1,968 research outputs found
Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars
Recent observations by XMM-Newton detected rotational pulsations in the total
brightness and spectrum of several neutron stars. To properly interpret the
data, accurate modeling of neutron star emission is necessary. Detailed
analysis of the shape and strength of the rotational variations allows a
measurement of the surface composition and magnetic field, as well as
constrains the nuclear equation of state. We discuss our models of the spectra
and light curves of two of the most observed neutron stars, RX J1856.5-3754 and
1E 1207.4-5209, and discuss some implications of our results and the direction
of future work.Comment: 5 pages, 6 figures; Proceedings of "40 Years of Pulsars", eds. C.
Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, submitte
I Want To Be There
https://digitalcommons.library.umaine.edu/mmb-vp/5185/thumbnail.jp
Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aqr
The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white
dwarf which is thought to expel most of the material streaming onto it.
Observations of AE Aqr have been obtained in the wavelength range of 5 - 70
microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space
Telescope. The spectral energy distribution reveals a significant excess above
the K4V spectrum of the donor star with the flux increasing with wavelength
above 12.5 microns. Superposed on the energy distribution are several hydrogen
emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared
spectrum above 12.5 microns can be interpreted as synchrotron emission from
electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding
clouds with an initial evolution timescale in seconds. However, too many
components must then be superposed to explain satisfactorily both the
mid-infrared continuum and the observed radio variability. Thermal emission
from cold circumbinary material can contribute, but it requires a disk
temperature profile intermediate between that produced by local viscous
dissipation in the disk and that characteristic of a passively irradiated disk.
Future high-time resolution observations spanning the optical to radio regime
could shed light on the acceleration process and the subsequent particle
evolution.Comment: 15 pages, 3 figures, accepted for publication in Ap
On the mass transfer in AE Aquarii
The observed properties of the close binary AE Aqr indicate that the mass
transfer in this system operates via the Roche lobe overflow mechanism, but the
material transferred from the normal companion is neither accreted onto the
surface of the white dwarf nor stored in a disk around its magnetosphere. As
previously shown, such a situation can be realized if the white dwarf operates
as a propeller. At the same time, the efficiency of the propeller action by the
white dwarf is insufficient to explain the rapid braking of the white dwarf,
which implies that the spin-down power is in excess of the bolometric
luminosity of the system. To avoid this problem we have simulated the
mass-transfer process in AE Aqr assuming that the observed braking of the white
dwarf is governed by a pulsar-like spin-down mechanism. We show that the
expected H_alpha Doppler tomogram in this case resembles the tomogram observed
from the system. We find that the agreement between the simulated and the
observed tomograms is rather good provided the mean value of the mass-transfer
rate ~5x10^16 g/s. Three spatially separated sources of H_alpha
emission can be distinguished within this approach. The structure of the
tomogram depends on the relative contributions of these sources to the H_alpha
emission and is expected to vary from night to night.Comment: 12 pages, 3 figures (6 eps files). Published in A&A. The paper with
high resolution images can be downloaded from
http://urania.it.nuigalway.ie/papers/ae_aqr.ps.g
Institutional requirements for optimal water quality management in arid urban areas
Summarizes Completion reports no. 45-47.AER72-73WRW-GVS-RCW-TLH28.Funded in part by the United States Department of the Interior, Office of Water Resources Research, as authorized by the Water Resources Research Act of 1964, and pursuant to Grant Agreement no. 14-31-0001-3567
Analysis of Small Water Management Structures in Irrigation Distribution Systems
The Irrigation and Drainage Research Conference conducted at Utah State University (ASCE, 1964) delineated many of the research needs regarding âSmall Low- Cost Hydraulic Structures for Conveyance and Distribution Systems,â which was one of the six topics considered at the conference. In discussing possibilities for accomplishing the recommended research, it was suggested by some panel members that a considerable portion of the work could be undertaken by graduate students, particularly at the Master of Science level. The intent of this report has been to sort through the large volume of literature in an attempt to define the specific research needs regarding small water management structure used in irrigation distribution systems. In particular, the emphasis has been to develop specific research topics which could be accomplished as a thesis by a graduate student at the Master of Science level
The spin periods and magnetic moments of white dwarfs in magnetic cataclysmic variables
We have used a model of magnetic accretion to investigate the rotational
equilibria of magnetic cataclysmic variables (mCVs). The results of our
numerical simulations demonstrate that there is a range of parameter space in
the P_spin / P_orb versus mu_1 plane at which rotational equilibrium occurs.
This has allowed us to calculate the theoretical histogram describing the
distribution of magnetic CVs as a function of P_spin / P_orb. We show that this
agrees with the observed distribution assuming that the number of systems as a
function of white dwarf magnetic moment is distributed approximately according
to N(mu_1) d mu_1 proportional to 1/mu_1 d mu_1. The rotational equilibria also
allow us to infer approximate values for the magnetic moments of all known
intermediate polars. We predict that intermediate polars with mu_1 > 5 x 10^33
G cm^3 and P_orb > 3h will evolve into polars, whilst those with mu_1 < 5 x
10^33 G cm^3 and P_orb > 3h will either evolve into low field strength polars
which are (presumably) unobservable, and possibly EUV emitters, or, if their
fields are buried by high accretion rates, evolve into conventional polars once
their magnetic fields re-surface when the mass accretion rate reduces. We
speculate that EX Hya-like systems may have low magnetic field strength
secondaries and so avoid synchronisation. Finally we note that the equilibria
we have investigated correspond to a variety of different types of accretion
flow, including disc-like accretion at small P_spin / P_orb values, stream-like
accretion at intermediate P_spin / P_orb values, and accretion fed from a ring
at the outer edge of the white dwarf Roche lobe at higher P_spin / P_orb
values.Comment: Accepted for publication in ApJ. Figre 3 included here as a low
resolution gif onl
The origins of Calcium-rich supernovae from disruptions of CO white-dwarfs by hybrid He-CO white-dwarfs
Calcium-rich (SN 2005E-like) explosions are very faint (typical -15.5, type I
supernovae (SNe) showing strong Ca-lines, mostly observed in old stellar
environments. Several models for such SNe had been explored and debated, but
non were able to consistently reproduce the observed properties of Ca-rich SNe,
nor their rates and host-galaxy distributions. Here we show that the
disruptions of low-mass carbon-oxygen (CO) white dwarfs (WDs) by hybrid
Helium-CO (HeCO) WDs during their merger could explain the origin and
properties of such SNe. We make use of detailed multi-dimensional
hydrodynamical-thermonuclear (FLASH) simulations models of HeCO-CO WD-WD
mergers to characterize such explosions. We find that the accretion of CO
material onto a HeCO-WD heats its He-shell and eventually leads to its "weak"
detonation and ejection and the production of a sub-energetic erg
Ca-rich SN, while leaving the CO core of the HeCO-WD intact as a hot remnant
white dwarf, possibly giving rise to X-ray emission as it cools down. We model
the detailed light-curve and spectra of such explosions to find an excellent
agreement with observations of Ia/c Ca-rich SNe. In these models most of the He
is burned, producing Ia/c Ca-rich SNe but higher He-enrichment levels might
potentially also explain the Ib Ca-rich SNe. We thereby provide a viable,
consistent model for the origins of Ca-rich. These findings can shed new light
on the role of Ca-rich in the chemical evolution of galaxies and the
intra-cluster medium, and their contribution to the observed 511 kev signal in
the Galaxy originating from positrons produced from decay.
Finally, the origins of such SNe points to the key-role of HeCO WDs as SN
progenitors, and their potential role as progenitors of other thermonuclear SNe
including normal type Ia SNe.Comment: Submitted to ApJ, comments are welcom
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