6,639 research outputs found
Influence of positional correlations on the propagation of waves in a complex medium with polydisperse resonant scatterers
We present experimental results on a model system for studying wave
propagation in a complex medium exhibiting low frequency resonances. These
experiments enable us to investigate a fundamental question that is relevant
for many materials, such as metamaterials, where low-frequency scattering
resonances strongly influence the effective medium properties. This question
concerns the effect of correlations in the positions of the scatterers on the
coupling between their resonances, and hence on wave transport through the
medium. To examine this question experimentally, we measure the effective
medium wave number of acoustic waves in a sample made of bubbles embedded in an
elastic matrix over a frequency range that includes the resonance frequency of
the bubbles. The effective medium is highly dispersive, showing peaks in the
attenuation and the phase velocity as functions of the frequency, which cannot
be accurately described using the Independent Scattering Approximation (ISA).
This discrepancy may be explained by the effects of the positional correlations
of the scatterers, which we show to be dependent on the size of the scatterers.
We propose a self-consistent approach for taking this "polydisperse
correlation" into account and show that our model better describes the
experimental results than the ISA
A Unified Picture of the FIP and Inverse FIP Effects
We discuss models for coronal abundance anomalies observed in the coronae of
the sun and other late-type stars following a scenario first introduced by
Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with
the partially neutral gas. Instead of considering wave heating of ions in this
location, we explore the effects on the upper chromospheric plasma of the wave
ponderomotive forces. These can arise as upward propagating waves from the
chromosphere transmit or reflect upon reaching the chromosphere-corona
boundary, and are in large part determined by the properties of the coronal
loop above. Our scenario has the advantage that for realistic wave energy
densities, both positive and negative changes in the abundance of ionized
species compared to neutrals can result, allowing both FIP and Inverse FIP
effects to come out of the model. We discuss how variations in model parameters
can account for essentially all of the abundance anomalies observed in solar
spectra. Expected variations with stellar spectral type are also qualitatively
consistent with observations of the FIP effect in stellar coronae.Comment: 25 pages, 4 figures, submitted to Ap
Observational Constraints on the Molecular Gas Content in Nearby Starburst Dwarf Galaxies
Using star formation histories derived from optically resolved stellar
populations in nineteen nearby starburst dwarf galaxies observed with the
Hubble Space Telescope, we measure the stellar mass surface densities of stars
newly formed in the bursts. By assuming a star formation efficiency (SFE), we
then calculate the inferred gas surface densities present at the onset of the
starbursts. Assuming a SFE of 1%, as is often assumed in normal star-forming
galaxies, and assuming that the gas was purely atomic, translates to very high
HI surface densities (~10^2-10^3 Msun pc^-2), which are much higher than have
been observed in dwarf galaxies. This implies either higher values of SFE in
these dwarf starburst galaxies or the presence of significant amounts of H_2 in
dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with
observations of more massive starbursts associated with merging galaxies),
still results in HI surface densities higher than observed in 10 galaxies.
Thus, these observations appear to require that a significant fraction of the
gas in these dwarf starbursts galaxies was in the molecular form at the onset
of the bursts. Our results imply molecular gas column densities in the range
10^19-10^21 cm^-2 for the sample. In those galaxies where CO observations have
been made, these densities correspond to values of the CO-H_2 conversion factor
(X_CO) in the range >3-80x10^20 cm^-2 (K km s^-1)^-1, or up to 40x greater than
Galactic X_CO values.Comment: 8 pages, 4 figures, 2 table
Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies
(Abridged) We combine data from The HI Nearby Galaxy Survey and the GALEX
Nearby Galaxy Survey to study the relationship between atomic hydrogen (HI) and
far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral
and 5 dwarf galaxies. In this regime, HI is likely to represent most of the ISM
and FUV emission to trace recent star formation with little bias due to
extinction, so that the two quantities closely trace the underlying
relationship between gas and star formation rate (SFR). The azimuthally
averaged HI and FUV intensities both decline with increasing radius in this
regime, with the scale length of the FUV profile typically half that of the HI
profile. Despite the mismatch in profiles, there is a significant spatial
correlation (at 15" resolution) between local FUV and HI intensities; near r25
this correlation is quite strong, in fact stronger than anywhere inside r25,
and shows a decline towards larger radii. The star formation efficiency (SFE) -
defined as the ratio of FUV/HI and thus the inverse of the gas depletion time -
decreases with galactocentric radius across the outer disks, though much
shallower than across the optical disks. On average, we find the gas depletion
times to be well above a Hubble time (~10^11 yr). We observe a clear
relationship between FUV/HI and HI column in the outer disks, with the SFE
increasing with increasing HI column. Despite observing systematic variations
in FUV/HI, we find no clear evidence for step-function type star formation
thresholds. When compared with results from inside r25, we find outer disk star
formation to be distinct in several ways: it is extremely inefficient
(depletion times of many Hubble times) with column densities and SFRs lower
than found anywhere inside the optical disks. It appears that the HI column is
one of, perhaps even the key environmental factor in setting the SFR in outer
galaxy disks.Comment: Accepted for Publication in The Astronomical Journa
The effective stability parameter for two-component galactic discs: Is 1/Q ~ 1/Q_stars + 1/Q_gas ?
The Wang-Silk approximation, 1/Q ~ 1/Q_stars + 1/Q_gas, is frequently used
for estimating the effective Q parameter in two-component discs of stars and
gas. Here we analyse this approximation in detail, and show how its accuracy
depends on the radial velocity dispersions and Toomre parameters of the two
components. We then propose a much more accurate but still simple approximation
for the effective Q parameter, which further takes into account the stabilizing
effect of disc thickness. Our effective Q parameter is a natural generalization
of Toomre's Q, and as such can be used in a wide variety of contexts, e.g. for
predicting star formation thresholds in galaxies or for measuring the stability
level of galactic discs at low and high redshifts.Comment: MNRAS, in pres
The Hydromagnetic Interior of a Solar Quiescent Prominence. I. Coupling between Force-balance and Steady Energy-transport
This series of papers investigates the dynamic interior of a quiescent
prominence revealed by recent {\it Hinode} and {\it SDO/AIA} high-resolution
observations. This first paper is a study of the static equilibrium of the
Kippenhahn-Schl\"{u}ter diffuse plasma slab, suspended vertically in a bowed
magnetic field, under the frozen-in condition and subject to a theoretical
thermal balance among an optically-thin radiation, heating, and field-aligned
thermal conduction. The everywhere-analytical solutions to this nonlinear
problem are an extremely restricted subset of the physically admissible states
of the system. For most values of the total mass frozen into a given bowed
field, force-balance and steady energy-transport cannot both be met without a
finite fraction of the total mass having collapsed into a cold sheet of zero
thickness, within which the frozen-in condition must break down. An exact,
resistive hydromagnetic extension of the Kippenhahn-Schl\"{u}ter slab is also
presented, resolving the mass-sheet singularity into a finite-thickness layer
of steadily-falling dense fluid. Our hydromagnetic result suggests that the
narrow, vertical prominence threads may be falling across magnetic
fields, with optically-thick cores much denser and ionized to much lower
degrees than conventionally considered. This implication is discussed in
relation to (i) the recent {\it SDO/AIA} observations of quiescent prominences
that are massive and yet draining mass everywhere in their interiors, (ii) the
canonical range of determined from spectral-polarimetric observations
of prominence magnetic fields over the years and (iii) the need for a more
realistic multi-fluid treatment.Comment: 45 pages, 14 figure
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