56,757 research outputs found
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
Uncertainties in Theoretical HeI Emissivities: HII Regions, Primordial Abundance, and Cosmological Recombination
A number of recent works in astronomy and cosmology have relied upon
theoretical He I emissivities, but we know of no effort to quantify the
uncertainties in the atomic data. We analyze and assign uncertainties to all
relevant atomic data, perform Monte Carlo analyses, and report standard
deviations in the line emissivities. We consider two sets of errors, which we
call "optimistic" and "pessimistic." We also consider three different
conditions, corresponding to prototypical Galactic and extragalactic H II
regions and the epoch of cosmological recombination. In the extragalactic H II
case, the errors we obtain are comparable to or larger than the errors in some
recent calculations, including those derived from CMB observations. We
demonstrate a systematic effect on primordial abundance calculations; this
effect cannot be reduced by observing a large number of objects. In the
cosmological recombination case, the errors are comparable to many of the
effects considered in recent calculations.Comment: 5 pages, 3 figures, accepted to MNRAS Letter
Reliable scaling exponent estimation of long-range correlated noise in the presence of random spikes
Detrended fluctuation analysis (DFA) has been used widely to determine
possible long-range correlations in data obtained from diverse settings. In a
recent study [1], uncorrelated random spikes superimposed on the long-range
correlated noise (LR noise) were found to affect DFA scaling exponent
estimates. In this brief communication, singular-value decomposition (SVD)
filter is proposed to minimize the effect random spikes superimposed on LR
noise, thus facilitating reliable estimation of the scaling exponents. The
effectiveness of the proposed approach is demonstrated on random spikes sampled
from normal and uniform distributions.Comment: 36 Pages, 20 Figure
Pumping up the [N I] nebular lines
The optical [N I] doublet near 5200 {\AA} is anomalously strong in a variety
of emission-line objects. We compute a detailed photoionization model and use
it to show that pumping by far-ultraviolet (FUV) stellar radiation previously
posited as a general explanation applies to the Orion Nebula (M42) and its
companion M43; but, it is unlikely to explain planetary nebulae and supernova
remnants. Our models establish that the observed nearly constant equivalent
width of [N I] with respect to the dust-scattered stellar continuum depends
primarily on three factors: the FUV to visual-band flux ratio of the stellar
population; the optical properties of the dust; and the line broadening where
the pumping occurs. In contrast, the intensity ratio [N I]/H{\beta} depends
primarily on the FUV to extreme-ultraviolet ratio, which varies strongly with
the spectral type of the exciting star. This is consistent with the observed
difference of a factor of five between M42 and M43, which are excited by an O7
and B0.5 star respectively. We derive a non-thermal broadening of order 5 km/s
for the [N I] pumping zone and show that the broadening mechanism must be
different from the large-scale turbulent motions that have been suggested to
explain the line-widths in this H II region. A mechanism is required that
operates at scales of a few astronomical units, which may be driven by thermal
instabilities of neutral gas in the range 1000 to 3000 K. In an appendix, we
describe how collisional and radiative processes are treated in the detailed
model N I atom now included in the Cloudy plasma code.Comment: ApJ in press. 8 pages of main paper plus 11 pages of appendices, with
13 figures and 12 table
All tree level scattering amplitudes in Chern-Simons theories with fundamental matter
We show that Britto-Cachazo-Feng-Witten (BCFW) recursion relations can be
used to compute all tree level scattering amplitudes in terms of
scattering amplitude in Chern-Simons
(CS) theory coupled to matter in fundamental representation. As a byproduct, we
also obtain a recursion relation for the CS theory coupled to regular fermions,
even though in this case standard BCFW deformations do not have a good
asymptotic behaviour. Moreover at large , scattering can be
computed exactly to all orders in 't Hooft coupling as was done in earlier
works by some of the authors. In particular, for theory, it
was shown that scattering is tree level exact to all orders
except in the anyonic channel arXiv:1505.06571, where it gets renormalized by a
simple function of 't Hooft coupling. This suggests that it may be possible to
compute the all loop exact result for arbitrary higher point scattering
amplitudes at large .Comment: RevTEX 4.1, 5 pages+6 Appendices, 7 figures; V2 Published versio
Dust Emission from Herbig Ae/Be stars - Evidence for Disks and Envelopes
IR and mm-wave emission from Herbig Ae/Be stars has produced conflicting
conclusions regarding the dust geometry in these objects. We show that the
compact dimensions of the mm-wave emitting regions are a decisive indication
for disks. But a disk cannot explain the spectral energy distribution (SED)
unless it is embedded in an extended envelope that (1) dominates the IR
emission and (2) provides additional disk heating on top of the direct stellar
radiation. Detailed radiative transfer calculations based on the simplest model
for envelope-embedded disks successfully fit the data from UV to mm wavelengths
and show that the disks have central holes. This model also resolves naturally
some puzzling results of IR imaging.Comment: 9 pages, 2 figures. accepted to ApJ
Herschel dust emission as a probe of starless cores mass: MCLD 123.5+24.9 of the Polaris Flare
We present newly processed archival Herschel images of molecular cloud MCLD
123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using
the spectral synthesis code Cloudy, we explore uncertainties in the derivation
of column densities, hence, masses of molecular cores from Herschel data. We
first consider several detailed grain models that predict far-IR grain
opacities. Opacities predicted by the models differ by more than a factor of
two, leading to uncertainties in derived column densities by the same factor.
Then we consider uncertainties associated with the modified blackbody fitting
process used by observers to estimate column densities. For high column density
clouds (N(H) 10 cm), this fitting technique can
underestimate column densities by about a factor of three. Finally, we consider
the virial stability of the five starless cores in MCLD 123.5+24.9. All of
these cores appear to have strongly sub-virial masses, assuming, as we argue,
that CO line data provide reliable estimates of velocity dispersions.
Evidently, they are not self-gravitating, so it is no surprise that they are
starless.Comment: ApJ, Accepted. Minor typographical errors corrected and figures 6 & 7
updated in v
Structure symmetry determination and magnetic evolution in
We use single-crystal neutron diffraction to determine the crystal structure
symmetry and the magnetic evolution in the rhodium doped iridates (). Throughout this doping range, the
crystal structure retains a tetragonal symmetry (space group ) with two
distinct magnetic Ir sites in the unit cell forming staggered
rotation. Upon Rh doping, the magnetic order is suppressed and the magnetic
moment of Ir is reduced from 0.21 /Ir for to 0.18 /Ir for . The magnetic structure at is different from
that of the parent compound while the moments remain in the basal plane.Comment: Accepted for publication in Phys. Rev.
Coexisting charge and magnetic orders in the dimer-chain iridate Ba5AlIr2O11
We have synthesized and studied single-crystal Ba5AlIr2O11 that features
dimer chains of two inequivalent octahedra occupied by tetravalent and
pentavalent ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes
a subtle structural phase transition near 210 K and a magnetic transition at
4.5 K; the latter transition is surprisingly resistant to applied magnetic
fields up to 12 T, but sensitive to modest applied pressure. All results
indicate that the phase transition at 210 K signals an enhanced charge order
that induces electrical dipoles and strong dielectric response near 210 K. It
is clear that the strong covalency and spin-orbit interaction (SOI) suppress
double exchange in Ir dimers and stabilize a novel magnetic state. The behavior
of Ba5AlIr2O11 therefore provides unique insights into the physics of SOI along
with strong covalency in competition with double exchange interactions of
comparable strength.Comment: 6 figures, 20 pages. arXiv admin note: text overlap with
arXiv:1505.0087
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