11,815 research outputs found
Herschel-PACS Measurements of Nitrogen Enrichment in Nebulae around Wolf-Rayet Stars
For three nebulae that have early-WN Wolf-Rayet exciting stars, NGC 6888, WR
8 and Abell 48, we have obtained Herschel-PACS line scans of the [N III] 57 um
and [O III] 88 micron lines, along with the 122 and 205 micron lines of [N II].
From the former two lines we have derived N/O abundance ratios,
equal to the overall N/O ratio under a wide range of nebular conditions. We
find that all of the nebulae observed possess significant nitrogen enrichment,
with derived N/O ratios greater than solar. The two nebulae with massive
Wolf-Rayet exciting stars, NGC 6888 and WR8 are found to have N/O ratios that
are enhanced by factors of 7 - 10 relative to the solar N/O ratio, consistent
with an origin as material ejected just before the onset of the Wolf-Rayet
phase. The other nebula, Abell 48, has recently been reclassified as a member
of the rare class of three planetary nebulae that have early-WN central stars
and are not of Peimbert Type I. We derive a nebular N/O ratio for it that is a
factor of 4 enhanced relative to solar and slightly above the range of N/O
values that have been measured for the other three members of its [WN]
planetary nebula class.Comment: 11 pages, 5 figures, MNRAS accepte
Optical imaging of resonant electrical carrier injection into individual quantum dots
We image the micro-electroluminescence (EL) spectra of self-assembled InAs
quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and
demonstrate optical detection of resonant carrier injection into a single QD.
Resonant tunneling of electrons and holes into the QDs at bias voltages below
the flat-band condition leads to sharp EL lines characteristic of individual
QDs, accompanied by a spatial fragmentation of the surface EL emission into
small and discrete light- emitting areas, each with its own spectral
fingerprint and Stark shift. We explain this behavior in terms of Coulomb
interaction effects and the selective excitation of a small number of QDs
within the ensemble due to preferential resonant tunneling paths for carriers.Comment: 4 page
Compton scattering of twisted light: angular distribution and polarization of scattered photons
Compton scattering of twisted photons is investigated within a
non-relativistic framework using first-order perturbation theory. We formulate
the problem in the density matrix theory, which enables one to gain new
insights into scattering processes of twisted particles by exploiting the
symmetries of the system. In particular, we analyze how the angular
distribution and polarization of the scattered photons are affected by the
parameters of the initial beam such as the opening angle and the projection of
orbital angular momentum. We present analytical and numerical results for the
angular distribution and the polarization of Compton scattered photons for
initially twisted light and compare them with the standard case of plane-wave
light
National Security and Immigration Policy: Reclaiming Terms, Measuring Success, and Setting Priorities
This paper offers a three-pronged framework for evaluating immigration-related security measures and tactics. First, does the measure or strategy relate to a legitimate security goal? Second, what does the measure propose to accomplish, will it be successful in achieving that end, and how can its success (or failure) be assessed? Third, is the measure worth the cost both in resources and opportunities foregone? The paper goes on to evaluate the US Government\u27s recent implementation of the US-VISIT program against this framework
Margaret D. Stock on Mexican Americans & the Law by Reynaldo Anaya Valencia, Sonia R. GarcĂa, Henry Flores, and JosĂ© Roberto Juárez, Jr. Tucson, AZ: The University of Arizona Press, 2004. 198pp.
A review of:
Mexican Americans & the Law by Reynaldo Anaya Valencia, Sonia R. GarcĂa, Henry Flores, and JosĂ© Roberto Juárez, Jr. Tucson, AZ: The University of Arizona Press, 2004. 198pp
PAHs and star formation in the HII regions of nearby galaxies M83 and M33
We present mid-infrared (MIR) spectra of HII regions within star-forming
galaxies M83 and M33. Their emission features are compared with Galactic and
extragalactic HII regions, HII-type galaxies, starburst galaxies, and
Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and
M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in
the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting
different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic
database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH
size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 m PAH
band depends on object type; however, there is no dependence on metallicity for
both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio
decreases with the hardness of the radiation field and galactocentric radius
(Rg), yet the ionization alone cannot account for the variation seen in all of
our sources; (v) the relative strength of PAH features does not change
significantly with increasing radiation hardness, as measured through the
[NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR
calibrations based on the tight correlation between the 6.2, 7.7, and 11.3
m PAH luminosities with the 24 m luminosity and the combination of
the 24 m and H luminosity; (vii) Based on the total luminosity
from PAH and FIR emission, we argue that extragalactic HII regions are more
suitable templates in modeling and interpreting the large scale properties of
galaxies compared to Galactic HII regions.Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRA
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