9,603 research outputs found
Do we really know how to derive the basic PNe parameters?
How well do we know the physical/chemical properties of PNe? 1D (CLOUDY) and
3D (MOCASSIN) photoionisation codes are used in this contribution to model the
PNe K 4-47 and NGC 7009 as an attempt to question whether or not the high Te
(higher than 21,000K) of the K 4-47's core and the N overabundance of the outer
knots in NGC 7009 are real.
These are very basic parameters, obtained for Galactic PNe, e.g. nearby
objects, even though with large uncertainties. Based on the comparison of the
modelling with, mainly, optical images and long-slit spectroscopic data, it is
suggested here that K 4-47 high Te can be explained if its core is composed of
a very dense and small inner region --that matches the radio measurements-- and
a lower density outer core --matching the optical observations. This approach
can account for the strong auroral emission lines [OIII]4363A and [NII]5755A
observed, and so for the high temperatures. This teaches us that the assumption
of a homogeneous distribution of the gas is completely wrong for the core of
such PN.
In the case of NGC 7009 a simple 3D model that reproduces the observed
geometry of this nebula is constructed. The aim of this modelling was to
explore the possibility that the enhanced [NII] emission observed in the outer
knots may be due to ionisation effects instead to a local N overabundance. Here
it is discussed the model that can best reproduce the observations employing a
homogeneous set of abundances throughout the nebula, not only for nitrogen but
also for all the other elements considered.Comment: 4 pages; talk presented in the international conference Planetary
Nebulae as Astronomical Tools; June 28 - July 02, 2005; Gdansk, Poland; Eds:
R. Szczerba, G. Stasinska, and S. K. Gorny; AIP Conference Proceeding
Soft-excess in ULX spectra: disc emission or wind absorption?
We assess the claim that Ultra-luminous X-ray sources (ULXs) host
intermediate-mass black holes (BH) by comparing the cool disc-blackbody model
with a range of other models, namelly a more complex physical model based on a
power-law component slightly modified at various energies by smeared
emission/absorption lines from highly-ionized gas. Our main conclusion is that
the presence of a soft excess, or a soft deficit, depends entirely on the
energy range to which we choose to fit the ``true'' power-law continuum; hence,
we argue that those components should not be taken as evidence for accretion
disc emission, nor used to infer BH masses. We speculate that bright ULXs could
be in a spectral state similar to (or an extension of) the steep-power-law
state of Galactic BH candidates, in which the disc is completely comptonized
and not directly detectable, and the power-law emission may be modified by the
surrounding, fast-moving, ionized gas.Comment: To appear in the proceedings of "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs. Observations",
Cefalu', Sicily, June 11-24, 2006 (AIP). Compilation needs specific AIP .clo,
.cls, .sty and .tex files (included along with the paper .tex file and
figures
Investigating the effects of the QCD dynamics in the neutrino absorption by the Earth's interior at ultrahigh energies
The opacity of the Earth to incident ultra high energy neutrinos is directly
connected with the behaviour of the neutrino - nucleon () cross
sections in a kinematic range utterly unexplored. In this work we investigate
how the uncertainties in due the different QCD dynamic models
modify the neutrino absorption while they travel across the Earth. In
particular, we compare the predictions of two extreme scenarios for the high
energy behaviour of the cross section, which are consistent with the current
experimental data. The first scenario considered is based on the solution of
the linear DGLAP equations at small- and large-, while the second one
take into account the unitarity effects in the neutrino - nucleon cross section
by the imposition of the Froissart bound behaviour in the nucleon structure
functions at large energies. Our results indicate that probability of
absorption and the angular distribution of neutrino events are sensitive to the
the QCD dynamics at ultra high energies.Comment: 6 pages, 3 figures. Improved version to be published in Physical
Review
Heavy Quark Production in Ultra High Energy Cosmic Ray Interactions
In this paper we present a comprehensive study of the heavy quark production
in ultra high energy cosmic ray interactions in the atmosphere considering that
the primary cosmic ray can be either a photon, neutrino or a proton. The
analysis is performed using a unified framework -- the dipole formalism --- and
the saturation effects, associated to the physical process of parton
recombination, are taken into account. We demonstrate that the contribution of
heavy quarks for cosmic ray interactions is in general non-negligible and can
be dominant depending of the process considered. Moreover, our results indicate
that new dynamical mechanisms should be included in order to obtain reliable
predictions for the heavy quark production in collisions at ultra high
cosmic ray energies.Comment: 8 pages, 5 figures. Enlarged version to be published in Astroparticle
Physic
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