14,191 research outputs found
Starburst-driven galactic winds: I. Energetics and intrinsic X-ray emission
We have performed an extensive hydrodynamical parameter study of
starburst-driven galactic winds, motivated by the latest observation data on
the best-studied starburst galaxy M82. We study how the wind dynamics,
morphology and X-ray emission depend on the host galaxy's ISM distribution,
starburst star formation history and strength, and presence and distribution of
mass-loading by dense clouds. We find that the soft X-ray emission from
galactic winds comes from low filling factor (ff < 2 per cent) gas, which
contains only a small fraction (f < 10 per cent) of the mass and energy of the
wind, irrespective of whether the wind models are strongly mass-loaded or not.
X-ray observations of galactic winds therefore do not directly probe the gas
that contains the majority of the energy, mass or metal-enriched gas in the
outflow. The soft X-ray emission comes from gas at a wide range different
temperatures and densities. Estimates of the physical properties of the hot gas
in starburst galaxies, based on fitting the standard simple spectral models to
existing X-ray spectra, should therefore be treated with extreme suspicion. The
majority of the thermal and kinetic energy of these winds is in a volume
filling hot, T approx 10^7 K, component which is extremely difficult to probe
observationally due to its low density and hence low emissivity. Most of the
total energy is in the kinetic energy of this hot gas, a factor which must be
taken into account when attempting to constrain wind energetics
observationally. We also find that galactic winds are efficient at transporting
large amounts of energy out of the host galaxy, in contrast to their
inefficiency at transporting mass out of star-forming galaxies. (Abridged)Comment: Accepted for publication in MNRAS. Letter page size postscript
available from http://adcam.pha.jhu.edu/~dks/dks_published.htm
Probing the wind-wind collision in Gamma Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization
We present a new analysis of an archived Chandra HETGS X-ray spectrum of the
WR+O colliding wind binary Gamma Velorum. The spectrum is dominated by emission
lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a
combination of broad-band spectral analysis and an analysis of line flux ratios
we infer a wide range of temperatures in the X-ray emitting plasma (~4-40 MK).
As in the previously published analysis, we find the X-ray emission lines are
essentially unshifted, with a mean FWHM of 1240 +/- 30 km/s. Calculations of
line profiles based on hydrodynamical simulations of the wind-wind collision
predict lines that are blueshifted by a few hundred km/s. The lack of any
observed shift in the lines may be evidence of a large shock-cone opening
half-angle (> 85 degrees), and we suggest this may be evidence of sudden
radiative braking. From the R and G ratios measured from He-like
forbidden-intercombination-resonance triplets we find evidence that the Mg XI
emission originates from hotter gas closer to the O star than the Si XIII
emission, which suggests that non-equilibrium ionization may be present.Comment: 22 pages, 14 figures. Accepted for publication in MNRA
A three dimensional model of the Venusian thermosphere with superrotation
An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory
A Survey for Federally Listed Grassland Birds at First Nations Reserves
We surveyed native and non-native grassland for federally listed grassland bird species at Reserves and/or Treaty Land Entitlements (TLE) belonging to five First Nations in the prairie ecozone of Canada. Ferruginous Hawk (Buteo regalis – of special concern), Loggerhead Shrike (Lanius ludovicianus – threatened), and Long-billed Curlew (Numenius americanus – of special concern) were observed at five of 335, three of 361, and five of 329 survey points within their respective geographic ranges. Sprague’s Pipit (Anthus spragueii – threatened) was observed at 69 of 361 survey points and accounted for 84% of sites with listed birds. Estimates of proportions of native and non-native grassland within 400 m of survey points indicated that Sprague’s Pipit preferred native grassland. Despite special effort, the Burrowing Owl (Athene culicularia – endangered) was not observed, although a nest from a previous year was encountered. We recorded the greatest number of species and individuals on reserves located in mixed and moist-mixed grasslands that contained large portions of native grassland. In many instances, field assessments revealed unsuitable habitat at sites that remote-sensing data classified as suitable
Design considerations for large space electric power systems
As power levels of spacecraft rise to the 50 to 100 kW range, it becomes apparent that low voltage (28 V) dc power distribution and management systems will not operate efficiently at these higher power levels. The concept of transforming a solar array voltage at 150 V dc into a 1000 V ac distribution system operating at 20 kHz is examined. The transformation is accomplished with series-resonant inverter by using a rotary transformer to isolate the solar array from the spacecraft. The power can then be distributed in any desired method such as three phase delta to delta. The distribution voltage can be easily transformed to any desired load voltage and operating frequency. The reasons for the voltage limitations on the solar array due to plasma interactions and the many advantages of a high voltage, high frequency at distribution system are discussed
Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph
We report a time-series analysis of satellite photometry of the non-radially
pulsating Oe star zeta Oph, principally using data from SMEI obtained
2003--2008, but augmented with MOST and WIRE results. Amplitudes of the
strongest photometric signals, at 5.18, 2.96, and 2.67/d, each vary
independently over the 6-year monitoring period (from ca. 30 to <2 mmag at
5.18/d), on timescales of hundreds of days. Signals at 7.19/d and 5.18/d have
persisted (or recurred) for around two decades. Supplementary spectroscopic
observations show an H-alpha emission episode in 2006; this coincided with
small increases in amplitudes of the three strongest photometric signals.Comment: MNRAS, in pres
XMM-Newton Spectroscopy of the Starburst Dominated Ultra Luminous Infrared Galaxy NGC 6240
We present new XMM-Newton observation of the Ultra Luminous Infrared Galaxy
(ULIRG) NGC 6240. We analyze the reflecting grating spectrometer (RGS) data,
and data from the other instruments, and find a starburst dominated 0.5-3 keV
spectrum with global properties resembling those observed in M82 but with a
much higher luminosity. We show that the starburst region can be divided into
an outer zone, beyond a radius of about 2.1 kpc, with a gas temperature of
about 10^7 K and a central region with temperatures in the range (2-6) x 10^7
K. The gas in the outer region emits most of the observed Oviii Lyman-alpha
line and the gas in the inner region the emission lines of higher ionization
ions, including a strong Fexxv line. We also identify a small inner part, very
close to the active nuclei, with typical Seyfert 2 properties including a large
amount of photoionized gas producing a strong Fe K-alpha 6.4 keV line. The
combined abundance, temperature and emission measure analysis indicates super
solar Ne/O, Mg/O, Si/O, S/O and possibly also Fe/O. The analysis suggests
densities in the range of (0.07-0.28) x epsilon^(-1/2) cm^(-3) and a total
thermal gas mass of about 4 x 10^8 x epsilon^(1/2) solar masses, where epsilon
is the volume filling factor. We used a simple model to argue that a massive
starburst with an age of about 2 x 10^7 years can explain most of the observed
properties of the source. NGC 6240 is perhaps the clearest case of an X-ray
bright luminous AGN, in a merger, whose soft X-ray spectrum is dominated by a
powerful starburst.Comment: 10 pages, 6 diagrams, accepted by ApJ, added a few minor change
Algebraic geometric methods for the stabilizability and reliability of multivariable and of multimode systems
The extent to which feedback can alter the dynamic characteristics (e.g., instability, oscillations) of a control system, possibly operating in one or more modes (e.g., failure versus nonfailure of one or more components) is examined
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