1,536 research outputs found
Effect of Mass Transfer on Aeroheating in Hypersonic Chemically Reacting Boundary Layers
Heat flux characterization of high-enthalpy boundary layer flows is key to optimize the performance and design of Thermal Protection System of next generation aerospace vehicles [1]. At atmospheric entry hypersonic speeds, ablation as well as surface catalycity impact boundary layer aeroheating. Out-gassing occurring from an ablative surface in planetary entry environment introduces a rich set of problems in thermodynamic, fluid dynamic, and material pyrolysis. Ablation leads to out-gassing and surface roughness, both of which are known to affect surface heating in hypersonic chemically reacting boundary layers via three main routes: gas blowing into the boundary layer from the wall, changing the surface heat transfer due to wall-flow chemical reactions, and modifying surface roughness via ablative processes
Preliminary Measurements of the Motion of Arcjet Current Channel Using Inductive Magnetic Probes
This paper covers the design and first measurements of non-perturbative, external inductive magnetic diagnostics for arcjet constrictors which can measure the motion of the arc current channel. These measurements of arc motion are motivated by previous simulations using the ARC Heater Simulator (ARCHeS), which predicted unsteady arc motion due to the magnetic kink instability. Measurements of the kink instability are relevant to characterizing motion of the enthalpy profile of the arcjet, the arcjet operational stability, and electrode damage due to associated arc detachment events. These first measurements indicate 4 mm oscillations at 0.5-2 kHz of the current profile
Development of a Three-Dimensional, Unstructured Material Response Design Tool
A preliminary verification and validation of a new material response model is presented. This model, Icarus, is intended to serve as a design tool for the thermal protection systems of re-entry vehicles. Currently, the capability of the model is limited to simulating the pyrolysis of a material as a result of the radiative and convective surface heating imposed on the material from the surrounding high enthalpy gas. Since the major focus behind the development of Icarus has been model extensibility, the hope is that additional physics can be quickly added. This extensibility is critical since thermal protection systems are becoming increasing complex, e.g. woven carbon polymers. Additionally, as a three-dimensional, unstructured, finite-volume model, Icarus is capable of modeling complex geometries. In this paper, the mathematical and numerical formulation is presented followed by a discussion of the software architecture and some preliminary verification and validation studies
Measuring Star-formation Rate and Far-Infrared Color in High-redshift Galaxies Using the CO (7-6) and [NII] 205 micron Lines
To better characterize the global star formation (SF) activity in a galaxy,
one needs to know not only the star formation rate (SFR) but also the
rest-frame, far-infrared (FIR) color (e.g., the 60-to-100 m color,
] of the dust emission. The latter probes the average intensity of
the dust heating radiation field and scales statistically with the effective
SFR surface density in star-forming galaxies including (ultra-)luminous
infrared galaxies [(U)LIRGs]. To this end, we exploit here a new spectroscopic
approach involving only two emission lines: CO\,(76) at 372 m and [NII]
at 205 m. For local (U)LIRGs, the ratios of the CO (76) luminosity
() to the total infrared luminosity (; 81000
m) are fairly tightly distributed (to within 0.12 dex) and show
little dependence on . This makes a good SFR
tracer, which is less contaminated by active galactic nuclei (AGN) than and may also be much less sensitive to metallicity than . Furthermore, the logarithmic [NII] 205 m to CO (76)
luminosity ratio is fairly steeply (at a slope of ) correlated with
, with a modest scatter (0.23 dex). This makes it a useful
estimator on with an implied uncertainty of 0.15 [or
4 K in the dust temperature () in the case of a
graybody emission with K and a dust emissivity index
]. Our locally calibrated SFR and estimators are shown
to be consistent with the published data of (U)LIRGs of up to 6.5.Comment: 6 pages, 3 figures, 1 table; accepted for publication in the ApJ
Lette
LINER/H II "Transition" Nuclei and the Nature of NGC 4569
Motivated by the discovery of young, massive stars in the nuclei of some
LINER/H II ``transition'' nuclei such as NGC 4569, we have computed
photoionization models to determine whether some of these objects may be
powered solely by young star clusters rather than by accretion-powered active
nuclei. The models were calculated with the photoionization code CLOUDY, using
evolving starburst continua generated by the the STARBURST99 code of Leitherer
et al. (1999). We find that the models are able to reproduce the emission-line
spectra of transition nuclei, but only for instantaneous bursts of solar or
higher metallicity, and only for ages of ~3-5 Myr, the period when the
extreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars.
For clusters younger than 3 Myr or older than 6 Myr, and for models with a
constant star-formation rate, the softer ionizing continuum results in an
emission spectrum more typical of H II regions. This model predicts that
Wolf-Rayet emission features should appear in the spectra of transition nuclei.
While such features have not generally been detected to date, they could be
revealed in observations having higher spatial resolution. Demographic
arguments suggest that this starburst model may not apply to the majority of
transition nuclei, particularly those in early-type host galaxies, but it could
account for some members of the transition class in hosts of type Sa and later.
The starburst models during the Wolf-Rayet-dominated phase can also reproduce
the narrow-line spectra of some LINERs, but only under conditions of
above-solar metallicity and only if high-density gas is present (n_e >~ 10^5
cm^{-3}). This scenario could be applicable to some ``Type 2'' LINERs which do
not show any clear signs of nonstellar activity.Comment: To appear in PASP. 22 pages, includes 9 figures, uses AASTeX v5.
Preliminary Measurements of the Motion of Arcjet Current Channel Using Inductive Magnetic Probes
This paper covers the design and first measurements of non-perturbative, external inductive magnetic diagnostics for arcjet constrictors which can measure the motion of the arc current-channel. These measurements of arc motion are motivated by previous simulations using the ARC Heater Simulator (ARCHeS), which predicted unsteady arc motion due to the magnetic kink instability. Measurements of the kink instability are relevant to characterizing motion of the enthalpy profile of the arcjet, the arcjet operational stability, and electrode damage due to associated arc detachment events. These first measurements indicate mm oscillations at 0.5-2 kHz of the current profile
Tracing PAHs and Warm Dust Emission in the Seyfert Galaxy NGC 1068
We present a study of the nearby Seyfert galaxy NGC 1068 using mid- and far-
infrared data acquired with the IRAC, IRS, and MIPS instruments aboard the
Spitzer Space Telescope. The images show extensive 8 um and 24 um emission
coinciding with star formation in the inner spiral approximately 15" (1 kpc)
from the nucleus, and a bright complex of star formation 47" (3 kpc) SW of the
nucleus. The brightest 8 um PAH emission regions coincide remarkably well with
knots observed in an Halpha image. Strong PAH features at 6.2, 7.7, 8.6, and
11.3 um are detected in IRS spectra measured at numerous locations inside,
within, and outside the inner spiral. The IRAC colors and IRS spectra of these
regions rule out dust heated by the AGN as the primary emission source; the
SEDs are dominated by starlight and PAH emission. The equivalent widths and
flux ratios of the PAH features in the inner spiral are generally consistent
with conditions in a typical spiral galaxy ISM. Interior to the inner spiral,
the influence of the AGN on the ISM is evident via PAH flux ratios indicative
of a higher ionization parameter and a significantly smaller mean equivalent
width than observed in the inner spiral. The brightest 8 and 24 um emission
peaks in the disk of the galaxy, even at distances beyond the inner spiral, are
located within the ionization cones traced by [O III]/Hbeta, and they are also
remarkably well aligned with the axis of the radio jets. Although it is
possible that radiation from the AGN may directly enhance PAH excitation or
trigger the formation of OB stars that subsequently excite PAH emission at
these locations in the inner spiral, the orientation of collimated radiation
from the AGN and star formation knots in the inner spiral could be
coincidental. (abridged)Comment: 20 pages, 11 figures; AJ, accepted; full resolution version available
at http://spider.ipac.caltech.edu/staff/jhhowell/astro/howelln1068.pd
Accretion Disc Wind Variability in the States of the Microquasar GRS 1915+105
Continuing our study of the role and evolution of accretion disc winds in the
microquasar GRS 1915+105, we present high-resolution spectral variability
analysis of the beta and gamma states with the Chandra High Energy Transmission
Grating Spectrometer. By tracking changes in the absorption lines from the
accretion disc wind, we find new evidence that radiation links the inner and
outer accretion discs on a range of time-scales. As the central X-ray flux
rises during the high-luminosity gamma state, we observe the progressive
over-ionization of the wind. In the beta state, we argue that changes in the
inner disc leading to the ejection of a transient 'baby jet' also quench the
highly-ionized wind from the outer disc. Our analysis reveals how the state,
structure, and X-ray luminosity of the inner accretion disc all conspire to
drive the formation and variability of highly-ionized accretion disc winds.Comment: Accepted for publication in MNRAS. 11 pages, 7 figures, uses
mn2e.cls. Comments welcom
Recommended from our members
Author Correction: Cryptic inoviruses revealed as pervasive in bacteria and archaea across Earth's biomes.
An amendment to this paper has been published and can be accessed via a link at the top of the paper
Sub-Arcsecond Mid-Infrared Observations of NGC 6240: Limitations of AGN-Starburst Power Diagnostics
In order to examine the relative importance of powerful starbursts and
Compton-thick AGNs in NGC 6240, we have obtained mid-infrared images and
low-resolution spectra of the galaxy with sub-arcsecond spatial resolution
using the Keck Telescopes. Despite the high spatial resolution (~200 pc) of our
data, no signature of the hidden AGNs has been detected in the mid-infrared.
The southern nucleus, which we show provides 80-90% of the total 8-25 um
luminosity of the system, has a mid-infrared spectrum and a mid-/far-infrared
spectral energy distribution consistent with starbursts. At the same time,
however, it is also possible to attribute up to 60% of the bolometric
luminosity to an AGN, consistent with X-ray observations, if the AGN is heavily
obscured and emits mostly in the far-infrared. This ambiguity arises because
the intrinsic variation of properties among a given galaxy population (e.g.,
starbursts) introduces at least a factor of a few uncertainty even into the
most robust AGN-starburst diagnostics. We conclude that with present
observations it is not possible to determine the dominant power source in
galaxies when AGN and starburst luminosities are within a factor of a few of
each other.Comment: 27 pages, 7 figures, 2 tables; Accepted for publication in A
- …