4,549 research outputs found
Continuum and Spectral Line Radiation from a Random Clumpy Medium
We present a formalism for continuum and line emission from random clumpy
media together with its application to problems of current interest, including
CO spectral lines from ensembles of clouds and radio emission from HII regions,
supernovae and star-forming regions. For line emission we find that the effects
of clump opacity on observed line ratios can be indistinguishable from
variations of intrinsic line strengths, adding to the difficulties in
determining abundances from line observations. Our formalism is applicable to
arbitrary distributions of cloud properties, provided the cloud volume filling
factor is small; numerical simulations show it to hold up to filling factors of
about 10%. We show that irrespective of the complexity of the cloud ensemble,
the radiative effect of clumpiness can be parametrized at each frequency by a
single multiplicative correction to the overall optical depth; this multiplier
is derived from appropriate averaging over individual cloud properties. Our
main finding is that cloud shapes have only a negligible effect on radiation
propagation in clumpy media; the results of calculations employing point-like
clouds are practically indistinguishable from those for finite-size clouds with
arbitrary geometrical shapes.Comment: ApJ, to be publishe
A Circumstellar Disc in a High-Mass Star Forming Region
We present an edge-on Keplerian disc model to explain the main component of
the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N.
The brightness distribution and spectrum of the line of bright masers are
successfully modeled with high amplification of background radio continuum
emission along velocity coherent paths through a maser disc. The bend seen in
the position-velocity diagram is a characteristic signature of differentially
rotating discs. For a central mass of 30 solar masses, suggested by other
observations, our model fixes the masing disc to have inner and outer radii of
about 350 AU and 1000 AU.Comment: 11 pages, accepted for publication in ApJ Letter
Continuum and Spectral Line Radiation from a Random Clumpy Medium
We present a formalism for continuum and line emission from random clumpy media together with its application to problems of current interest, including CO spectral lines from ensembles of clouds and radio emission from H ii regions, supernovae, and star-forming regions. For line emission, we find that the effects of clump opacity on observed line ratios can be indistinguishable from variations of intrinsic line strengths, adding to the difficulties in determining abundances from line observations. Our formalism is applicable to arbitrary distributions of cloud properties, provided the cloud volume filling factor is small; numerical simulations show it to hold up to filling factors of ~10%. We show that irrespective of the complexity of the cloud ensemble, the radiative effect of clumpiness can be parameterized at each frequency by a single multiplicative correction to the overall optical depth; this multiplier is derived from appropriate averaging over individual cloud properties. Our main finding is that cloud shapes have only a negligible effect on radiation propagation in clumpy media; the results of calculations employing point-like clouds are practically indistinguishable from those for finite-sized clouds with arbitrary geometrical shapes
Resolution of the Compact Radio Continuum Sources in Arp220
We present 2 cm and 3.6 cm wavelength very long baseline interferometry
images of the compact radio continuum sources in the nearby ultra-luminous
infrared galaxy Arp220. Based on their radio spectra and variability
properties, we confirm these sources to be a mixture of supernovae (SNe) and
supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both
wavelengths. The SNe generally only have upper size limits. In contrast all the
SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent
with them having just entered their Sedov phase while embedded in an
interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the
diameter-luminosity correlation for SNRs (and so also on the diameter-surface
brightness relation) and extend these correlations to very small sources. The
data are consistent with the relation L {\propto} D^{-9/4}. Revised
equipartition arguments adjusted to a magnetic field to relativistic particle
energy density ratio of 1% combined with a reasonable synchrotron-emitting
volume filling factor of 10% give estimated magnetic field strengths in the SNR
shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from
compression of ambient ISM fields and must instead be internally generated. We
set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in
relativistic particles, 2%-20% of the explosion kinetic energy, is consistent
with estimates from models that fit the IR-radio correlation in compact
starburst galaxies.Comment: 16 pages, 5 figure
A thin ring model for the OH megamaser in IIIZw35
We present a model for the OH megamaser emission in the starburst galaxy
IIIZw35. The observed diffuse and compact OH maser components in this source
are explained by a single phase of unsaturated clumpy gas distributed in a thin
ring structure and amplifying background continuum. We emphasize the importance
of clumpiness in the OH masing medium, an effect that has not been fully
appreciated previously.
The model explains why multiple bright spots are seen only at the ring
tangents while smoother emission is found elsewhere. Both the observed velocity
gradients and the line to continuum ratios around the ring enquire a geometry
where most of the seed photons come from a continuum emission which lies
outside the OH ring. To explain both the OH and continuum brightness, free-free
absorbing gas is required along the ring axis to partially absorb the far side
of the ring. It is proposed that the required geometry arises from an inwardly
propagating ring of starburst activity
Mass wasting triggered by seasonal CO<sub>2</sub> sublimation under Martian atmospheric conditions: Laboratory experiments
Sublimation is a recognized process by which planetary landscapes can be modified. However, interpretation of whether sublimation is involved in downslope movements on Mars and other bodies is restricted by a lack of empirical data to constrain this mechanism of sediment transport and its influence on landform morphology. Here we present the first set of laboratory experiments under Martian atmospheric conditions which demonstrate that the sublimation of CO2 ice from within the sediment body can trigger failure of unconsolidated, regolith slopes and can measurably alter the landscape. Previous theoretical studies required CO2 slab ice for movements, but we find that only frost is required. Hence, sediment transport by CO2 sublimation could be more widely applicable (in space and time) on Mars than previously thought. This supports recent work suggesting CO2 sublimation could be responsible for recent modification in Martian gullies
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