dwarf velocity semi-amplitude of K_WD = 18.6+/-0.5km/s, and with the velocity
semi-amplitude of the red dwarf companion (K_RD = 75.2+/-3.1 km/s), we estimate
the mass ratio to be q = 0.25+/-0.01. Adopting a spectroscopic mass
determination for the white dwarf, we infer a low secondary mass of M_RD =
0.14+/-0.01 M_solar. For EUVE J2013+400 we determine a white dwarf velocity
semi-amplitude of K_WD = 36.7+/-0.7 km/s. The FUSE observations of EUVE
J0720-317 cover approximately 30% of the binary period and combined with the
HST GHRS measurements (Vennes et al. 1999, ApJ 523, 386), we update the binary
properties. FUSE observations of Feige 24 cover approximately 60% of the orbit
and we combine this data set with HST STIS (Vennes et al. 2000, ApJ, 544, 423)
data to update the binary properties.Comment: Accepted for publication in Ap
Finding White Dwarfs with Transit Searches
We make predictions for the rate of discovery of eclipsing white dwarf-main
sequence (WD-MS) binaries in terrestrial-planet transit searches, taking the
planned Kepler and Eddington missions as examples. We use a population
synthesis model to characterize the Galactic WD-MS population, and we find
that, despite increased noise due to stellar variability compared with the
typical planetary case, discovery of >100 non-accreting, eclipsing WD-MS
systems is likely using Kepler and Eddington, with periods of 2-20 days and
transit amplitudes of |delta m|~0.0003-0.00003 magnitudes. Follow-up
observations of these systems could accurately test the theoretical white dwarf
mass-radius relation or theories of binary star evolution.Comment: 5 pages, 3 figures, one table, accepted for publication in ApJ. Minor
changes to Galactic model and to discusion section; conclusions unchange
Using New Submillimetre Surveys to Identify the Evolutionary Status of High-z Galaxies
This paper describes a key submillimetre survey which we are currently
conducting to address some of the outstanding questions in cosmology - how, at
what epoch and over what period of time did massive galaxies form at high
redshift? A summary of the technical feasibility of future submillimetre
observations with new ground-based, airborne and satellite telescopes is also
presented.Comment: 6 pages, 3 postscript figures, LaTex uses Kluwer book style file
crckapb10.sty, to appear in "Observational Cosmology with the New Radio
Surveys", 13-15 January 1997, Puerto de la Cruz, Tenerife, M.Bremer,
N.Jackson, I.Perez-Fournon (eds.), Kluwe
On the Binding Energy Parameter of Common Envelope Evolution. Dependency on the Definition of the Stellar Core Boundary during Spiral-in
According to the standard picture for binary interactions, the outcome of
binaries surviving the evolution through a common envelope (CE) and spiral-in
phase is determined by the internal structure of the donor star at the onset of
the mass transfer, as well as the poorly-known efficiency parameter, eta_CE},
for the ejection of the H-envelope of the donor. In this Research Note we
discuss the bifurcation point which separates the ejected, unprocessed H-rich
material from the inner core region of the donor (the central part of the star
which will later contract to form a compact object). We demonstrate that the
exact location of this point is very important for evaluating the binding
energy parameter, lambda, which is used to determine the post-CE orbital
separation. Here we compare various methods to define the bifurcation point
(core/envelope boundary) of evolved stars with masses 4, 7, 10 and 20 M_sun. We
consider the specific nuclear energy production rate profile, the change in the
mass-density gradient (Bisscheroux 1998), the inner region containing less than
10% hydrogen, the method suggested by Han et al. (1994) and the entropy
profile. We also calculated effective polytropic index profiles. The entropy
profile method measures the convective boundary (at the onset of flatness in
the specific entropy) which is not equivalent to the core boundary for RGB
stars. Hence, this method is not applicable for RGB stars, unless the actual
bifurcation point of a CE is located at the bottom of the outer convection zone
(resulting in larger values of lambda and larger post-CE orbital separations).
On the AGB, where highly degenerate and condensed cores are formed, we find
good agreement between the various methods, except for massive (20 M_sun)
stars.Comment: 4 pages, 1 figure, A&A in pres
Inhibitory effect of the reversal agents V-104, GF120918 and Pluronic L61 on MDR1 Pgp-, MRP1- and MRP2-mediated transport
The human multidrug transporter MDR1 P-glycoprotein and the multidrug resistance proteins MRP1 and MRP2 transport a range of cytotoxic drugs, resulting in multidrug resistance in tumour cells. To overcome this form of drug resistance in patients, several inhibitors (reversal agents) of these transporters have been isolated. Using polarized cell lines stably expressing human MDR1, MRP1 or MRP2 cDNA, and 2008 ovarian carcinoma cells stably expressing MRP1 cDNA, we have investigated in this study the specificity of the reversal agents V-104 (a pipecolinate derivative), GF120918 (an acridone carboxamide derivative also known as GG918), and Pluronic L61 (a (poly)oxypropethylene and (poly)oxypropylene block copolymer). Transport experiments with cytotoxic drugs with polarized cell lines indicate that all three compounds efficiently inhibit MDR1 Pgp. Furthermore, V-104 partially inhibits daunorubicin transport by MRP1 but not vinblastine transport by MRP2. V-104 reverses etoposide resistance of 2008/MRP1 cells, whereas GF120918 does not reverse resistance due to MRP1. V-104 partially inhibits the export of the organic anion dinitrophenyl S -glutathione by MDCKII-MRP1 but not by MDCKII-MRP2 cells. Unexpectedly, export of the organic anion calcein by MDCKII-MRP1 and MDCKII-MRP2 cells is stimulated by Pluronic L61, probably because it relieves the block on entry of calcein AM into the cell by endogenous MDR1 Pgp. © 2000 Cancer Research Campaig
Jet Acceleration by Tangled Magnetic Fields
We explore the possibility that extragalactic radio jets might be accelerated
by highly disorganized magnetic fields that are strong enough to dominate the
dynamics until the terminal Lorentz factor is reached. Following the
twin-exhaust model by Blandford & Rees (1974), the collimation under this
scenario is provided by the stratified thermal pressure from an external
medium. The acceleration efficiency then depends on the pressure gradient of
that medium. In order for this mechanism to work there must be continuous
tangling of the magnetic field, changing the magnetic equation of state away
from pure flux freezing (otherwise conversion of Poynting flux to kinetic
energy flux is suppressed). This is a complementary approach to models in which
the plasma is accelerated by large scale ordered fields. We include a simple
prescription for magnetic dissipation, which leads to tradeoffs among
conversion of magnetic energy into bulk kinetic energy, random particle energy,
and radiation. We present analytic dynamical solutions of such jets, assess the
effects of radiation drag, and comment on observational issues, such as the
predicted polarization and synchrotron brightness. Finally, we try to make the
connection to observed radio galaxies and gamma-ray bursts.Comment: 15 pages, 10 figures, accepted for publication in Ap
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