8,538 research outputs found

### An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law

We combine new HST/ACS observations and existing data to investigate the
wavelength dependence of NIR extinction. Previous studies suggest a power-law
form, with a "universal" value of the exponent, although some recent
observations indicate that significant sight line-to-sight line variability may
exist. We show that a power-law model provides an excellent fit to most NIR
extinction curves, but that the value of the power, beta, varies significantly
from sight line-to-sight line. Therefore, it seems that a "universal NIR
extinction law" is not possible. Instead, we find that as beta decreases, R(V)
[=A(V)/E(B-V)] tends to increase, suggesting that NIR extinction curves which
have been considered "peculiar" may, in fact, be typical for different R(V)
values. We show that the power law parameters can depend on the wavelength
interval used to derive them, with the beta increasing as longer wavelengths
are included. This result implies that extrapolating power law fits to
determine R(V) is unreliable. To avoid this problem, we adopt a different
functional form for NIR extinction. This new form mimics a power law whose
exponent increases with wavelength, has only 2 free parameters, can fit all of
our curves over a longer wavelength baseline and to higher precision, and
produces R(V) values which are consistent with independent estimates and
commonly used methods for estimating R(V). Furthermore, unlike the power law
model, it gives R(V)'s that are independent of the wavelength interval used to
derive them. It also suggests that the relation R(V) = -1.36 E(K-V)/E(B-V) -
0.79 can estimate R(V) to +/-0.12. Finally, we use model extinction curves to
show that our extinction curves are in accord with theoretical expectations.Comment: To appear in the Astrophysical Journa

### A recalibration of IUE NEWSIPS low dispersion data

While the low dispersion IUE NEWSIPS data products represent a significant
improvement over original IUE SIPS data, they still contain serious systematic
effects which compromise their utility for certain applications. We show that
NEWSIPS low resolution data are internally consistent to only 10-15% at best,
with the majority of the problem due to time dependent systematic effects. In
addition, the NEWSIPS flux calibration is shown to be inconsistent by nearly
10%.
We examine the origin of these problems and proceed to formulate and apply
algorithms to correct them to ~ 3% level -- a factor of 5 improvement in
accuracy. Because of the temporal systematics, transforming the corrected data
to the IUE flux calibration becomes ambiguous. Therefore, we elect to transform
the corrected data onto the HST FOS system. This system is far more
self-consistent, and transforming the IUE data to it places data from both
telescopes on a single system.
Finally, we argue that much of the remaining 3% systematic effects in the
corrected data is traceable to problems with the NEWSIPS intensity
transformation function (ITF). The accuracy could probably be doubled by
rederiving the ITF.Comment: Submitted to ApJ Supplement, 35 pages, 13 figures, LaTeX - AASTEX
aas2pp4.st

### Field localization on a brane intersection in anti-de Sitter spacetime

We discuss the localization of scalar, fermion, and gauge field zero modes on
a $3-$brane that resides at the intersection of two $4-$branes in
six-dimensional anti-de Sitter space. This set-up has been introduced in the
context of brane world models and, higher-dimensional versions of it, in string
theory. In both six- and ten-dimensional cases, it has been shown that
four-dimensional gravity can be reproduced at the intersection, due to the
existence of a massless, localized graviton zero-mode. However, realistic
scenarios require also the Standard Model to be localized on the $3-$brane. In
this paper, we discuss under which conditions a higher-dimensional field
theory, propagating on the above geometry, can have a zero-mode sector
localized at the intersection and find that zero modes can be localized only if
masses and couplings to the background curvature satisfy certain relations. We
also consider the case when other 4-branes cut the bulk at some distance from
the intersection and argue that, in the probe brane approximation, there is no
significant effect on the localization properties at the $3-$brane. The case of
bulk fermions is particularly interesting, since the properties of the geometry
allow localization of chiral modes independently.Comment: 13 pages, 3 figures, the version to be published in PR

### An analysis of the shapes of interstellar extinction curves. VII Milky Way spectrophotometric optical-through-ultraviolet extinction and its R-dependence

We produce a set of 72 NIR-through-UV extinction curves by combining new Hubble Space Telescope/STIS optical spectrophotometry with existing International Ultraviolet Explorer spectrophotometry (yielding gapless coverage from 1150 to 10000 ?) and NIR photometry. These curves are used to determine a new, internally consistent NIR-through-UV Milky Way mean curve and to characterize how the shapes of the extinction curves depend on R(V). We emphasize that while this dependence captures much of the curve variability, considerable variation remains that is independent of R(V). We use the optical spectrophotometry to verify the presence of structure at intermediate wavelength scales in the curves. The fact that the optical-through-UV portions of the curves are sampled at relatively high resolution makes them very useful for determining how extinction affects different broadband systems, and we provide several examples. Finally, we compare our results to previous investigations

Recommended from our members

### The effect of weld residual stresses and their re-distribution with crack growth during fatigue under constant amplitude loading

In this work the evolution of the residual stresses in a MIG-welded 2024-T3 aluminium alloy M(T) specimen during in situ fatigue crack growth at constant load amplitude has been measured with neutron diffraction. The plastic relaxation and plasticity-induced residual stresses associated with the fatigue loading were found to be small compared with the stresses arising due to elastic re-distribution of the initial residual stress field. The elastic re-distribution was modelled with a finite element simulation and a good correlation between the experimentally-determined and the modelled stresses was found. A significant mean stress effect on the fatigue crack growth rate was seen and this was also accurately predicted using the measured initial residual stresses

### Variable Interstellar Absorption toward the Halo Star HD 219188 - Implications for Small-Scale Interstellar Structure

Within the last 10 years, strong, narrow Na I absorption has appeared at
v_sun ~ -38 km/s toward the halo star HD 219188; that absorption has continued
to strengthen, by a factor 2-3, over the past three years. The line of sight
appears to be moving into/through a relatively cold, quiescent intermediate
velocity (IV) cloud, due to the 13 mas/yr proper motion of HD 219188; the
variations in Na I probe length scales of 2-38 AU/yr. UV spectra obtained with
the HST GHRS in 1994-1995 suggest N(H_tot) ~ 4.8 X 10^{17} cm^{-2}, ``halo
cloud'' depletions, n_H ~ 25 cm^{-3}, and n_e ~ 0.85-6.2 cm^{-3} (if T ~ 100 K)
for the portion of the IV cloud sampled at that time. The relatively high
fractional ionization, n_e/n_H >~ 0.034, implies that hydrogen must be
partially ionized. The N(Na I)/N(H_tot) ratio is very high; in this case, the
variations in Na I do not imply large local pressures or densities.Comment: 12 pages; aastex; to appear in ApJ

### An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards

We present a new method for deriving UV-through-IR extinction curves, based
on the use of stellar atmosphere models to provide estimates of the intrinsic
(i.e., unreddened) stellar spectral energy distributions (SEDs), rather than
unreddened (or lightly reddened) standard stars. We show that this
``extinction-without-standards'' technique greatly increases the accuracy of
the derived extinction curves and allows realistic estimations of the
uncertainties. An additional benefit of the technique is that it simultaneously
determines the fundamental properties of the reddened stars themselves, making
the procedure valuable for both stellar and interstellar studies. We
demonstrate how the extinction-without-standards curves make it possible to: 1)
study the uniformity of extinction in localized spatial regions with
unprecedented precision; 2) determine the relationships between different
aspects of curve morphology; 3) produce high quality extinction curves from low
color excess sightlines; and 4) derive reliable extinction curves for mid-late
B stars, thereby increasing spatial coverage and allowing the study of
extinction in open clusters and associations dominated by such stars. The
application of this technique to the available database of UV-through-IR SEDs,
and to future observations, will provide valuable constraints on the nature of
interstellar grains and on the processes which modify them, and will enhance
our ability to remove the multi-wavelength effects of extinction from
astronomical energy distributions.Comment: To appear in the Astronomical Journal (September 2005); 28 pages
including 9 figure

### An analysis of the shapes of interstellar extinction curves : VIII : the optical extinction structure

New Hubble Space Telescope/STIS optical spectra were obtained for a sample of early-type stars with existing International Ultraviolet Explorer UV spectra. These data were used to construct optical extinction curves whose general properties are discussed elsewhere. In this paper, we identify extinction features in the curves that are wider than diffuse interstellar bands (DIBs) but narrower than the well known broadband variability. This intermediate scale structure, or ISS, contains distinct features whose peaks can contribute a few percent to 20% of the total extinction. Most of the ISS variation can be captured by three principal components. We model the ISS with three Drude profiles and show that their strengths and widths vary from one sight line to another, but their central positions are stable, near 4370, 4870, and 6300 A. The very broad structure (VBS) in optical curves appears to be a minimum between the 4870 and 6300 A absorption peaks. We find relations among the fit parameters and provide a physical interpretation of them in terms of a simplistic grain model. Finally, we note that the strengths of the 4370 and 4870 A features are correlated to the strength of the 2175 A UV bump, but that the 6300 A feature is not, and that none of the ISS features are related to R(V). However, we verify that the broadband curvature of the continuous optical extinction is strongly related to R(V)

- â€¦