990 research outputs found
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
OPserver: interactive online-computations of opacities and radiative accelerations
Codes to compute mean opacities and radiative accelerations for arbitrary
chemical mixtures using the Opacity Project recently revised data have been
restructured in a client--server architecture and transcribed as a subroutine
library. This implementation increases efficiency in stellar modelling where
element stratification due to diffusion processes is depth dependent, and thus
requires repeated fast opacity reestimates. Three user modes are provided to
fit different computing environments, namely a web browser, a local workstation
and a distributed grid.Comment: 5 pages, 1 figur
Ising tricriticality and the dilute A model
Some universal amplitude ratios appropriate to the peturbation
of the c=7/10 minimal field theory, the subleading magnetic perturbation of the
tricritical Ising model, are explicitly demonstrated in the dilute A model,
in regime 1.Comment: 8 pages, LaTeX using iop macro
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
Excitation of the 3.071mm Hyperfine Line in Li-Like 57-Fe in Astrophysical Plasmas
As noted first by Sunyaev & Churazov (1984), the 3.071 mm hyperfine line from
might be observable in astrophysical plasmas. We assess the
atomic processes which might contribute to the excitation of this line. We
determine the intensity of the hyperfine line from an isothermal, coronal
plasma in collisional ionization equilibrium and for a coronal plasma cooling
isobarically due to its own radiation. Comparisons of the hyperfine line to
other lines emitted by the same ion, Fe, are shown to be useful for
deriving the isotopic fraction of Fe. We calculate the ratios of the
hyperfine line to the 2s--2p EUV lines at 192 \AA and 255 \AA, and the 2s--3p
X-ray doublet at 10.6 \AA.Comment: 28 pages text+figures, Accepted to ApJ in Jan 98, also at
http://www.astro.virginia.edu/~nld2n/research.htm
An HST Search for Lyman Continuum Emission From Galaxies at z=1.1--1.4
If enough of their Lyman limit continuum escapes, star-forming galaxies could
be significant contributors to the cosmic background of ionizing photons. To
investigate this possibility, we obtained the first deep imaging in the far
ultraviolet of eleven bright blue galaxies at intermediate redshift
(z=1.1--1.4). NO Lyman continuum emission was detected. Sensitive,
model-independent, upper limits of typically 2 x 10**-19 erg/sec/cm2/Ang were
obtained for the ionizing flux escaping from these normal galaxies. This
corresponds to lower limits on the observed ratio of 1500 to 700Ang flux of 150
up to 1000. Based on a wide range of stellar synthesis models, this suggests
that less than 6%, down to less than 1%, of the available ionizing flux emitted
by hot stars is escaping these galaxies. The magnitude of this spectral break
at the Lyman l imit confirms that the basic premise of `Lyman break' searches
for galaxies at high redshift can also be applied at intermediate redshifts.
This implies that the integrated contribution of galaxies to the UV cosmic
background at z around 1.2 is less than 15%, and may be less than 2%.Comment: 20 manuscript pages, which includes two tables and two figures. To be
published in 1 December 2003 issue of The Astrophysical Journa
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
Direct observations of a complex coronal web driving highly structured slow solar wind
The solar wind consists of continuous streams of charged particles that
escape into the heliosphere from the Sun, and is split into fast and slow
components, with the fast wind emerging from the interiors of coronal holes.
Near the ecliptic plane, the fast wind from low-latitude coronal holes is
interspersed with a highly structured slow solar wind, the source regions and
drivers of which are poorly understood. Here we report extreme-ultraviolet
observations that reveal a spatially complex web of magnetized plasma
structures that persistently interact and reconnect in the middle corona.
Coronagraphic white-light images show concurrent emergence of slow wind streams
over these coronal web structures. With advanced global MHD coronal models, we
demonstrate that the observed coronal web is a direct imprint of the magnetic
separatrix web (S-web). By revealing a highly dynamic portion of the S-web, our
observations open a window into important middle-coronal processes that appear
to play a key role in driving the structured slow solar wind.Comment: This version of the article has been accepted for publication, after
peer review but is not the Version of Record and does not reflect
post-acceptance improvements, or any corrections. The Version of Record is
available online at: http://dx.doi.org/10.1038/s41550-022-01834-
New HST Observations of the Halo Gas of NGC 3067: Limits on the Extragalactic Ionizing Background at Low Redshift and the Lyman Continuum Escape Fraction
We present UV spectroscopy from HST/GHRS and reanalyze existing H_alpha
images of the quasar/galaxy pair 3C 232/NGC 3067 and of the halo gas associated
with NGC 3067. The spectra permit measurement of, or limits on, the column
densities of Fe I, Fe II, Mg I, and Mg II in the absorbing cloud. Two distinct
models of the extragalactic radiation field are considered: (1) the ionizing
spectrum is dominated by a power-law extragalactic continuum, and (2) the
power-law spectrum contains a Lyman break, implying enhanced flux longward of
912 A relative to the hydrogen-ionizing flux. The H_alpha images constrain the
escape fraction of Lyman continuum photons from the galaxy to f_esc <= 0.02.
With the assumption that the cloud is shielded from all galactic contributions,
we can constrain the intensity and shape of the extragalactic continuum. For an
AGN-dominated power-law extragalactic spectrum, we derive a limit on the
extragalactic ionizing flux Phi_ion >= 2600 photons cm^-2 s^-1, or I_0 >=
10^-23 erg cm^-2 s^-1 Hz^-1 sr^-1 for an ionizing spectrum with power-law index
of 1.8 and a cloud of constant density. When combined with previous upper
limits from the absence of H_alpha recombination emission from intergalactic
clouds, our observations require 2600 <= Phi_ion <= 10000 photons cm^-2 s^-1.
We show that if galactic contributions to the incident radiation are important,
it is difficult to constrain Phi_ion. These results demonstrate that galactic
halo opacities and their wavelength dependence are crucial to understanding the
abundance of low-ionization metals in the IGM.Comment: 25 Pages LaTex, 8 PostScript Figures, accepted for publication in AJ,
Nov. 99 issu
Near-IR spectroscopy of PKS1549-79: a proto-quasar revealed?
We present a near-IR spectrum of the nearby radio galaxy PKS1549-79 (z=0153).
These data were taken with the aim of testing the idea that this object
contains a quasar nucleus that is moderately extinguished, despite evidence
that its radio jet points close to our line-of-sight. We detect broad Paschen
Alpha emission (FWHM ~1745 km/s), relatively bright continuum emission, and a
continuum slope consistent with a reddened quasar spectrum (3.1 < Av < 7.3),
all emitted by an unresolved point source. Therefore we conclude that we have,
indeed, detected a hidden quasar nucleus in PKS1549-79. Combined with previous
results, these observations are consistent with the idea that PKS1549-79 is a
young radio source in which the cocoon of debric left over from the triggering
events has not yet been swept aside by circumnuclear outflows.Comment: 6 pages, 4 figures, accepted for publication in MNRA
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