3,188 research outputs found
The Structure and C=C Vibrational Frequencies of the all- trans Polyenes C2nH2n+2(n=2-15), C2nH2n(Me)2(n=2-13), and C2nH2n(tert-Butyl)2(n=2-5): Computational Results
Carbon-carbon bond lengths and C=C vibrational frequencies are reported for the linear, all-trans unsubstituted C2nH2n+2 (n=2-15), methyl capped C2nH2nMe2 (n=2-13), and tert-butyl capped C2nH2n(tert-butyl)2 (n=2-5) polyenes (C2h) calculated at the B3LYP/6-311++G(d,p) level. The C=C/C-C bond length alternation remains evident at this level for the unsubstituted and methyl capped polyenes as the chain length increases; the center-most difference in the length of the C-C/C=C bonds is ~0.06 AÌ for C30H32 and C26H26Me2. The Ag, in-phase, harmonic C=C Raman frequency for the unsubstituted polyenes decreases from 1699.2 cm-1 (n = 2) to 1528.9 cm-1 (n=15); the anharmonic frequency decreases from 1651.5 cm-1 (n = 2) to 1547.7 cm-1 (n = 8). The harmonic C=C frequency for the methyl capped polyenes decreases from 1717.9 cm-1 (n = 2) to 1539.6 cm- 1 (n= 13), and the anharmonic C=C frequency decreases from 1675.0 cm-1 (n = 2) to 1562.8 cm-1 (n = 7)
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
Analytic models and forward scattering from accelerator to cosmic-ray energies
Analytic models for hadron-hadron scattering are characterized by analytical
parametrizations for the forward amplitudes and the use of dispersion relation
techniques to study the total cross section and the
parameter. In this paper we investigate four aspects related to the application
of the model to and scattering, from accelerator to cosmic-ray
energies: 1) the effect of different estimations for from
cosmic-ray experiments; 2) the differences between individual and global
(simultaneous) fits to and ; 3) the role of the
subtraction constant in the dispersion relations; 4) the effect of distinct
asymptotic inputs from different analytic models. This is done by using as a
framework the single Pomeron and the maximal Odderon parametrizations for the
total cross section. Our main conclusions are the following: 1) Despite the
small influence from different cosmic-ray estimations, the results allow us to
extract an upper bound for the soft pomeron intercept: ;
2) although global fits present good statistical results, in general, this
procedure constrains the rise of ; 3) the subtraction constant as
a free parameter affects the fit results at both low and high energies; 4)
independently of the cosmic-ray information used and the subtraction constant,
global fits with the odderon parametrization predict that, above GeV, becomes greater than , and
this result is in complete agreement with all the data presently available. In
particular, we infer at GeV and
at 500 GeV (BNL RHIC energies).Comment: 16 pages, 7 figures, aps-revtex, wording changes, corrected typos, to
appear in Physical Review
The Dwarf Galaxy Population of the Dorado group down to Mv=-11
We present V and I CCD photometry of suspected low-surface brightness dwarf
galaxies detected in a survey covering ~2.4 deg^2 around the central region of
the Dorado group of galaxies. The low-surface brightness galaxies were chosen
based on their sizes and magnitudes at the limiting isophote of 26.0V\mu. The
selected galaxies have magnitudes brighter than V=20 (Mv=-11 for an assumed
distance to the group of 17.2 Mpc), with central surface brightnesses \mu0>22.5
V mag/arcsec^2, scale lengths h>2'', and diameters > 14'' at the limiting
isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four
of them are large very low-surface brightness galaxies that were detected on a
smoothed image, after masking high surface brightness objects. Monte Carlo
simulations performed to estimate completeness, photometric uncertainties and
to evaluate our ability to detect extended low-surface brightness galaxies show
that the completeness fraction is, on average, > 80% for dwarf galaxies with
and 22.5<\mu0<25.5 V mag/arcsec^2, for the range of sizes
considered by us (D>14''). The V-I colors of the dwarf candidates vary from
-0.3 to 2.3 with a peak on V-I=0.98, suggesting a range of different stellar
populations in these galaxies. The projected surface density of the dwarf
galaxies shows a concentration towards the group center similar in extent to
that found around five X-ray groups and the elliptical galaxy NGC1132 studied
by Mulchaey and Zabludoff (1999), suggesting that the dwarf galaxies in Dorado
are probably physically associated with the overall potential well of the
group.Comment: 32 pages, 16 postscript figures and 3 figures in GIF format, aastex
v5.0. To appear in The Astronomical Journal, January 200
Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation
Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in
the radial density profile at some distance from the center. Using 22 galaxies
with known HI distributions as an example, we discuss the question of whether
this effect can be associated exclusively with external ionizing radiation, as
is commonly assumed. We show that before the surface density reaches
(the same for
galaxies of different types), it is hard to expect the gas to be fully ionized
by background radiation. For two of 13 galaxies with a sharp drop in the HI
profile, the "steepening" can actually be caused by ionization. At the same
time, for the remaining galaxies, the observed cutoff in the radial HI profile
is closer to the center than if it was a consequence of ionization by
background radiation and, therefore, it should be caused by other factors.Comment: 15 pages, 6 figure
Visualizing individual microtubules using bright-field microscopy
Microtubules are filament-shaped, polymeric proteins (~25 nm in diameter)
involved in cellular structure and organization. We demonstrate the imaging of
individual microtubules using a conventional bright-field microscope, without
any additional phase or polarization optics. Light scattered by microtubules is
discriminated through extensive use of digital image-processing, thus removing
background, reducing noise and enhancing contrast. The setup builds on a
commercial microscope, with the inclusion of a minimal and inexpensive set of
components, suitable for implementation in the student laboratory. We show how
this technique can be applied to a demonstrative biophysical assay, by tracking
the motions of microtubules driven by the motor protein kinesin
Semi-Analytical Models for Lensing by Dark Halos: I. Splitting Angles
We use the semi-analytical approach to analyze gravitational lensing of
quasars by dark halos in various cold dark matter (CDM) cosmologies, in order
to determine the sensitivity of the prediction probabilities of images
separations to the input assumptions regarding halos and cosmologies. The mass
function of dark halos is assumed to be given by the Press-Schechter function.
The mass density profile of dark halos is alternatively taken to be the
singular isothermal sphere (SIS), the Navarro-Frenk-White (NFW) profile, or the
generalized NFW profile. The cosmologies include: the Einstein-de Sitter model
(SCDM), the open model (OCDM), and the flat \Lambda-model (LCDM). As expected,
we find that the lensing probability is extremely sensitive to the mass density
profile of dark halos, and somewhat less so to the mean mass density in the
universe, and the amplitude of primordial fluctuations. NFW halos are very much
less effective in producing multiple images than SIS halos. However, none of
these models can completely explain the current observations: the SIS models
predict too many large splitting lenses, while the NFW models predict too few
small splitting lenses. This indicates that there must be at least two
populations of halos in the universe. A combination of SIS and NFW halos can
reasonably reproduce the current observations if we choose the mass for the
transition from SIS to NFW to be ~ 10^{13} solar masses. Additionally, there is
a tendency for CDM models to have too much power on small scales, i.e. too much
mass concentration; and it appears that the cures proposed for other apparent
difficulties of CDM would help here as well, an example being the warm dark
matter (WDM) variant which is shown to produce large splitting lenses fewer
than the corresponding CDM model by one order of magnitude.Comment: 46 pages, including 13 figures. Revised version with significant
improvemen
Evolution and biogeography of the Pavetteae tribe
With over 700 species, the Pavetteae are one o f the largest tribes in the subfamily Dialypetalanthoideae (formerly: Ixoroideae). Pavetteae representatives are characterized by a high morphological variation, especially regarding the reproductive characters (e.g., number of seeds per fruit, seed type and placentation). Representatives of the tribe occur throughout the Paleotropics in humid and dry vegetation types. In the Asian-Pacific regiĂłn ca 300 species are currently described whereas on continental Africa and Madagascar (and Western Indian Ocean Islands) ca 350 and ca 80 species are present, respectively. Recently many new genera have been recognized among the Malagasy Pavetteae species (e.g. Tulearia). In addition, it is clear that within the paleotropical genus Tarenna there will be a split into different genera. Not only is the taxonomic history of the tribe rather complicated, also the biogeographical history and evolutionary patterns of the tribe remain understudied to date. Molecular phylogenetics combined with age inference methods, diversificaron analyses and ancestral area reconstruction clearly indicate that current diversity of the Pavetteae is the result of several dispersal events within the Paleotropical region. Moreover, several shifts in ecological preference have likely driven the evolutionary history within the Pavetteae
The Relationship Between Stellar Light Distributions of Galaxies and their Formation Histories
A major problem in extragalactic astronomy is the inability to distinguish in
a robust, physical, and model independent way how galaxy populations are
related to each other and to their formation histories. A similar, but
distinct, and also long standing question is whether the structural appearances
of galaxies, as seen through their stellar light distributions, contain enough
physical information to offer this classification. We argue through the use of
240 images of nearby galaxies that three model independent parameters measured
on a single galaxy image reveal its major ongoing and past formation modes, and
can be used as a robust classification system. These parameters quantitatively
measure: the concentration (C), asymmetry (A) and clumpiness (S) of a galaxy's
stellar light distribution. When combined into a three dimensional `CAS' volume
all major classes of galaxies in various phases of evolution are cleanly
distinguished. We argue that these three parameters correlate with important
modes of galaxy evolution: star formation and major merging activity. This is
argued through the strong correlation of Halpha equivalent width and broad band
colors with the clumpiness parameter, the uniquely large asymmetries of 66
galaxies undergoing mergers, and the correlation of bulge to total light
ratios, and stellar masses, with the concentration index. As an obvious goal is
to use this system at high redshifts to trace evolution, we demonstrate that
these parameters can be measured, within a reasonable and quantifiable
uncertainty, with available data out to z ~ 3 using the Hubble Space Telescope
GOODS ACS and Hubble Deep Field images.Comment: ApJS, in press, 30 pages, Figures 15 and 16 are in color. For a full
resolution version, please go to http://www.astro.caltech.edu/~cc/cas.p
Gas flow and dark matter in the inner parts of early-type barred galaxies
This paper presents the dynamical simulations run in the potential derived
from the light distribution of 5 late-type barred spiral galaxies. The aim is
to determine whether the mass distribution together with the hydrodynamical
simulations can reproduce the observed line-of-sight velocity curves and the
gas morphology in the inner regions of the sample barred galaxies. The light
distribution is obtained from the -band and the -band combined together.
The M/L is determined using population synthesis models. The observations and
the methodology of the mass distribution modelling are presented in a companion
paper. The SPH models using the stellar mass models obtained directly from the
-band light distributions give a good representation of the gas distribution
and dynamics of the modelled galaxies, supporting the maximum disk assumption.
This result indicates that the gravitational field in the inner region is
mostly provided by the stellar luminous component. When 40% of the total mass
is transferred to an axisymmetric dark halo, the modelled kinematics clearly
depart from the observed kinematics, whereas the departures are negligible for
dark mass halos of 5% and 20% of the total mass. This result sets a lower limit
for the contribution of the luminous component of about 80%, which is in
agreement with the maximum disk definition of the stellar mass contribution to
the rotation curve (about 85%10).Comment: 28 pages, 30 figures. Accepted for publication in A&A on 17/05/2004.
High resolution figures on publicatio
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