392 research outputs found
Estimating Peak Demand for Beach Parking Spaces
The United States Army Corps of Engineers planning guidance stipulates that in order for local beach communities to qualify for Federal cost share funds for Hurricane and Storm Damage Reduction beach renourishment projects, the community must provide public beach access and parking to satisfy peak demand. This study presents a method for estimating peak demand for beach parking spaces in the presence of parking constraints. A Tobit regression model is developed to estimate the number of parking spaces that would be necessary to meet unconstrained demand on a given percentage of peak demand days. For example, the model can be used to estimate the number of parking spaces that would be adequate to meet peak demand on 90% of peak parking days. The Tobit model provides a promising framework for estimating peak parking demand under constrained parking conditions, a situation that characterizes most beach communities.
An analysis of ultraviolet spectra of Extreme Helium Stars and new clues to their origins
Abundances of about 18 elements including the heavy elements Y and Zr are
determined from Hubble Space Telescope Space Telescope Imaging Spectrograph
ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179,
V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of
the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The
abundance analyses is done using LTE line formation and LTE model atmospheres
especially constructed for these EHe stars. The stellar parameters derived from
an EHe's UV spectrum are in satisfactory agreement with those derived from its
optical spectrum. Adopted abundances for the seven EHes are from a combination
of the UV and optical analyses. Published results for an additional ten EHes
provide abundances obtained in a nearly uniform manner for a total of 17 EHes,
the largest sample on record. The initial metallicity of an EHe is indicated by
the abundance of elements from Al to Ni; Fe is adopted to be the representative
of initial metallicity. Iron abundances range from approximately solar to about
one-hundredth of solar. Clues to EHe evolution are contained within the H, He,
C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for
some stars is close to the predicted initial abundance yet the N abundance
indicates almost complete conversion of initial C, N, and O to N by the
CNO-cycles; (ii) three of the seven stars with UV spectra show a strong
enhancement of Y and Zr attributable to an s-process. The observed compositions
are discussed in light of expectations from accretion of a He white dwarf by a
CO white dwarf.Comment: 126 pages, 15 figures, 20 Tables, accepted for publication in the Ap
pVHL modification by NEDD8 is required for fibronectin matrix assembly and suppression of tumor development
Modeling Multi-Wavelength Stellar Astrometry. II. Determining Absolute Inclinations, Gravity Darkening Coefficients, and Spot Parameters of Single Stars with SIM Lite
We present a novel technique to determine the absolute inclination of single
stars using multi-wavelength sub-milliarcsecond astrometry. The technique
exploits the effect of gravity darkening, which causes a wavelength-dependent
astrometric displacement parallel to a star's projected rotation axis. We find
this effect is clearly detectable using SIM Lite for various giant stars and
rapid rotators, and present detailed models for multiple systems using the
REFLUX code. We also explore the multi-wavelength astrometric reflex motion
induced by spots on single stars. We find that it should be possible to
determine spot size, relative temperature, and some positional information for
both giant and nearby main-sequence stars utilizing multi-wavelength SIM Lite
data. This data will be extremely useful in stellar and exoplanet astrophysics,
as well as supporting the primary SIM Lite mission through proper
multi-wavelength calibration of the giant star astrometric reference frame, and
reduction of noise introduced by starspots when searching for extrasolar
planets.Comment: 8 pages, 7 figures, 4 tables. Accepted for publication in the
Astrophysical Journa
Effect of room temperature transport vials on DNA quality and phylogenetic composition of faecal microbiota of elderly adults and infants
Background: Alterations in intestinal microbiota have been correlated with a growing number of diseases. Investigating the faecal microbiota is widely used as a non-invasive and ethically simple proxy for intestinal biopsies. There is an urgent need for collection and transport media that would allow faecal sampling at distance from the processing laboratory, obviating the need for same-day DNA extraction recommended by previous studies of freezing and processing methods for stool. We compared the faecal bacterial DNA quality and apparent phylogenetic composition derived using a commercial kit for stool storage and transport (DNA Genotek OMNIgene GUT) with that of freshly extracted samples, 22 from infants and 20 from older adults. Results: Use of the storage vials increased the quality of extracted bacterial DNA by reduction of DNA shearing. When infant and elderly datasets were examined separately, no differences in microbiota composition were observed due to storage. When the two datasets were combined, there was a difference according to a Wilcoxon test in the relative proportions of Faecalibacterium, Sporobacter, Clostridium XVIII, and Clostridium XlVa after 1 week's storage compared to immediately extracted samples. After 2 weeks' storage, Bacteroides abundance was also significantly different, showing an apparent increase from week 1 to week 2. The microbiota composition of infant samples was more affected than that of elderly samples by storage, with significantly higher Spearman distances between paired freshly extracted and stored samples (
Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds
The B-type pulsators known as \beta Cephei and Slowly Pulsating B (SPB) stars
present pulsations driven by the \kappa mechanism, which operates thanks to an
opacity bump due to the iron group elements. In low-metallicity environments
such as the Magellanic Clouds, \beta Cep and SPB pulsations are not expected.
Nevertheless, recent observations show evidence for the presence of B-type
pulsator candidates in both galaxies. We seek an explanation for the excitation
of \beta Cep and SPB modes in those galaxies by examining basic input physics
in stellar modelling: i) the specific metal mixture of B-type stars in the
Magellanic Clouds; ii) the role of a potential underestimation of stellar
opacities. We first derive the present-day chemical mixtures of B-type stars in
the Magellanic Clouds. Then, we compute stellar models for that metal mixture
and perform a non-adiabatic analysis of these models. In a second approach, we
simulate parametric enhancements of stellar opacities due to different iron
group elements. We then study their effects in models of B stars and their
stability. We find that adopting a representative chemical mixture of B stars
in the Small Magellanic Cloud cannot explain the presence of B-type pulsators
there. An increase of the opacity in the region of the iron-group bump could
drive B-type pulsations, but only if this increase occurs at the temperature
corresponding to the maximum contribution of Ni to this opacity bump. We
recommend an accurate computation of Ni opacity to understand B-type pulsators
in the Small Magellanic Cloud, as well as the frequency domain observed in some
Galactic hybrid \beta Cep-SPB stars.Comment: 16 pages, 12 figures. Accepted for publication in MNRA
Effect of human leukocyte antigen heterozygosity on infectious disease outcome: The need for allele-specific measures
BACKGROUND: Doherty and Zinkernagel, who discovered that antigen presentation is restricted by the major histocompatibility complex (MHC, called HLA in humans), hypothesized that individuals heterozygous at particular MHC loci might be more resistant to particular infectious diseases than the corresponding homozygotes because heterozygotes could present a wider repertoire of antigens. The superiority of heterozygotes over either corresponding homozygote, which we term allele-specific overdominance, is of direct biological interest for understanding the mechanisms of immune response; it is also a leading explanation for the observation that MHC loci are extremely polymorphic and that these polymorphisms have been maintained through extremely long evolutionary periods. Recent studies have shown that in particular viral infections, heterozygosity at HLA loci was associated with a favorable disease outcome, and such findings have been interpreted as supporting the allele-specific overdominance hypothesis in humans. METHODS: An algebraic model is used to define the expected population-wide findings of an epidemiologic study of HLA heterozygosity and disease outcome as a function of allele-specific effects and population genetic parameters of the study population. RESULTS: We show that overrepresentation of HLA heterozygotes among individuals with favorable disease outcomes (which we term population heterozygote advantage) need not indicate allele-specific overdominance. On the contrary, partly due to a form of confounding by allele frequencies, population heterozygote advantage can occur under a very wide range of assumptions about the relationship between homozygote risk and heterozygote risk. In certain extreme cases, population heterozygote advantage can occur even when every heterozygote is at greater risk of being a case than either corresponding homozygote. CONCLUSION: To demonstrate allele-specific overdominance for specific infections in human populations, improved analytic tools and/or larger studies (or studies in populations with limited HLA diversity) are necessary
Evolution of an adaptive behavior and its sensory receptors promotes eye regression in blind cavefish
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