122 research outputs found
Discovery of new low-excitation planetary nebulae
We report a multi-wavelength study of four new planetary nebula (PN)
candidates selected from the INT/WFC Photometric Ha Survey of the Northern
Galactic Plane (IPHAS) and Deep Sky Hunter (DSH) catalogues. We present
mid-resolution optical spectra of these PNs. The PN status of our sample was
confirmed by optical narrow-band images and mid-resolution spectra. Based on
the locations of these objects in the log (Ha/[N II]) versus log (Ha/[S II])
diagnostic diagram, we conclude that these sources are evolved lowexcitation
PNs. The optical and infrared appearances of these newly discovered PNs are
discussed. Three of the new nebulae studied here are detected in infrared and
have low infrared-to-radio flux ratios, probably suggesting that they are
evolved. Furthermore, we derive the dynamical ages and distances of these
nebulae and study the spectral energy distribution for one of them with
extensive infrared archival data.Comment: 11 pages, 9 figures, 6 tables, accepted for publication on Astronomy
& Astrophysic
A Revisit to Quadratic Programming with One Inequality Quadratic Constraint via Matrix Pencil
The quadratic programming over one inequality quadratic constraint (QP1QC) is
a very special case of quadratically constrained quadratic programming (QCQP)
and attracted much attention since early 1990's. It is now understood that,
under the primal Slater condition, (QP1QC) has a tight SDP relaxation (PSDP).
The optimal solution to (QP1QC), if exists, can be obtained by a matrix rank
one decomposition of the optimal matrix X? to (PSDP). In this paper, we pay a
revisit to (QP1QC) by analyzing the associated matrix pencil of two symmetric
real matrices A and B, the former matrix of which defines the quadratic term of
the objective function whereas the latter for the constraint. We focus on the
\undesired" (QP1QC) problems which are often ignored in typical literature:
either there exists no Slater point, or (QP1QC) is unbounded below, or (QP1QC)
is bounded below but unattainable. Our analysis is conducted with the help of
the matrix pencil, not only for checking whether the undesired cases do happen,
but also for an alternative way to compute the optimal solution in comparison
with the usual SDP/rank-one-decomposition procedure.Comment: 22 pages, 0 figure
An Optical-Infrared Study of the Young Multipolar Planetary Nebula NGC 6644
High-resolution HST imaging of the compact planetary nebula NGC 6644 has
revealed two pairs of bipolar lobes and a central ring lying close to the plane
of the sky. From mid-infrared imaging obtained with the Gemini Telescope, we
have found a dust torus which is oriented nearly perpendicular to one pair of
the lobes. We suggest that NGC 6644 is a multipolar nebula and have constructed
a 3-D model which allows the visualization of the object from different lines
of sight. These results suggest that NGC 6644 may have similar intrinsic
structures as other multipolar nebulae and the phenomenon of multipolar
nebulosity may be more common than previously believed.Comment: 31 pages, 13 figures, accepted for publication in Ap
Thermal Dissipation and Variability in Electrical Breakdown of Carbon Nanotube Devices
We study high-field electrical breakdown and heat dissipation from carbon
nanotube (CNT) devices on SiO2 substrates. The thermal "footprint" of a CNT
caused by van der Waals interactions with the substrate is revealed through
molecular dynamics (MD) simulations. Experiments and modeling find the
CNT-substrate thermal coupling scales proportionally to CNT diameter and
inversely with SiO2 surface roughness (~d/{\Delta}). Comparison of diffuse
mismatch modeling (DMM) and data reveals the upper limit of thermal coupling
~0.4 W/K/m per unit length at room temperature, and ~0.7 W/K/m at 600 C for the
largest diameter (3-4 nm) CNTs. We also find semiconducting CNTs can break down
prematurely, and display more breakdown variability due to dynamic shifts in
threshold voltage, which metallic CNTs are immune to; this poses a fundamental
challenge for selective electrical breakdowns in CNT electronics
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