4,913 research outputs found
Properties of the ionized gas in HH202. I: Results from integral field spectroscopy with PMAS
We present results from integral field spectroscopy with the Potsdam
multi-Aperture Spectrograph of the head of the Herbig-Haro object HH 202 with a
spatial sampling of 1"x1". We have obtained maps of different emission lines,
physical conditions --such as electron temperature and density-- and ionic
abundances from recombination and collisionally excited lines. We present the
first map of the Balmer temperature and of the temperature fluctuation
parameter, t^2. We have calculated the t^2 in the plane of the sky, which is
substantially smaller than that determined along the line of sight. We have
mapped the abundance discrepancy factor of O^{2+}, ADF(O^{2+}), finding its
maximum value at the HH 202-S position. We have explored the relations between
the ADF(O^{2+}) and the electron density, the Balmer and [O III] temperatures,
the ionization degree as well as the t^2 parameter. We do not find clear
correlations between these properties and the results seem to support that the
ADF and t^2 are independent phenomena. We have found a weak negative
correlation between the O^{2+} abundance determined from recombination lines
and the temperature, which is the expected behaviour in an ionized nebula,
hence it seems that there is not evidence for the presence of super-metal rich
droplets in H II regions.Comment: 12 pages, 11 figures. Accepted for publication in MNRA
Structure formation during the collapse of a dipolar atomic Bose-Einstein condensate
We investigate the collapse of a trapped dipolar Bose-Einstein condensate.
This is performed by numerical simulations of the Gross-Pitaevskii equation and
the novel application of the Thomas-Fermi hydrodynamic equations to collapse.
We observe regimes of both global collapse, where the system evolves to a
highly elongated or flattened state depending on the sign of the dipolar
interaction, and local collapse, which arises due to dynamically unstable
phonon modes and leads to a periodic arrangement of density shells, disks or
stripes. In the adiabatic regime, where ground states are followed, collapse
can occur globally or locally, while in the non-adiabatic regime, where
collapse is initiated suddenly, local collapse commonly occurs. We analyse the
dependence on the dipolar interactions and trap geometry, the length and time
scales for collapse, and relate our findings to recent experiments.Comment: In this version (the published version) we have slightly rewritten
the manuscript in places and have corrected some typos. 15 pages and 13
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
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
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
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
Aerosol information content analysis of multi-angle high spectral resolution measurements and its benefit for high accuracy greenhouse gas retrievals
New generations of space-borne spectrometers for the retrieval of atmospheric abundances of greenhouse gases require unprecedented accuracies as atmospheric variability of long-lived gases is very low. These instruments, such as GOSAT and OCO-2, typically use a high spectral resolution oxygen channel (O_2 A-band) in addition to CO_2 and CH_4 channels to discriminate changes in the photon path-length distribution from actual trace gas amount changes. Inaccurate knowledge of the photon path-length distribution, determined by scatterers in the atmosphere, is the prime source of systematic biases in the retrieval. In this paper, we investigate the combined aerosol and greenhouse gas retrieval using multiple satellite viewing angles simultaneously. We find that this method, hitherto only applied in multi-angle imagery such as from POLDER or MISR, greatly enhances the ability to retrieve aerosol properties by 2–3 degrees of freedom. We find that the improved capability to retrieve aerosol parameters significantly reduces interference errors introduced into retrieved CO_2 and CH_4 total column averages. Instead of focussing solely on improvements in spectral and spatial resolution, signal-to-noise ratios or sampling frequency, multiple angles reduce uncertainty in space based greenhouse gas retrievals more effectively and provide a new potential for dedicated aerosols retrievals
Cloud fragmentation and proplyd-like features in HII regions imaged by HST
We have analyzed HST ACS and WFPC2 new and archival images of eight HII
regions to look for new proto-planetary disks (proplyds) similar to those found
in the Orion Nebula. We find a wealth of features similar in size (though many
are larger) to the bright cusps around the Orion Nebula proplyds. None of them,
however, contains a definitive central star. From this, we deduce that the new
cusps may not be proplyds, but instead are fragments of molecular cloud
material. Out of all the features found in the eight HII regions examined, only
one, an apparent edge-on silhouette in M17, may have a central star. This
feature might join the small number of bona fide proplyds found outside the
Orion Nebula, in M8, M20 and possibly in M16. In line with the results found
recently by Smith et al. (2005), the paucity of proplyds outside the Orion
Nebula, may be explained by their transient nature as well as by the specific
environmental conditions under whichthey can be observed.Comment: 51 pages; 19 figures; 5 tables. Accepted by A
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