15,072 research outputs found
The outer rotation curve of the Milky Way
A straightforward determination of the circular-speed curve vc(R) of the
Milky Way suggests that near the Sun, vc starts to rise approximately linearly
with R. If this result were correct, the Galactic mass density would have to be
independent of radius at R ~> R0. We show that the apparent linear rise in v_c
arises naturally if the true circular-speed curve is about constant or gently
falling at R0 1.25 R0
are actually concentrated into a ring of radius ~1.6 R0.Comment: 3 pages, LaTeX, uses mn.sty, 5 .ps figures, submitted to MNRA
Dynamical Models for the Milky Way
The only way to map the Galaxy's gravitational potential and
the distribution of matter that produces it is by modelling the dynamics of
stars and gas. Observations of the kinematics of gas provide key information
about gradients of within the plane, but little information about the
structure of out of the plane. Traditional Galaxy models {\em assume},
for each of the Galaxy's components, arbitrary flattenings, which together with
the components' relative masses yield the model's equipotentials. However, the
Galaxy's isopotential surfaces should be {\em determined\/} directly from the
motions of stars that move far from the plane. Moreover, from the kinematics of
samples of such stars that have well defined selection criteria, one should be
able not only to map at all positions, but to determine the distribution
function of each stellar population studied. These
distribution functions will contain a wealth of information relevant to the
formation and evolution of the Galaxy. An approach to fitting a wide class of
dynamical models to the very heterogeneous body of available data is described
and illustrated.Comment: 10 pages, LaTeX, style file and 4 figures included. Invited talk
presented at the meeting ``Formation of the Galactic Halo ... Inside and
Out'', Tucson, October 9-11. Full .ps file available at
ftp://ftp.physics.ox.ac.uk/pub/local/users/dehnen/MilkyWayModels.ps.g
Mass models of the Milky Way
A parameterized model of the mass distribution within the Milky Way is fitted
to the available observational constraints. The most important single parameter
is the ratio of the scale length R_d* of the stellar disk to R0. The disk and
bulge dominate v_c(R) at R<R0 only for R_d*/R0< 0.3. Since the only knowledge
we have of the halo derives from studies like the present one, we allow it to
contribute to the density at all radii. When allowed this freedom, however, the
halo causes changes in assumptions relating to R << R0 to affect profoundly the
structure of the best-fitting model at R >> R0. For example, changing the disk
slightly from an exponential surface-density profile significantly changes the
form of v_c(R) at R >> R0, where the disk makes a negligible contribution to
v_c. Moreover, minor changes in the constraints can cause the halo to develop a
deep hole at its centre that is not physically plausible. These problems call
into question the proposition that flat rotation curves arise because galaxies
have physically distinct halos rather than outwards-increasing mass-to-light
ratios.
The mass distribution of the Galaxy and the relative importance of its
various components will remain very uncertain until more observational data can
be used to constrain mass models. Data that constrain the Galactic force field
at z > R and at R > R0 are especially important.Comment: 10 pages, LaTeX, mn.sty, 5 .ps figures, accepted by MNRAS major
revision involving new cepheid & hipparcos dat
Lyapunov Exponents for the Intermittent Transition to Chaos
The dependence of the Lyapunov exponent on the closeness parameter,
, in tangent bifurcation systems is investigated. We study and
illustrate two averaging procedures for defining Lyapunov exponents in such
systems. First, we develop theoretical expressions for an isolated tangency
channel in which the Lyapunov exponent is defined on single channel passes.
Numerical simulations were done to compare theory to measurement across a range
of values. Next, as an illustration of defining the Lyapunov
exponent on many channel passes, a simulation of the intermittent transition in
the logistic map is described. The modified theory for the channels is
explained and a simple model for the gate entrance rates is constructed. An
important correction due to the discrete nature of the iterative flow is
identified and incorporated in an improved model. Realistic fits to the data
were made for the Lyapunov exponents from the logistic gate and from the full
simulation. A number of additional corrections which could improve the
treatment of the gates are identified and briefly discussed.Comment: 25 pages LaTeX and 12 separate ps figure
A Revised Parallax and its Implications for RX J185635-3754
New astrometric analysis of four WFPC2 images of the isolated neutron star RX
J185635-3754 show that its distance is 117 +/- 12 pc, nearly double the
originally published distance. At the revised distance, the star's age is 5 x
10^5 years, its space velocity is about 185 km/s, and its radiation radius
inferred from thermal emission is approximately 15 km, in the range of many
equations of state both with and without exotic matter. These measurements
remove observational support for an extremely soft equation of state. The
star's birthplace is still likely to be in the Upper Sco association, but a
connection with zeta Oph is now unlikely.Comment: submitted to ApJ Letter
Axially Uniform Magnetic Field-Modulation Excitation for Electron Paramagnetic Resonance in Rectangular and Cylindrical Cavities by Slot Cutting
In continuous-wave (CW) Electron Paramagnetic Resonance (EPR) a low-frequency time-harmonic magnetic field, called field modulation, is applied parallel to the static magnetic field and incident on the sample. Varying amplitude of the field modulation incident on the sample has consequences on spectral line-shape and line-height over the axis of the sample. Here we present a method of coupling magnetic field into the cavity using slots perpendicular to the sample axis where the slot depths are designed in such a way to produce an axially uniform magnetic field along the sample. Previous literature typically assumes a uniform cross-section and axial excitation due to the wavelength of the field modulation being much larger than the cavity. Through numerical analysis and insights obtained from the eigenfunction expansion of dyadic Green’s functions, it is shown that evanescent standing-wave modes with complex cross-sections are formed within the cavity. From this analysis, a W-band (94 GHz) cylindrical cavity is designed where modulation slots are optimized to present a uniform 100 kHz field modulation over the length of the sample
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