14,054 research outputs found
Chandra Reveals Variable Multi-Component X-ray Emission from FU Orionis
FU Orionis is the prototype of a class of eruptive young stars (``FUors'')
characterized by strong optical outbursts. We recently completed an exploratory
survey of FUors using XMM-Newton to determine their X-ray properties, about
which little was previously known. The prototype FU Ori and V1735 Cyg were
detected. The X-ray spectrum of FU Ori was found to be unusual, consisting of a
cool moderately-absorbed component plus a hotter component viewed through an
absorption column density that is an order of magnitude higher. We present here
a sensitive (99 ks) follow-up X-ray observation of FU Ori obtained at higher
angular resolution with Chandra ACIS-S. The unusual multi-component spectrum is
confirmed. The hot component is centered on FU Ori and dominates the emission
above 2 keV. It is variable (a signature of magnetic activity) and is probably
coronal emission originating close to FU Ori's surface viewed through cool gas
in FU Ori's strong wind or accretion stream. In contrast, the X-ray centroid of
the soft emission below 2 keV is offset 0.20 arcsec to the southeast of FU Ori,
toward the near-IR companion (FU Ori S). This offset amounts to slightly less
than half the separation between the two stars. The most likely explanation for
the offset is that the companion contributes significantly to the softer X-ray
emission below 2 keV (and weakly above 2 keV). The superimposed X-ray
contributions from FU Ori and the companion resolve the paradox posed by
XMM-Newton of an apparently single X-ray source viewed through two different
absorption columns.Comment: 21 pages, 3 tables, 6 figure
New X-ray Detections of WNL Stars
Previous studies have demonstrated that putatively single nitrogen-type
Wolf-Rayet stars (WN stars) without known companions are X-ray sources.
However, almost all WN star X-ray detections so far have been of earlier WN2 -
WN6 spectral subtypes. Later WN7 - WN9 subtypes (also known as WNL stars) have
proved more difficult to detect, an important exception being WR 79a (WN9ha).
We present here new X-ray detections of the WNL stars WR 16 (WN8h) and WR 78
(WN7h). These new results, when combined with previous detections, demonstrate
that X-ray emission is present in WN stars across the full range of spectral
types, including later WNL stars. The two WN8 stars observed to date (WR 16 and
WR 40) show unusually low X-ray luminosities (Lx) compared to other WN stars,
and it is noteworthy that they also have the lowest terminal wind speeds
(v_infty). Existing X-ray detections of about a dozen WN stars reveal a trend
of increasing Lx with wind luminosity Lwind = (1/2) M_dot v_infty^2, suggesting
that wind kinetic energy may play a key role in establishing X-ray luminosity
levels in WN stars.Comment: 20 pages, 5 figure
Development of lanthanum nickelate as a cathode for use in intermediate temperature solid oxide fuel cells
The performance of lanthanum nickelate, La2NiO4+δ (LNO), as a cathode in IT-SOFCs with the electrolyte cerium gadolinium oxide, Ce0.9Gd0.1O2−δ (CGO), has been investigated by AC impedance spectroscopy of symmetrical cells. A significant reduction in the area specific resistance (ASR) has been achieved with a layered cathode structure consisting of a thin compact LNO layer between the dense electrolyte and porous electrode. This decrease in ASR is believed to be a result of contact at the electrolyte/cathode boundary enhancing the oxygen ion transfer to the electrolyte. An ASR of 1.0 Ω cm2 at 700 °C was measured in a symmetrical cell with this layered structure, compared to an ASR of 7.4 Ω cm2 in a cell without the compact layer. In addition, further improvements were observed by enhancing the cell current collection and it is anticipated that a symmetrical cell consisting of a layered structure with adequate current collection would lower these ASR values further
A model of large volumetric capacitance in graphene supercapacitors based on ion clustering
Electric double layer supercapacitors are promising devices for high-power
energy storage based on the reversible absorption of ions into porous,
conducting electrodes. Graphene is a particularly good candidate for the
electrode material in supercapacitors due to its high conductivity and large
surface area. In this paper we consider supercapacitor electrodes made from a
stack of graphene sheets with randomly-inserted "spacer" molecules. We show
that the large volumetric capacitances C > 100 F/cm^3 observed experimentally
can be understood as a result of collective intercalation of ions into the
graphene stack and the accompanying nonlinear screening by graphene electrons
that renormalizes the charge of the ion clusters.Comment: 13 pages, 5 figures; additional discussion and supporting
calculations adde
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
Unbiased bases (Hadamards) for 6-level systems: Four ways from Fourier
In quantum mechanics some properties are maximally incompatible, such as the
position and momentum of a particle or the vertical and horizontal projections
of a 2-level spin. Given any definite state of one property the other property
is completely random, or unbiased. For N-level systems, the 6-level ones are
the smallest for which a tomographically efficient set of N+1 mutually unbiased
bases (MUBs) has not been found. To facilitate the search, we numerically
extend the classification of unbiased bases, or Hadamards, by incrementally
adjusting relative phases in a standard basis. We consider the non-unitarity
caused by small adjustments with a second order Taylor expansion, and choose
incremental steps within the 4-dimensional nullspace of the curvature. In this
way we prescribe a numerical integration of a 4-parameter set of Hadamards of
order 6.Comment: 5 pages, 2 figure
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