3,661 research outputs found
Toward the total synthesis of spirastrellolide A. Part 3: Intelligence gathering and preparation of a ring-expanded analogue
Different methods for the formation of the C.25–C.26 bond of spirastrellolide A (1) are evaluated that might qualify for the end game of the projected total synthesis, with emphasis on metathetic ways to forge the macrocyclic frame
The dependence of HII region properties on global and local surface brightness within galaxy discs
Using B, R, and H-alpha images of roughly equal-sized samples of low surface
brightness (LSB) and high surface brightness (HSB) galaxies (~40 galaxies
apiece), we have explored the dependence of HII region properties on local and
global disc surface brightness. We have done this by constructing co-added HII
region luminosity functions (LFs) according to local and central disc surface
brightness and fitting Schechter functions to these LFs. The results show that
the shape of the HII region LF within LSB galaxies does not change noticeably
as different limiting (i.e., mu>mu_lim) local surface brightness values are
used. However, the LFs for HSB galaxies have larger values of L_* and are less
steep at the faint-end than those of LSB galaxies for limiting B-band local
surface brightness values as faint as mu_B,lim~23-24. Both the LFs and the data
for individual HII regions show that luminous (log L>39 ergs/s) HII regions are
much more common within HSB discs than within LSB discs, implying that the
newly formed star clusters are also larger. Taking this into account along with
the results of Monte Carlo simulations, the shapes of the LFs imply that the
regions within LSB discs and those within the LSB areas of HSB discs are
relatively old (~5 Myr) while the regions within HSB discs for mu_B<24 are
significantly younger (<1 Myr). Since the majority of the LSB galaxies do not
have noticeable spiral arms and the majority of the HSB galaxies do, this may
indicate a transition within HSB discs from spiral arm-driven star formation to
a more locally driven, possibly sporadic form of star formation at mu_B~24, a
transition that does not appear to occur within LSB discs.Comment: Accepted to MNRA
The Energy of a Plasma in the Classical Limit
When \lambda_{T} << d_{T}, where \lambda_{T} is the de Broglie wavelength and
d_{T}, the distance of closest approach of thermal electrons, a classical
analysis of the energy of a plasma can be made. In all the classical analysis
made until now, it was assumed that the frequency of the fluctuations \omega <<
T (k_{B}=\hbar=1). Using the fluctuation-dissipation theorem, we evaluate the
energy of a plasma, allowing the frequency of the fluctuations to be arbitrary.
We find that the energy density is appreciably larger than previously thought
for many interesting plasmas, such as the plasma of the Universe before the
recombination era.Comment: 10 pages, 2 figures, accepted for publication in Phys.Rev.Let
Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors -- with Applications from Black Holes to Planets
We propose a new approach to Bayesian prior probability distributions
(priors) that can improve orbital solutions for low-phase-coverage orbits,
where data cover less than approximately 40% of an orbit. In instances of low
phase coverage such as with stellar orbits in the Galactic center or with
directly-imaged exoplanets, data have low constraining power and thus priors
can bias parameter estimates and produce under-estimated confidence intervals.
Uniform priors, which are commonly assumed in orbit fitting, are notorious for
this. We propose a new observable-based prior paradigm that is based on
uniformity in observables. We compare performance of this observable-based
prior and of commonly assumed uniform priors using Galactic center and
directly-imaged exoplanet (HR 8799) data. The observable-based prior can reduce
biases in model parameters by a factor of two and helps avoid under-estimation
of confidence intervals for simulations with less than about 40% phase
coverage. Above this threshold, orbital solutions for objects with sufficient
phase coverage such as S0-2, a short-period star at the Galactic center with
full phase coverage, are consistent with previously published results. Below
this threshold, the observable-based prior limits prior influence in regions of
prior dominance and increases data influence. Using the observable-based prior,
HR 8799 orbital analyses favor lower eccentricity orbits and provide stronger
evidence that the four planets have a consistent inclination around 30 degrees
to within 1-sigma. This analysis also allows for the possibility of
coplanarity. We present metrics to quantify improvements in orbital estimates
with different priors so that observable-based prior frameworks can be tested
and implemented for other low-phase-coverage orbits.Comment: Published in AJ. 23 pages, 14 figures. Monte Carlo chains are
available in the published article, or are available upon reques
Phase transition in the collisionless regime for wave-particle interaction
Gibbs statistical mechanics is derived for the Hamiltonian system coupling
self-consistently a wave to N particles. This identifies Landau damping with a
regime where a second order phase transition occurs. For nonequilibrium initial
data with warm particles, a critical initial wave intensity is found: above it,
thermodynamics predicts a finite wave amplitude in the limit of infinite N;
below it, the equilibrium amplitude vanishes. Simulations support these
predictions providing new insight on the long-time nonlinear fate of the wave
due to Landau damping in plasmas.Comment: 12 pages (RevTeX), 2 figures (PostScript
Hubble Space Telescope Planetary Camera Images of NGC 1316
We present HST Planetary Camera V and I~band images of the central region of
the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by
a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an
assumed distance of 16.9 Mpc, the deprojected luminosity density reaches \sim
2.0 \times 10^3 L_{\sun} pc.
Outside the inner two or three arcseconds, a constant mass-to-light ratio of
is found to fit the observed line width measurements. The
line width measurements of the center indicate the existence of either a
central dark object of mass 2 \times 10^9 M_{\sun}, an increase in the
stellar mass-to-light ratio by at least a factor of two for the inner few
arcseconds, or perhaps increasing radial orbit anisotropy towards the center.
The mass-to-light ratio run in the center of NGC 1316 resembles that of many
other giant ellipticals, some of which are known from other evidence to harbor
central massive dark objects (MDO's).
We also examine twenty globular clusters associated with NGC 1316 and report
their brightnesses, colors, and limits on tidal radii. The brightest cluster
has a luminosity of 9.9 \times 10^6 L_{\sun} (), and the
faintest detectable cluster has a luminosity of 2.4 \times 10^5 L_{\sun}
(). The globular clusters are just barely resolved, but their core
radii are too small to be measured. The tidal radii in this region appear to be
35 pc. Although this galaxy seems to have undergone a substantial merger
in the recent past, young globular clusters are not detected.Comment: 21 pages, latex, postscript figures available at
ftp://delphi.umd.edu/pub/outgoing/eshaya/fornax
The Effects of Starburst Activity on Low Surface Brightness Disk Galaxies
Although numerous simulations have been done to understand the effects of
intense bursts of star formation on high surface brightness galaxies, few
attempts have been made to understand how localized starbursts would affect
both the color and surface brightness of low surface brightness (LSB) galaxies.
To remedy this, we have run 53 simulations involving bursts of star formation
activity on LSB galaxies, varying both the underlying galaxy properties and the
parameters describing the starbursts. We discovered that although changing the
total color of a galaxy was fairly straightforward, it was virtually impossible
to alter a galaxy's central surface brightness and thereby remove it from the
LSB galaxy classification without placing a high (and fairly artificial)
threshold for the underlying gas density. The primary effect of large amounts
of induced star formation was to produce a centralized core (bulge) component
which is generally not observed in LSB galaxies. The noisy morphological
appearance of LSB galaxies as well as their noisy surface brightness profiles
can be reproduced by considering small bursts of star formation that are
localized within the disk. The trigger mechanism for such bursts is likely
distant/weak tidal encounters. The stability of disk central surface brightness
to these periods of star formation argues that the large space density of LSB
galaxies at z = 0 should hold to substantially higher redshifts.Comment: 38 pages, 5 figures, 4 tables, tarred and compressed Also available
on http://guernsey.uoregon.edu/~kare
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