547 research outputs found
Parallel-to-serial biphase-data converter
Data converter produces a serial biphase output signal from parallel input data. Alternate bits are loaded into a shift register in complement form so that the bits appear at the end of the shift register in a true-complement form sequence
On the Role of Disks in the Formation of Stellar Systems: A Numerical Parameter Study of Rapid Accretion
We study rapidly accreting, gravitationally unstable disks with a series of
global, three dimensional, numerical experiments using the code ORION. In this
paper we conduct a numerical parameter study focused on protostellar disks, and
show that one can predict disk behavior and the multiplicity of the accreting
star system as a function of two dimensionless parameters which compare the
disk's accretion rate to its sound speed and orbital period. Although
gravitational instabilities become strong, we find that fragmentation into
binary or multiple systems occurs only when material falls in several times
more rapidly than the canonical isothermal limit. The disk-to-star accretion
rate is proportional to the infall rate, and governed by gravitational torques
generated by low-m spiral modes. We also confirm the existence of a maximum
stable disk mass: disks that exceed ~50% of the total system mass are subject
to fragmentation and the subsequent formation of binary companions.Comment: 16 pages, 12 figures, submitte
The contribution of supernova remnants to the galactic cosmic ray spectrum
The supernova paradigm for the origin of galactic cosmic rays has been deeply
affected by the development of the non-linear theory of particle acceleration
at shock waves. Here we discuss the implications of applying such theory to the
calculation of the spectrum of cosmic rays at Earth as accelerated in supernova
remnants and propagating in the Galaxy. The spectrum is calculated taking into
account the dynamical reaction of the accelerated particles on the shock, the
generation of magnetic turbulence which enhances the scattering near the shock,
and the dynamical reaction of the amplified field on the plasma. Most
important, the spectrum of cosmic rays at Earth is calculated taking into
account the flux of particles escaping from upstream during the Sedov-Taylor
phase and the adiabatically decompressed particles confined in the expanding
shell and escaping at later times. We show how the spectrum obtained in this
way is well described by a power law in momentum with spectral index close to
-4, despite the concave shape of the instantaneous spectra of accelerated
particles. On the other hand we also show how the shape of the spectrum is
sensible to details of the acceleration process and environment which are and
will probably remain very poorly known.Comment: 19 pages, 8 figures, published version (references updated
A Chandra Observation of Supernova Remnant G350.1-0.3 and Its Central Compact Object
We present a new Chandra observation of supernova remnant (SNR) G350.1-0.3.
The high resolution X-ray data reveal previously unresolved filamentary
structures and allow us to perform detailed spectroscopy in the diffuse regions
of this SNR. Spectral analysis demonstrates that the region of brightest
emission is dominated by hot, metal-rich ejecta while the ambient material
along the perimeter of the ejecta region and throughout the remnant's western
half is mostly low-temperature, shocked interstellar/circumstellar medium
(ISM/CSM) with solar-type composition. The data reveal that the emission
extends far to the west of the ejecta region and imply a lower limit of 6.6 pc
on the diameter of the source (at a distance of 4.5 kpc). We show that
G350.1-0.3 is likely in the free expansion (ejecta-dominated) stage and
calculate an age of 600-1200 years. The derived relationship between the shock
velocity and the electron/proton temperature ratio is found to be entirely
consistent with that of other SNRs. We perform spectral fits on the X-ray
source XMMU J172054.5-372652, a candidate central compact object (CCO), and
find that its spectral properties fall within the typical range of other CCOs.
We also present archival 24 um data of G350.1-0.3 taken with the Spitzer Space
Telescope during the MIPSGAL galactic survey and find that the infrared and
X-ray morphologies are well-correlated. These results help to explain this
remnant's peculiar asymmetries and shed new light on its dynamics and
evolution
Non-linear diffusive acceleration of heavy nuclei in supernova remnant shocks
We describe a semi-analytical approach to non-linear diffusive shock
acceleration in the case in which nuclei other than protons are also
accelerated. The structure of the shock is determined by the complex interplay
of all nuclei, and in turn this shock structure determines the spectra of all
components. The magnetic field amplification upstream is described as due to
streaming instability of all nuclear species. The amplified magnetic field is
then taken into account for its dynamical feedback on the shock structure as
well as in terms of the induced modification of the velocity of the scattering
centers that enters the particle transport equation. The spectra of accelerated
particles are steep enough to be compared with observed cosmic ray spectra only
if the magnetic field is sufficiently amplified and the scattering centers have
high speed in the frame of the background plasma. We discuss the implications
of this generalized approach on the structure of the knee in the all-particle
cosmic ray spectrum, which we interpret as due to an increasingly heavier
chemical composition above eV. The effects of a non trivial chemical
composition at the sources on the gamma ray emission from a supernova remnant
when gamma rays are of hadronic origin are also discussed.Comment: 23 pages, 5 figures, minor changes to reflect the published versio
Children and parents’ perspectives of the impact of the COVID-19 pandemic on Ontario children’s physical activity, play, and sport behaviours
Background: The COVID-19 pandemic and associated public health measures have resulted in the closure of many physical activity-supporting facilities. This study examined Ontario parents’ and children’s perspectives of COVID-19’s impact on children’s physical activity behaviours, return to play/sport during COVID-19, as well as barriers/facilitators to getting active amid extended closures of physical activity venues. Methods: Parents/guardians of children aged 12 years and under living in Ontario, Canada were invited to participate in an interview. 12 parent/guardian and 9 child interviews were conducted via Zoom between December 2020 – January 2021, were audio-recorded, and transcribed verbatim. Thematic content analysis was undertaken to identify pronounced themes. Results: Themes for both parent and child interviews fell into one of three categories: 1) barriers and facilitators for getting children active amid COVID-19 closures of physical activity-supporting facilities; 2) changes in children’s activity levels; and, 3) perspectives on return to play/sport during and post-pandemic. Various subthemes were identified and varied between parents and children. The most common facilitator for dealing with children’s inactivity voiced by parents/guardians was getting active outdoors. Parents/guardians noted their willingness to have their children return to play/sport in the community once deemed safe by public health guidelines, and children’s willingness to return stemmed primarily from missing their friends and other important authority figures (e.g., coaches) and sporting events (e.g., tournaments). Conclusions: Findings from this study could inform families of feasible and realistic strategies for increasing children’s physical activity during community closures, while also providing public health experts with information regarding what supports, or infrastructure might be needed during future lockdown periods and/or pandemics
An adjustable law of motion for relativistic spherical shells
A classical and a relativistic law of motion for an advancing shell are
deduced applying the thin layer approximation. A new parameter connected with
the quantity of absorbed matter in the expansion is introduced; this allows of
matching theory and observation.Comment: 15 pages, 10 figures and article in press; Central European Journal
of Physics 201
Fall-back crust around a quark-nova compact remnant I: The degenerate shell case with applications to SGRs, AXPs and XDINs
We explore the formation and evolution of debris ejected around quark stars
in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters
(SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes
as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the
fall-back (crust) material. Our model can account for many of the observed
features of SGRs and AXPs such as: (i) the two types of bursts (giant and
regular); (ii) the spin-up and spin-down episodes during and following the
bursts with associated persistant increases in ; (iii) the energetics
of the boxing day burst, SGR180620; (iv) the presence of an Iron line as
observed in SGR190014; (v) the correlation between the far-Infrared and the
X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard
X-ray component observed in SGRs during the giant bursts, and (vii) the
discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also
find a natural evolutionary relationship between SGRs and AXPs in our model
which predicts that only the youngest SGRs/AXPs are most likely to exhibit
strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are
also accounted for in our model such as, (i) the two-component blackbody
spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical
emission.Comment: submitted to Ap
- …