322 research outputs found
Three-Dimensional Ionisation, Dust RT and Chemical Modelling of Planetary Nebulae
The assumption of spherical symmetry is not justified for the vast majority
of PNe. The interpretation of spatially-resolved observations cannot rely
solely on the application of 1D codes, which may yield incorrect abundances
determinations resulting in misleading conclusions. The 3D photoionisation code
MOCASSIN (Monte CAarlo SimulationS of ionised Nebulae) is designed to remedy
these shortcomings. The 3D transfer of both primary and secondary radiation is
treated self-consistently without the need of approximations. The code was
benchmarked and has been applied to the study of several PNe. The current
version includes a fully self-consistent radiative transfer treatment for dust
grains mixed within the gas, taking into account the microphysics of dust-gas
interactions within the geometry-independent Monte Carlo transfer. The new code
provides an excellent tool for the self-consistent analysis of dusty ionised
regions showing asymmetries and/or density and chemical inhomogeneities. Work
is currently in progress to incorporate the processes that dominate the thermal
balance of photo-dissociation regions (PDRs), as well as the formation and
destruction processes for all the main molecular species.Comment: 3 pages, to appear in Proc. IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge
Univ. Press
The long-term evolution of photoevaporating transition discs with giant planets
Photo-evaporation and planet formation have both been proposed as mechanisms
responsible for the creation of a transition disc. We have studied their
combined effect through a suite of 2d simulations of protoplanetary discs
undergoing X-ray photoevaporation with an embedded giant planet. In a previous
work we explored how the formation of a giant planet triggers the dispersal of
the inner disc by photo-evaporation at earlier times than what would have
happened otherwise. This is particularly relevant for the observed transition
discs with large holes and high mass accretion rates that cannot be explained
by photo-evaporation alone. In this work we significantly expand the parameter
space investigated by previous simulations. In addition, the updated model
includes thermal sweeping, needed for studying the complete dispersal of the
disc. After the removal of the inner disc the disc is a non accreting
transition disc, an object that is rarely seen in observations. We assess the
relative length of this phase, to understand if it is long lived enough to be
found observationally. Depending on the parameters, especially on the X-ray
luminosity of the star, we find that the fraction of time spent as a
non-accretor greatly varies. We build a population synthesis model to compare
with observations and find that in general thermal sweeping is not effective
enough to destroy the outer disc, leaving many transition discs in a relatively
long lived phase with a gas free hole, at odds with observations. We discuss
the implications for transition disc evolution. In particular, we highlight the
current lack of explanation for the missing non-accreting transition discs with
large holes, which is a serious issue in the planet hypothesis.Comment: 11 pages, 5 figures; accepted by MNRA
The interplay between X-ray photoevaporation and planet formation
We assess the potential of planet formation instigating the early formation
of a photoevaporation driven gap, up to radii larger than typical for
photoevaporation alone. For our investigation we make use of hydrodynamics
models of photoevaporating discs with a giant planet embedded. We find that, by
reducing the mass accretion flow onto the star, discs that form giant planets
will be dispersed at earlier times than discs without planets by X-ray
photoevaporation. By clearing the portion of the disc inner of the planet
orbital radius, planet formation induced photoevaporation (PIPE) is able to
produce transition disc that for a given mass accretion rate have larger holes
when compared to standard X-ray photoevaporation. This constitutes a possible
route for the formation of the observed class of accreting transition discs
with large holes, which are otherwise difficult to explain by planet formation
or photoevaporation alone. Moreover, assuming that a planet is able to filter
dust completely, PIPE produces a transition disc with a large hole and may
provide a mechanism to quickly shut down accretion. This process appears to be
too slow however to explain the observed desert in the population of transition
disc with large holes and low mass accretion rates.Comment: 11 pages, 10 figures, accepted by MNRAS on 31/12/201
Mocassin: A fully three-dimensional Monte Carlo photoionization code
The study of photoionized environments is fundamental to many astrophysical
problems. Up to the present most photoionization codes have numerically solved
the equations of radiative transfer by making the extreme simplifying
assumption of spherical symmetry. Unfortunately very few real astronomical
nebulae satisfy this requirement. To remedy these shortcomings, a
self-consistent, three-dimensional radiative transfer code has been developed
using Monte Carlo techniques. The code, Mocassin, is designed to build
realistic models of photoionized nebulae having arbitraries geometry and
density distributions with both the stellar and diffuse radiation fields
treated self-consistently. In addition, the code is capable of tretating on or
more exciting stars located at non-central locations. The gaseous region is
approximated by a cuboidal Cartesian grid composed of numerous cells. The
physical conditions within each grid cell are determined by solving the thermal
equilibrium and ionization balance equations This requires a knowledge of the
local primary and secondary radiation fields, which are calculated
self-consistently by locally simulating the individual processes of ionization
and recombination. The main structure and computational methods used in the
Mocassin code are described in this paper. Mocassin has been benchmarked
against established one-dimensional spherically symmetric codes for a number of
standard cases, as defined by the Lexington/Meudon photoionization workshops
(Pequignot et al., 1986; Ferland et al., 1995; Pequignot et al.,
2001)\citep{pequignot86,ferland95, pequignot01}. The results obtained for the
benchmark cases are satisfactory and are presented in this paper. A performance
analysis has also been carried out and is discussed here.Comment: 17 pages, 4 figures, 1 appendix Changes: appendix adde
Potato R1 resistance gene confers resistance against Phytophthora infestans in transgenic tomato plants
Tomato is challenged by several pathogens which cause loss of production. One such pathogen is the oomycete Phytophthora infestans which is able to attack all the aerial parts of the plant. Although a wide range of resistance sources are available, genetic control of this disease is not yet successful. Pyramiding R-genes through genetic transformation could be a straightforward way to produce tomato and potato lines carrying durable resistance to P. infestans. In this work the R1 potato gene was transferred into tomato lines. The tomato transgenic lines were analyzed by using q-RT-PCR and progeny segregation to determine the gene copy number. To test the hypothesis that R1 represents a specifically regulated R-gene, transgenic tomato plants were inoculated with P. infestans isolate 88133 and IPO. All the plants containing the R1 gene were resistant to the late blight isolate IPO-0 and susceptible to isolate 88133. These results provide evidence for specific activation of the R1 gene during pathogen challenge. Furthermore, evidence for enhancement of PR-1 gene expression during P. infestans resistance response was obtained
Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra
We present coefficients for the calculation of the continuous emission spectra of HI, HeI and HeII due to electron-ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the n=20 threshold of hydrogen (36.5um), and for temperatures 100 K <= Te <=10^5 K. The emission coefficients for HeI are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1% in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionisation/plasma codes and should aid with the interpretation of spectra from the very cold ionised gas phase inferred to exist in a number of gaseous clouds
Disc Clearing of Young Stellar Objects: Evidence for Fast Inside-out Dispersal
The time-scale over which and the modality by which young stellar objects
(YSOs) disperse their circumstellar discs dramatically influences the eventual
formation and evolution of planetary systems. By means of extensive radiative
transfer (RT) modelling, we have developed a new set of diagnostic diagrams in
the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the
classification of the evolutionary stage of YSOs from photometric observations.
Our diagrams allow the differentiation of sources with unevolved (primordial)
discs from those evolving according to different clearing scenarios (e.g.
homologous depletion vs. inside-out dispersal), as well as from sources that
have already lost their disc. Classification of over 1500 sources in 15 nearby
star-forming regions reveals that approximately 39 % of the sources lie in the
primordial disc region, whereas between 31 % and 32 % disperse from the
inside-out and up to 22 % of the sources have already lost their disc. Less
than 2 % of the objects in our sample lie in the homogeneous draining regime.
Time-scales for the transition phase are estimated to be typically a few 10^5
years independent of stellar mass. Therefore, regardless of spectral type, we
conclude that currently available infrared photometric surveys point to fast
(of order 10 % of the global disc lifetime) inside-out clearing as the
preferred mode of disc dispersal.Comment: 31 pages, 21 figures, 6 tables, accepted for publication in MNRA
HYPERION: An open-source parallelized three-dimensional dust continuum radiative transfer code
HYPERION is a new three-dimensional dust continuum Monte-Carlo radiative
transfer code that is designed to be as generic as possible, allowing radiative
transfer to be computed through a variety of three-dimensional grids. The main
part of the code is problem-independent, and only requires an arbitrary
three-dimensional density structure, dust properties, the position and
properties of the illuminating sources, and parameters controlling the running
and output of the code. HYPERION is parallelized, and is shown to scale well to
thousands of processes. Two common benchmark models for protoplanetary disks
were computed, and the results are found to be in excellent agreement with
those from other codes. Finally, to demonstrate the capabilities of the code,
dust temperatures, SEDs, and synthetic multi-wavelength images were computed
for a dynamical simulation of a low-mass star formation region. HYPERION is
being actively developed to include new features, and is publicly available
(http://www.hyperion-rt.org).Comment: Accepted for publication in Astronomy & Astrophysics. HYPERION is
being prepared for release at the start of 2012, but you can already sign up
to the mailing list at http://www.hyperion-rt.org to be informed once it is
available for downloa
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