928 research outputs found
Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph
Context: ASPIICS is a novel externally occulted solar coronagraph, which will
be launched onboard the PROBA-3 mission of the European Space Agency. The
external occulter will be placed on the first satellite approximately 150 m
ahead of the second satellite that will carry an optical instrument. During 6
hours per orbit, the satellites will fly in a precise formation, constituting a
giant externally occulted coronagraph. Large distance between the external
occulter and the primary objective will allow observations of the white-light
solar corona starting from extremely low heights 1.1RSun. Aims: To analyze
influence of optical ghost images formed inside the telescope and develop an
algorithm for their removal. Methods: We implement the optical layout of
ASPIICS in Zemax and study the ghost behaviour in sequential and non-sequential
regimes. We identify sources of the ghost contributions and analyze their
geometrical behaviour. Finally we develop a mathematical model and software to
calculate ghost images for any given input image. Results: We show that ghost
light can be important in the outer part of the field of view, where the
coronal signal is weak, since the energy of bright inner corona is
redistributed to the outer corona. However the model allows to remove the ghost
contribution. Due to a large distance between the external occulter and the
primary objective, the primary objective does not produce a significant ghost.
The use of the Lyot spot in ASPIICS is not necessary.Comment: 14 pages, 13 figure
Low-Altitude Reconnection Inflow-Outflow Observations during a 2010 November 3 Solar Eruption
For a solar flare occurring on 2010 November 3, we present observations using
several SDO/AIA extreme-ultraviolet (EUV) passbands of an erupting flux rope
followed by inflows sweeping into a current sheet region. The inflows are soon
followed by outflows appearing to originate from near the termination point of
the inflowing motion - an observation in line with standard magnetic
reconnection models. We measure average inflow plane-of-sky speeds to range
from ~150-690 km/s with the initial, high-temperature inflows being the
fastest. Using the inflow speeds and a range of Alfven speeds, we estimate the
Alfvenic Mach number which appears to decrease with time. We also provide
inflow and outflow times with respect to RHESSI count rates and find that the
fast, high-temperature inflows occur simultaneously with a peak in the RHESSI
thermal lightcurve. Five candidate inflow-outflow pairs are identified with no
more than a minute delay between detections. The inflow speeds of these pairs
are measured to be 10^2 km/s with outflow speeds ranging from 10^2-10^3 km/s -
indicating acceleration during the reconnection process. The fastest of these
outflows are in the form of apparently traveling density enhancements along the
legs of the loops rather than the loop apexes themselves. These flows could
either be accelerated plasma, shocks, or waves prompted by reconnection. The
measurements presented here show an order of magnitude difference between the
retraction speeds of the loops and the speed of the density enhancements within
the loops - presumably exiting the reconnection site.Comment: 31 pages, 13 figures, 1 table, Accepted to ApJ (expected publication
~July 2012
Physical Conditions in the Inner Narrow-Line Region of the Seyfert 2 Galaxy NGC 1068
The physical conditions in the inner narrow line region (NLR) of the Seyfert
2 galaxy, NGC 1068, are examined using ultraviolet and optical spectra and
photoionization models. The spectra are Hubble Space Telescope (HST) Harchive
data obtained with the Faint Object Spectrograph (FOS). We selected spectra of
four regions, taken through the 0.3" FOS aperture, covering the full FOS 1200A
to 6800A waveband. Each region is approximately 20 pc in extent, and all are
within 100 pc of the apparent nucleus of NGC 1068. The spectra show similar
emission-line ratios from wide range of ionization states for the most abundant
elements. After extensive photoionization modeling, we interpret this result as
an indication that each region includes a range of gas densities, which we
included in the models as separate components. Supersolar abundances were
required for several elements to fit the observed emission line ratios. Dust
was included in the models but apparently dust to gas fraction varies within
these regions. The low ionization lines in these spectra can be best explained
as arising in gas that is partially shielded from the ionizing continuum.
Although the predicted line ratios from the photoionization models provide a
good fit to the observed ratios, it is apparent that the model predictions of
electron temperatures in the ionized gas are too low. We interpret this as an
indication of additional collisional heating due to shocks and/or energetic
particles associated with the radio jet that traverses the NLR of NGC 1068. The
density structure within each region may also be the result of compression by
the jet.Comment: 38 pages, Latex, includes 5 figures (postscript), to appear in Ap
Magnetic Correlation Length and Universal Amplitude of the Lattice E_8 Ising Model
The perturbation approach is used to derive the exact correlation length
of the dilute A_L lattice models in regimes 1 and 2 for L odd. In regime
2 the A_3 model is the E_8 lattice realisation of the two-dimensional Ising
model in a magnetic field h at T=T_c. When combined with the singular part f_s
of the free energy the result for the A_3 model gives the universal amplitude
as in precise agreement with the result
obtained by Delfino and Mussardo via the form-factor bootstrap approach.Comment: 7 pages, Late
A New Look At Carbon Abundances In Planetary Nebulae. III. DDDM1, IC 3568, IC4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009
This paper is the third in a series reporting on a study of carbon abundances
in a carefully chosen sample of planetary nebulae representing a large range in
progenitor mass and metallicity. We make use of the IUE Final Archive database
containing consistently-reduced spectra to measure line strengths of C III]
1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC
3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, & NGC 7009. We combine the IUE
data with line strengths from optical spectra obtained specifically to match
the IUE slit positions as closely as possible, to determine values for the
abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] 1909/C II
4267 is found to be effective for merging UV and optical spectra when He II
1640/4686 is unavailable. Our abundance determination method includes a 5-level
program whose results are fine-tuned by corrections derived from detailed
photoionization models constrained by the same set of emission lines. All
objects appear to have subsolar levels of O/H, and all but one show N/O levels
above solar. In addition, the seven planetary nebulae span a broad range in C/O
values. We infer that many of our objects are matter bounded, and thus the
standard ionization correction factor for N/O may be inappropriate for these
PNe. Finally, we estimate C/O using both collisionally-excited and
recombination lines associated with C+2 and find the well established result
that abundances from recombination lines usually exceed those from
collisionally-excited lines by several times.Comment: 36 pages, 7 tables, 2 figures, latex. Tables and figures supplied as
two separate postscript files. Accepted for publication in Ap
Theoretical He I Emissivities in the Case B Approximation
We calculate the He I case B recombination cascade spectrum using improved
radiative and collisional data. We present new emissivities over a range of
electron temperatures and densities. The differences between our results and
the current standard are large enough to have a significant effect not only on
the interpretation of observed spectra of a wide variety of objects but also on
determinations of the primordial helium abundance.Comment: Accepted to ApJ
Evidence for a Physically Compact Narrow-Line Region in the Seyfert 1 Galaxy NGC 5548
We have combined HST/FOS and ground-based spectra of the Seyfert 1 galaxy NGC
5548 to study the narrow emission lines over the 1200 -- 10,000 angstrom
region. All of the spectra were obtained when the broad emission line and
continuum fluxes were at an historic low level, allowing us to accurately
determine the contribution of the narrow-line region (NLR) to the emission
lines. We have generated multicomponent photoionization models to investigate
the relative strength of the high ionization lines compared to those in Seyfert
2 galaxies, and the weakness of the narrow Mg II 2800 line. We present evidence
for a high ionization component of NLR gas that is very close to the nucleus
(~1 pc). This component must be optically thin to ionizing radiation at the
Lyman edge (tau = 2.5) to avoid producing [O I] and Mg II in a partially
ionized zone. The very high ionization lines (N V, [Ne V], [Fe VII], [Fe X])
are stronger than the predictions of our standard model, and we show that this
may be due to supersolar abundances and/or a ``blue bump'' in the extreme
ultraviolet (although recent observations do not support the latter). An outer
component of NLR gas (at only ~70 pc from the continuum source) is needed to
produce the low ionization lines. We show that the outer component may contain
dust, which further reduces the Mg II flux by depletion and by absorption of
the resonance photons after multiple scatterings. We show that the majority of
the emission in the NLR of NGC 5548 must arise within about ~70 pc from the
nucleus. Thus, the NLR in this Seyfert 1 galaxy is very physically compact,
compared to the typical NLR in Seyfert 2 galaxies.Comment: 38 pages, Latex, includes 2 figures (postscript), to appear in Ap
Molecular simulation of silica/surfactant self-assembly in the synthesis of periodic mesoporous silicas
Understanding the synthesis of periodic mesoporous silica (PMS) is crucial for a more efficient use of these materials and is a necessary first step toward a rational design strategy for the templated synthesis of porous solids. In this paper, the early stages of the synthesis process of PMS materials are simulated directly by molecular dynamics, using realistic atomistic models. It is the first time that such computationally demanding calculations have been attempted. By comparing the self-assembly of cationic surfactants in the presence and absence of silicates, we are able to show that silica promotes the formation of larger aggregates than in a simple surfactant/water solution. The formation of these larger micelles is explained by a strong interaction of the silicate molecules with the surfactant head groups. This strong interaction increases the local concentration of silica at the surface of the micelles, which induces the formation of more condensed silicate species. The surfactant/silica structures observed here are potentially important intermediates in PMS synthesis
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