1,658 research outputs found
A note on the cyclic evolution of the pulsar magnetosphere
Positive and negative pulsar breaking indices suggest that some fraction of
the pulsar spindown torque undergoes a cyclic evolution. The observed strong
correlation of `anomalous' breaking indices with pulsar age implies that the
characteristic periodicity timescale is in the range 100 to 10,000 years
depending on the fraction of the spindown torque that undergoes cyclic
evolution, 1 to 100% respectively. We argue that the longest variability
timescale is consistent with a neutron star magnetic cycle similar to the solar
cycle.Comment: Accepted for publication in Astronomy & Astrophysic
Pitch Angle Restrictions in Late Type Spiral Galaxies Based on Chaotic and Ordered Orbital Behavior
We built models for low bulge mass spiral galaxies (late type as defined by
the Hubble classification) using a 3-D self-gravitating model for spiral arms,
and analyzed the orbital dynamics as a function of pitch angle, going from
10 to 60. Testing undirectly orbital self-consistency, we search
for the main periodic orbits and studied the density response. For pitch angles
up to approximately , the response supports closely the potential
permitting readily the presence of long lasting spiral structures. The density
response tends to "avoid" larger pitch angles in the potential, by keeping
smaller pitch angles in the corresponding response. Spiral arms with pitch
angles larger than , would not be long-lasting structures but
rather transient. On the other hand, from an extensive orbital study in phase
space, we also find that for late type galaxies with pitch angles larger than
, chaos becomes pervasive destroying the ordered phase space
surrounding the main stable periodic and quasi-periodic orbits and even
destroying them. This result is in good agreement with observations of late
type galaxies, where the maximum observed pitch angle is .Comment: ApJL accepted (12 pages, 3 figures
Asymptotic Orbits in Barred Spiral Galaxies
We study the formation of the spiral structure of barred spiral galaxies,
using an -body model. The evolution of this -body model in the adiabatic
approximation maintains a strong spiral pattern for more than 10 bar rotations.
We find that this longevity of the spiral arms is mainly due to the phenomenon
of stickiness of chaotic orbits close to the unstable asymptotic manifolds
originated from the main unstable periodic orbits, both inside and outside
corotation. The stickiness along the manifolds corresponding to different
energy levels supports parts of the spiral structure. The loci of the disc
velocity minima (where the particles spend most of their time, in the
configuration space) reveal the density maxima and therefore the main
morphological structures of the system. We study the relation of these loci
with those of the apocentres and pericentres at different energy levels. The
diffusion of the sticky chaotic orbits outwards is slow and depends on the
initial conditions and the corresponding Jacobi constant.Comment: 17 pages, 24 figure
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