89 research outputs found
A New Measurement of the Average FUV Extinction Curve
We have measured the extinction curve in the far-ultraviolet wavelength
region of (900 -- 1200 A) using spectra obtained with the Berkeley EUV/FUV
spectrometer during the ORFEUS-I and the ORFEUS-II missions in 1993 and 1996.
From the complete sample of early-type stars observed during these missions,
we have selected pairs of stars with the same spectral type but different
reddenings to measure the differential FUV extinction. We model the effects of
molecular hydrogen absorption and exclude affected regions of the spectrum to
determine the extinction from dust alone. We minimize errors from inaccuracies
in the cataloged spectral types of the stars by making our own determinations
of spectral types based on their IUE spectra. We find substantial scatter in
the curves of individual star pairs and present a detailed examination of the
uncertainties and their effects on each extinction curve. We find that, given
the potentially large uncertainties inherent in using the pair method at FUV
wavelengths, a careful analysis of measurement uncertainties is critical to
assessing the true dust extinction. We present a new measurement of the average
far-ultraviolet extinction curve to the Lyman limit; our new measurement is
consistent with an extrapolation of the standard extinction curve of Savage &
Mathis (1979).Comment: 13 pages text, 7 figures 4 tables. Sent as gzipped tar, with ms.tex
and 7 figure
Correcting for the Effects of Interstellar Extinction
This paper addresses the issue of how best to correct astronomical data for
the wavelength-dependent effects of Galactic interstellar extinction. The main
general features of extinction from the IR through the UV are reviewed, along
with the nature of observed spatial variations. The enormous range of
extinction properties found in the Galaxy, particularly in the UV spectral
region, is illustrated. Fortunately, there are some tight constraints on the
wavelength dependence of extinction and some general correlations between
extinction curve shape and interstellar environment. These relationships
provide some guidance for correcting data for the effects of extinction.
Several strategies for dereddening are discussed along with estimates of the
uncertainties inherent in each method. In the Appendix, a new derivation of the
wavelength dependence of an average Galactic extinction curve from the IR
through the UV is presented, along with a new estimate of how this extinction
law varies with the parameter R = A(V)/E(B-V). These curves represent the true
monochromatic wavelength dependence of extinction and, as such, are suitable
for dereddening IR--UV spectrophotometric data of any resolution, and can be
used to derive extinction relations for any photometry system.Comment: To appear in PASP (January 1999) 14 pages including 4 pages of
figures Uses emulateapj style. PASP, in press (January 1999
Dust properties along anomalous extinction sightlines. II. Studying extinction curves with dust models
The large majority of extinction sight lines in our Galaxy obey a simple
relation depending on one parameter, the total-to-selective extinction
coefficient, Rv. Different values of Rv are able to match the whole extinction
curve through different environments so characterizing normal extinction
curves. In this paper more than sixty curves with large ultraviolet deviations
from their best-fit one parameter curve are analyzed. These curves are fitted
with dust models to shed light into the properties of the grains, the processes
affecting them, and their relations with the environmental characteristics. The
extinction curve models are reckoned by following recent prescriptions on grain
size distributions able to describe one parameter curves for Rv values from 3.1
to 5.5. Such models, here extended down to Rv=2.0, allow us to compare the
resulting properties of our deviating curves with the same as normal curves in
a self-consistent framework, and thus to recover the relative trends overcoming
the modeling uncertainties. Such curves represent the larger and homogeneous
sample of anomalous curves studied so far with dust models. Results show that
the ultraviolet deviations are driven by a larger amount of small grains than
predicted for lines of sight where extinction depends on one parameter only.
Moreover, the dust-to-gas ratios of anomalous curves are lower than the same
values for no deviating lines of sight. Shocks and grain-grain collisions
should both destroy dust grains, so reducing the amount of the dust trapped
into the grains, and modify the size distribution of the dust, so increasing
the small-to-large grain size ratio. Therefore, the extinction properties
derived should arise along sight lines where shocks and high velocity flows
perturb the physical state of the interstellar medium living their signature on
the dust properties. (Abridged version)Comment: 31 pages,12 figures; accepted for publication in A&
Radiative Transfer Analysis of Far-UV Background Observations Obtained with the Far-Ultraviolet Space Telescope (FAUST)
In 1992 the Far-Ultraviolet Space Telescope (FAUST) provided measurements of
the ultraviolet (140-180nm) diffuse sky background at high, medium, and low
Galactic latitudes. A significant fraction of the detected radiation was found
to be of Galactic origin, resulting from scattering by dust in the diffuse
interstellar medium. To simulate the radiative transfer in the Galaxy, we
employed a Monte Carlo model which utilized a realistic, non-isotropic
radiation field based on the measured fluxes (at 156nm) and positions of 58,000
TD-1 stars, and a cloud structure for the interstellar medium. The comparison
of the model predictions with the observations led to a separation of the
Galactic scattered radiation from an approximately constant background,
attributed to airglow and extragalactic radiation, and to a well constrained
determination of the dust scattering properties. The derived dust albedo a =
0.45 +/- 0.05 is substantially lower than albedos derived for dust in dense
reflection nebulae and star-forming regions, while the phase function asymmetry
g = 0.68 +/- 0.10 is indicative of a strongly forward directed phase function.
We show the highly non-isotropic phase function to be responsible, in
conjunction with the non-isotropic UV radiation field, for the wide range of
observed correlations between the diffusely scattered Galactic radiation and
the column densities of neutral atomic hydrogen. The low dust albedo is
attributed to a size distribution of grains in the diffuse medium with average
sizes smaller than those in dense reflection nebulae.Comment: 35 pages, 10 figures included, to be published in the Ap
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Feâi, and Feâii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsinâi, is typical for A and F stars and ranges from 8 to about 280 km sâ1, with a mean of 134 km sâ1
Ultraviolet Extinction Properties in the Milky Way
We have assembled a homogeneous database of 417 ultraviolet (UV) extinction
curves for reddened sightlines having International Ultraviolet Explorer (IUE)
spectra. We have combined these with optical and 2MASS photometry allowing
estimates of the ratio of total-to-selective extinction, R(V), for the entire
sample. Fitzpatrick-Massa (FM) parameters have also been found for the entire
sample. This is the largest study of parameterized UV extinction curves yet
published and it covers a wide range of environments, from dense molecular
clouds to the diffuse interstellar medium (ISM), with extinctions A(V) ranging
from 0.50 to 4.80. It is the first to extend far beyond the solar neighborhood
and into the Galaxy at large, with 30 sightlines having distances > 5 kpc.
Previously, the longest sightlines with FM parameters and R(V) extended ~ 1
kpc. We find that (1.) the CCM extinction law applies for 93% of the
sightlines, implying that dust processing in the Galaxy is efficient and
systematic; (2.) the central wavelength of the 2175 A bump is constant; (3.)
the 2175 A bump width is dependent on environment. Only four sightlines show
systematic deviations from CCM, HD 29647, 62542, 204827, and 210121. These
sightlines all sample dense, molecule-rich clouds. The new extinction curves
and values of R(V) allow us to revise the CCM law.Comment: 32 pages, 12 figure
Effect of manipulation of primary tumour vascularity on metastasis in an adenocarcinoma model
One explanation for the clinical association between tumour vascularity and probability of metastasis is that increased primary tumour vascularity enhances haematogenous dissemination by offering greater opportunity for tumour cell invasion into the circulation (intravasation). We devised an experimental tumour metastasis model that allowed manipulation of primary tumour vascularity with differential exposure of the primary and metastatic tumour site to angiogenic agents. We used this model to assess the effects of local and systemic increases in the level of the angiogenic agent basic fibroblast growth factor on metastasis. BDIX rats with implanted hind limb K12/TR adenocarcinoma tumours received either intratumoural or systemic, basic fibroblast growth factor or saline infusion. Both intratumoural and systemic basic fibroblast growth factor infusion resulted in significant increases in tumour vascularity, blood flow and growth, but not lung metastasis, compared with saline-infused controls. Raised basic fibroblast growth factor levels and increase in primary tumour vascularity did not increase metastasis. The clinical association between tumour vascularity and metastasis is most likely to arise from a metastatic tumour genotype that links increased tumour vascularity with greater metastatic potential
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