37,604 research outputs found
Nature of W51e2: Massive Cores at Different Phases of Star Formation
We present high-resolution continuum images of the W51e2 complex processed
from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3 mm and the
Very Large Array (VLA) at 7 and 13 mm. We also made line images and profiles of
W51e2 for three hydrogen radio recombination lines (H26\alpha, H53\alpha, and
H66\alpha) and absorption of two molecular lines of HCN(4-3) and CO(2-1). At
least four distinct continuum components have been detected in the 3" region of
W51e2 from the SMA continuum images at 0.85 and 1.3 mm with resolutions of
0.3"x0.2" and 1.4"x0.7", respectively. The west component, W51e2-W, coincides
with the UC HII region reported from previous radio observations. The H26\alpha
line observation reveals an unresolved hyper-compact ionized core (<0.06" or
<310 AU) with a high electron temperature of 1.2x10^4 K, with corresponding
emission measure EM>7x10^{10} pc cm^{-6} and electron density N_e>7x10^6
cm^{-3}. The inferred Lyman continuum flux implies that the HII region W51e2-W
requires a newly formed massive star, an O8 star or a cluster of B-type stars,
to maintain the ionization. The east component, W51e2-E, has a total mass of
~140 M_{\sun} according to our SED analysis and a large infall rate of >
1.3x10^{-3} M_{\sun}yr^{-1} inferred from the absorption of HCN. W51e2-E
appears to be the accretion center in W51e2 and to host one or more growing
massive proto-stars. Located 2" northwest from W51e2-E, W51e2-NW is not
detected in the continuum emission at \lambda>=7 mm. Along with the maser
activities previously observed, our analysis suggests that W51e2-NW is at an
earlier phase of star formation. W51e2-N is located 2" north of W51e2-E and has
only been detected at 1.3 mm with a lower angular resolution (~1"), suggesting
that it is a primordial, massive gas clump in the W51e2 complex.Comment: 10 pages, 5 figures, 3 table, accepted for publication in Ap
Pi pi scattering lengths at O(p^6) revisited
This article completes a former work where part of the O(p^6) low-energy
constants entering in the pi pi scattering were estimated. Some resonance
contributions were missed in former calculations and slight differences
appeared with respect to our outcome. Here, we provide the full results for all
the contributing O(p^6) couplings. We also perform a reanalysis of the hadronic
inputs used for the estimation (resonance masses, widths...). Their reliability
was checked together with the impact of the input uncertainties on the
determinations of the chiral couplings and the scattering lengths a^I_J. Our
outcome is found in agreement with former works though with slightly larger
errors. However, the effect in the final values of the a^I_J is negligible
after combining them with the other uncertainties. Based on this consistency,
we conclude that the previous scattering length determinations seem to be
rather solid and reliable, with the cO(p^6) low-energy constants quite under
control. Nevertheless, the uncertainties found in the present work point out
the limitation on further improvements unless the precision of the O(p^6)
couplings is properly increased.Comment: 19 pages. Improved treatment of the a0 decay width and update of the
numerical outcomes. Final version published in Phys. Rev. D
(10.1103/PhysRevD.79.096006
Second-harmonic generation in graded metallic films
We study the effective second-harmonic generation (SHG) susceptibility in
graded metallic films by invoking the local field effects exactly, and further
numerically demonstrate that the graded metallic films can serve as a novel
optical material for producing a broad structure in both the linear and SHG
response and an enhancement in the SHG signal.Comment: 10 pages, 2 EPS figures. Minor revision
Coding theorems for turbo code ensembles
This paper is devoted to a Shannon-theoretic study of turbo codes. We prove that ensembles of parallel and serial turbo codes are "good" in the following sense. For a turbo code ensemble defined by a fixed set of component codes (subject only to mild necessary restrictions), there exists a positive number γ0 such that for any binary-input memoryless channel whose Bhattacharyya noise parameter is less than γ0, the average maximum-likelihood (ML) decoder block error probability approaches zero, at least as fast as n -β, where β is the "interleaver gain" exponent defined by Benedetto et al. in 1996
Coincidence Problem in Cyclic Phantom Models of the Universe
We examine cyclic phantom models for the universe, in which the universe is
dominated sequentially by radiation, matter, and a phantom dark energy field,
followed by a standard inflationary phase. Since this cycle repeats endlessly,
the Universe spends a substantial portion of its lifetime in a state for which
the matter and dark energy densities have comparable magnitudes, thus
ameliorating the coincidence problem. We calculate the fraction of time that
the universe spends in such a coincidental state and find that it is nearly the
same as in the case of a phantom model with a future big rip. In the limit
where the dark energy equation of state parameter, w, is close to -1, we show
that the fraction of time, f, for which the ratio of the dark energy density to
the matter density lies between r_1 and r_2, is f = -(1+w) ln [(\sqrt{r_2} +
\sqrt{1+r_2})/(\sqrt{r_1} + \sqrt{1+r_1})].Comment: 4 pages, no figures, discussion and references adde
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