4,967 research outputs found
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
Merged ionization/dissociation fronts in planetary nebulae
The hydrogen ionization and dissociation front around an ultraviolet
radiation source should merge when the ratio of ionizing photon flux to gas
density is sufficiently low and the spectrum is sufficiently hard. This regime
is particularly relevant to the molecular knots that are commonly found in
evolved planetary nebulae, such as the Helix Nebula, where traditional models
of photodissociation regions have proved unable to explain the high observed
luminosity in H_2 lines. In this paper we present results for the structure and
steady-state dynamics of such advection-dominated merged fronts, calculated
using the Cloudy plasma/molecular physics code. We find that the principal
destruction processes for H_2 are photoionization by extreme ultraviolet
radiation and charge exchange reactions with protons, both of which form H_2^+,
which rapidly combines with free electrons to undergo dissociative
recombination. Advection moves the dissociation front to lower column densities
than in the static case, which vastly increases the heating in the partially
molecular gas due to photoionization of He^0, H_2, and H^0. This causes a
significant fraction of the incident bolometric flux to be re-radiated as
thermally excited infrared H_2 lines, with the lower excitation pure rotational
lines arising in 1000 K gas and higher excitation H_2 lines arising in 2000 K
gas, as is required to explain the H_2 spectrum of the Helix cometary knots.Comment: 4 pages, accepted by ApJL, scheduled December 20 issu
Alternative derivation of the Feigel effect and call for its experimental verification
A recent theory by Feigel [Phys. Rev. Lett. {\bf 92}, 020404 (2004)] predicts
the finite transfer of momentum from the quantum vacuum to a fluid placed in
strong perpendicular electric and magnetic fields. The momentum transfer arises
because of the optically anisotropic magnetoelectric response induced in the
fluid by the fields. After summarising Feigel's original assumptions and
derivation (corrected of trivial mistakes), we rederive the same result by a
simpler route, validating Feigel's semi-classical approach. We then derive the
stress exerted by the vacuum on the fluid which, if the Feigel hypothesis is
correct, should induce a Poiseuille flow in a tube with maximum speed m/s (2000 times larger than Feigel's original prediction). An experiment
is suggested to test this prediction for an organometallic fluid in a tube
passing through the bore of a high strength magnet. The predicted flow can be
measured directly by tracking microscopy or indirectly by measuring the flow
rate (ml/min) corresponding to the Poiseuille flow. A second
experiment is also proposed whereby a `vacuum radiometer' is used to test a
recent prediction that the net force on a magnetoelectric slab in the vacuum
should be zero.Comment: 20 pages, 1 figures. revised and improved versio
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
Anisotropic and long-range vortex interactions in two-dimensional dipolar Bose gases
We perform a theoretical study into how dipole-dipole interactions modify the
properties of superfluid vortices within the context of a two-dimensional
atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts
like a giant anti-dipole, changing the density profile and generating an
effective dipolar potential centred at the vortex core whose most slowly
decaying terms go as and . These effects modify
the vortex-vortex interaction which, in particular, becomes anisotropic for
dipoles polarized in the plane. Striking modifications to vortex-vortex
dynamics are demonstrated, i.e. anisotropic co-rotation dynamics and the
suppression of vortex annihilation.Comment: PRL accepted, 6 pages, 5 figure
Classical versus quantum dynamics of the atomic Josephson junction
We compare the classical (mean-field) dynamics with the quantum dynamics of
atomic Bose-Einstein condensates in double-well potentials. The quantum
dynamics are computed using a simple scheme based upon the Raman-Nath
equations. Two different methods for exciting a non-equilbrium state are
considered: an asymmetry between the wells which is suddenly removed, and a
periodic time oscillating asymmetry. The first method generates wave packets
that lead to collapses and revivals of the expectation values of the
macroscopic variables, and we calculate the time scale for these revivals. The
second method permits the excitation of a single energy eigenstate of the
many-particle system, including Schroedinger cat states. We also discuss a band
theory interpretation of the energy level structure of an asymmetric
double-well, thereby identifying analogies to Bloch oscillations and Bragg
resonances. Both the Bloch and Bragg dynamics are purely quantum and are not
contained in the mean-field treatment.Comment: 31 pages, 14 figure
Collective excitation frequencies and stationary states of trapped dipolar Bose-Einstein condensates in the Thomas-Fermi regime
We present a general method for obtaining the exact static solutions and
collective excitation frequencies of a trapped Bose-Einstein condensate (BEC)
with dipolar atomic interactions in the Thomas-Fermi regime. The method
incorporates analytic expressions for the dipolar potential of an arbitrary
polynomial density profile, thereby reducing the problem of handling non-local
dipolar interactions to the solution of algebraic equations.
We comprehensively map out the static solutions and excitation modes,
including non-cylindrically symmetric traps, and also the case of negative
scattering length where dipolar interactions stabilize an otherwise unstable
condensate. The dynamical stability of the excitation modes gives insight into
the onset of collapse of a dipolar BEC. We find that global collapse is
consistently mediated by an anisotropic quadrupolar collective mode, although
there are two trapping regimes in which the BEC is stable against quadrupole
fluctuations even as the ratio of the dipolar to s-wave interactions becomes
infinite. Motivated by the possibility of fragmented BEC in a dipolar Bose gas
due to the partially attractive interactions, we pay special attention to the
scissors modes, which can provide a signature of superfluidity, and identify a
long-range restoring force which is peculiar to dipolar systems. As part of the
supporting material for this paper we provide the computer program used to make
the calculations, including a graphical user interface.Comment: 23 pages, 11 figure
Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
[Abridged] We use new HST and archived images to clarify the nature of the
knots in the Helix Nebula. We employ published far infrared spectrophotometry
and existing 2.12 micron images to establish that the population distribution
of the lowest ro-vibrational states of H2 is close to the distribution of a gas
in LTE at 988 +- 119 K. We derive a total flux from the nebula in H2 lines and
compare this with the power available from the central star for producing this
radiation. We establish that neither soft X-rays nor FUV radiation has enough
energy to power the H2 radiation, only the stellar EUV radiation shortward of
912 Angstrom does. Advection of material from the cold regions of the knots
produces an extensive zone where both atomic and molecular hydrogen are found,
allowing the H2 to directly be heated by Lyman continuum radiation, thus
providing a mechanism that can explain the excitation temperature and surface
brightness of the cusps and tails. New images of the knot 378-801 reveal that
the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas
zone. This firmly establishes that the "tail" structure is an ionization
bounded radiation shadow behind the optically thick core of the knot. A unique
new image in the HeII 4686 Angstrom line fails to show any emission from knots
that might have been found in the He++ core of the nebula. We also re-examined
high signal-to-noise ratio ground-based telescope images of this same inner
region and found no evidence of structures that could be related to knots.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full resolution version is available at
http://www.ifront.org/wiki/Helix_Nebula_2007_Pape
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