4,643 research outputs found
Chandra X-ray Observation of a Mature Cloud-Shock Interaction in the Bright Eastern Knot Region of Puppis A
We present Chandra X-ray images and spectra of the most prominent cloud-shock
interaction region in the Puppis A supernova remnant. The Bright Eastern Knot
(BEK) has two main morphological components: (1) a bright compact knot that
lies directly behind the apex of an indentation in the eastern X-ray boundary
and (2) lying 1' westward behind the shock, a curved vertical structure (bar)
that is separated from a smaller bright cloud (cap) by faint diffuse emission.
Based on hardness images and spectra, we identify the bar and cap as a single
shocked interstellar cloud. Its morphology strongly resembles the ``voided
sphere'' structures seen at late times in Klein et al.'s experimental
simulations of cloud-shock interactions, when the crushing of the cloud by
shear instabilities is well underway. We infer an interaction time of roughly 3
cloud-crushing timescales, which translates to 2000-4000 years, based on the
X-ray temperature, physical size, and estimated expansion of the shocked cloud.
This is the first X-ray identified example of a cloud-shock interaction in this
advanced phase. Closer to the shock front, the X-ray emission of the compact
knot in the eastern part of the BEK region implies a recent interaction with
relatively denser gas, some of which lies in front of the remnant. The complex
spatial relationship of the X-ray emission of the compact knot to optical [O
III] emission suggests that there are multiple cloud interactions occurring
along the line of sight.Comment: 22 pages LaTeX with multiple figures, to appear in Ap
Interplay between Nitrogen Dopants and Native Point Defects in Graphene
To understand the interaction between nitrogen dopants and native point
defects in graphene, we have studied the energetic stability of N-doped
graphene with vacancies and Stone-Wales (SW) defect by performing the density
functional theory calculations. Our results show that N substitution
energetically prefers to occur at the carbon atoms near the defects, especially
for those sites with larger bond shortening, indicating that the defect-induced
strain plays an important role in the stability of N dopants in defective
graphene. In the presence of monovacancy, the most stable position for N dopant
is the pyridinelike configuration, while for other point defects studied (SW
defect and divacancies) N prefers a site in the pentagonal ring. The effect of
native point defects on N dopants is quite strong: While the N doping is
endothermic in defect-free graphene, it becomes exothermic for defective
graphene. Our results imply that the native point defect and N dopant attract
each other, i.e., cooperative effect, which means that substitutional N dopants
would increase the probability of point defect generation and vice versa. Our
findings are supported by recent experimental studies on the N doping of
graphene. Furthermore we point out possibilities of aggregation of multiple N
dopants near native point defects. Finally we make brief comments on the effect
of Fe adsorption on the stability of N dopant aggregation.Comment: 10 pages, 5 figures. Figure 4(g) and Figure 5 are corrected. One
additional table is added. This is the final version for publicatio
The Radial Structure of the Cygnus Loop Supernova Remnant --- Possible evidence of a cavity explosion ---
We observed the North-East (NE) Limb toward the center region of the Cygnus
Loop with the ASCA Observatory. We found a radial variation of electron
temperature (kTe) and ionization timescale (log(\tau)) whereas no variation
could be found for the abundances of heavy elements. In this paper, we
re-analyzed the same data set and new observations with the latest calibration
files. Then we constructed the precise spatial variations of kTe, log(\tau),
and abundances of O, Ne, Mg, Si, and Fe over the field of view (FOV). We found
a spatial variation not only in kTe and in log(\tau) but also in most of heavy
elements. As described in Miyata et al. (1994), values of kTe increase and
those of log(\tau) decrease toward the inner region. We found that the
abundance of heavy elements increases toward the inner region. The radial
profiles of O, Ne, and Fe show clear jump structures at a radius of 0.9 Rs,
where Rs is the shock radius. Outside of 0.9 Rs, abundances of all elements are
constant. On the contrary, inside of 0.9 Rs, abundances of these elements are
20--30 % larger than those obtained outside of 0.9 Rs. The radial profile of
kTe also shows the jump structure at 0.9 Rs. This means that the hot and metal
rich plasma fills the volume inside of 0.9 Rs. We concluded that this jump
structure was the possible evidence for the pre-existing cavity produced by the
precursor. If the ejecta fills inside of 0.9 Rs, the total mass of the ejecta
was roughly 4\Msun. We then estimated the main-sequence mass to be roughly
15\Msun, which supports the massive star in origin of the Cygnus Loop supernova
remnant and the existence of a pre-existing cavity.Comment: 37 pages, 14 figures. Accepted for publication of Ap
Quantum phase transitions in alternating spin-(1/2, 5/2) Heisenberg chains
The ground state spin-wave excitations and thermodynamic properties of two
types of ferrimagnetic chains are investigated: the alternating spin-1/2
spin-5/2 chain and a similar chain with a spin-1/2 pendant attached to the
spin-5/2 site. Results for magnetic susceptibility, magnetization and specific
heat are obtained through the finite-temperature Lanczos method with the aim in
describing available experimental data, as well as comparison with theoretical
results from the semiclassical approximation and the low-temperature
susceptibility expansion derived from Takahashi's modified spin-wave theory. In
particular, we study in detail the temperature vs. magnetic field phase diagram
of the spin-1/2 spin-5/2 chain, in which several low-temperature quantum phases
are identified: the Luttinger Liquid phase, the ferrimagnetic plateau and the
fully polarized one, and the respective quantum critical points and crossover
lines
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