1,122 research outputs found
OH emission from cometary knots in planetary nebulae
We model the molecular emission from cometary knots in planetary nebulae
(PNe) using a combination of photoionization and photodissociation region (PDR)
codes, for a range of central star properties and gas densities. Without the
inclusion of ionizing extreme ultraviolet (EUV) radiation, our models require
central star temperatures to be near the upper limit of the range
investigated in order to match observed H and OH surface brightnesses
consistent with observations - with the addition of EUV flux, our models
reproduce observed OH surface brightnesses for .
For , the predicted OH surface brightness is much
lower, consistent with the non-detection of this molecule in PNe with such
central star temperatures. Our predicted level of H emission is somewhat
weaker than commonly observed in PNe, which may be resolved by the inclusion of
shock heating or fluorescence due to UV photons. Some of our models also
predict ArH and HeH rotational line emission above detection
thresholds, despite neither molecule having been detected in PNe, although the
inclusion of photodissociation by EUV photons, which is neglected by our
models, would be expected to reduce their detectability.Comment: Accepted by MNRAS, 11 pages, 15 figures. Author accepted manuscript.
Accepted on 24/04/18. Deposited on 27/04/1
Modelling the ArH emission from the Crab Nebula
We have performed combined photoionization and photodissociation region (PDR)
modelling of a Crab Nebula filament subjected to the synchrotron radiation from
the central pulsar wind nebula, and to a high flux of charged particles; a
greatly enhanced cosmic ray ionization rate over the standard interstellar
value, , is required to account for the lack of detected [C I]
emission in published Herschel SPIRE FTS observations of the Crab Nebula. The
observed line surface brightness ratios of the OH and ArH transitions
seen in the SPIRE FTS frequency range can only be explained with both a high
cosmic ray ionization rate and a reduced ArH dissociative recombination
rate compared to that used by previous authors, although consistent with
experimental upper limits. We find that the ArH/OH line strengths and
the observed H vibration-rotation emission can be reproduced by model
filaments with cm,
and visual extinctions within the range found for dusty globules in the Crab
Nebula, although far-infrared emission from [O I] and [C II] is higher than the
observational constraints. Models with cm
underpredict the H surface brightness, but agree with the ArH and
OH surface brightnesses and predict [O I] and [C II] line ratios consistent
with observations. These models predict HeH rotational emission above
detection thresholds, but consideration of the formation timescale suggests
that the abundance of this molecule in the Crab Nebula should be lower than the
equilibrium values obtained in our analysis.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on
05/09/2017. Deposited on 05/09/1
The apparent anticorrelation between the mass opacity of interstellar dust and the surface density of interstellar gas
Recent analyses of Herschel observations suggest that in nearby disc galaxies the dust mass opacity at 500μm, κ500, decreases with increasing gas surface density, ΣISM. This apparent anticorrelation between κ500 and ΣISM is opposite to the behaviour expected from theoretical dust evolution models; in such models, dust in denser, cooler regions (i.e. regions of increased ΣISM) tends to grow and therefore to have increased κ500. We show, using a toy model, that the presence of a range of dust temperatures along the line of sight can lead to spuriously low estimated values of κ500. If in regions of higher ΣISM the range of dust temperatures extends to lower values (as seems likely), the magnitude of this effect may be sufficient to explain the apparent anticorrelation between κ500 and ΣISM. Therefore there may not be any need for spatial variation in the intrinsic dust properties that run counter to theoretical expectations
Molecular line signatures of cloud-cloud collisions
Collisions between interstellar gas clouds are potentially an important
mechanism for triggering star formation. This is because they are able to
rapidly generate large masses of dense gas. Observationally, cloud collisions
are often identified in position-velocity (PV) space through bridging features
between intensity peaks, usually of CO emission. Using a combination of
hydrodynamical simulations, time-dependent chemistry, and radiative transfer,
we produce synthetic molecular line observations of overlapping clouds that are
genuinely colliding, and overlapping clouds that are just chance
superpositions. Molecules tracing denser material than CO, such as NH and
HCN, reach peak intensity ratios of and with respect to CO in the
`bridging feature' region of PV space for genuinely colliding clouds. For
overlapping clouds that are just chance superpositions, the peak NH and HCN
intensities are co-located with the CO intensity peaks. This represents a way
of confirming cloud collisions observationally, and distinguishing them from
chance alignments of unrelated material.Comment: 7 pages, 4 figures, MNRAS accepte
Line emission from filaments in molecular clouds
Filamentary structures are often identified in column density maps of
molecular clouds, and appear to be important for both low- and high-mass star
formation. Theoretically, these structures are expected to form in regions
where the supersonic cloud-scale turbulent velocity field converges. While this
model of filament formation successfully reproduces several of their properties
derived from column densities, it is unclear whether it can also reproduce
their kinematic features. We use a combination of hydrodynamical, chemical and
radiative transfer modelling to predict the emission properties of these
dynamically-forming filaments in the CO, HCN and NH
rotational lines. The results are largely in agreement with observations; in
particular, line widths are typically subsonic to transonic, even for filaments
which have formed from highly supersonic inflows. If the observed filaments are
formed dynamically, as our results suggest, no equilibrium analysis is
possible, and simulations which presuppose the existence of a filament are
likely to produce unrealistic results.Comment: 9 pages, 9 figures. MNRAS accepte
An efficient method for determining the chemical evolution of gravitationally collapsing prestellar cores
We develop analytic approximations to the density evolution of prestellar
cores, based on the results of hydrodynamical simulations. We use these
approximations as input for a time-dependent gas-grain chemical code to
investigate the effects of differing modes of collapse on the molecular
abundances in the core. We confirm that our method can provide reasonable
agreement with an exact numerical solution of both the hydrodynamics and
chemistry while being significantly less computationally expensive, allowing a
large grid of models varying multiple input parameters to be run. We present
results using this method to illustrate how the chemistry is affected not only
by the collapse model adopted, but also by the large number of unknown physical
and chemical parameters. Models which are initially gravitationally unstable
predict similar abundances despite differing densities and collapse timescales,
while ambipolar diffusion produces more extended inner depleted regions which
are not seen in observations of prestellar cores. Molecular observations are
capable of discriminating between modes of collapse despite the unknown values
of various input parameters. We also investigate the evolution of the ambipolar
diffusion timescale for a range of collapse modes, metallicities and cosmic ray
ionization rates, finding that it remains comparable to or larger than the
collapse timescale during the initial stages for all models we consider, but
becomes smaller at later evolutionary stages. This confirms that ambipolar
diffusion is an important process for diffuse gas, but becomes less significant
as cores collapse to higher densities.Comment: Accepted by AJ. Author accepted manuscript. Accepted 29/05/2018,
deposited 05/06/201
The state of the upper mantle beneath Southern Africa
We present a new upper mantle seismic model for southern Africa based on the fitting of a large (3622 waveforms) multi-mode surface wave data set with propagation paths significantly shorter (≤ 6000 km) than those in globally-derived surface wave models. The seismic lithosphere beneath the cratonic region of southern Africa in this model is about 175 ± 25 km thick, consistent with other recent surface wave models, but significantly thinner than indicated by teleseismic body-wave tomography. We determine the in situ geotherm from kimberlite nodules from beneath the same region and find the thermal lithosphere model that best fits the nodule data has a mechanical boundary layer thickness of 186 km and a thermal lithosphere thickness of 204 km, in very good agreement with the seismic measurement. The shear wave velocity determined from analyzes of the kimberlite nodule compositions agree with the seismic shear wave velocity to a depth of not, vert, similar150 km. However, the shear wave velocity decrease at the base of the lid seen in the seismic model does not correspond to a change in mineralogy. Recent experimental studies of the shear wave velocity in olivine as a function of temperature and period of oscillation demonstrate that this wave speed decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is close to the melting temperature (within not, vert, similar100 °C)
Cogeneration Technology Alternatives Study (CTAS). Volume 5: Cogeneration systems results
The use of various advanced energy conversion systems is examined and compared with each other and with current technology systems for savings in fuel energy, costs, and emissions in individual plants and on a national level. About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. The methodology and results of matching the cogeneration energy conversion systems to approximately 50 industrial processes are described. Results include fuel energy saved, levelized annual energy cost saved, return on investment, and operational factors relative to the noncogeneration base cases
Cogeneration Technology Alternatives Study (CTAS). Volume 2: Analytical approach
The use of various advanced energy conversion systems were compared with each other and with current technology systems for their savings in fuel energy, costs, and emissions in individual plants and on a national level. The ground rules established by NASA and assumptions made by the General Electric Company in performing this cogeneration technology alternatives study are presented. The analytical methodology employed is described in detail and is illustrated with numerical examples together with a description of the computer program used in calculating over 7000 energy conversion system-industrial process applications. For Vol. 1, see 80N24797
The mass, location and heating of the dust in the Cassiopeia A supernova remnant
We model the thermal dust emission from dust grains heated by synchrotron
radiation and by particle collisions, under conditions appropriate for four
different shocked and unshocked gas components of the Cassiopeia A (Cas A)
supernova remnant (SNR). By fitting the resulting spectral energy distributions
(SEDs) to the observed SNR dust fluxes, we determine the required mass of dust
in each component. We find the observed SED can be reproduced by of silicate grains, the majority of which is in the unshocked
ejecta and heated by the synchrotron radiation field. Warmer dust, located in
the X-ray emitting reverse shock and blastwave regions, contribute to the
shorter wavelength infrared emission but make only a small fraction of the
total dust mass. Carbon grains can at most make up of the total
dust mass. Combined with estimates for the gas masses, we obtain dust-to-gas
mass ratios for each component, which suggest that the condensation efficiency
in the ejecta is high, and that dust in the shocked ejecta clumps is well
protected from destruction by sputtering in the reverse shock.Comment: Accepted by MNRAS, 14 pages, 6 figures. Author accepted manuscript.
Accepted on 05/02/2019. Deposited on 05/02/201
- …