955 research outputs found
Passivation of Grid-Following VSCs: A Comparison Between Active Damping and Multi-Sampled PWM
This article compares different strategies used to enhance the stability properties of grid-following voltage-source converters (VSCs). Because of digital delays, VSC admittance exhibits a nonpassive zone, which introduces negative damping and may destabilize the grid-connected operation. It is shown that typically used active damping (AD) strategies only bring positive impact up to a certain frequency, while deteriorating admittance properties around and above the Nyquist frequency. Multi-sampled pulsewidth modulation (MS-PWM) greatly extends the passive admittance region, using only a single-loop current controller. Experimental admittance measurements are performed on a single-phase VSC, up to twice the switching frequency. Subsequently, different grid-connected scenarios are tested to show that MS-PWM retains stable operation, where AD methods cause instability. This article also offers analytic modeling and experimental measurements of noise propagation for compared strategies. It is shown that derivative-based AD is not highly sensitive; however, MS-PWM offers additional noise suppression
Chromatic numbers for facet colouring of some generalized associahedra
The chromatic number of a proper colouring of the facets of certain classes
of generalized associahedra is studied. The exact values are obtained for the
permutohedron and the associahedron, while lower and upper bounds are
established for the -dimensional cyclohedron, astrohedron and simple
permutoassociahedron
Spectral energy distributions of quasars selected in the mid-infrared
We present preliminary results on fitting of SEDs to 142 z>1 quasars selected
in the mid-infrared. Our quasar selection finds objects ranging in extinction
from highly obscured, type-2 quasars, through more lightly reddened type-1
quasars and normal type-1s. We find a weak tendency for the objects with the
highest far-infrared emission to be obscured quasars, but no bulk systematic
offset between the far-infrared properties of dusty and normal quasars as might
be expected in the most naive evolutionary schemes. The hosts of the type-2
quasars have stellar masses comparable to those of radio galaxies at similar
redshifts. Many of the type-1s, and possibly one of the type-2s require a very
hot dust component in addition to the normal torus emission.Comment: 4 pages, 2 figures, to appear in the proceedings of The Spectral
Energy Distribution of Galaxies, Preston, September 2011, eds R.J. Tuffs &
C.C. Popesc
The location of an active nucleus and a shadow of a tidal tail in the ULIRG Mrk 273
Analysis of data from the Chandra X-ray Observatory for the double nucleus ULIRG Mrk 273 reveals an absorbed hard X-ray source coincident with the southwest nucleus, implying that this unresolved, near-infrared source is where an active nucleus resides, while the northern nuclear region contains a powerful starburst that dominates the far infrared luminosity. There is evidence of a slight image extension in the 6–7 keV band, where an Fe K line is present, towards the northern nucleus. A large-scale, diffuse emission nebula detected in soft X-rays contains a dark lane that spatially coincides with a high surface-brightness tidal tail extending ~50 arcsec (40 kpc) to the south. The soft X-ray source is likely located behind the tidal tail, which absorbs X-ray photons along the line of sight. The estimated column density of cold gas in the tidal tail responsible for shadowing the soft X-rays is N_H ≥ 6 × 10^(21) cm^(-2), consistent with the tidal tail having an edge-on orientation
Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars
We present Hubble Space Telescope images, and spectral energy distributions
from optical to infrared wavelengths for a sample of six 0.3<z<0.8 type-2
quasars selected in the mid-infrared using data from the Spitzer Space
Telescope. All the host galaxies show some signs of disturbance. Most seem to
possess dusty, star-forming disks. The disk inclination, estimated from the
axial ratio of the hosts, correlates with the depth of the silicate feature in
the mid-infrared spectra, implying that at least some of the reddening towards
the AGN arises in the host galaxy. The star formation rates in these objects,
as inferred from the strengths of the PAH features and far-infrared continuum,
range from 3-90 Msun/yr, but are mostly much larger than those inferred from
the [OII]3727 emission line luminosity, due to obscuration. Taken together with
studies of type-2 quasar hosts from samples selected in the optical and X-ray,
this is consistent with previous suggestions that two types of extinction
processes operate within the type-2 quasar population, namely a component due
to the dusty torus in the immediate environment of the AGN, and a more extended
component due to a dusty, star forming disk.Comment: 5 pages, 3 figures. Accepted by ApJ Letter
The influence of gibberellic acid and paclobutrazol on induction of somatic embryogenesis in wild type and hairy root cultures of Centaurium erythraea Gillib.
The effects of exogenous gibberellic acid (GA3) and paclobutrazol on induction of somatic embryos in wild type and hairy root culture of Centaurium erythraea Gillib. were investigated. Both compoundswere incorperated into 1/2 MS medium at 6 concentrations (0.01, 0.03, 0.1, 0.3, 1.0 and 3.0 ìM). Wild type root and hairy root explants cultured in the presence of GA3 at all tested concentrations under 16-h photoperiod or in the darkness decreased the number of somatic embryos that were produced. Paclobutrazol (0.3 mM) induced the largest number (19.7, 16.5) of somatic embryos in wild type and hairy root cultures, respectively. Rooting of plants derived from somatic embryos as achieved on ½MSmedium. These results indicate that paclobutrazol is beneficial for somatic embryo induction and formation in wild type and hairy root culture
Sensitive VLBI Observations of the z = 4.7 QSO BRI 1202-0725
We present sensitive phase-referenced VLBI results on the radio continuum
emission from the z=4.7 double source BRI 1202-0725. The observations were
carried out at 1425 MHz using the Very Long Baseline Array (VLBA), the phased
Very Large Array (VLA), and the Green Bank Telescope (GBT). Our sensitive VLBI
images of BRI 1202-0725 at 0.25 x 0.14 arcsec resolution show a continuum
structure in each of its two components. Fitting Gaussian models to these
continuum structures yield total flux densities of 315 +/- 38 and 250 +/- 39
microJy, for the northern and the southern components, respectively. The
estimated intrinsic brightness temperatures of these continuum structures are
about 2 x 10^4 K. Neither component is detected at the full VLBI resolution (29
mas x 7 mas), with a 4 sigma point source upper limit of 40 microJy/beam, or an
upper limit to the intrinsic brightness temperature of 6.7 x 10^5 K. The
highest angular resolution with at least a 4sigma detection is about 85 mas. At
this resolution, the images reveal a single continuum feature in the northern
component of BRI 1202-0725, and two continuum features in the southern
component, separated by 320 mas. This is similar to the structures seen in the
high resolution images of the CO emission. The extent of the observed continuum
sources at 1.4 GHz and the derived brightness temperatures are consistent with
nuclear starbursts. Moreover, the absence of any compact high-brightness
temperature source suggests that thereis no radio-loud AGN in BRI 1202-0725.Comment: 19 pages, 5 figures. Accepted for publication in A
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