4,245 research outputs found
Recommended from our members
Opportunities for and challenges to further reductions in the âspecific powerâ rating of wind turbines installed in the United States
A wind turbineâs âspecific powerâ rating relates its capacity to the swept area of its rotor in terms of Watt per square meter. For a given generator capacity, specific power declines as rotor size increases. In land-rich but capacity-constrained wind power markets, such as the United States, developers have an economic incentive to maximize megawatt-hours per constrained megawatt, and so have favored turbines with ever-lower specific power. To date, this trend toward lower specific power has pushed capacity factors higher while reducing the levelized cost of energy. We employ geospatial levelized cost of energy analysis across the United States to explore whether this trend is likely to continue. We find that under reasonable cost scenarios (i.e. presuming that logistical challenges from very large blades are surmountable), low-specific-power turbines could continue to be in demand going forward. Beyond levelized cost of energy, the boost in market value that low-specific-power turbines provide could become increasingly important as wind penetration grows
Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891
We present infrared spectroscopy from the Spitzer Space Telescope at one disk
position and two positions at a height of 1 kpc from the disk in the edge-on
spiral NGC 891, with the primary goal of studying halo ionization. Our main
result is that the [Ne III]/[Ne II] ratio, which provides a measure of the
hardness of the ionizing spectrum free from the major problems plaguing optical
line ratios, is enhanced in the extraplanar pointings relative to the disk
pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for
the effects of radiation field hardening, we find that this trend cannot be
reproduced by any plausible photo-ionization model, and that a secondary source
of ionization must therefore operate in gaseous halos. We also present the
first spectroscopic detections of extraplanar PAH features in an external
normal galaxy. If they are in an exponential layer, very rough emission
scale-heights of 330-530 pc are implied for the various features. Extinction
may be non-negligible in the midplane and reduce these scale-heights
significantly. There is little significant variation in the relative emission
from the various features between disk and extraplanar environment. Only the
17.4 micron feature is significantly enhanced in the extraplanar gas compared
to the other features, possibly indicating a preference for larger PAHs in the
halo.Comment: 35 pages in ApJ preprint format, 8 figures, accepted for publication
in ApJ. Minor change to Introduction to give appropriate credit to earlier,
related wor
Chandra Observation of the Edge-on Galaxy NGC 3556 (M 108): Violent Galactic Disk-halo Interaction Revealed
We present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral
NGC 3556, together with a multiwavelength analysis of various discrete X-ray
sources and diffuse X-ray features. Among 33 discrete X-ray sources detected
within the I_B = 25 mag per square arcsec isophote ellipse of the galaxy, we
identify a candidate for the galactic nucleus, an ultraluminous X-ray source
that might be an accreting intermediate-mass black hole, a possible X-ray
binary with a radio counterpart, and two radio-bright giant HII regions. We
detect large amounts of extraplanar diffuse X-ray emission, which extends about
10 kpc radially in the disk and >~ 4 kpc away from the galactic plane. The
diffuse X-ray emission exhibits significant substructures, possibly
representing various blown-out superbubbles or chimneys of hot gas heated in
massive star forming regions. This X-ray-emitting gas has temperatures in the
range of ~ 2-7 x 10^6 K and has a total cooling rate of ~ 2 x 10^40 erg/s. The
energy can be easily supplied by supernova blast-waves in the galaxy. These
results demonstrate NGC 3556 as being a galaxy undergoing vigorous disk-halo
interaction. The halo in NGC 3556 is considerably less extended, however, than
that of NGC 4631, in spite of many similarities between the two galaxies. This
may be due to the fact that NGC 3556 is isolated whereas NGC 4631 is
interacting. Thus NGC 3556 presents a more pristine environment for studying
the disk-halo interaction.Comment: 30 pages, 12 figures. To appear in ApJ. Please see
http://www.astro.umass.edu/~wqd/papers/n3556/n3556.pdf for a high resolution
versio
Molecular Gas in the Edge-On Galaxy NGC 4013
Our OVRO observations at 300 pc resolution of the molecular gas disk in the edge-on spiral galaxy NGC 4013 show no evidence for extraplanar material at our sensitivity limit. The observed molecular gas kinematics are in agreement with gas motion in a barred potential
Memento Mori: The development and validation of the Death Reflection Scale
Despite its potential for advancing organizational behavior (OB) research, the topic of death awareness has been vastly understudied. Moreover, research on death awareness has predominantly focused on the anxietyâprovoking aspect of deathârelated cognitions, thus overlooking the positive aspect of death awareness, death reflection. This gap is exacerbated by the lack of a valid research instrument to measure death reflection. To address this issue, we offer a systematic conceptualization of death reflection, develop the Death Reflection Scale, and assess its psychometric properties across four studies. Further, using a sample of 268 firefighters, we examine whether death reflection buffers the detrimental impact of mortality cues at work on employee wellâbeing and safety performance. Results provide strong support for the psychometric properties of the Death Reflection Scale. Further, moderation analysis indicates death reflection weakens the negative effect of mortality cues on firefighters' safety performance. Overall, these findings suggest the newly developed Death Reflection Scale will prove useful in future research on deathârelated cognitions
The Lifetime of Grand Design
The lifetime of the structure in grand design spiral galaxies is
observationally ill-determined, but is essentially set by how accurately the
pattern's rotation can be characterized by a single angular pattern speed. This
paper derives a generalized version of the Tremaine-Weinberg method for
observationally determining pattern speeds, in which the pattern speed is
allowed to vary arbitrarily with radius. The departures of the derived pattern
speed from a constant then provides a simple metric of the lifetime of the
spiral structure. Application of this method to CO observations of NGC 1068
reveal that the pattern speed of the spiral structure in this galaxy varies
rapidly with radius, and that the lifetime of the spiral structure is
correspondingly very short. If this result turns out to be common in
grand-design spiral galaxies, then these features will have to be viewed as
highly transient phenomena.Comment: 6 pages, 3 figures, accepted for publication in MNRA
Geometric scaling in high-energy QCD at nonzero momentum transfer
We show how one can obtain geometric scaling properties from the
Balitsky-Kovchegov (BK) equation. We start by explaining how, this property
arises for the b-independent BK equation. We show that it is possible to extend
this model to the full BK equation including momentum transfer. The saturation
scale behaves like max(q,Q_T) where q is the momentum transfer and Q_T a
typical scale of the target.Comment: 4 pages, 2 figures. Talk given by G. Soyez at the "Rencontres de
Moriond", 12-19 March 2005, La Thuile, Ital
Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo
We present imaging Fabry-Perot observations of Halpha emission in the nearly
edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial
density profile along the major axis by examining position-velocity (PV)
diagrams from the Fabry-Perot data cube as well as a CO 2-1 data cube from the
literature. PV diagrams constructed parallel to the major axis are used to
examine changes in azimuthal velocity as a function of height above the
midplane. The results of this analysis reveal the presence of a vertical
gradient in azimuthal velocity. The magnitude of this gradient is approximately
1 km/s/arcsec, or about 8 km/s/kpc, though a higher value of the gradient may
be appropriate in localized regions of the halo. The evidence for an azimuthal
velocity gradient is much stronger for the approaching half of the galaxy,
although earlier slit spectra are consistent with a gradient on both sides.
There is evidence for an outward radial redistribution of gas in the halo. The
form of the rotation curve may also change with height, but this is not
certain. We compare these results with those of an entirely ballistic model of
a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity
which is shallower than the observed gradient, indicating that an additional
mechanism is required to further slow the rotation speeds in the halo.Comment: 18 pages, 18 figures. Uses emulateapj.cls. Accepted for publication
in Ap
- âŠ