98 research outputs found
On the Perils of Curve-of-Growth Analysis: Systematic Abundance Underestimates for the Gas in Gamma-Ray Burst Host Galaxies
We examine the practice of deriving interstellar medium (ISM) abundances from
low-resolution spectroscopy of GRB afterglows. We argue that the multi-ion
single-component curve-of-growth analysis technique systematically
underestimates the column densities of the metal-line profiles commonly
observed for GRB. This systematic underestimate is accentuated by the fact that
many GRB line-profiles (e.g. GRB 050730, GRB 050820, GRB 051111) are comprised
of `clouds' with a bi-modal distribution of column density. Such line-profiles
may be characteristic of a sightline which penetrates both a high density
star-forming region and more distant, ambient ISM material. Our analysis
suggests that the majority of abundances reported in the literature are
systematically underestimates and that the reported errors are frequently
over-optimistic. Further, we demonstrate that one cannot even report precise
relative abundances with confidence. The implications are profound for our
current understanding on the metallicity, dust-to-gas ratio, and chemical
abundances of the ISM in GRB host galaxies. For example, we argue that all but
a few sightlines allow for the gas to have at least solar metallicity. Finally,
we suggests new approaches for constraining the abundances.Comment: 9 pages, 9 figures. Accepted to Ap
The Interaction of Laser Energy with Ureter Tissues in a Long Term Investigation
This study investigates tissue responses after laser irradiation of the rabbit ureter, which serves as an experimental model for rectourogenital fistulae of children. Twenty-five rabbit ureters were irradiated intraluminally by a Nd:YAG laser 1320 nm (2 Watt, 20 seconds and 3 Watt, 8 seconds) via an applicator with radialsymmetrical light distribution. Immediately, 2 weeks, 4 weeks, 8 weeks, and 16 weeks after irradiation, the ureters were X-rayed with contrast solution and prepared for light and transmission electron microscopy. For the parameters employed, no apparent morphological differences could be observed. Immediately, the central laser zone showed a transmural therrnonecrosis prevailed by cellular destruction, condensed ground substance and occlusion of most vascular lumina. Peripheral laser zones displayed urothelial vacuolations. Between 2 and 16 weeks, urothelial regeneration and ingrowth of granulation tissue caused a luminal stenosis or occlusion followed by transformation into scar tissue. In some peripheral laser zones, a hydroureter with marked luminal dilatation developed. We conclude that the ureter is occluded if the expanding force of the growing scar tissue exceeds the hydrostatic pressure of the obstructed urine. A laser occlusion of rectourogenital fistulae will be easier to achieve since fistula occlusion does not entail an obstruction of the urine flow
VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager
In this paper we describe in detail the implementation and main properties of
a new inversion code for the polarized radiative transfer equation (VFISV: Very
Fast inversion of the Stokes vector). VFISV will routinely analyze pipeline
data from the Helioseismic and Magnetic Imager (HMI) on-board of the Solar
Dynamics Observatory (SDO). It will provide full-disk maps (40964096
pixels) of the magnetic field vector on the Solar Photosphere every 10 minutes.
For this reason VFISV is optimized to achieve an inversion speed that will
allow it to invert 16 million pixels every 10 minutes with a modest number
(approx. 50) of CPUs. Here we focus on describing a number of important
details, simplifications and tweaks that have allowed us to significantly speed
up the inversion process. We also give details on tests performed with data
from the spectropolarimeter on-board of the Hinode spacecraft.Comment: 23 pages, 9 figures (2 color). Submitted for publication to Solar
Physic
Rotation of planet-harbouring stars
The rotation rate of a star has important implications for the detectability,
characterisation and stability of any planets that may be orbiting it. This
chapter gives a brief overview of stellar rotation before describing the
methods used to measure the rotation periods of planet host stars, the factors
affecting the evolution of a star's rotation rate, stellar age estimates based
on rotation, and an overview of the observed trends in the rotation properties
of stars with planets.Comment: 16 pages, 4 figures: Invited review to appear in 'Handbook of
Exoplanets', Springer Reference Works, edited by Hans J. Deeg and Juan
Antonio Belmont
On the Dynamic Stability of Cool Supergiant Atmospheres
We have developed a new formalism to compute the thermodynamic coefficient
Gamma1 in the theory of stellar and atmospheric stability. We generalize the
classical derivation of the first adiabatic index, which is based on the
assumption of thermal ionization and equilibrium between gas and radiation
temperature, towards an expression which incorporates photo-ionization due to
radiation with a temperature T_rad different from the local kinetic gas
temperature.Our formalism considers the important non-LTE conditions in the
extended atmospheres of supergiant stars. An application to the Kurucz grid of
cool supergiant atmospheres demonstrates that models with T_rad =~ T_eff
between 6500 K and 7500 K become most unstable against dynamic perturbations,
according to Ledoux' stability integral . This results from Gamma1 and
acquiring very low values, below 4/3, throughout the entire stellar
atmosphere, which causes very high gas compression ratios around these
effective temperatures. Based on detailed NLTE-calculations, we discuss
atmospheric instability of pulsating massive yellow supergiants, like the
hypergiant rho Cas (Ia+), which exist in the extension of the Cepheid
instability strip, near the Eddington luminosity limit.Comment: 54 pages including figures and the Appendix, 7 figures, Accepted for
The Astrophysical Journal, Main Journal, 558, Sept. 200
The identification of HCN and HNC in Carbon Stars: Model Atmospheres, Synthetic Spectra and Fits to Observations in the 2.7-4.0 micron Region
Model carbon star atmospheres and synthetic spectra have been calculated
using the recent HCN/HNC vibration rotation linelist of Harris et al. (2002)
ApJ, 578, 657. The calculations are repeated using only HCN lines and show that
HNC has a significant effect upon the temperature, density and optical depth of
a stellar atmosphere.
We fit synthetic spectra in the 2.7-4.0 micron region to observed ISO spectra
of the carbon stars WZ Cas and TX Psc obtained by Aoki et al. (1998), A&A, 340,
222. These fits allow us to identify absorption by HNC in the spectrum of WZ
Cas at 2.8-2.9 microns, and to determine new independent estimates of effective
temperature and log(Nc)/log(No). The findings reported here indicate that
absorption by both HCN and HNC is needed to fully explain the observed stellar
spectra and represent the first identification of HNC in a star. Q branch
absorption by the HCN , and ,
bands at 3.55 and 3.86 microns respectively, are identified in
the spectrum of WZ Cas.Comment: 13 pages, 9 figure
On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon
We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D
(PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to
high-precision interferometric data from the VLTI Interferometer (K-band) and
from the Mark III interferometer (500 nm and 800 nm). These data sets provide a
test of theoretical wavelength dependent limb-darkening predictions. The work
of Allende Prieto et al. has shown that the temperature structure from a
spatially and temporally averaged 3-D hydrodynamical model produces
significantly less limb darkening at 500 nm relative to the temperature
structure of a 1-D MARCS model atmosphere with a standard mixing-length
approximation for convection. Our direct fits to the interferometric data
confirm this prediction. A 1-D ATLAS 12 model with ``approximate overshooting''
provides the required temperature gradient. We show, however, that 1-D models
cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective
temperatures in the range constrained by the measured bolometric flux and
angular diameter. We find that a good match to the full spectral energy
distribution can be obtained with a composite model consisting of a weighted
average of twelve 1-D model atmospheres based on the surface intensity
distribution of a 3-D granulation simulation. We emphasize that 1-D models with
overshooting may realistically represent the mean temperature structure of
F-type stars like Procyon, but the same models will predict redder colors than
observed because they lack the multicomponent temperature distribution expected
for the surfaces of these stars.Comment: 24 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Effective temperatures, rotational velocities, microturbulent velocities and abundances in the atmospheres of the Sun,. HD1835 and HD10700
We describe our procedure to determine effective temperatures, rotational
velocities, microturbulent velocities, and chemical abundances in the
atmospheres of Sun-like stars. We use independent determinations of iron
abundances using the fits to the observed Fe I and Fe II atomic absorption
lines. We choose the best solution from the fits to these spectral features for
the model atmosphere that provides the best confidence in the determined log
N(Fe), Vt, and vsini. First, we compute the abundance of iron for a set of
adopted microturbulent velocities. To determine the most self-consistent
effective temperature and microturbulent velocity in any star's atmosphere, we
used an additional constraint where we minimise the dependence of the derived
abundances of Fe I and Fe II on the excitation potential of the corresponding
lines. We analyse the spectra of the Sun and two well known solar type stars,
HD1835 and HD10700 to determine their abundances, microturbulent velocity and
rotational velocity. For the Sun abundances of elements obtained from the fits
of their absorption features agree well enough (+/- 0.1 dex) with the known
values for the Sun. We determined a rotational velocity of vsini = 1.6 +/- 0.3
km/s for the spectrum of the Sun as a star. For HD1835 the self-consistent
solution for Fe I and Fe II lines log N(Fe)=+0.2 was obtained with a model
atmosphere of 5807/4.47/+0.2 andmicroturbulent velocity Vt = 0.75 km/s, and
leads to vsini = 7.2 0.5 km/s. For HD10700 the self-consistent solution
log N(Fe) = -4.93 was obtained using a model atmosphere of 5383/4.59/-0.6and
microturbulent velocity Vt = 0.5 km/s. The Fe I and Fe II lines give rise to a
vsini = 2.4 +/- 0.4 km/s. Using the Teff found from the ionisation equilibrium
parameters for all three stars, we found abundances of a number of other
elements: Ti, Ni, Ca, Si, Cr. ... Abriged.Comment: 11 pages, 7 tables, 5 figs, to appear in MNRA
Conidiation Color Mutants of Aspergillus fumigatus Are Highly Pathogenic to the Heterologous Insect Host Galleria mellonella
The greater wax moth Galleria mellonella has been widely used as
a heterologous host for a number of fungal pathogens including Candida
albicans and Cryptococcus neoformans. A positive
correlation in pathogenicity of these yeasts in this insect model and animal
models has been observed. However, very few studies have evaluated the
possibility of applying this heterologous insect model to investigate virulence
traits of the filamentous fungal pathogen Aspergillus
fumigatus, the leading cause of invasive aspergillosis. Here, we have
examined the impact of mutations in genes involved in melanin biosynthesis on
the pathogenicity of A. fumigatus in the G.
mellonella model. Melanization in A. fumigatus confers
bluish-grey color to conidia and is a known virulence factor in mammal models.
Surprisingly, conidial color mutants in B5233 background that have deletions in
the defined six-gene cluster required for DHN-melanin biosynthesis caused
enhanced insect mortality compared to the parent strain. To further examine and
confirm the relationship between melanization defects and enhanced virulence in
the wax moth model, we performed random insertional mutagenesis in the Af293
genetic background to isolate mutants producing altered conidia colors. Strains
producing conidia of previously identified colors and of novel colors were
isolated. Interestingly, these color mutants displayed a higher level of
pathogenicity in the insect model compared to the wild type. Although some of
the more virulent color mutants showed increased resistance to hydrogen
peroxide, overall phenotypic characterizations including secondary metabolite
production, metalloproteinase activity, and germination rate did not reveal a
general mechanism accountable for the enhanced virulence of these color mutants
observed in the insect model. Our observations indicate instead, that
exacerbated immune response of the wax moth induced by increased exposure of
PAMPs (pathogen-associated molecular patterns) may cause self-damage that
results in increased mortality of larvae infected with the color mutants. The
current study underscores the limitations of using this insect model for
inferring the pathogenic potential of A. fumigatus strains in
mammals, but also points to the importance of understanding the innate immunity
of the insect host in providing insights into the pathogenicity level of
different fungal strains in this model. Additionally, our observations that
melanization defective color mutants demonstrate increased virulence in the
insect wax moth, suggest the potential of using melanization defective mutants
of native insect fungal pathogens in the biological control of insect
populations
Pressure Ionization and Line Merging in Strongly Coupled Plasmas Produced by 100-fs Laser Pulses
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