4,049 research outputs found
The analysis of solar models: Neutrinos and oscillations
Tests of solar neutrino flux and solar oscillation frequencies were used to assess standard stellar structure theory. Standard and non-standard solar models are enumerated and discussed. The field of solar seismology, wherein the solar interior is studied from the measurement of solar oscillations, is introduced
Molecular footprint of drug-selective pressure in a human immunodeficiency virus transmission chain
Known human immunodeficiency virus (HIV) transmission histories are invaluable models for investigating the evolutionary and transmission dynamics of the virus and to assess the accuracy of phylogenetic reconstructions. Here we have characterized an HIV-1 transmission chain consisting of nine infected patients, almost all of whom were treated with antiviral drugs at later stages of infection. Partial pol and env gp41 regions of the HIV genome were directly sequenced from plasma viral RNA for at least one sample from each patient. Phylogenetic analyses in pol using likelihood methods inferred an evolutionary history not fully compatible with the known transmission history. This could be attributed to parallel evolution of drug resistance mutations resulting in the incorrect clustering of multidrug-resistant virus. On the other hand, a fully compatible phylogenetic tree was reconstructed from the env sequences. We were able to identify and quantify the molecular footprint of drug-selective pressure in pol using maximum likelihood inference under different codon substitution models. An increased fixation rate of mutations in the HIV population of the multidrug-resistant patient was demonstrated using molecular clock modeling. We show that molecular evolutionary analyses, guided by a known transmission history, can reveal the presence of confounding factors like natural selection and caution should be taken when accurate descriptions of HIV evolution are required.status: publishe
High resolution study of associated C IV absorption systems in NGC 5548
We present the results of a careful analysis of associated absorption systems
toward NGC 5548. Most of the well resolved narrow components in the associated
system, defined by the Lyman alpha, C IV and N V profiles, show velocity
separation similar (to within 10~\kms) to the C IV doublet splitting. We
estimate the chance probability of occurrence of such pairs with velocity
separation equal to C IV doublet splitting to be . Thus it is
more likely that most of the narrow components are line-locked with C IV
doublet splitting. This will mean that the radiative acceleration plays an
important role in the kinematics of the absorbing clouds. We build grids of
photoionization models and estimate the radiative acceleration due to all
possible bound-bound transitions. We show that the clouds producing absorption
have densities less than , and are in the outer regions of the
broad emission line region (BLR). We note that the clouds which are line-locked
cannot produce appreciable optical depths of O VII and O VIII, and hence cannot
be responsible for the observed ionized edges, in the soft X-ray. We discuss
the implications of the presence of optically thin clouds in the outer regions
of the BLR to the models of broad emission lines.Comment: 21 pages, latex (aasms4 style), incluedes 4 ps figures. To appear in
Astrophysical Journa
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
Los hongos de colombia - vii: leotiaceae - iv: hymenoscyphus caudatus and related species from colombia and adjacent regions
The present paper redescribes and illustrates five common species of Hymenoscyphus, H. caudatus, H. serotinus, H. affin. scutulus, H. Iasiopodtum and amp; H.sclerogenus, from Colombia and adjacent regions in South America. Emphasis is given to discussions of anatomical and morphological variability and host specificity of the species included. Inoperculate Discomycetes are generally small Ascomycetes producing an apothecium as their sexual fruiting structure. The largest order, the Helotiales, contains 5 families: Dermateaceae, Hyaloscyphaceae, Orbiliaceae, Sclerotiniaceae and Leotiaceae, In recent years there have been several monographic and submonographic studies on selected genera within these families, but no definitive studies on the principal family, Leotiaceae (previously referred to as the Helotiaceae), has been prepared. These studies have resulted in a better understanding of some of the family limits, but most of the generic limits still remain vague and confused. This is especially true for Hymenoscyphus (previously referred to by most workers as Helotium), which for decades has been the depository for small Discomycetes of uncertain affinities. White (1942, 1942-a, 1943, 1944) began to work systematically through the species of Hymenoscyphus and to bring order out of the existing chaos. His carefully prepared descriptions, meticulously drawn illustrations, and long synonymies were the real important contribution to the understanding of the genus. The most recent attempts to studyHymenoscyphus have been by Dennis, who summarized his works in 1964 (Dennis, 1964). However, as Dennis himself points out, he was unable to come to a sound understanding and arrangement of the species because of the extent of the problems, and the resulting treatment is rather artificial. He relied heavily upon substrate and geographical relations, rather than true morphological similarities. We agree with Dennis, that this has resulted in the separation of closely related taxa and an "unnecessary proliferation of names". Although Dennis contributions were extraordinary in terms of the number of described species he examined and in his literature searches, he did not have at his disposal necessary tropical collections for a complete comprehension of the individual species and/or the entire genus. Further, as pointed out by Dumont (1976), because of limited collections Dennis was unable to understand the full morphological and geographical variation of certain species and recognized several taxa, when in the final analysis only one existed. Many of these deficiencies are apparent in his monumental work on "The Fungus Flora of Venezuela and Adjacent Countries" (Dennis, 1970). InHymenoscyphus for example, he included only seven species (in his 1964 treatment he included more than 70 species for the world), and of these three, H. crocatus , H. caracassensis and H.atrosubiculatum have been removed from the genus by Dumont (1974) and Dumont and Pal (1978). Arendholz (1979) has recently provided a very important contribution to the understanding of several species of leaf inhabiting species ofHymenoscyphus. Again, as with previous workers, his work stressed temperate collections and temperate species. The purpose of the present study is to report the findings of some of our tropical collecting, to expand the understanding of five species of Hymenoscyphus which appear to be common in northern South America, to discuss the morphological variability observed, and to comment on the host or substrate specificity (or lack of it) within certain taxa. The materials and methods used during these studies are the same as those reported by Dumont (1972). The following is a key to the five species reported herein.The present paper redescribes and illustrates five common species of Hymenoscyphus, H. caudatus, H. serotinus, H. affin. scutulus, H. Iasiopodtum and amp; H. sclerogenus, from Colombia and adjacent regions in South America. Emphasis is given to discussions of anatomical and morphological variability and host specificity of the species included. Inoperculate Discomycetes are generally small Ascomycetes producing an apothecium as their sexual fruiting structure. The largest order, the Helotiales, contains 5 families: Dermateaceae, Hyaloscyphaceae, Orbiliaceae, Sclerotiniaceae and Leotiaceae, In recent years there have been several monographic and submonographic studies on selected genera within these families, but no definitive studies on the principal family, Leotiaceae (previously referred to as the Helotiaceae), has been prepared. These studies have resulted in a better understanding of some of the family limits, but most of the generic limits still remain vague and confused. This is especially true for Hymenoscyphus (previously referred to by most workers as Helotium), which for decades has been the depository for small Discomycetes of uncertain affinities. White (1942, 1942-a, 1943, 1944) began to work systematically through the species of Hymenoscyphus and to bring order out of the existing chaos. His carefully prepared descriptions, meticulously drawn illustrations, and long synonymies were the real important contribution to the understanding of the genus. The most recent attempts to study Hymenoscyphus have been by Dennis, who summarized his works in 1964 (Dennis, 1964). However, as Dennis himself points out, he was unable to come to a sound understanding and arrangement of the species because of the extent of the problems, and the resulting treatment is rather artificial. He relied heavily upon substrate and geographical relations, rather than true morphological similarities. We agree with Dennis, that this has resulted in the separation of closely related taxa and an "unnecessary proliferation of names". Although Dennis contributions were extraordinary in terms of the number of described species he examined and in his literature searches, he did not have at his disposal necessary tropical collections for a complete comprehension of the individual species and/or the entire genus. Further, as pointed out by Dumont (1976), because of limited collections Dennis was unable to understand the full morphological and geographical variation of certain species and recognized several taxa, when in the final analysis only one existed. Many of these deficiencies are apparent in his monumental work on "The Fungus Flora of Venezuela and Adjacent Countries" (Dennis, 1970). In Hymenoscyphus for example, he included only seven species (in his 1964 treatment he included more than 70 species for the world), and of these three, H. crocatus , H. caracassensis and H. atrosubiculatum have been removed from the genus by Dumont (1974) and Dumont and Pal (1978). Arendholz (1979) has recently provided a very important contribution to the understanding of several species of leaf inhabiting species of Hymenoscyphus. Again, as with previous workers, his work stressed temperate collections and temperate species. The purpose of the present study is to report the findings of some of our tropical collecting, to expand the understanding of five species of Hymenoscyphus which appear to be common in northern South America, to discuss the morphological variability observed, and to comment on the host or substrate specificity (or lack of it) within certain taxa. The materials and methods used during these studies are the same as those reported by Dumont (1972). The following is a key to the five species reported herein
Radar Detection of High Concentrations of Ice Particles - Methodology and Preliminary Flight Test Results
High Ice Water Content (HIWC) has been identified as a primary causal factor in numerous engine events over the past two decades. Previous attempts to develop a remote detection process utilizing modern commercial radars have failed to produce reliable results. This paper discusses the reasons for previous failures and describes a new technique that has shown very encouraging accuracy and range performance without the need for any hardware modifications to industrys current radar designs. The performance of this new process was evaluated during the joint NASA/FAA HIWC RADAR II Flight Campaign in August of 2018. Results from that evaluation are discussed, along with the potential for commercial application, and development of minimum operational performance standards for a future commercial radar product
On the Origin of Broad Fe K alpha and Hi H alpha Lines in AGN
We examine the properties of the Fe emission lines that arise near 6.4 keV in
the ASCA spectra of AGN. Our emphasis is on the Seyfert 1 galaxies where broad
and apparently complex Fe K alpha emission is observed. We consider various
origins for the line but focus on the pros and cons for line emitting accretion
disk models. We develop a simple model of an illuminated disk capable of
producing both X-ray and optical lines from a disk. The model is able to
reproduce the observed Fe K alpha FWHM ratio as well as the radii of maximum
emissivity implied by the profile redshifts. The overall profile shapes however
do not fit well the predictions of our disk illumination model nor do we derive
always consistent disk inclinations for the two lines. We conclude that the
evidence for and against an accretion disk origin for the Fe K alpha emission
is equal at best. The bulk of the data requires a very disparate set of line
fits which shed little light on a coherent physical model. We briefly consider
alternatives to disk emission models and show that a simple bicone model can
reproduce the FE line profiles equally well.Comment: 29 pages, 6 tables, 6 figures. Submitted for publication in the
Astrophysical Journal part
Average UV Quasar Spectra in the Context of Eigenvector 1: A Baldwin Effect Governed by Eddington Ratio?
We present composite UV spectra for low redshift Type 1 AGN binned to exploit
the information content of the Eigenvector 1 (E1) parameter space. Composite
spectra allow a decomposition of the CIV1549 line profile - one of the
strongest high-ionization lines. The simplest CIV decomposition into narrow
(NLR), broad (BLR) and very broad (VBLR) components suggests that different
components have an analog in Hb with two major exceptions. VBLR emission is
seen only in population B (FWHM(Hb)>4000 km/s) sources. A blue
shifted/asymmetric BLR component is seen only in pop. A (FWHM(Hb)<4000 km/s)
HIL such as CIV. The blueshifted component is thought to arise in a wind or
outflow. Our analysis suggests that such a wind can only be produced in pop. A
(almost all radio-quiet) sources where the accretion rate is relatively high.
Comparison between broad UV lines in radio-loud (RL) and radio-quiet (RQ)
sources shows few significant differences. Clear evidence is found for a narrow
CIV component in most radio-loud sources. We find also some indirect
indications that the black hole (BH) spin, rather than BH mass or accretion
rate is a key trigger in determining whether an object will be RL or RQ. We
find a ten-fold decrease in EW CIV with Eddington ratio (decreasing from ~1 to
\~0.01) while NV shows no change. These trends suggest a luminosity-independent
"Baldwin effect" where the physical driver may be the Eddington ratio.Comment: 39 pages, 6 figures. To appear in Ap
Tri-axial Octupole Deformations and Shell Structure
Manifestations of pronounced shell effects are discovered when adding
nonaxial octupole deformations to a harmonic oscillator model. The degeneracies
of the quantum spectra are in a good agreement with the corresponding main
periodic orbits and winding number ratios which are found by classical
analysis.Comment: 10 pages, Latex, 4 postscript figures, to appear in JETP Letter
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