2,370 research outputs found
Chandra view of Kes 79: a nearly isothermal SNR with rich spatial structure
A 30 ks \chandra ACIS-I observation of Kes 79 reveals rich spatial
structures, including many filaments, three partial shells, a loop and a
``protrusion''. Most of them have corresponding radio features. Regardless of
the different results from two non-equilibrium ionization (NEI) codes,
temperatures of different parts of the remnant are all around 0.7 keV, which is
surprisingly constant for a remnant with such rich structure. If thermal
conduction is responsible for smoothing the temperature gradient, a lower limit
on the thermal conductivity of 1/10 of the Spitzer value can be derived.
Thus, thermal conduction may play an important role in the evolution of at
least some SNRs. No spectral signature of the ejecta is found, which suggests
the ejecta material has been well mixed with the ambient medium. From the
morphology and the spectral properties, we suggest the bright inner shell is a
wind-driven shell (WDS) overtaken by the blast wave (the outer shell) and
estimate the age of the remnant to be 6 kyr for the assumed dynamics.
Projection is also required to explain the complicated morphology of Kes 79.Comment: 12 pages, 6 figures (3 in color), ApJ, in press, April 20, 200
Are the Models for Type Ia Supernova Progenitors Consistent with the Properties of Supernova Remnants?
We explore the relationship between the models for progenitor systems of Type
Ia supernovae and the properties of the supernova remnants that evolve after
the explosion. Most models for Type Ia progenitors in the single degenerate
scenario predict substantial outflows during the presupernova evolution.
Expanding on previous work, we estimate the imprint of these outflows on the
structure of the circumstellar medium at the time of the supernova explosion,
and the effect that this modified circumstellar medium has on the evolution of
the ensuing supernova remnant. We compare our simulations with the
observational properties of known Type Ia supernova remnants in the Galaxy
(Kepler, Tycho, SN 1006), the Large Magellanic Cloud (0509-67.5, 0519-69.0,
N103B), and M31 (SN 1885). We find that optically thick outflows from the white
dwarf surface (sometimes known as accretion winds) with velocities above 200
km/s excavate large low-density cavities around the progenitors. Such large
cavities are incompatible with the dynamics of the forward shock and the X-ray
emission from the shocked ejecta in all the Type Ia remnants that we have
examined.Comment: To appear in ApJ. 17 pages, 10 figures, emulateap
Tails of the Unexpected: The Interaction of an Isothermal Shell with a Cloud
A new mechanism for the formation of cometary tails behind dense clouds or
globules is discussed. Numerical hydrodynamical models show that when a dense
shell of swept-up matter overruns a cloud, material in the shell is focussed
behind the cloud to form a tail. This mode of tail formation is completely
distinct from other methods, which involve either the removal of material from
the cloud, or shadowing from a strong, nearby source of ionization. This
mechanism is relevant to the cometary tails seen in planetary nebulae and to
the interaction of superbubble shells with dense clouds.Comment: 6 pages, 6 figures, accepted for publication in MNRAS letter
Temperature Control of Node Appearance and Initiation in Soybean
Soybean demand remains strong and continues to grow as a source of protein and oil for food and feed. Soybean production is expanding into cooler and warmer environments, thus, it becomes critical to expand the current knowledge about the influence of temperature on soybean. Temperature is the main environmental factor effecting node appearance rate (NAR) and node initiation rate (NIR), which are key parameters controlling soybean growth and development. This study aims to assess the response of NAR and NIR to temperature and investigate the coordination between these two processes under controlled and field conditions. Two greenhouse experiments with four contrasting mean temperatures (15-26ºC) and two field experiments with different sowing dates (April 23 to June 19) were conducted using maturity groups 2.1 and 3.0. The number of appeared nodes was measured every 2 to 7 days from sowing to ca. R5 to estimate NAR (nodes d-1). Stem apex was dissected every 4 to 7 days from sowing to ca. R3 to estimate NIR (nodes d-1). A co-ordination model was fitted between the number of initiated nodes and number of appeared nodes. Significant relationship was found in response to temperature of NIR and NAR. A constant plastochron of 36ºCd and dual value for phyllochron (83 and 58ºCd) depending on ontogeny was observed, with base temperature of ca. 10ºC for both processes. There is a strong two-phase co-ordination between node initiation and node appearance. This work established the response of NAR and NIR to temperature which could improve prediction of phenology, leaf area, and yield by the current soybean simulation models.
Advisor: Patricio Grassin
Spatial patterns and biodiversity in rock-paper-scissors models with regional unevenness
Climate changes may affect ecosystems destabilising relationships among
species. We investigate the spatial rock-paper-scissors models with a regional
unevenness that reduces the selection capacity of organisms of one species. Our
results show that the regionally weak species predominates in the local
ecosystem, while spiral patterns appear far from the region, where individuals
of every species play the rock-paper-scissors game with the same strength.
Because the weak species controls all local territory, it is attractive for the
other species to enter the local ecosystem to conquer the territory. However,
our stochastic simulations show that the transitory waves formed when organisms
of the strong species reach the region are quickly destroyed because of local
strength unbalance in the selection game rules. Computing the effect of the
topology on population dynamics, we find that the prevalence of the weak
species becomes more significant if the transition of the selection capacity to
the area of uneven rock-paper-scissors rules is smooth. Finally, our findings
show that the biodiversity loss due to the arising of regional unevenness is
minimised if the transition to the region where the cyclic game is unbalanced
is abrupt. Our results may be helpful to biologists in comprehending the
consequences of changes in the environmental conditions on species coexistence
and spatial patterns in complex systems.Comment: 17 pages, 7 figure
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